The unintended consequences of reductions in regional
anthropogenic sulfur dioxide (SO2) emissions implemented to protect
human health are poorly understood. SO2 decreases began in the 1970s in
the US and Europe and are expected to continue into the future, while recent
emissions decreases in China are also projected to continue. In addition to
the well-documented climate effects (warming) from reducing aerosols,
tropospheric oxidation is impacted via aerosol modification of photolysis
rates and radical sinks. Impacts on the hydroxyl radical and other trace
constituents directly affect climate and air quality metrics such as surface
ozone levels. We use the Geophysical Fluid Dynamics Laboratory Atmospheric
Model version 3 nudged towards National Centers for Environmental Prediction
(NCEP) reanalysis wind velocities to estimate the impact of SO2
emissions from the US, Europe, and China by differencing a
control simulation with an otherwise identical simulation in which 2015
anthropogenic SO2 emissions are set to zero over one of the regions.
Springtime sulfate aerosol changes occur both locally to the emission region
and also throughout the Northern Hemispheric troposphere, including remote
oceanic regions and the Arctic. Hydroperoxy (HO2) radicals are directly
removed via heterogeneous chemistry on aerosol surfaces, including sulfate,
in the model, and we find that sulfate aerosol produced by SO2
emissions from the three individual northern mid-latitude regions strongly
reduces both HO2 and hydroxyl (OH) by up to 10 % year-round
throughout most of the troposphere north of 30∘ N latitude.
Regional SO2 emissions significantly increase nitrogen oxides
(NOx) by about 5 %–8 % throughout most of the free troposphere in the
Northern Hemisphere by increasing the NOx lifetime as the heterogeneous
sink of HO2 on sulfate aerosol declines. Despite the NOx
increases, tropospheric ozone decreases at northern mid-latitudes by 1 %–4 %
zonally averaged and by up to 5 ppbv in summertime surface air over China,
where the decreases in HO2 and OH suppress O3 production. Since
2015 anthropogenic SO2 emissions in China exceed those in the US or
Europe, the oxidative response is greatest for the China perturbation
simulation. Chemical effects of aerosols on oxidation (reactive uptake)
dominate over radiative effects (photolysis rates), the latter of which are
only statistically significant locally for the large perturbation over
China. We find that the SO2 emissions decrease in China, which has yet
to be fully realized, will have the largest impact on oxidants and related
species in the Northern Hemisphere free troposphere compared to future
decreases in Europe or the US. Our results bolster previous calls for a
multipollutant strategy for air pollution mitigation to avoid the
unintended consequence of aerosol removal leading to surface ozone increases
that offset or mask surface ozone gains achieved by regulation of other
pollutants, especially in countries where current usage of high-sulfur
emitting fuels may be phased out in the future.
Introduction
Understanding and constraining tropospheric oxidants such as the hydroxyl
radical (OH) remains a key challenge of direct relevance to understanding
the oxidizing power of the atmosphere, radiative forcing, and surface air
quality. Despite the critical role in atmospheric chemistry, OH abundances
differ widely among chemistry–climate and chemical transport models (Stevenson
et al., 2020; Zhao et al., 2019). In addition, global annual mean OH
response to historical anthropogenic emission changes (all species) between
the preindustrial and the present-day ranged from a 12.7 % decrease to a
14.6 % increase across 17 global models (Naik
et al., 2013b), with similar discrepancies across simulations of future
composition and climate (Voulgarakis et
al., 2013). These differences between model estimates of OH suggest major
knowledge gaps in our understanding of the drivers of OH. One potential
driver of tropospheric oxidant changes that has not received sufficient
study is aerosols, which can uptake radical species (chemical effect) and
scatter or absorb incoming solar radiation (radiative effect), thereby
impacting OH and other important chemical species (Jacob,
2000; Wild et al., 2000).
Anthropogenic emissions of sulfur dioxide (SO2), a precursor to sulfate
aerosol, have significantly decreased in the US and Europe for
the last several decades and are projected to continue to decline (Riahi
et al., 2011; Vuuren et al., 2011; Westervelt et al., 2015). In China,
emissions of anthropogenic aerosols began to decline in about 2013 after
increasing for decades (Fontes et al.,
2017; Li et al., 2017; Samset et al., 2018). Previous research has indicated
that these past and forthcoming emission changes have the potential to
influence the tropospheric oxidation capacity on both a regional and global
basis (Dentener and
Crutzen, 1993; Dickerson et al., 1997; Martin et al., 2003). The aerosol
decreases in China were associated with subsequent increases in summertime
surface ozone (O3) in China, attributed to a reduction in the sink of
radical species such as the hydroperoxyl radical (HO2) that promote
O3 production (Li et al., 2019b).
