Introduction
Comprehensive atmospheric chemistry transport models must constantly evolve
to address the increasing complexity arising from emerging applications that
treat multi-pollutant interactions on urban to hemispheric spatial scales and
hourly to annual temporal scales. To assist with the design of emission
control strategies that comply with more stringent air quality standards,
such models must accurately simulate ambient pollutant levels across the
entire spectrum ranging from background to extreme concentrations. The
adverse impacts of airborne pollutants are not confined to a region or even
a continent (NRC, 2009). Both observational (e.g., Andrea et al., 1988;
Fishman et al., 1991; Jaffe et al., 1999; Zhang et al., 2008; Uno et al.,
2009) and modeling studies (e.g., Jacob et al., 1999; Fiore et al., 2009;
HTAP, 2010) have demonstrated that pollutants near the Earth's surface can be
convectively lofted to higher altitudes where strong winds can efficiently
transport them from one continent to another, thereby impacting air quality
on intercontinental to global scales.
As air quality standards are tightened, the need to quantify the
contributions of long-range transport to local pollution becomes increasingly
important. Limited-area models such as the Community Multiscale Air Quality
(CMAQ) (Byun and Schere, 2006; Foley et al., 2010; Appel et al., 2017) have
played a central role in guiding the development and implementation of the
National Ambient Air Quality Standards (NAAQS). These models are now being
routinely applied to examine variability in surface-level air pollutants
across the continental United States over annual cycles. Since transport is efficient in
the free troposphere (FT) and since simulations over continental scales and annual
cycles provide sufficient opportunity for “atmospheric turnover”, i.e.,
exchange between the FT and the boundary layer (BL), it can be
argued that accurate simulation of the variability in free-tropospheric
pollutant concentrations is important for the model's ability to capture the
variability in surface-level concentrations, especially at moderate to low
concentrations. Based on typical advective timescales, it can further be
postulated that in limited-area chemistry transport models, this
free-tropospheric variability in simulated concentrations is largely dictated
by the specification of lateral boundary conditions (LBCs). This is
exemplified in Fig. 1, which illustrates the influence of LBC specification on
simulated surface-level concentrations across a typical regional modeling
domain covering the contiguous United States; the space- and time-varying LBCs themselves
were derived from the global Integrated Forecasting System of the European Center
for Medium-Range Weather Forecasts (ECMWF) (Flemming et al., 2015). In these
calculations, three tracer species were added to CMAQ to track the ozone
(O3) LBC specified for three vertical zones: (i) surface to
750 hPa, nominally representing the atmospheric BL,
(ii) 750–250 hPa representing the FT, and
(iii) 250–50 hPa (the model top) representing possible stratospheric
influences (see Mathur et al., 2008). These tracer species were subject to
transport processes associated with 3-D advection, turbulent mixing in the
vertical and horizontal diffusion, and cloud transport on resolved and
sub-grid scales. The tracer species were deposited at the surface using
space- and time-varying deposition velocity estimates for O3, and
they were also subjected to wet scavenging and rainout processes that mimic
modeled O3. If the tracer background is defined as the amount of the
tracer imported into the regional domain, then the sum of the three simulated
tracers can be viewed as the modeled background for this species. Since we
did not include chemical sinks for the tracers and since the intent here is
to assess the relative influence of LBC, we examine the distribution of the
normalized concentrations (normalized by the domain maximum value across all
seasons) in Fig. 1a–d. Significant spatial and seasonal variability is seen
in the estimated tracer background levels, with higher normalized
concentration values in the high-elevation regions of the intermountain
west. Additionally, higher background levels are estimated in the warmer
seasons, with the highest levels during spring. More importantly, the free-tropospheric concentrations dominate the surface background levels
(Fig. 1e–h). During summer, across most of the continental United States more than half
(and up to 90 %) of the tracer background originated in the FT. Though
significant seasonal variability is noted in the FT fractional contributions
to surface-level background concentrations, the contributions are still
substantial during other seasons and expectedly the highest contributions are
seen in the high-elevation regions of the western United States across all seasons.
These results clearly illustrate the importance of accurately characterizing
the long-range transport that occurs in the FT and its
influence on surface background pollution levels via subsidence and
entrainment into the BL. It can be expected that pollutants with
atmospheric lifetimes greater than a few days would exhibit similar
characteristics, thereby highlighting the need to accurately characterize
long-range transport influences on regional model simulations spanning
seasonal to annual timescales.
Impact of lateral boundary conditions (LBCs) on simulated seasonal
surface-level concentrations. (a–d) Spatial
variation in seasonal-mean surface concentrations normalized by the maximum value within the model domain across all seasons.
(e–h) Fractional contribution of free-tropospheric (FT) LBCs (specified between 750 and 250 hPa) to the total
LBC-derived concentrations at the surface. Seasons are defined as winter (December–February), spring (March–May), summer
(June–August), and fall (September–November).
One approach to capturing the effects of long-range transport in regional
models is through deriving space- and time-varying LBCs from global chemistry
transport models. However, efforts linking regional- and global-scale models
have met with mixed success because biases in the global model can propagate
and influence regional calculations and often confound interpretation of
model results (e.g., Tang et al., 2008; Schere et al., 2012). Additionally,
inconsistencies in process representations, species mapping, and grid
structures could also introduce errors in the model linkage if not examined
and handled carefully. A modeling framework is thus needed wherein
interactions between processes occurring on various spatial and temporal
scales can be consistently examined. Expanding comprehensive regional models
to the hemispheric scale enables a consistent representation of atmospheric
processes across spatial and temporal scales. Motivated by this need, the
applicability of the CMAQ modeling system has been extended to hemispheric
scales through systematic investigation of key model processes and attributes
influencing simulated distributions of O3, fine particulate matter
(PM2.5), and precursor species. The hemispheric modeling system
also facilitates the examination of linked air pollution–climate across a region
in the context of the changing global atmosphere.
Section 2 overviews the key CMAQ model structural attributes and process
representations that were refined to fully simulate the Northern Hemisphere.
Section 3 summarizes a variety of applications with the hemispheric CMAQ
configuration, highlights the model performance relative to a variety of
observational data sets, and identifies aspects that would benefit from
further model development. A variety of surface, aloft, and remotely sensed
observations used to guide and evaluate the model changes are presented in
Sects. 2 and 3. Lastly, Sect. 4 summarizes the current model state and
discusses future development and applications of the hemispheric CMAQ.
(a) The Northern Hemisphere modeling domain discretized
using a 108 km resolution grid. The shaded region shows the extent of
the typical continental US nested domain discretized using
a 12 km resolution horizontal grid. (b) Comparison of two
layer configurations used to discretize the vertical extent ranging from the
surface to 50 hPa.
Model setup and process enhancements
Atmospheric chemistry and transport of trace species occur across the
continuum of spatial and temporal scales. For instance, transport across
intercontinental to hemispheric scales occurs over timescales ranging from
days to months, which influences the distribution of trace species with
lifetimes within this range. Transport on these scales can also influence
shorter-lived radical budgets through chemical reactions involving
intermediate-lived species, especially reservoir species such as organic
nitrates. Thus, the expansion of CMAQ to hemispheric scales
required reexamination of process
representations and grid structures so that interactions amongst various
processes occurring over the disparate scales is adequately captured. The key
changes to CMAQ that were considered in this effort are summarized below.
Impact of layer configuration on simulated mean O3 vertical
profiles for August 2006 at selected locations for a case involving zero-out
of emissions across the United States: (a) Trinidad Head, CA;
(b) Boulder, CO; (c) Huntsville, AL; and
(d) Narragansett, RI.
Domain and grid configuration
CMAQ's governing three-dimensional equations for species mass conservation
and moment dynamics (number, surface area, and volume) describing modes of
particulate size distribution are cast in generalized coordinates (see Mathur
et al., 2005; Byun and Schere, 2006). This formulation allows CMAQ to
accommodate horizontal map projections and vertical coordinates from various
meteorological models. This flexibility enables CMAQ to be used on
a horizontal domain covering the Northern Hemisphere set on a polar
stereographic projection (Fig. 2a) without altering CMAQ or its input–output
file structure. Polar stereographic projections are also used in the Danish
Eulerian Hemispheric Model (Brandt et al., 2012) and a hemispheric version of
the CHIMERE model (Mailler et al., 2017). Current WRF and CMAQ hemispheric
applications have utilized a horizontal discretization of a 187×187
grid configuration with a grid spacing of 108 km and
terrain-following σ vertical coordinate vertical coordinate system.
