ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-13647-2015Oceanic bromoform emissions weighted by their ozone depletion potentialTegtmeierS.stegtmeier@geomar.dehttps://orcid.org/0000-0001-9206-3161ZiskaF.PissoI.https://orcid.org/0000-0002-0056-7897QuackB.VeldersG. J. M.https://orcid.org/0000-0002-6597-7088YangX.https://orcid.org/0000-0002-3838-9758KrügerK.https://orcid.org/0000-0002-0636-9488GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, GermanyNorwegian Institute for Air Research (NILU), Kjeller, NorwayNational Institute for Public Health and the Environment, Bilthoven,
the NetherlandsBritish Antarctic Survey, Cambridge, UKUniversity of Oslo, Oslo, NorwayS. Tegtmeier (stegtmeier@geomar.de)10December20151523136471366312March201526May201517November201527November2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/15/13647/2015/acp-15-13647-2015.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/15/13647/2015/acp-15-13647-2015.pdf
At present, anthropogenic
halogens and oceanic emissions of very short-lived substances (VSLSs) both
contribute to the observed stratospheric ozone depletion. Emissions of the
long-lived anthropogenic halogens have been reduced and are currently
declining, whereas emissions of the biogenic VSLSs are expected to increase
in future climate due to anthropogenic activities affecting oceanic
production and emissions. Here, we introduce a new approach for assessing the
impact of oceanic halocarbons on stratospheric ozone by calculating their
ozone depletion potential (ODP)-weighted emissions. Seasonally and spatially
dependent, global distributions are derived within a case-study framework for
CHBr3 for the period 1999–2006. At present, ODP-weighted emissions of
CHBr3 amount up to 50 % of ODP-weighted anthropogenic emissions of
CFC-11 and to 9 % of all long-lived ozone depleting halogens. The
ODP-weighted emissions are large where strong oceanic emissions coincide with
high-reaching convective activity and show pronounced peaks at the Equator
and the coasts with largest contributions from the Maritime Continent and
western Pacific Ocean. Variations of tropical convective activity
lead to seasonal shifts in the spatial distribution of the trajectory-derived
ODP with the updraught mass flux, used as a proxy for trajectory-derived ODP,
explaining 71 % of the variance of the ODP distribution. Future climate
projections based on the RCP 8.5 scenario suggest a 31 % increase of the
ODP-weighted CHBr3 emissions by 2100 compared to present values. This
increase is related to a larger convective updraught mass flux in the upper
troposphere and increasing emissions in a future climate. However, at the
same time, it is reduced by less effective bromine-related ozone depletion
due to declining stratospheric chlorine concentrations. The comparison of the
ODP-weighted emissions of short- and long-lived halocarbons provides a new
concept for assessing the overall impact of oceanic halocarbon emissions on
stratospheric ozone depletion for current conditions and future projections.
Introduction
The overall abundance of ozone-depleting substances in the atmosphere has
been decreasing since the beginning of the 21st century as a result of the
successful implementation of the 1987 Montreal Protocol and its later
adjustments and amendments (Carpenter and Reimann, 2014). In contrast to the
long-lived halocarbons, the halogenated very short-lived substances (VSLSs)
with chemical lifetimes of less than 6 months are not controlled by the
Montreal Protocol and are even suggested to increase in the future (Hepach et
al., 2014; Hossaini et al., 2015). Brominated VSLSs are known to have large
natural sources; however, evidence has emerged that their oceanic production
and emissions are enhanced through anthropogenic activities which are
expected to increase in the future (Leedham et al., 2013; Ziska et al.,
2015). At present, oceanic VSLSs provide a significant contribution to the
stratospheric bromine budget (Carpenter and Reimann, 2014). In the future,
the decline of anthropogenic chlorine and bromine will further increase the
relative impact of oceanic VSLSs on stratospheric chemistry. The amount of
ozone loss for given bromine emission, however, is expected to decrease due
to decreasing stratospheric chlorine concentrations and thus a less efficient
BrO / ClO ozone loss cycle (Yang et al., 2014). Furthermore, the impacts
of climate change on surface emissions, troposphere-to-stratosphere
transport, stratospheric chemistry and residence time will change the role of
VSLSs (Pyle et al., 2007; Hossaini et al., 2012). While stratospheric ozone
depletion due to long-lived halocarbons is expected to level off and reverse
(Austin and Butchart, 2003), assessing oceanic VSLSs and their impact on
stratospheric ozone in a future changing climate remains a challenge.