Using a model and observations, the authors found that a 40 % decrease in
fine particulate matter (PM2.5) in China between 2013 and 2017 led to
an increasing ozone trend of up to 3 ppb per year in eastern China and was a
more important factor than NOx emissions reductions over the same time
period (Li
et al., 2019a, b). These findings confirm earlier modeling work and
point to an important role for aerosol impacts on tropospheric oxidation
with implications for surface O3 concentrations, especially over China (Li
et al., 2018; Lou et al., 2014).
On a global scale, the impact of aerosols on tropospheric oxidants has
received little attention. Often, aerosol impacts are assumed to be
negligible in constraining present and future OH concentrations (Voulgarakis
et al., 2013). Primary production of OH depends on the amount of water vapor
and O(1D) present (formed via O3 photolysis) and is the dominant
pathway of OH formation in most locations except for high latitudes (Spivakovsky
et al., 2000). Secondary production involves reactions of HO2 or
RO2 (organic peroxy) radicals generated from oxidation of volatile
organic compounds (VOCs) or carbon monoxide (CO) with nitric oxide (NO),
which regenerates OH. Concentrations of these atmospheric constituents and
certain meteorological factors such as absolute humidity, temperature, and
ultraviolet radiation are thought to predominantly control OH abundance (Spivakovsky
et al., 2000). However, by differencing a Goddard Earth Observing System
Chemistry Transport Model (GEOS-Chem) control simulation of late 1990s
atmospheric composition with a sensitivity simulation in which the offline
global aerosols are excluded, Martin et al. (2003) find
that the presence of all aerosols decreases OH by 9 % globally and
5 %–35 % in the Northern Hemisphere boundary layer. The authors also find
15–45 ppbv decreases in boundary layer O3 over India in March
associated with the presence of all aerosols compared to all aerosols
removed. In a similar global study, Tie et al. (2005) use the
Model for Ozone and Related Chemical Tracers version 2 (MOZART-2) to show
that the net effect of all aerosols (natural and anthropogenic) reduces
HOx (defined as OH + HO2) and O3 by 30 % and 20 %,
respectively, improving on past methodology (e.g., Martin et al., 2003) by
calculating aerosol abundances interactively. Past studies only considered
global distributions of aerosols and often focused on natural aerosols such
as dust or sea salt (Bian and
Zender, 2003; Liao et al., 2003). The impact of rapidly changing spatially
heterogeneous anthropogenic aerosol abundances on tropospheric OH and
O3 in response to regional air pollution control programs is thus an
open question.
We expand on past studies by considering sulfate aerosol decrease via
SO2 emissions reductions within individual regions (China, Europe, and
the US), and quantify the local and remote impacts of changing these
emissions on atmospheric HOx, NOx, and O3 concentrations on
a seasonal basis within a chemistry–climate model nudged to observed
meteorology. We focus on anthropogenic SO2 emissions, which have
decreased most dramatically in many regions compared to
anthropogenically sourced carbonaceous aerosols or natural aerosols such as
dust and sea salt. We seek mechanistic understanding of the interactions
between aerosols, oxidants, and radical species and photolysis rates over
different regions and in different seasons. We consider two main pathways
through which aerosols can affect oxidation: modification of photolysis
rates via extinction of incoming solar radiation (radiative effect) and
heterogeneous uptake of radical species onto aerosol surfaces (chemical
effect). Finally, we consider the impact of anthropogenic SO2 emissions
reductions on boreal summertime surface O3 concentrations in China,
Europe, and the US.
Boreal springtime (MAM) mean percent change in sulfate
concentration between a control simulation and a perturbation simulation in
which anthropogenic SO2 emissions are removed over a certain region:
(a, b) US, (c, d) Europe, and (e, f) China. Hatching denotes statistical
significance according to a Student's t test at the 95 % confidence level.