Current regional modeling applications with CMAQ typically utilize 35 layers
of variable thickness to resolve the model vertical extent between the
surface and 50 hPa. Longer-term calculations over the Northern
Hemisphere must be able to capture potential impacts of
stratosphere–troposphere exchange (STE) as well as that between the FT and
the BL. At altitudes above 10 km (Fig. 2b), the 35-layer
configuration has relatively coarse resolution with layer thickness
> 1.5 km and the topmost layer is nearly 4 km deep. To
improve the representation of three-dimensional transport processes on
modeled vertical profiles, the vertical resolution employed in hemispheric
CMAQ calculations is increased. The revised layer structure uses 44 layers,
with significantly finer resolution above the BL (Fig. 2b) to better
represent long-range transport in the FT, STE processes, and influences from
cloud mixing on both the sub-grid and resolved scales. The impacts of using
these alternate layer configurations are illustrated in Fig. 3 for a case in
which only emissions across the United States were zeroed out to isolate the
impacts of model vertical resolution on representing the downward transport
of pollutants in the region. Both the 35-layer and 44-layer model simulations
were initialized with the same conditions in mid-February 2006, utilized
a constant potential vorticity (PV) scaling to specify O3 in the
model top layer (discussed further in Sect. 2.5), and were driven by
meteorological information from the Weather Research and Forecasting (WRF)
model simulations using the respective layer configurations (discussed in
Sect. 2.2). Figure 3 shows systematically higher simulated O3 below
10 km in the 35-layer configuration compared to the 44-layer
configuration, indicating that the coarser vertical resolution will likely
overestimate the downward transport of both long-range transport effects as
well as stratospheric influences.
Coupling of WRF and CMAQ and initialization
To minimize interpolation error and to avoid introducing mass imbalances,
hemispheric simulations with CMAQ inherit the projection and grid structure
from the WRF model, which provides the driving meteorological fields. In
applications presented in Sect. 3, meteorological inputs for grid nudging
used in WRF over the Northern Hemisphere domain were derived from the
NCEP/NCAR Reanalysis data (Kalnay et al., 1996), which have 2.5∘
spatial and 6 h temporal resolution; other reanalysis products such as
Global Forecast System (GFS) can also be used instead. Surface reanalysis
based on a fusion of the NCEP/NCAR Reanalysis and NCEP Automated Data
Processing (ADP) Operational Global Surface Observations on the WRF grid
using the NCAR distributed objective analysis tool OBSGRID
(http://www2.mmm.ucar.edu/wrf/users/docs/user_guide_V3.1/users_guide_chap7.htm#techniques)
is used for the indirect soil moisture and temperature nudging in the
Pleim–Xiu land surface model (Pleim and Gilliam, 2009). The WRF
configuration over the Northern Hemisphere also used MODIS land use
classification with 20 categories, RRTMG shortwave radiation (SWR) and
longwave radiation schemes (Iacono et al., 2008), and the ACM2 planetary BL
model (Pleim, 2007). WRF's simulation of hourly surface temperature, relative
humidity, and wind speed and direction was evaluated by Xing et al. (2015a)
through comparison with observations from NOAA's National Centers for
Environmental Information (NCEI) Integrated Surface Data and no significant
bias in the meteorological fields was detected. WRF and CMAQ can be run
either in the traditional off-line sequential manner or in the coupled mode
with or without aerosol feedback effects (Mathur et al., 2010; Wong et al.,
2012).
Comparisons of simulated average vertical profiles of O3
with ozonesonde measurements at Trinidad Head, California,
USA: (a) March 2006 (4 months after start of simulation) and (b) August 2006 (9 months after start of
simulation). Also shown is ±1 SD of the observed mixing ratios. “Profile IC” uses the default profile for initialization as in
regional CMAQ applications, “Clean IC” is the case in which the model is spun up from clean conditions, and “Revised NTR” is the
simulation with “Clean IC” with updates to the physical and chemical sinks for the species NTR representing organic nitrates.
Expectedly, the application over expanded space and timescales necessitates
closer attention to model chemical initialization, especially in the FT wherein typical residence times for most atmospheric pollutants of
concern are long enough so that initial conditions can persist. If the FT is
poorly represented, model predictions within the BL will be
adversely affected. Thus, unlike regional simulations with CMAQ, which are
initialized with a prescribed vertical profile for different species or with
concentration fields derived from global chemistry transport models, for
hemispheric applications it is recommended that CMAQ be initialized to
“clean” tropospheric background values and allowed to build up based on the
model emissions, physics, and chemistry. The impact of these different
initializations is illustrated through comparisons of the cases denoted
Profile IC and Clean IC in Fig. 4, which compares monthly mean model and
observed O3 profiles at Trinidad Head, CA. In the Profile IC case,
a vertical O3 profile that monotonically increased from
35 ppb at the surface to 100 ppb at model top was used for
initialization. The clean IC case initialized O3 at 30 ppb
through the model column. In both cases, the hemispheric model simulations
were initiated on 1 November 2015. Initial conditions for all other chemical
species were based on clean tropospheric conditions prescribed in Byun and
Ching (1999). Large overestimations in O3 through the troposphere are
noted for the Profile IC simulation and arise from the profile used to
initialize O3 in the mid-troposphere. In contrast, the Clean IC
case, wherein the model was initialized to clean tropospheric background
values and allowed to build up based on the model physics and chemistry,
resulted in much better agreement with the measured profile during spring
(Fig. 4a); however, by summer, overestimations developed (discussed further
in Sect. 2.4.1). Note that the Clean IC case also utilized a fixed PV scaling for O3 at the model top, described further in
Sect. 2.5. Also, the similarity in simulated O3 profiles for the Clean
IC and Profile IC cases by August, 9 months after the start of the
simulation, suggests the diminishing impact of initialization. Based on these
results, the inherent seasonality in atmospheric transport and chemistry, and
practices employed in previous global chemistry transport model applications
(e.g., Fiore et al., 2009), a spin-up of 12 months from clean tropospheric
conditions is recommended for new CMAQ applications over the Northern
Hemisphere. Additional future studies would be helpful to further constrain
this spin-up period recommendation.
Similar to regional applications, chemical boundary conditions also need to
be specified along the discrete lateral boundaries of the hemispheric domain.
In current applications, these are set to the same values as in the Clean IC case
discussed above. Note that the boundaries of the hemispheric domain (shown in
Fig. 2) are in the area encircling the Earth near the Equator. Because of the
presence of the intertropical convergence zone in this region, the
mixing of air masses originating in the Northern and Southern hemispheres
occurs relatively slowly, with exchange times of typically about 1 year
(e.g., Jacob et al., 1987). Since the atmospheric lifetimes of most modeled
species are significantly shorter, any impacts of chemical lateral boundary
condition specification are typically confined to the lower latitudes and do
not propagate into the domain. Additional model sensitivity tests should
however be conducted in the future to quantify any likely seasonal influence
of LBC specification on model predictions in lower-latitude regions of the
Northern Hemisphere.
Emissions
Specifying emissions across the Northern Hemisphere is challenging because
the distributions and compositions of emissions across the globe are rapidly
changing and because emissions are poorly quantified in many regions. In
addition, simulations of CMAQ on broader spatial scales are influenced by
emissions from marine environments (which are less prominent in
regional and continental applications) and intercontinental transport of other
sources (e.g., windblown dust). Changes to emissions used by CMAQ for the
Northern Hemisphere application are described below.
Global emission inventories
Two primary sources of global emission estimates have been used in
hemispheric CMAQ applications to date. The first is based on a global
emission inventory compiled by Argonne National Laboratory in support of the
ARCTAS pre-mission planning and includes estimates for anthropogenic,
international shipping, and biomass burning
(http://bio.cgrer.uiowa.edu/arctas/emission.html). This inventory was
used in early testing of the hemispheric CMAQ model (e.g., Mathur et al.,
2014) and will be referred to as the ARCTAS inventory in subsequent
discussions. More recent applications have relied on year-specific estimates
from the EDGAR (Emission Database for Global Atmospheric Research,
version 4.2; European Commission, 2011) database, which reports emissions for
17 anthropogenic sectors and large-scale biomass burning on a 0.1∘×0.1∘ resolution grid. Since EDGARv4.2 provides only
PM10 emissions, PM2.5 emissions were estimated by
deriving the ratio of PM2.5 to PM10 from the 2000–2005
EDGAR HTAP (Hemispheric Transport of Air Pollution, version 1) inventory
(Janssens-Maenhout et al., 2012) and then applying this ratio to partition
EDGARv4.2 PM10 emissions into PM2.5 and
PM2.5–10 (Xing et al., 2015a). In applications to date, biogenic
volatile organic compound (VOC; Guenther et al., 1995) and lightning
NOx (Price et al., 1997) emissions were obtained from GEIA
(Global Emissions Inventory Activity; http://www.geiacenter.org). The
monthly biogenic VOC emissions were further temporalized to hourly resolution
for each simulation day. Monthly lightning NOx emissions
were distributed evenly to each hour of each simulation day. Xing
et al. (2015a) further describe the processing of global emission inventories
for CMAQ, including temporalization of the annual estimates to hourly model
inputs, vertical distributions of anthropogenic and lightning emissions, and
speciation of PM2.5 and non-methane volatile organic compound
emissions to model primary aerosol constituents and gas-phase species.