Over the last years, there has been
increasing evidence from observational (e.g., Dorf et al., 2006; Sioris et
al., 2006) and modeling (e.g., Warwick et al., 2006; Liang et al., 2010;
Tegtmeier et al., 2012) studies that VSLSs provide a significant contribution
to stratospheric total bromine (Bry). The current best-estimate range of
2–8 ppt (Carpenter and Reimann, 2014) includes observation-derived
estimates of 2.9 ppt (Sala et al., 2014) and model-derived estimates of
4 ppt (Hossaini et al., 2013), 4.5–6 ppt (Aschmann and Sinnhuber, 2013)
and 7.7 ppt (Liang et al., 2014). Brominated VSLSs reduce ozone in the lower
stratosphere with current estimates of a 3–11 % contribution to ozone
depletion (Hossaini et al., 2015) or a 2–10 % contribution (Braesicke et
al., 2013; Yang et al., 2014). Through the relatively large impact of VSLSs
on ozone in the lower stratosphere, VSLSs contribute -0.02 W m-2 to
global radiative forcing (Hossaini et al., 2015) (∼ 6 % of the
0.33 W m-2 from all halocarbons of ozone-depleting
substances).
The most abundant bromine-containing VSLSs are dibromomethane
(CH2Br2) and bromoform (CHBr3) with potentially important
source regions in tropical, subtropical and shelf waters (Quack et al.,
2007). The contribution of VSLSs to stratospheric bromine in form of organic
source gases or inorganic product gases depends strongly on the efficiency
of troposphere-to-stratosphere transport relative to the photochemical loss
of the source gases and to the wet deposition of the product gases.
Uncertainties in the contribution of VSLSs to stratospheric halogen loading
mainly result from uncertainties in the emission inventories (e.g., Hossaini
et al., 2013) and from uncertainties in the modeled transport and wet
deposition processes (e.g., Schofield et al., 2011).
The relative contribution of individual halocarbons to stratospheric ozone
depletion is often quantified by the ozone depletion potential (ODP) defined
as the time-integrated ozone depletion resulting from a unit mass emission of
that substance relative to the ozone depletion resulting from a unit mass
emission of CFC-11 (CCl3F) (Wuebbles, 1983). Independent of the total
amount of the substance emitted, the ODP describes only the potential but not
the actual damaging effect of the substance to the ozone layer, relative to
that of CFC-11. The ODP, traditionally defined for anthropogenic long-lived
halogens, is a well-established and extensively used measure and plays an
important role in the Montreal Protocol for control metrics and reporting of
emissions. Some recent studies have applied the ODP concept to VSLSs (e.g.,
Brioude et al., 2010; Pisso et al., 2010), which have also natural sources.
Depending on the meteorological conditions, only a fraction of the originally
released VSLSs reaches the
stratosphere. As a consequence, the ODP of a VSLS is not one number as for
the long-lived halocarbons but needs to be quantified as a function of time
and location of emission. ODPs of VSLSs have been estimated based on Eulerian
(Wuebbles et al., 2001) and Lagrangian (Brioude et al., 2010; Pisso et al.,
2010) studies, showing strong geographical and seasonal variations, in
particular within the tropics. The studies demonstrated that the ODPs of
VSLSs are to a large degree determined by the efficiency of vertical
transport from the surface to the stratosphere and that uncertainties in the
ODPs arise mainly from uncertainties associated with the representation of
convection.
Combining the emission strength and the ozone-destroying capabilities of a
substance in a meaningful way can be achieved by calculating the ODP-weighted
emissions. For the long-lived halocarbons, global ODP-weighted emissions can
be calculated as the product of two numbers, their mean global emissions and
their ODPs (e.g., Velders et al., 2007; Ravishankara et al., 2009). For the
VSLSs, however, the concept of ODP-weighted emissions has not yet been
applied. To do so requires combining estimates of the emissions with the
ODPs, both of which are highly variable in space and time. Among the
brominated VSLSs, the calculation of CHBr3 ODP-weighted emissions is now
possible, since global emission
inventories (Ziska et al., 2013) and global ODP maps (Pisso et al., 2010)
has become available. ODP-weighted emissions provide insight in where and when
CHBr3 is emitted that impacts stratospheric ozone. Furthermore, in a
globally averaged framework, the ODP-weighted emissions allow comparisons of
the impact of past, present and future long- and short-lived halocarbon
emissions. The ODP-weighted emissions for the anthropogenic component of the
CHBr3 emission budget cannot be calculated, since no reliable estimates
of anthropogenic contributions are available at the moment. The concept is
introduced here for the available total emission inventory.