Heterogeneous reactive uptake coefficients for several reactions in
GFDL-AM3.
ReactionUptakecoefficient (γ)HO2→H2O2 or H2O0.2N2O5→2.0HNO30.1NO3→1.0HNO30.1NO2→0.5HNO30.0001
Boreal springtime (MAM) mean percent change in OH (a, c, e)
and HO2(b, d, f) between a control simulation and a perturbation
simulation in which anthropogenic SO2 emissions are removed over a
certain region: (a, b) US, (c, d) Europe, and (e, f) China. Hatching denotes
statistical significance according to a Student's t test at the 95 %
confidence level.
Model and simulations
We use the National Oceanic and Atmospheric Administration Geophysical Fluid
Dynamics Laboratory Atmospheric Model version 3 (GFDL-AM3), which is the
atmosphere-only component of the GFDL-coupled climate model, CM3 (Donner et al., 2011). The
model has been rigorously evaluated against observations in previous work,
including against surface observations of O3 over the US, Europe, and
China (Donner
et al., 2011; Naik et al., 2013a, Westervelt et al., 2019). Paulot et al. (2016) evaluate sulfate concentrations in GFDL-AM3 over the US (Interagency
Monitoring of Protected Visual Environments, IMPROVE) and Europe (European
Monitoring and Evaluation Programme, EMEP) and find a normalized mean bias
of 0.07 in model surface concentrations compared against IMPROVE and a -0.43
mean normalized bias over Europe against EMEP. The model has 48 vertical
layers from the surface up to about 0.01 hPa and a six-face cubed-sphere
grid with 48 cells along each edge (C48), which is regridded to a 2∘
latitude by 2.5∘ longitude Cartesian grid. Emissions of anthropogenic
trace gases and aerosols for year 2015 emissions are from the Representative
Concentration Pathway 8.5 (RCP8.5) scenario (Riahi et al.,
2011). The tropospheric chemical mechanism for aerosols and gas-phase
species follows the work of Horowitz et al. (2003, 2007) with updates to photolysis,
radical uptake by aerosols and convective wet scavenging of aerosols. The
Fast-JX module (Bian et
al., 2003; Wild et al., 2000) calculates the impact of online aerosols and
clouds on photolysis rates and actinic fluxes implemented into GFDL-AM3
according to Mao et al. (2013b).
Heterogeneous uptake of radical species is simulated according to Mao et al. (2013b) and Mao et al. (2013a) using a first order
reactive uptake rate constant k (Eq. 1):
k=-reDg+4γν-1A,
where re is the aerosol effective radius (m), Dg is the gas-phase
molecular diffusion coefficient, ν is the mean molecular speed of the
gas, and A is the aerosol surface area per unit volume of air. Here we set
the heterogeneous reactive uptake coefficient (γ) of HO2 to 0.2
instead of the value of 1.0 in Mao et al. (2013a). Though estimates of γ are uncertain, recent
literature suggests such high values of 1.0 are not supported by
observations and that the parameter is likely closer to 0.2 (Abbatt
et al., 2012; Li et al., 2019a, b; Taketani et al., 2012). Taketani et al. (2012) recommend a
middle γ value of 0.24 based on measurements at two high-altitude
sites in China. Reactive uptake coefficients for all other reactions
including N2O5, NO3, and NO2 are shown in Table 1, taken
from Jacob
(2000). We allow uptake of HO2, N2O5, NO3, and NO2
onto all aerosol types, including sulfate, black carbon, organic carbon, sea
salt, and dust using the same coefficients for each composition. We also
include updates to convective wet scavenging of aerosols in the form of
finer vertical discretization of convective updraft plumes, resulting in
improvements in aerosol budgets (Paulot et al.,
2016). Horizontal wind velocities are nudged using a pressure-dependent
technique towards reanalysis values from the National Centers for
Environmental Prediction Global Forecast System (NCEP GFS; Lin et al., 2012). Further model description and
model evaluation against observations can be found in Donner et al. (2011), Naik et al. (2013a), and Rasmussen
et al. (2012).
Boreal springtime (MAM) mean percent change in NOx between a
control simulation and a perturbation simulation in which anthropogenic
SO2 emissions are removed over a certain region: (a) US, (b) Europe,
and (c) China. Hatching denotes statistical significance according to a
Student's t test at the 95 % confidence level.