Emissions of NO from soil or SO2 from volcanos are not considered in
the applications presented here. It should also be noted that several efforts
are underway to harmonize regional emission estimates and incorporate them
into global emission inventories with improved spatial and temporal
resolution (e.g., Janssens-Maenhout et al., 2015). Furthermore, the SMOKE
modeling system typically used to prepare emissions for regional CMAQ
applications has recently been updated to support hemispheric CMAQ
applications to allow for a more streamlined implementation of the various
emission processing steps described above (Eyth et al., 2016).
Windblown dust
The windblown dust emission parameterization employed in CMAQ (Tong et al.,
2008) was adapted for hemispheric applications by making two primary
modifications. First, the mapping for land use categories representing
potentially erodible dust sources was updated to map the categories of the
MODIS land use types used in the hemispheric WRF-CMAQ configuration. Second,
the threshold friction velocity (above which dust emissions occur due to wind
action) for desert regions was reduced to mobilize sufficient episodic dust
emissions over the Sahara. The original value for threshold friction
velocity, derived from the work of Gillette et al. (1980), was based on data
from the Mojave Desert. However, Li et al. (2007) suggest a much lower (about
half) threshold friction velocity based on dust samples from the northern
China desert. Fu et al. (2014) found that the default threshold friction
velocity for loose, fine-grained soil with low surface roughness was too high
for Asian dust sources and that reducing it to the Li et al. (2007) values
yielded much better agreement of simulated airborne dust relative to
observations. We found similar underestimations in PM2.5
concentrations and aerosol optical depth (AOD) over the Sahara with the default values and have thus
followed an approach similar to Fu et al. (2014) in the hemispheric CMAQ
applications presented here. Concurrent with the development of this paper,
a newer physics-based windblown dust emission parameterization was developed
and implemented in CMAQ, and that parameterization includes a dynamic
relation for the surface roughness length relevant to small-scale dust
generation processes (Foroutan et al., 2017). The new dust emission
parameterization is currently being tested for hemispheric applications and
will be available in future public releases of CMAQ.
Emissions in marine environments
Natural emissions of particulate matter and gas-phase species from the oceans
can impact air quality in coastal regions, influence global burdens of
atmospheric trace species and radiative budgets, and modulate lifetimes of
tropospheric O3 thereby influencing its long-range transport.
A detailed representation of sea-spray particle emissions and chemistry is
already available in CMAQ (Kelly et al., 2010), and it can be used for
hemispheric-scale applications without any modifications.
Reactive halogen emissions can play an important role in dictating lifetimes
of O3 in marine environments. Parameterizations to estimate marine
emissions of bromine- and iodine-containing compounds for the three categories
(halocarbons, inorganic bromine, and inorganic iodine) were developed for
inclusion in the hemispheric CMAQ. The halocarbons include five bromocarbons
(CHBr3, CH2Br2, CH2BrCl, CHBrCl2,
CHBr2Cl) and four iodocarbons (CH3I, CH2ICl,
CH2IBr, CH2I2). The halocarbon emissions are estimated
using monthly average climatological chl a concentrations derived from the
Moderate Resolution Imaging Spectroradiometer (MODIS). Sarwar et al. (2015)
provide details on estimating halogen emission and comparisons with other
existing estimates.
Enhancements to gas-phase chemistry
The 2005 Carbon Bond Mechanism with updated toluene chemistry (CB05TU; Sarwar
et al., 2011), commonly used in regional CMAQ applications, was also used for
initial hemispheric-scale applications. Important enhancements to CB05TU were
implemented to improve (1) its ability to represent multi-day chemistry
associated with cycling of NOx through reservoir organic
nitrate species in the mechanism and (2) representation of chemical sinks for
tropospheric O3 due to halogen-mediated chemistry in marine
environments. Additionally, the more detailed RACM2 mechanism has also been
implemented (Sarwar et al., 2013) to facilitate its use in follow-on
hemispheric applications.
Organic nitrate lifetime
Organic nitrates form during the atmospheric photo-degradation of
hydrocarbons in the presence of nitrogen oxides (NOx)
through reactions of peroxy-alkyl radicals (RO2) with NO as well as
through reactions with NO3, and they act as a reservoir for oxides of
nitrogen. In the CB05TU mechanism, the species NTR is used to represent organic nitrates.
Depending on its modeled lifetime, NTR can potentially redistribute
NOx from source regions to
NOx-sensitive remote areas where additional ozone may be
produced. Representing inert and reservoir organic nitrate species in
condensed mechanisms used in chemistry transport models is challenging (e.g.,
Kasibhatla et al., 1997) since they can dramatically influence simulated
O3 and NOy distributions. In the CB05TU
implementation in CMAQ, the chemical sinks for NTR include photolysis
(producing NO2) and reaction with OH (producing HNO3).
Additionally, defining a Henry's law constant for a single lumped species
representing several alkyl nitrates such as NTR is challenging. In previous
CMAQ versions, the Henry's law constant for peroxyacetyl nitrate (PAN) was also used for NTR,
resulting in its very slow removal either through scavenging by clouds or
through dry deposition at the Earth's surface. However, the Henry's law
constants for several alkyl nitrates and hydroxyalkyl nitrates have been
suggested to be much higher (some comparable to HNO3), especially
those that are of biogenic origin (Shepson et al., 1996; Treves and Rudich,
2003). On the hemispheric scale, organic nitrates formed from isoprene are
the largest contributor to the simulated tropospheric NTR burden and can
consequently modulate the simulated tropospheric O3 burden. Based on
recent work by Xie et al. (2013), we updated the rate constant for the
NTR + OH reaction to that for isoprene nitrates. The Henry's law constant
for NTR was also mapped to that of HNO3, thereby enhancing wet
scavenging of NTR. Additionally, the dry deposition velocity for NTR was
mapped to that for HNO3. Collectively, these changes result not only
in faster NOx recycling from NTR but also faster removal
of NTR through the enhancement of its dry deposition and wet scavenging
physical sinks.
The impacts of these changes to representing NTR in CMAQ on simulated
O3 distributions are illustrated in Fig. 4, which presents
a comparison of monthly mean profiles of simulated O3 mixing ratios
for various cases with ozonesonde measurements at Trinidad Head, California,
a site nominally representing inflow conditions to North America. The
comparisons shown in Fig. 4 illustrate the relatively large effects of
modulating the resultant NTR burden on the simulated O3 distribution
through much of the lower to mid-troposphere, especially during summer when
isoprene emissions are high. In limited-area calculations with the CB05TU
mechanism, it is likely that the NTR produced is transported out of the
regional domains before it can significantly alter O3 production.
However, over the spatial and temporal scales of northern hemispheric
calculations, NOx recycled from NTR can modulate the
simulated background O3; consequently, accurate characterization of
its sources and sinks becomes critical. Thus, the hemispheric calculations
provide a framework for examining the role of various physical and chemical
processes in atmospheric chemical budgets in a consistent manner.
Additional improvements to representing NTR in the CB05TU mechanism in CMAQ
are underway. In particular, replacing the single alkyl nitrate species (NTR)
in CB05TU with seven species to better capture the range of chemical
reactivity and Henry's law constants (and thus the physical sinks) is being
investigated (Schwede et al., 2014; Appel et al., 2017). Early comparisons of
this expanded treatment with the simpler changes discussed above suggest that
the approximations invoked through mapping the OH reactivity to isoprene
nitrates (from Xie et al., 2013) and mimicking NTR's wet and dry removal
rates to HNO3, yield simulated O3 distributions similar to
the ones obtained from the expanded treatment.
Representation of marine environments
More than half of the Northern Hemisphere is covered by oceans. To accurately
represent the intercontinental transport of pollutants, it is important to
accurately represent how continental air masses evolve as they traverse the
vast oceanic regions. The fate of O3 in marine environments directly
affects inflow to continental regions and background O3
concentrations. Though O3 photolysis in the presence of high water
vapor results in chemical O3 loss and is well quantified, additional
loss of O3 in these environments through deposition as well as
chemical reactions with halogens emitted from the ocean is expected (Vogt
et al., 1999; Read et al., 2008) but still uncertain and not represented in
most tropospheric models. In expanding CMAQ to hemispheric scales, particular
attention was devoted to the role of deposition and halogen chemistry in
marine environments, which can serve as sinks for O3 exported from
continental outflow and in transit to other regions via long-range transport.
An enhanced O3 deposition treatment that accounts for the interaction
of iodide in seawater with O3 was implemented (Sarwar et al., 2015)
and found to increase deposition velocities in marine environments by an
order of magnitude. In addition, the gas-phase chemical mechanisms were
expanded to include 25 chemical reactions involving 7 chlorine species
(Sarwar et al., 2012), 39 chemical reactions involving 14 bromine species,
and 53 chemical reactions involving 17 iodine species (Sarwar et al., 2015).