We compile ODP-weighted emissions of CHBr3 in form of the seasonal and
annual mean distribution in order to assess the overall impact of oceanic
CHBr3 emissions on stratospheric ozone. First, we introduce the new
approach of calculating ODP-weighted VSLS emissions, taking into account the
high spatial variability of oceanic emission and ODP fields (Sect. 2). Maps
and global mean values of ODP-weighted CHBr3 emissions for present-day
conditions are given in Sect. 3. The method and application are introduced
for CHBr3 within a case-study framework and can be applied to all VSLSs
where emissions and ODP are available at a spatial resolution necessary to
describe their variability. In Sect. 4, we demonstrate that ODP fields of
short-lived gases can be estimated based on the convective mass flux from
meteorological reanalysis data and develop a proxy for the ODP of CHBr3.
We use this method to derive long-term time series of ODP-weighted CHBr3
emissions for 1979–2013 based on ERA-Interim data in Sect. 5. Model-derived
ODP-weighted CHBr3 emissions for present conditions are introduced in
Sect. 6. Based on model projections of climate scenarios, the future
development of the ODP-weighted CHBr3 emissions is analyzed in Sect. 7.
This approach provides a new tool for an assessment of future growing
biogenic VSLSs and declining chlorine emissions in the form of a direct comparison
of the global-averaged ODP-weighted emissions of short- and long-lived
halocarbons.
Data and methodsCHBr3 emissions
The present-day global emission scenario from Ziska et al. (2013) is a
bottom-up estimate of the oceanic CHBr3 fluxes. Emissions are estimated
using global surface concentration maps generated from the atmospheric and
oceanic in situ measurements of the HalOcAt (Halocarbons in the Ocean and
Atmosphere) database project (https://halocat.geomar.de). The in situ
measurements collected between 1989 and 2011 were classified based on
physical and biogeochemical characteristics of the ocean and atmosphere and
extrapolated to a global 1∘× 1∘ grid with the
ordinary least square regression technique. Based on the concentration maps,
the oceanic emissions were calculated with the transfer coefficient
parameterization of Nightingale et al. (2000) adapted to CHBr3 (Quack
and Wallace, 2003). The concentration maps represent climatological fields
covering the time period 1989–2011. The emissions are calculated as a
6-hourly time series based on meteorological ERA-Interim data (Dee et al.,
2011) for 1979–2013 under the assumption that the constant concentration
maps can be applied to the complete time period (Ziska et al., 2013). Recent
model studies showed that atmospheric CHBr3 derived from the Ziska et
al. (2013) bottom-up emission inventory agrees better with tropical
atmospheric measurements then the CHBr3 model estimates derived from top-down emission
inventories (Hossaini et al., 2013).
Future emission estimates are calculated based on the present-day
(1989–2011) climatological concentration maps and future estimates of global
sea surface temperature, pressure, winds and salinity (Ziska et al., 2015).
The meteorological parameters are model output from the Community Earth
System Model version 1 – Community Atmospheric Model version 5 (CESM1-CAM5)
(Neale et al., 2010) runs based on the Representative Concentration Pathway
(RCP) 8.5 scenarios conducted within phase 5 of the Coupled Model
Intercomparison Project (CMIP5) (Taylor et al., 2012). The CESM1-CAM5 model
has been chosen since it provides model output for all the parameters
required to calculate future VSLS emissions and future ODP estimates
(Sect. 2.2). Comparisons have shown that the global emissions based on
historical CESM1-CAM5 meteorological data agree well with emissions based on
ERA-Interim fields (Ziska et al., 2015). For the time period 2006–2100, the
global monthly mean emissions are calculated based on the monthly mean
meteorological input parameters from CESM1-CAM5 and the fixed atmospheric and
oceanic concentrations from Ziska et al. (2013) following the
parameterization of the air–sea gas
exchange coefficient from Nightingale et al. (2000). The future global
CHBr3 emissions increase by about 30 % until 2100 for the CESM1-CAM5
RCP 8.5 simulation. These derived changes of the future VSLS emissions are
only driven by projected changes in the meteorological and marine surface
parameters, in particular, by changes in surface wind and sea surface
temperature. The respective contributions of wind and temperature changes to
the future emission increase can vary strongly depending on the region (Ziska
et al., 2015). The future emissions do not take into account possible changes
of the oceanic concentrations, since no reliable estimates of future oceanic
halocarbon production and loss processes exist so far.