We conduct a two-year (2014–2015) nudged control simulation in which
emissions of aerosols and their precursors follow RCP8.5 and contrast it
with three perturbations: one in which all anthropogenic SO2 emissions
are set to zero over the US (30–50∘ N, 70–125∘ W) ,
all anthropogenic SO2 emissions are set to zero over Europe
(35–70∘ N, 15∘ W–55∘ E), and all anthropogenic SO2 emissions are set to
zero over China (15–50∘ N,
95–130∘ E). SO2 is oxidized by
the hydroxyl radical in the gas phase and by ozone and hydrogen peroxide in
clouds to form sulfate aerosol, which is a dominant component of total
aerosol in GFDL-AM3 (Westervelt
et al., 2015, 2017). We separately subtract each regional SO2
perturbation simulation from the control simulation, thereby isolating the
impact of regional SO2 emissions (and subsequent sulfate formation) on
tropospheric oxidants and related species. We test for statistical
significance using a Student's t test on seasonal mean responses with the
null hypothesis being that the difference between the control and the
perturbation simulation is zero. Only the full year of 2015 is used for
analysis to allow for a full year of initialization. SO2 perturbations
from our simulations are 10.8, 12.4, and 16.2 Tg SO2 y-1 for US,
Europe, and China, respectively.
The global annual mean OH for the 2015 control simulation is 7.0×105 molecules cm-3, which is within the range of the 14 Atmospheric
Chemistry and Climate Model Intercomparison Project (ACCMIP) for year 2000
and 14 Chemistry Climate Model Initiative (CCMI) models (Voulgarakis
et al., 2013; Zhao et al., 2019) for years 2000–2010. The global annual
tropospheric burden of O3 in the 2015 control simulation is 356 Tg,
which compares well to the year 2000 O3 burden mean across the ACCMIP
models of 337±23 Tg (Young
et al., 2013).
Boreal springtime (MAM) mean percent change in O3 between a
control simulation and a perturbation simulation in which anthropogenic
SO2 emissions are removed over a certain region: (a) US, (b) Europe,
and (c) China. Hatching denotes statistical significance according to a
Student's t test at the 95 % confidence level.
Results
Figure 1 shows the percent increase in seasonal (March–April–May, MAM)
sulfate concentrations at the surface (right column) and at altitude (left
column) due to the presence of all anthropogenic US SO2 (first row),
all European SO2 (second row), and all Chinese SO2 (third row)
based on year 2015 anthropogenic emissions. Additional seasons are shown in
the Supplement (Figs. S1–S3). The zeroing of 2015 SO2 emissions in each
location results in the largest relative perturbation in China, where
emissions are highest, followed by Europe and the US. Sulfate increases are
largest closest to the source region, but all three regional simulations
show statistically significant remote impacts both horizontally and
vertically in the atmosphere, as evidenced by the spatial and zonal plots in
Fig. 1. Emissions from US, Europe, and China perturbations all significantly
increase sulfate throughout the troposphere up to 200 hPa and higher towards
the North Pole, with the largest increases of up to 30 %–40 % resulting from
the China SO2 perturbation. Transport to the Arctic is a common feature
in all three perturbations and is consistent with previous studies on
aerosol transport to the Arctic (Shindell
et al., 2008; Stohl, 2006, Yang et al., 2017, 2018; Ren et al.,
2020). The US perturbation impacts sulfate concentrations significantly at
the surface and at altitude over the North Atlantic Ocean, while emissions
from China exert a heavy influence over the Pacific reaching all of the way
to the western US. European SO2 emissions have widespread influence on
the Northern Hemisphere, but especially in the Arctic, the Mediterranean,
and northern Africa. In all cases, sulfate changes are nearly entirely
confined to the Northern Hemisphere.