Alternate gas-phase mechanism
As discussed in Sect. 2.4.1, characterizing multi-day chemistry and
long-lived reservoir species is important for representing the long-range
transport of pollutants and their distributions on hemispheric scales. To
enable practical model applications over extended spatial and temporal
scales, the chemical mechanisms must be sufficiently condensed to run
efficiently while faithfully representing the chemistry over the space and
timescales modeled. However, the impacts on model predictions of using
different condensation rules and assumptions on species lumping and
intermediate compounds are largely unquantified. To enable such investigation
in the future over the hemispheric scale, an alternate chemical mechanism,
RACM2 has also been implemented and tested in the hemispheric CMAQ (Sarwar
et al., 2013). The RACM2 mechanism is designed to simulate remote to polluted
conditions from the Earth's surface through the upper troposphere (Goliff
et al., 2013). It consists of 363 chemical reactions including 33 photolytic
reactions among 120 chemical species.
Representing impacts of stratosphere–troposphere exchange on O3 distributions
Though the role of cross-tropopause transport of O3 is acknowledged
as a significant contributor to the tropospheric O3 budget, the
distribution of O3 in the troposphere that originates from the
stratosphere is still uncertain. Tightening O3 NAAQS and decreasing
amounts of photochemically derived O3 due to continuously declining
anthropogenic precursor emissions across large parts of North America and
Europe now puts greater emphasis on accurately characterizing the fraction of
O3 in the troposphere, especially at the surface, that is of
stratospheric origin. For instance, Roelofs and Lelieveld (1997), using
a global chemistry transport model, estimated that stratospheric contributions
to surface O3 varied between 10 and 60 % depending on season and
location. Clearly this fraction varies spatially and seasonally in response
to the tropopause height, and perhaps even more episodically, from deep
intrusion events associated with weather patterns and frontal movement.
PV has been shown to be a robust indicator of air mass
exchange between the stratosphere and the troposphere, with strong positive
correlation with O3 and other trace species transported from the
stratosphere to the upper troposphere (Danielsen, 1968). Numerous modeling
studies have used this correlation to develop scaling factors that specify
O3 in the modeled upper troposphere–lower stratosphere (UTLS) based
on estimated PV. The reported O3 / PV ratios (e.g., Ebel et al.,
1991; Carmichael et al., 1998; McCaffery et al., 2004), however, exhibit a wide
range, 20–100 ppbPVu-1
(1 PV unit =10-6 m2 Kkg-1s-1), and vary
as a function of location, altitude, and season. Based on extensive ozonesonde
measurements available during the summer of 2006 from the IONS network
(http://croc.gsfc.nasa.gov/intexb/ions06.html) and PV fields from the
WRF model matched to the location and time of the ozonesonde launch, we
examined the UTLS O3–PV correlation at sites across North America. At
12 sites with a sufficient number (11 or greater) of launches during
August 2006, strong linear correlations (r2>0.7) were noted, with
slopes of the linear regression varying between 20 and
39 ppbPVu-1 (Mathur et al., 2008). Based on this analysis, in
the initial implementation of STE impacts on tropospheric O3 in
hemispheric CMAQ, we scale the space- and time-varying model-estimated PV in
the topmost model layer with a scaling factor of 20 ppbPVu-1 to
specify O3 at the model top. This initial conservative choice for the
O3 / PV ratio was in part dictated by lack of additional
information on seasonality and also by the relatively coarse model resolution
in the UTLS. As indicated in Fig. 3, layer configuration influences the
representation of STE and subsequent simulation of 3-D O3
distributions. Thus, the initial conservative choice of
20 ppbPVu-1 was motivated by the desire to reduce any possible
effects of excessive and artificial downward entrainment associated with
inadequate vertical model resolution.
To overcome some of these challenges and to develop a more robust
representation of STE impacts, we have recently developed a dynamic
O3–PV function based on 21-year ozonesonde records from the World Ozone
and Ultraviolet Radiation Data Centre (WOUDC) with corresponding PV values
from WRF-CMAQ simulation across the Northern Hemisphere from 1990 to 2010.
Analyses of PV and ozonesonde data suggest strong spatial and seasonal
variations in O3 / PV ratios, which exhibit large values in the
upper layers and in high-latitude regions, with the highest values in spring and
the lowest values in autumn over an annual cycle. The new generalized
parameterization, detailed in Xing et al. (2016a), can dynamically represent
O3 in the UTLS across the Northern Hemisphere. The implementation of
the new function significantly improves CMAQ's simulation of UTLS O3
in both magnitude and seasonality compared to observations, which results in
a more accurate simulation of the vertical distribution of O3 across
the Northern Hemisphere (Xing et al., 2016a). These hemispheric O3
fields can then be used to derive more realistic vertically and temporally
varying LBCs for regional nested model calculations.
Hemispheric-scale applications, analysis, and evaluation
The hemispheric CMAQ model is now being used for a variety of process-based
air pollution studies across the Northern Hemisphere over seasonal (e.g.,
Mathur et al., 2014; Sarwar et al., 2014) to decadal (Xing et al., 2015a)
timescales. In this section we present an overview of these diverse evolving
applications with the hemispheric CMAQ model. In some instances, the
applications have been detailed before, but the analysis summarized here
builds upon that previous work and those distinctions are stated in the
individual application discussion. Detailed comparisons of model predictions
of pollutant concentrations (and radiative properties) with corresponding
observations are conducted to establish credibility in the model's use in
these applications that range from representing episodic long-range pollutant
transport to quantifying long-term trends in air pollution across the
Northern Hemisphere, to emerging applications examining air pollution–climate
interactions. Model applications are performed over the hemispheric domain
and 44-layer structure illustrated in Fig. 2. The CMAQ configuration is based
on version 5.0 (CMAQv5.0) with the process updates detailed in Sect. 2. Two
sets of applications are analyzed and evaluated in the subsequent discussion:
(1) a 21-year simulation over 1990–2010 (in Sect. 3.5 and 3.6) and
(2) process sensitivity studies for the calendar year 2006, which were each
initialized in September 2005 using fields from the prior 21-year simulation
set (in Sect. 3.1–3.4).
Comparing model predictions with measurements from the 2006 INTEX-B campaign
The Intercontinental Chemical Transport Experiment-B (INTEX-B) was
a NASA-led, multi-partner atmospheric field experiment conducted in the
spring of 2006. A major objective of the second phase of the campaign during
17 April–15 May 2006 was characterizing the long-range transport and
evolution of Asian pollution and implications for air quality across western
North America (Singh et al., 2009). Airborne measurements of a variety of
trace species were made over the remote Pacific as well as along the inflow
region to western North America from aircrafts with extensive measurement instruments onboard and
provide a unique data set to test and evaluate the ability of hemispheric
CMAQ to represent the 3-D structure of air pollutants as they are transported
long distances across the Pacific Ocean to eventually impact US background
pollution levels.
The NASA DC-8 flights during 17 April–1 May 2006 were based out of Honolulu,
Hawaii, and sampled the subtropical Pacific, while the flights during
4–15 May 2006 based out of Anchorage, Alaska, sampled the troposphere over
the subarctic Pacific region. Figure 5a–d present comparisons of modeled
and observed O3 mixing ratios along selected flight paths of the DC-8
aircraft; several of these flights were designed to sample aged and fresh
Asian pollution over the Pacific (Table 5a in Singh et al., 2009). Modeled
mixing ratios were extracted by flying the aircraft through the 3-D
modeling domain; the spatial locations of the aircraft were mapped to the
model grid, whereas hourly model output was temporally interpolated to the
time of the measurement. Figure 5 shows results from three different CMAQ
configurations aimed at isolating the impacts of STE and marine halogen
chemistry on simulated 3-D O3 distributions. Differences between the
simulations denoted “PV + Halogen” and “Halogen, NoPV” are used to
estimate the O3 sources due to modeled STE processes, while
differences between the simulations PV + Halogen and “PV,
NoHalogen” help quantify the model O3 sinks due to halogen chemistry
in marine environments. Note that the simulation PV, NoHalogen employed
the CB05TU mechanism, while the other two simulations employed a version of
the CB05TU mechanism augmented with the halogen chemistry discussed in
Sect. 2.4.2. Comparisons of model predictions from the PV + Halogen
simulation with observations along the various flight paths suggest that the
model exhibits skill in capturing the vertical variations in O3
observed in the region. The simulation that did not employ any PV scaling
(green trace in Fig. 5) systematically underestimates O3. The
improved comparisons with measurements along the different flight paths for
the PV + Halogen simulation suggest that O3 in the lower to mid-troposphere in this region during this period is often influenced by sizeable
contributions from the stratosphere, and these enhancements are generally
captured by the simulation employing the PV scaling.