We analyze the impact of sulfate changes on atmospheric oxidation capacity,
starting in Fig. 2 with OH (left column) and HO2 (right column) for
each of the three regional perturbations (rows of Fig. 2). Sulfate aerosol
surfaces directly uptake HO2 radicals as described in Sect. 2,
resulting in significant decreases of HO2 and OH (via their rapid
cycling). For each perturbation, decreases in both OH and HO2 occur
throughout most of the Northern Hemisphere up to about 200 hPa vertically
during the boreal spring (MAM). The largest decreases in OH and HO2
occur in spring for each of the perturbations, followed by winter
(December–January–February, DJF), autumn (September–October–November, SON),
and summer (June–July–August, JJA). These additional seasons are plotted in
Figs. S4–S6 in the Supplement. In MAM, SO2 emissions over the US decrease OH and HO2
by about 5 % within the US planetary boundary layer. In the
mid-troposphere (400–600 hPa), OH decreases are 5 % or greater and are
located spatially above the Arctic. For the Europe SO2 and China
SO2 cases during MAM, the Arctic middle troposphere OH decreases are
larger in percent change (>10 %) than the local changes near
the surface (∼8 %). The presence of 2015 China SO2
emissions also decreases OH and HO2 by about 10 % over the North
Pacific Ocean middle troposphere (about 400–600 hPa) in the model. By
comparing the first row of Fig. 2 with the second and third rows, we find
that the zonal structure of the OH and HO2 response to anthropogenic
SO2 emissions is very similar across the three regional
perturbations, while the magnitude is largest in response to China SO2
emissions, followed by Europe SO2 and US SO2. We conclude that
regional SO2 emissions may have stronger impacts remotely than locally,
and OH may be relatively more sensitive to aerosol changes in the Arctic and
remote oceans at higher altitudes where its production is more limited.
In Fig. 3 we plot spring (MAM) changes in NOx (defined as NO +
NO2) concentrations in response to anthropogenic SO2 emissions in
the US, Europe, and China. While HO2 and OH strongly decreased in
response to SO2 emissions, NOx significantly increases throughout
most of the Northern Hemisphere. In the model, aerosols can take up NO2
directly but with a very low reaction probability (0.0001; Table 1), such
that little uptake actually occurs and is easily offset by feedbacks onto
other chemical reactions involving NOx. Instead, reduction in the sinks
of NOx via OH (nitric acid formation) during the day and uptake of
NO3 at night dominates the response to SO2 emission changes,
increasing NOx in the model as OH decreases. At night, NOx is
removed by reaction with the nitrate radical (NO3), which forms
dinitrogen pentoxide (N2O5) (Chang
et al., 2011; Jacob, 2000). Sulfate aerosols are effective at removing
NO3 via reactive uptake (reaction probability of 0.1), slowing down
this nighttime NOx sink and thus increasing NOx abundance. This
hindering of day and night NOx sinks is most effective during MAM and
DJF in the Northern Hemisphere mid-troposphere (Fig. 3a–c for MAM;
additional seasons shown in Figs. S7–S9 in the Supplement). Mid-tropospheric Northern
Hemisphere NOx increases reach about 7 %–8 % in response to Chinese
SO2 emissions specifically, with smaller effects for both US and Europe
SO2 perturbations. NOx at the surface increases slightly less at
about 5 %–7 % depending on the regional emissions perturbation, though these
changes are still statistically significant. N2O5 is removed by
aerosols also with a reaction probability of 0.1, although several previous
studies have used smaller reactive uptake coefficients for N2O5 (Evans
and Jacob, 2005; Holmes et al., 2019; Macintyre and Evans, 2010; McDuffie et
al., 2019) based on more recent laboratory experiments, but only find
impacts on mean tropospheric O3 burden of 2 %–4 %. Using a box
modeling approach, McDuffie et al. (2019) find a median γ for
N2O5 of 0.076, reasonably close to our assumed value of 0.1.
Summertime (JJA) surface O3 change (in ppbv) between a control
simulation and a perturbation simulation in which anthropogenic SO2
emissions are removed over a certain region: (a) US, (b) Europe, and (c)
China. Hatching denotes statistical significance according to a Student's
t test at the 95 % confidence level.
Boreal springtime (MAM) mean percent change in photolysis rates
(jO(1D), left column; jNO2, right column) between a control
simulation and a perturbation simulation in which anthropogenic SO2
emissions are removed over a certain region: (a, b) US, (c ,d) Europe, and
(e, f) China. Hatching denotes statistical significance according to a
Student's t test at the 95 % confidence level.
In most of the Northern Hemisphere troposphere, O3 decreases in MAM by
up to 4 % in response to US, European, or Chinese SO2 emissions
increases in the model (Fig. 4), mostly coinciding with regions of large
HOx decreases (Fig. 2), despite the increase in NOx (Fig. 3).