Comparison of simulated 3-D distributions of O3 mixing
ratios with observations from the DC-8 aircraft during the
INTEX-B field campaign: (a–d) comparison of observed O3 and simulated values from various model configurations
along flight tracks on specific days; (e) comparison of model and observed campaign average O3 vertical profile for
flights over the subtropical Pacific during 17 April–1 May 2006; (f) comparison of model and observed campaign average
O3 vertical profile for flights over the subarctic Pacific during 1–15 May 2006. Also shown in panels (e) and
(f) is ±1 SD for the observed values. Also shown in panels (a–d) is the aircraft altitude along the flight path.
The model's ability to simulate the regionally averaged vertical profiles
sampled by the aircraft over the subtropical and subarctic Pacific is
illustrated in Fig. 5e and f, respectively. In constructing these composite
average vertical O3 profiles, the observed and the modeled data were
averaged within each 500 m vertical bin and over all the flights in
that region; the figure also shows the SD for the observations. These vertical
profiles represent the mean conditions that occurred over the subtropical and
subarctic Pacific during the study period. The model tracks the composite
average gradients within the lower and upper troposphere in these regions and
accurately simulates that there is higher O3 in the subarctic
Pacific upper troposphere relative to the subtropical Pacific. Also apparent
in these comparisons is the systematic and large underestimation of
O3 throughout the troposphere in the simulation that did not account
for any contributions due to STE processes. The much-closer agreement of the
observed composite profile with that derived from the simulations with the PV
scaling further suggests that on average ∼ 10 ppb (or greater)
of the O3 in the troposphere over the Pacific during spring could be
of stratospheric origin. Thus, O3 in air masses entering western North
America is comprised of both anthropogenic contributions due to long-range
transport of aged pollution from Asia and central America and
a natural stratospheric component. The composite O3 vertical profile
during this period derived from the hemispheric CMAQ is within the range of
those predicted by other global atmospheric chemistry models illustrated in
Singh et al. (2009).
Simulation of the impact of transpacific transport on 3-D distributions
of SO42- aerosol on 21
April 2006. (a) Flight path of C-130 aircraft color-coded by hour (UTC). (b) Simulated SO42-
distribution at 4 km altitude on 21 April 2006 at 21:00 UTC. (c) Comparison of modeled and observed
SO42- aerosol concentrations along C-130 flight path.
Anthropogenic emissions from Asia are often lifted into the FT
and transported across the Pacific to North America in 5–10 days (e.g., Jaffe
et al., 1999). Enhancements to free-tropospheric SO42-
measurements over northwest North America have been attributed to Asian
sources (e.g., Andreae et al., 1988; Barrie et al., 1994). Increasing
SO2 emissions in Asia could potentially increase the amount of
SO42- imported to North America and impact local efforts to reduce
regional haze and improve visibility in national parks. Consequently,
developing tools that accurately characterize the long-range transport from
source regions and the amount of aerosols (both natural and anthropogenic) in
air masses imported into a region is needed. To assess the ability of the
hemispheric CMAQ model to represent airborne SO42- levels and
gradients off the Pacific coast of North America, we compared model
predictions of SO42- (total of SO42- in the Aitken and
accumulation modes) distributions with measurements taken during the INTEX-B
study. In addition to SO42- measurements from bulk aerosol filters
on the DC-8, measurements from the Particle Into Liquid Sampler (PILS) onboard the C-130 aircraft (11 flights) were also analyzed to evaluate the
simulated SO42- distributions within both the BL and
FT over the eastern Pacific and western North America. Analysis
of the evolution of the MODIS AOD retrievals during
mid-April 2006 (van Donkelaar et al., 2008) documents the development and
transport of an Asian plume to western north America, and transects of a C-130
flight on 21 April 2006 during 20:00–23:00 UTC sampled this plume in the
FT off the coast of the western United States. Figure 6 illustrates the
simulation of this episodic Asian plume transport event. The flight path
(color-coded by UTC time) and sampling region are shown in Fig. 6a. Simulated
transport features of the Asian SO42- plume in the model layer at
approximately 4 km altitude at 21:00 UTC on 21 April are illustrated
in Fig. 6b while Fig. 6c presents space- and time-matched comparisons of the
model results with measurements along the C-130 flight path. Both the MODIS
retrievals (in van Donkelaar et al., 2008) and model simulations in Fig. 6b
show the export of SO42- from East Asia and its eastward transport
across the Pacific Ocean to the western coast of North America. As
illustrated in Fig. 6c, SO42- levels > 1 µgm-3
were often measured in the FT. Both the observations and model
show these enhanced SO42- levels at altitudes of 4–6 km,
which in conjunction with the large-scale simulated SO42-
distributions in Fig. 6b, suggest that CMAQ captures the SO42-
enhancements in the FT associated with this episodic event.
Some discrepancies in the space- and time-matched model and observed concentrations
are also apparent in Fig. 6c, which likely result from the relatively coarse
(108 km) horizontal grid resolution employed in the model
calculations.
Comparisons of campaign-average composite vertical profiles for
SO42- for all the DC-8 and C-130 flights are shown in Fig. 7a and
b, respectively. Relative to the observations, CMAQ tends to overestimate
mean SO42- levels, especially in the lower troposphere, as seen in
the comparisons with the bulk filter measurements from the DC-8. It should be
noted that the C-130 PILS measurements represent SO42- mass only
for particle sizes <1 µm, while the model values that are
total mass in the Aitken and accumulation modes, nominally represent
particle sizes <2.5 µm. This discrepancy in particle size
cutoffs between the measured and modeled SO42- in part
contributed to the systematic overestimation relative to the C-130 PILS
measurements. In their comparisons with model results, van Donkelaar
et al. (2008), for instance, used a factor of 1.4 to scale the PILS
SO42- during INTEX-B to account for particle size restrictions.
Using a similar scaling here would result in a much closer comparison of the
composite measured and modeled SO42- profiles in Fig. 7.
Comparison of modeled and observed campaign-average
SO42- vertical profiles: (a) against measurements
from the DC-8 aircraft and (b) against measurements from the C-130 aircraft. Also shown is ±1 SD for both observed and
modeled values.
Transatlantic transport of a Saharan dust plume, 29 July–3 August
2006, as simulated by hemispheric CMAQ. Shown in the
panels is the difference in daily-average PM2.5 concentrations (µg m-3) between CMAQ simulations with
and without considering dust emissions.
Episodic intercontinental transport of Saharan dust and impact on US air quality
Some of the earliest recognition of long-range transport of air pollutants,
dating back almost a century, was through observations of intercontinental
transport of dust (Husar, 2004). North Africa is one of the largest sources
of windblown dust, and the frequent transport of Saharan dust across the
North Atlantic Ocean to the Caribbean has long been studied (e.g., Prospero
and Carlson, 1972). Transatlantic transport of major Saharan dust outbreaks
can episodically influence tropospheric particulate matter loading in the
southeastern United States. The ability of the hemispheric CMAQ to simulate such
long-range transport events is investigated through analysis of a Saharan
dust transport event in late July–early August 2006. The simulated
development and transatlantic transport of a Saharan dust plume during this
period is illustrated in Fig. 8, which presents daily average enhancements in
PM2.5 concentrations attributable to windblown dust. Large amounts
of dust lofted into the atmosphere were transported west across the Atlantic,
eventually impacting the Gulf of Mexico coast region of the United States. Surface-level
PM2.5 measured in the US Gulf of Mexico states showed enhanced values as seen
in the average concentrations across monitoring sites in Florida (Fig. 9b).
The impact of simulated transatlantic transport of Saharan dust on
daily-average surface PM2.5 in the Gulf of Mexico
region. (a) Average change in bias in daily average surface PM2.5 (µgm-3) at AQS monitor
locations between CMAQ simulations with and without considering dust emissions, 29 July–2 August 2006. Negative changes in bias
denote improvement in model performance by including Saharan dust emissions and representing their transatlantic
transport. (b) Comparison of modeled and observed daily-average surface PM2.5 averaged over all AQS monitor
locations in Florida.
Impact of dynamic-PV scaling on surface-level seasonal mean maximum
daily 8 h average O3 (MD8O3) mixing
ratios (ppb) estimated as the difference between the simulation with and without the dynamic-PV formulation.
A demonstration of CMAQ's ability to simulate episodic long-range Saharan
dust transport is shown using comparisons with surface-level PM2.5
measurements at the Air Quality System (AQS) monitors in the Gulf of Mexico states
(Fig. 9). The average change in bias in modeled PM2.5 between
simulations with and without dust emissions is shown in Fig. 9a, which
indicates a reduction in bias in the simulation incorporating the impact of
Saharan dust emissions and transport. Collectively, the analysis here and in
Sect. 3.1 demonstrate that the hemispheric CMAQ modeling system can
represent,
with reasonable skill, the impacts of episodic transatlantic (Figs. 8 and 9)
and transpacific (Figs. 6 and 7) transport events on air pollution over
North America.