O3 decreases are the largest in response to Chinese anthropogenic
SO2 emissions, owing to the larger SO2 perturbation compared to
the US and Europe emissions perturbations. The O3 increases in the
upper troposphere are mostly not significant. We examine model diagnostics
of gross ozone production (the sum of HO2+NO and all RO2+NO
reaction pathways) and O3 loss (which includes reaction of O3 with
HOx and with alkenes, plus O3 photolysis followed by
O1D+H2O) to interpret further the O3 decrease. While both
O3 production (PO3) and loss (LO3) rates decline (Figs. S10
and S11 in the Supplement), production decreases more strongly than loss, lowering
O3 concentrations. We confirm that transport of O3 from other
latitudes is unlikely to contribute much to the modeled O3 response as
the change in zonal mean advective or convective tendency in O3 (Fig. S12 in the Supplement) is far smaller than the chemical production and loss terms (Fig. S10 in the Supplement).
The O3 production and loss rates decrease most strongly in the lower
troposphere over the source regions (Figs. S10 and S11 in the Supplement) while the
O3 decreases (Fig. 4) propagate more widely through the free
troposphere, indicating reduced export from these source regions. Additional
seasons for O3 change are shown in Figs. S13–S15 in the Supplement.
We find here that the decline in HOx and its impact on PO3
outweighs the aerosol-induced increases in NOx and decreases in
O3–HOx sinks, even during summer in all three source regions. We
show the response of summertime surface 8 h maximum daily average (MDA)
O3 to increasing anthropogenic SO2 emissions in the US, Europe,
and China in Fig. 5. Increasing sulfate aerosol increases the sink of
HO2 radicals and thus slows down O3 production (Fig. S11 in the Supplement),
resulting in surface O3 concentration decreases, which are largest and
mostly confined to the emissions source region. Sulfate aerosol can also
reduce NO2 and O3 photolysis rates. The combined effect of sulfate
aerosol on changes in photolysis rates and heterogeneous chemistry is a
statistically significant decrease of about 5 ppbv over most of eastern
China, Korea, and Japan when Chinese SO2 emissions are introduced, a
decrease of about 3 ppbv over the eastern US for US SO2 emissions, and
a decrease of about 3 ppbv over Eurasia for the Europe SO2 emissions
perturbation. Changes in similar magnitude have been reported over China
using both a chemistry transport model and observations (Li
et al., 2019). Large sulfate decreases have occurred since the 1970s in both
Europe and the US. The SO2 perturbation in our study (zero-out 2015
level emissions) is 10.8, 12.4, and 16.2 Tg SO2 y-1 in the US,
Europe, and China, respectively. These results imply that the sulfate
decreases from clean air regulations and technologies have had the
unintended consequence of driving O3 up by a few ppb during the
summertime in the US and Europe. While this may be a small amount of the
total surface O3 concentration and not entirely outside the range of
typical variability, our study only considers the impact of sulfate aerosol
and not carbonaceous aerosols, which make up greater than 50 % of the
total aerosol mass in many environments (Jiminez et al., 2009).
Additionally, even O3 changes on the order of 3–5 ppbv may be important
for holistically meeting tightening air quality standards. NOx
emissions have also decreased dramatically over roughly the same time period
and have likely more than offset any O3 increase from decreasing
sulfate. However, the full potential of possible O3 improvement via
NOx and anthropogenic volatile organic carbon (VOC) decreases may have
been partially masked by sulfate decreases. These findings highlight the
importance of a multipollutant strategy for effective clean air regulation.
Finally, in Fig. 6 we plot the relative change in MAM O3→O(1D) and
NO2 photolysis rates, denoted jO(1D) and jNO2 in response to
SO2 emissions in each region. Photolysis of both species is slightly
influenced by changing SO2 emissions, especially over China in response
to China's SO2 emissions, where decreases in both photolysis rates are
about 7 %. For each of the perturbations, especially the US and Europe
cases, changes in photolysis rates rarely rise above the noise, which is
likely caused by meteorological factors such as slight changes in cloud
cover. We conclude that while radiative effects via photolysis are
non-negligible, they are significantly less important than chemical effects
for aerosol impacts on oxidation, consistent with previous findings (Li et
al., 2019).