Assessing the influence of stratosphere–troposphere exchange on surface O3
The analysis of 3-D O3 distributions from model sensitivity
simulations relative to aircraft measurements, discussed in Sect. 3.1,
indicated the influence of STE processes on tropospheric O3
distributions over the Pacific during the INTEX-B study period. To further
analyze impacts of STE on tropospheric and surface-level O3 over
different seasons and regions, two simulations for the calendar year 2006
were conducted with the hemispheric CMAQ: with and without the dynamic
PV-scaling approach discussed in Sect. 2.5. Figure 10 presents the simulated
seasonal average influence of STE processes on daily maximum 8 h average
(DM8) O3, estimated as the difference between the simulations with
and without the dynamic PV-scaling parameterization. As can be seen, the
amount of O3 at the surface that is of stratospheric origin varies
substantially both spatially as well as seasonally. As expected,
high-latitude regions typically show greater influence of STE at the surface.
Also, the contributions to surface O3 from STE are much higher during
spring and winter when height of the tropopause is lower (e.g., Elbern et al.,
1998) and the stratospheric influence can penetrate far down to the lower
troposphere (e.g., Wang et al., 2002).
Figure 11 presents an evaluation of the PV-scaling parameterization for
representing the seasonal impacts of STE processes on surface DM8O3
relative to measurements from the CASTNET monitoring network in the United States.
A third simulation of 2006 was conducted using a constant
O3–PV scaling factor of 20 ppbPVu-1 rather than the
dynamic scaling approach. The model-estimated stratospheric contribution to
surface DM8O3 at the CASTNET locations can be estimated as the
difference between the DM8O3 from the simulations with and without
the dynamic PV scaling. The bias in the DM8O3 predictions was
computed at each location for the simulations with constant-PV and dynamic-PV
parameterizations. A reduction in bias between these two simulations is
a relative measure of the improvements in surface O3 predictions from
using the dynamic-PV parameterization. Figure 11 correlates the seasonal
average of this bias change with the estimated stratospheric contribution.
The calculated seasonal means at each location were restricted to days with
observed DM8O3>40 ppb. This helps screen out days when
low O3 may not be captured due to model grid resolution or other
process limitations and limited the analysis to periods in which STE influences
were likely greater. A strong linear relationship is noted in Fig. 11 between
the bias change and estimated stratospheric contribution. Across all seasons
and at most locations, the dynamic-PV parameterization reduced the bias in
predicted surface DM8O3 relative to the constant-PV scaling. More
importantly, when the estimated stratospheric contribution to surface
O3 is high, greater reductions in model DM8O3 error are
realized through the use of the dynamic-PV scaling parameterization,
demonstrating the ability of the PV-based parameterization in representing
the effects of STE on surface O3 levels and its seasonal and spatial
variability. Additionally, the improvements in model predictions (i.e.,
reduction in model bias) of DM8O3 are also greater during spring and
winter when the stratospheric contributions are higher (Fig. 10). These
evaluation results help build further confidence in the use of the dynamic-PV
scaling parameterization in the hemispheric CMAQ model and for representing
the impact of STE processes on surface O3 levels.
Impact of dynamic-PV scaling
parameterization on model performance for surface-level seasonal mean maximum
daily 8 h average
O3 (MD8O3) at CASTNET sites in the United States. The model-estimated stratospheric contribution is estimated as the difference
between the simulation with the dynamic-PV scaling and one without. The bias change is estimated as the difference between the
absolute bias in the simulation with a constant-PV (20 PV) scaling and the absolute bias in the dynamic-PV scaling
simulations. Positive changes in bias represent reduction in bias due to dynamic PV, while negative changes represent an increase in
bias in simulated surface MD8O3. Seasonal means are computed based on model-observed pairs when the observed
MD8O3 > 40 ppb.
Differences in simulated O3 distributions using the RACM2
and CB05TU gas-phase chemical mechanisms in the hemispheric
CMAQ model. (a) Mean difference (RACM2 minus CB05TU) in monthly mean surface-level O3 (ppb) for
August 2006. Comparisons of mean (for August 2006) O3 mixing ratio vertical profiles simulated with the CB05TU and RACM2
mechanisms with ozonesonde measurements at (b) Sable Island, Nova Scotia; (c) Trinidad Head, California; and
(d) Hilo, Hawaii. RACM2_HAL is from a simulation in which the RACM2 mechanism was augmented with halogen chemistry
(described in Sect. 2.4.2).
Comparison of O3 predictions using the RACM and CB05 mechanisms
As mentioned in Sect. 2.4.3, the RACM2 is also available as an alternate and
more detailed representation of gas-phase atmospheric chemistry for
hemispheric-scale CMAQ applications. A detailed comparison of the CB05TU and
RACM2 predictions for regional
scale applications over the continental United States was described in Sarwar et al. (2013). A brief summary of comparisons of tropospheric
O3 predictions using the two mechanisms in hemispheric CMAQ is
presented in Fig. 12. The simulations were conducted for May–August 2006 and
were initialized using chemical fields from an existing longer-term
simulation. Differences in predictions of surface-level monthly mean
O3 mixing ratios across the Northern Hemisphere using the RACM2 and
CB05TU mechanisms are illustrated in Fig. 12a for August 2006. Note that the
simulations denoted CB05TU and RACM2 did not include representation of
halogen chemistry. In the simulation using the RACM2, higher O3 is
noted in polluted regions (regions with higher NOx in
Fig. 13e), but lower values are seen in the remote areas. These differences
arise primarily due to higher rates of NOx recycling from
organic nitrates and more active organic chemistry in RACM2.
Comparison of observed (left) and model (right) changes in
NO2 vertical column density (VCD) across the Northern
Hemisphere. Panel (a) shows 2003 SCIAMACHY NO2 VCD;
(b) 2010 SCIAMACHY NO2 VCD; (c) SCIAMACHY
NO2 VCD trend; (d) 2003 CMAQ NO2 VCD; (e)
2010 CMAQ NO2 VCD; (f) CMAQ NO2 VCD trend. VCD is
units of 1015 moleculescm-2, and VCD trend is in units of
1015 moleculescm-2year-1.
To further assess the impacts of using different chemical mechanisms on 3-D
O3 predictions, modeled O3 distributions were compared with
ozonesonde measurements at Sable Island, Nova Scotia; Trinidad Head,
California; and Hilo, Hawaii. Comparisons of monthly mean O3 vertical
profiles simulated using different chemical mechanisms with corresponding
observed profiles are shown in Fig. 12b–d. Also shown are predictions with
a model configuration in which the RACM2 mechanism was augmented with the
halogen chemistry described in Sect. 2.4.2. At Sable Island, which often
receives outflow from the US northeast corridor, RACM2 overpredicts
O3 at lower altitudes. The higher O3 relative to CB05TU in
the North American outflow is likely due to the faster NOx
recycling in RACM2 as discussed earlier. At Trinidad Head, both RACM2 and
CB05TU overestimate O3 near the surface, though RACM2 is closer to
the observations at altitudes of 1000–3000 m. In contrast, at Hilo
CB05TU overestimates O3, and RACM2 is much closer to the observed
profile. In general, the addition of halogen chemistry in RACM2 helps reduce
the overestimation at lower altitudes. At altitudes > 1 km, the
RACM2 O3 predictions are generally in closer agreement with the
observations at all three sites. These mixed performance results indicate
that neither mechanism is necessarily better suited over the other for
hemispheric-scale calculations. Nevertheless, analysis with both the CB05TU
and RACM2 demonstrate the importance of NOx recycling from
isoprene nitrates and halogen chemistry on simulated O3
distributions. Additional analyses of NOy partitioning and
HOx predictions is needed to gain further insights into the
reasons for the differences between the behaviors of the two mechanisms.
Simulating long-term trends in tropospheric composition
Over the past 2 decades significant and contrasting changes have occurred
in anthropogenic air pollutant emissions across the globe. Emission control
measures implemented in North America and western Europe have led to
improvements in air quality in these regions. In contrast, due to increasing
energy demand associated with rapidly growing economies and population, many
regions in Asia and Africa are experiencing a dramatic increase in emissions
of pollutants. These spatially heterogeneous emission trends across the globe
have not only resulted in contrasting changes in human exposure levels to air
pollution (e.g., Wang et al., 2017) but also likely impact long-range
transport patterns and influence background air pollution levels in
receptor regions. Accurate characterization of these changes in the chemical
state of the troposphere (and potential influences on atmospheric dynamics)
is needed to guide future control measures aimed at protecting human and
environmental health. To assess these contrasting changes in air pollution
levels, the hemispheric CMAQ was used to simulate trends in air quality
across the Northern Hemisphere over a 21-year period (1990–2010). Year-specific emission inputs were derived from the Emission Database for Global
Atmospheric Research (EDGAR, version 4.2) database (European Commission,
2011) as discussed earlier in Sect. 2.3.1 and detailed in Xing
et al. (2015a).