Summary and conclusions
Using the updated GFDL-AM3 nudged chemistry–climate model with online
aerosol heterogeneous chemistry and interactions with radiation, we estimate
the impact of Northern Hemisphere mid-latitude regional anthropogenic
SO2 emissions on tropospheric OH, HO2, O3, and NOx.
Regional SO2 emissions perturbations lead to significant changes to
sulfate aerosol in far-reaching regions of the world, particularly in the
Arctic and the mid- and upper troposphere. OH and HO2 decrease
throughout the northern hemisphere mid-troposphere by up to 10 %, which in
turn increase NOx concentrations by at least 5 %. NOx is not
efficiently removed by heterogeneous reactions on aerosols, while species
that contribute to NOx sinks such as OH (via HO2 uptake) and
NO3 are efficiently removed, slowing down the NOx sink and
increasing NOx concentrations. However, any influence of NOx
increases on tropospheric O3 are overwhelmed by HO2 decreases, and
the resulting decrease in O3 production offsets decreases in O3
sinks, resulting in up to 4 % decrease in O3 in the free troposphere
and at the surface. Aerosols impact oxidation primarily through
heterogeneous reactive uptake pathways over photolysis pathways.
Surface ozone decreases by 3 to 5 ppbv in response to the introduction of
regional SO2 emissions. If SO2 emissions decline in developing
regions of the world such as South Asia and sub-Saharan Africa, a goal
attained through air quality improvements to protect human health, there
could be an unintended increase in surface O3 concentrations.
Decreasing surface O3 in these regions will require a multipollutant
approach in which NOx and VOCs are simultaneously decreased with
aerosols in order to offset the effect of decrease in aerosols and their
precursors. While SO2 and NOx emissions decreases coincided to
some extent in the US, end-of-pipe technologies at power plants allow for
control of SO2 and NOx individually, and other sources of fine
particulate matter (PM2.5) such as waste burning and vehicle emissions
will have a similar effect on ozone as sulfate aerosols. PM2.5 and
SO2 have decreased dramatically in recent years in the US and Europe,
such that O3 improvements may have been partially masked by the aerosol
impact. SO2 perturbations from our simulations are 10.8, 12.4, and 16.2 Tg yr-1 for US, Europe, and China, respectively, which result in a 3 ppbv surface ozone response over the US and Europe, and a 5 ppbv surface
ozone response over China, where SO2 emissions are the largest in 2015.
Model overprediction of surface O3 over urban areas in China
(Westervelt et al., 2019) likely make this 5 ppbv change an upper estimate
of the surface O3 response to China SO2 emissions.
Future work is needed to improve estimates of reactive uptake of HO2
and other radical species by aerosols, as great uncertainty still exists
surrounding this parameter as well as the dependence of aerosol composition
on reactive uptake parameters (George et al.,
2013). We focus here on anthropogenic aerosols as they are changing rapidly
and expected to continue to change. Previous work finds a large influence of
Saharan dust aerosols on oxidation (Tie et al., 2005). Regions
of biomass burning such as Africa and South America are also potential
contributors to aerosol-driven oxidation changes. In order to avoid
trading one problem for another in areas of the world that are
experiencing both rapid emissions changes and high exposures to air
pollutants, we must better understand the impact of aerosols on atmospheric
photochemistry.
Code availability
The code for GFDL-AM3 is available here: https://www.gfdl.noaa.gov/am3/ (Geophysical Fluid Dynamics Lab, 2021).
Data availability
Data are available here: https://figshare.com/articles/dataset/Concentration_data_for_aerosol_impact_on_oxidants/13331066 (Westervelt,
2020).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-6799-2021-supplement.
Author contributions
DMW wrote the manuscript, created all figures, and conducted all
simulations. AMF and DMW originally conceived the project. CBB assisted with
model setup and output analysis. GC developed the model for use at LDEO.
All authors contributed to editing the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Bryan Duncan and Melanie Follette-Cook of NASA GSFC for their helpful
conversations.
Financial support
This research has been supported by the National Aeronautics and Space Administration, Earth Sciences Division (grant no. NNX17AG40G).
Review statement
This paper was edited by Qiang Zhang and reviewed by two anonymous referees.
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