Satellite-based tropospheric NO2 measurements now provide valuable
observable information on the changing emission patterns and air quality
across the globe (e.g., Richter et al., 2005; van der A et al., 2008). To
determine if the model can capture the impact of these changing emissions on
tropospheric composition, trends in model tropospheric vertical column
densities (VCDs) of NO2
were compared with those derived from radiances measured by the satellite
instruments GOME (Global Ozone Monitoring Experiment) and SCIAMACHY (Scanning
Imaging Absorption spectrometer for Atmospheric CartograpHY). Note that the
model NO2 column is estimated by integrating the predicted
NO2 fields through the model column from the surface to
∼ 50 hPa and did not utilize an averaging kernel. GOME
NO2 observations are available from 1995 to 2003, while SCIAMACHY
NO2 retrievals have been available since 2002. Figure 13a, b, d, and
e compare annual mean tropospheric vertical column NO2 for the
calendar years 2003 and 2010 derived from SCIAMACHY retrievals and
hemispheric CMAQ. Spatial distributions of the estimated 2003–2010 trends in
NO2 VCD from SCIAMACHY and the model are presented in Fig. 13c and f,
respectively.
Figure 13 shows that the spatial distributions of NO2 VCD across the
Northern Hemisphere are generally well correlated between CMAQ and SCIAMACHY,
with higher NO2 in the industrial and urban areas of North America,
Europe, and Asia. Some discrepancies are noted in central Africa where CMAQ
simulates higher tropospheric NO2 in the Central African Republic and
its northern border with Chad. Trends in NOx emissions
derived from the EDGAR inventory show a similar increasing trend in this
region (see Fig. 2b in Xing et al., 2015a) and indicate that this discrepancy
is associated with the underlying emission data set used in these
simulations. In contrast, SCIAMACHY distributions in the region show a signal
associated with biomass burning in the savanna region of Africa both in 2003
and 2010 – the spatial extent of which is not captured by the model.
Comparison of 2003–2010 trends in tropospheric NO2 between the CMAQ
simulations and SCIAMACHY also indicate that the model captures increases in
eastern China, and many cities in India and the Middle East as well as the
decreases across the eastern United States, southern California, and western
Europe. Figure 14 presents comparisons of time series of regional-average
monthly mean variations in tropospheric NO2 simulated by the model
with corresponding values based on the GOME and SCIAMACHY retrievals for
three regions: eastern China, United States, and Europe (see Fig. 3 of Xing
et al., 2015a for sub-domain extents). The domain-mean seasonal variability
in tropospheric NO2 (as represented by the GOME and SCIAMACHY
retrievals) is captured reasonably well by the model with a cool season
maximum and warm season minimum. The model accurately simulates the amplitude
of this variability for the United States as well as its interannual
variability. For East Asia the model underestimates the peak values. In
contrast for Europe, relative to both GOME and SCIAMACHY, the model
consistently overestimates the summertime minimum values. Note that these
simulations did not account for aerosol radiative feedback effects, which,
due to scattering and absorption of incoming solar radiation, reduce the
amount of radiation impinging the Earth's surface. The resulting
stabilization can reduce BL ventilation and increase surface-level
concentrations. As shown in Xing et al. (2015b, c) these effects are
especially important in polluted environments such as East Asia.
Consequently, some of the underestimation in tropospheric NO2 over
East Asia during the cooler months (when particulate matter pollution is the
highest) could also arise from ignoring the aerosol direct radiative effects
on simulated concentrations.
Comparison of changes in regional- and monthly-average modeled
NO2 vertical column density with satellite retrievals
from SCIAMACHY and GOME for (a) East Asia, (b) the United States, and (c) Europe.
In addition to NOx, anthropogenic emissions of SO2
and VOCs have also been reduced significantly in the United States. To assess
the impact of these precursor emission changes on trends in concentrations of
secondarily produced species, we compared model-simulated trends in ambient
O3 and aerosol SO42- with those inferred from measurements
at the CASTNET monitors. Figure 15 presents comparisons of model and observed
trends in average summer, daily maximum, 8 h average O3 and average
summer weekly-average SO42- at each CASTNET monitor site. Trends
are estimated as the slope of the linear regression of these concentration
metrics for the 21-year period. Also shown in Fig. 15 are results from an
additional 21-year simulation with CMAQ that used
a 36 km resolution regional domain
focused on the contiguous United States (see Fig. 2). This regional
simulation used an updated long-term emission inventory for the United States
(Xing et al., 2013) and was driven by space- and time-varying lateral
boundary conditions from the hemispheric CMAQ simulations for 1990–2010 (Gan
et al., 2015a). Figure 15 shows that both the hemispheric-scale and the
nested regional model capture the decreasing trends in both DM8O3 and
SO42- as well as the spatial variability in the magnitude of the
trends across the CASTNET sites, though the hemispheric model tends to
underestimate the magnitude of the trends by 25–47 %. However, the finer
resolution of the nested simulation in conjunction with the updated emission
inventory better captures the observed trends in surface-level DM8O3
and SO42- as indicated by slopes closer to unity. This suggests
the need to further explore finer-resolution model calculations with the
hemispheric CMAQ. As computing resources increase in the future it may be
possible to conduct hemispheric-scale simulations utilizing grid spacing
finer than the 108 km utilized here.
Comparison model and observed summertime (JJA) 1990–2010 trends at
CASTNET monitoring sites in the United States for (a) JJA
average daily maximum 8 h average O3 and (b) JJA average SO42-. Model results from the 108 km
resolution hemispheric CMAQ simulation are shown in blue, while results from a 36 km resolution nested model calculation over
the contiguous United States are shown in red.
Assessment of representation of aerosol direct radiative effects
Both aerosol scattering and absorption reduce the SWR
impinging on the Earth's surface. The variability in surface solar radiation
plays a prominent role in the Earth's climate system as it contributes to the
modulation of the surface temperature, intensity of the hydrological cycle,
and potentially the net ecosystem productivity (Wild, 2009). Observed trends
in solar radiation reaching the Earth's surface are very likely associated
with changes in aerosol and aerosol-precursor emissions governed by economic
development and air pollution regulations, which have modulated the trends in
regional and global tropospheric aerosol burden over the past 2 decades
(see Wild, 2009; Streets et al., 2009). Surface solar radiation “dimming”
and “brightening” effects respectively dampen and enhance the warming
trends induced by greenhouse gasses; thus, it is essential to accurately
characterize these trends and quantify the role of regional variability in
tropospheric aerosol burdens on these trends.
Simulations over the Northern Hemisphere can also be conducted using
a two-way coupled WRF-CMAQ configuration (Wong et al., 2012), in which
CMAQ-simulated aerosol composition and size distribution are used to estimate
their optical properties, which are then fed back to the WRF radiation
module to influence the radiation simulated by WRF. Thus, the effects of
aerosol scattering and absorption of incoming radiation can further impact
the simulated atmospheric dynamics (BL heights, temperature, simulated
resolved, and sub-grid-scale clouds), which then impact emission rates,
transport and dispersion, deposition, and temperature- and actinic-flux-dependent chemical rate
constants. The aerosol optical properties in the two-way
coupled WRF-CMAQ are calculated using the BHCOAT coated-sphere module
approach (Bohren and Huffman, 1983), i.e., particles in the Aitken and
accumulation model are assumed to have a core composed of elemental carbon
with a shell coating of other species. The aerosol optic calculations in
WRF-CMAQ have been evaluated against field measurements as detailed in Gan
et al. (2015b).
In addition to the long-term (1990–2010) simulations discussed in Sect. 3.5,
additional feedback simulations were conducted over the Northern Hemisphere
with the two-way coupled WRF-CMAQ configuration for the summer months (June,
July, and August) of this 21-year period. Comparison of these two sets of
simulations (with and without aerosol feedbacks) provides an indication of
the impact of the aerosol direct radiative effects and an assessment
of its trends associated with the changing tropospheric aerosol burden over
the multi-decadal period. Figure 16 examines the modeled and observed
relationships between changes in AOD (representing
changes in the tropospheric aerosol burden) and changes in clear-sky surface
SWR using regional monthly averages for eastern China,
Europe, and the eastern United States. Figure 16 examines 2000–2010, when satellite-based data
were available. Monthly regional averages of SWR and AOD values were calculated for
each of the summer months (June, July, and August). To minimize the influence
of month-to-month variability, monthly averaged SWR and AOD were
deseasonalized by subtracting the average of 11 year data for the
corresponding month. Additionally, we used 24 h averaged SWR but AOD at noon
(local time) for model values to be consistent with the observation-derived
data from satellite retrievals (Xing et al., 2015b). The relationship between
these deseasonalized values (or anomaly) of SWR and AOD for each summer month
in the 2001–2010 period is examined in Fig. 16 for model simulations
both with and without direct aerosol feedback effects. Also shown in Fig. 16 is
the corresponding observed relationship between the similarly estimated AOD
anomaly and SWR anomaly derived from retrievals from the MODIS and the Clouds
and the Earth's Radiant Energy System (CERES; Wielicki et al., 1998)
instruments, respectively. Note that the CERES mission estimates clear-sky
surface SWR through radiative transfer calculations using satellite-retrieved
surface, cloud, and aerosol properties as input (Kato et al., 2013), which
have also been shown to agree with surface observations (Wild et al., 2013).
Relationship between regional and monthly average (only summer
months) changes in aerosol optical depth and changes in clear-sky surface
shortwave radiation during the 2001–2010 period for (a) eastern China, (b) Europe, and (c) the eastern
United States.
Observed values are shown in grey, CMAQ calculations with direct aerosol radiative feedbacks in red, and CMAQ calculations without
aerosol radiative feedbacks in blue. Also indicated are the slope and correlation coefficient (R) for the individual linear
regressions. For each data set (model or observed) there are 33 values, corresponding to each summer month over the 11-year
(2001–2010) period. The anomaly is estimated by subtracting the corresponding 11-year average for that month.
All three regions show a strong relationship between observed changes in AOD
and clear-sky surface SWR, with reductions in SWR associated with increases
in AOD and increases in SWR with reductions in AOD. This observational
comparison clearly suggests that as tropospheric aerosol burden increases,
scattering and absorption reduces the amount of surface SWR. The magnitude of
these changes is comparatively larger for eastern China than for Europe and
the eastern United States due to the higher levels of tropospheric particulate matter in
eastern
China. In all three regions, the model simulation without direct aerosol
feedback fails to capture the changes in SWR and its association with AOD. In
contrast the model simulation incorporating the aerosol direct radiative
effects replicates the relationship between the observed AOD and SWR changes
during the 2000–2010 period in all three regions as reflected by the higher
R2 and slopes of the linear regression closer to those inferred from the
observed data. These results suggest that the hemispheric two-way coupled
WRF-CMAQ configuration can represent the differing regional changes in
surface SWR with contrasting changes in regional aerosol burden. Accordingly,
this tool could also be used to examine chemistry–climate interactions on
hemispheric to regional scales.
Summary and concluding remarks
The applicability of the CMAQ modeling
system was extended to the entire Northern Hemisphere to enable consistent
examination of interactions between atmospheric processes occurring on
various spatial and temporal scales. Improvements were made in model process
representation (stratospheric O3 influences, representation of
NOx recycling through organic nitrates, halogen chemistry
in marine environments, deposition over water), structure (model vertical
extent and layer resolution), and input data sets (allocation of global
emission estimates). These improvements to CMAQ were investigated and
evaluated through comparison of model predictions with measurements from
surface, aloft, remote sensing, and specialized field campaign platforms.
Comparisons with measurements from the INTEX-B field campaign indicate that
hemispheric CMAQ captures the mean variability in O3 and
SO42- distributions observed over the tropical and subarctic
Pacific regions and episodic transport of Asian pollution across the Pacific
as indicated by comparisons of model and observed SO42- along
specific flight tracks. The ability to capture the development and evolution
of intercontinental transport (i.e., the lofting of pollutants in the source
region, multiday transport in the FT and subsequent subsidence
and mixing down to the surface in receptor regions) is also demonstrated by
evaluating a transatlantic Saharan dust transport event and its
contributions to elevated surface PM2.5 in the US Gulf of Mexico region.
These results suggest that regional CMAQ applications can now be driven by
space- and time-varying lateral boundary conditions derived from consistent
hemispheric applications, enabling examination of air quality across the
United States
in the context of the changing global atmosphere.
The hemispheric CMAQ model can reproduce historical trends in tropospheric air
pollution, as shown by comparing simulated results with surface and
remote-sensing observation-derived records during 1990–2010. Trends in
modeled tropospheric NO2 vertical column distributions agree with
those inferred from GOME and SCIAMACHY retrievals and indicate the
contrasting and heterogeneous changes in emissions across the Northern
Hemisphere, with increases in rapidly developing regions of Asia and
decreases in Europe and North America resulting from implementation of
control measures. Additionally, comparisons with observed trends at the US
CASTNET monitors indicate that though the model captures the resultant
decreasing trends in surface-level air pollution (for O3 and
SO42-) in the United States, the current configuration underestimated (by
25–47 %) the magnitude of the trend at some monitoring locations. The
underestimation in the magnitude of the trend is however significantly
reduced in a nested regional simulation utilizing finer horizontal grid
resolution and updated historical regional emissions. The changing emission
patterns across the Northern Hemisphere will likely influence future
long-range pollutant transport patterns and potentially impact background
pollution levels in receptor regions. The hemispheric CMAQ model provides
a framework to explore such changing impacts on air pollution exposure. For
instance, Wang et al. (2017) estimated trends in PM2.5 premature
mortality during 1990–2010 using hemispheric CMAQ predictions and show that
correlations between population and PM2.5 have become weaker in
Europe and North America due to air pollution controls but stronger in East
Asia due to deteriorating air quality.
Analysis of aerosol optical and radiative effects inferred from the two-way
coupled WRF-CMAQ applications over the Northern Hemisphere also indicate the
association between changing tropospheric aerosol burden and clear-sky
surface SWR. In rapidly developing regions such as East Asia,
the increasing tropospheric aerosol burden results in greater scattering and
absorption by aerosols, and that acts to reduce the amount of clear-sky
surface SWR. In contrast, increases in clear-sky surface
SWR are noted in regions with declining tropospheric aerosol
burden, where emission controls have been more active during that period.
The two-way coupled WRF-CMAQ configuration that incorporates direct aerosol
radiative effects captures these contrasting observed changes in clear-sky
SWR across the Northern Hemisphere during 2000–2010, with
brightening in the United States and western Europe and dimming in eastern China. Using
these modeling results, Xing et al. (2016b) show that because of reduced
atmospheric mixing resulting from direct aerosol feedbacks, air pollutants
become more concentrated locally, especially in highly polluted and populated
regions. Thus, modulation of air pollution due to direct aerosol effects also
translates into an additional human health dividend in regions (e.g., the United States and
Europe) with air pollution control measures but a penalty for regions (e.g.,
East Asia) witnessing rapid deterioration in air quality.
Analysis of three-dimensional O3 distributions across the Northern
Hemisphere from model sensitivity simulations and comparisons with surface
and aloft measurements also highlights the need to better quantify the
contribution of STE processes on surface O3. A nontrivial
contribution of up to ∼ 10 ppb from the stratosphere to
seasonal mean surface-level O3 mixing ratios is inferred from the
current applications. An accurate characterization of this contribution is
essential for source attribution of background O3. Since measurements
of 3-D O3 distributions alone are insufficient to directly quantify
this contribution, model estimates need to be better constrained. To that
end,
additional CMAQ simulations that explore the sensitivity of STE process
representation to model vertical extent and vertical grid resolution are
warranted. Model calculations presented here also indicate the possible
influence of horizontal grid resolution on model evaluation results.
Hemispheric CMAQ simulations to date have employed a horizontal grid
resolution of 108 km, which is insufficient to resolve local
gradients. Emerging environmental problems will likely require the
simultaneous characterization of air pollution on local to global scales.
Variable-resolution nonuniform grids can simultaneously and accurately
resolve local gradients and large-scale features in air pollutant
distributions (e.g., Odman and Russell, 1991; Mathur et al., 1992; Srivastava
et al., 2000). The emergence of variable-resolution atmospheric dynamical
models (e.g., Skamarock et al., 2012) provides opportunities to develop
comprehensive atmospheric modeling systems that seamlessly represent the
scale interactions on urban to global scales. The use of such approaches
could improve the representation of scale interactions in air pollution
modeling.
Several efforts are underway to harmonize regional emission estimates and
incorporate them into global emission inventories with improved spatial and
temporal resolution (e.g., Janssens-Maenhout et al., 2015). It can be expected
that future improvements in the performance of hemispheric CMAQ will also be
realized through improvements in these underlying global emission inventories
used to drive model calculations. Additional improvements in sector-specific
emissions and additional details on chemical speciation of the emissions will
also lend themselves to the use of more detailed chemical mechanisms such as
RACM2, which explicitly treat the chemistry of longer-lived species (e.g.,
acetone) that are important for the chemistry of the upper troposphere, and help
further assess the relative benefits of the use of different chemical
mechanisms on hemispheric scales. Predictions of a variety of atmospheric
pollutant species from hemispheric CMAQ are also being compared to those from
other modeling systems (Hogrefe et al., 2015) through the activities of Air
Quality Model Evaluation International Initiative (AQMEII). The adequacies
and inadequacies of the lateral boundary conditions derived from hemispheric
CMAQ to drive regional CMAQ simulations are being further analyzed through
comparisons with those from other large-scale models and observations
(Hogrefe et al., 2017) and will also guide the future evolution of the
hemispheric CMAQ.