CHBr3 trajectory-derived ODP
The ozone depletion potential is a measure of a substance's destructive
effect to the ozone layer relative to the reference substance CFC-11 (Wuebbles, 1983). ODPs
of long-lived halogen compounds can be calculated based on the change in
total ozone per unit mass emission of this compound using atmospheric
chemistry-transport models. Alternatively, the ODP of a long-lived species
X can be estimated by a semiempirical approach (Solomon et al., 1992):
ODPX=MCFC-11MXαnBr+nCl3τXτCFC-11,
where τ is the global atmospheric lifetime, M is the molecular
weight, n is the number of halogen atoms and α is the effectiveness
of ozone loss by bromine relative to ozone loss by chlorine. In contrast to
the long-lived halocarbons, for VSLSs the tropospheric transport timescale
plays a dominant role for the calculation of their ODP and the concept of a
global lifetime τX cannot be adapted. Therefore, the global lifetime
needs to be replaced by an expression weighting the fraction of VSLSs reaching
the tropopause and their subsequent residence time in the stratosphere.
Following a method previously developed specifically for VSLSs, the ODP of
CHBr3 is calculated as a function of location and time of emission
(xe,te) based on ERA-Interim-driven FLEXPART trajectories
(Pisso et al., 2010). Based on the trajectory calculations, the fraction of
VSLSs reaching the tropopause and the stratospheric residence time are
derived. Owing to the different timescales and processes in the troposphere
and stratosphere, the estimates are based on separate ensembles of
trajectories quantifying the transport in both regions. The tropospheric
trajectory ensembles are used to determine the fraction of VSLSs reaching the
tropopause at different injection points (y,s). The subsequent residence
time in the stratosphere is quantified from stratospheric trajectory
ensembles run for a longer time period (20 years). ODPs as a function of
location and time of emission were obtained from Eq. (1) where the expression
∫te∞∫ΩσrXΩTstratdyds replaces τX. This expression integrated in time
s starting at the emission time te and throughout the surface
Ω (representing the tropopause) is estimated from the tropospheric
and stratospheric trajectory ensembles. Tropospheric transport appears as the
probability σy,s;xe,te of
injection at (y,s) in Ω, while physicochemical processes in the troposphere appear as the
injected proportion of total halogen emitted rXΩy,s;xe,te. Stratospheric transport is taken
into account by Tstraty,s, which expresses the
stratospheric residence time of a parcel injected at the tropopause at
(y,s). An ozone depletion efficiency factor of 60 is used for bromine
(Sinnhuber et al., 2009). A more detailed derivation of the approximations
and parameterizations including a discussion of the errors involved can be
found in Pisso et al. (2010).
CHBr3 mass-flux-derived ODP
While present-day ODP estimates for VSLSs based on ERA-Interim are available
(e.g., Pisso et al., 2010), the trajectory-based method has not been applied
to future model scenarios so far. Therefore, we attempt to determine an ODP
proxy easily available from climate model output, which can be used to derive
future estimates of the ODP fields. In general, the ODP of a VSLS as a
function of time and location of emission is determined by tropospheric and
stratospheric chemistry and transport processes. It has been shown, however,
that the effect of spatial variations in the stratospheric residence time on
the ODP is relatively weak (Pisso et al., 2010). We identify a pronounced
relationship between the ODP of CHBr3 and deep convective activity,
which demonstrates that for such short-lived substances the ODP variability
is mostly determined by tropospheric transport processes. Based on the
identified relationship we develop a proxy for the ODP of CHBr3 based on
the ERA-Interim convective upward mass flux. For the available
trajectory-derived ODP fields, we determine a linear fit a0,a1 with residual r in a least-square sense:
y=a0+a1x+r.
The dependent variable y is the trajectory-based ODP prescribed as a
vector of all available monthly mean ODP values comprising 26 months of data
re-gridded to the ERA-Interim standard resolution of
1∘× 1∘. The independent variable x is a
vector of the ERA-Interim monthly mean updraught mass flux between 250 and
80 hPa with a 1∘× 1∘ resolution for the same
months. The fit coefficients a0,a1 are used to
calculate the ODP proxy y^:
y^=a0+a1x.
The fit scores a coefficient of determination of 0.71 conveying that our ODP
proxy (called mass-flux-derived ODP from now on) explains 71 % of the
variance of the original trajectory-derived ODP fields for the time period
1999–2006. We find good agreement between the trajectory-derived and the
mass-flux-derived ODP and ODP-weighted CHBr3 emissions (see Sects. 4 and
5 for details). In order to extend the ODP-weighted CHBr3 emissions
beyond 1999 and 2006, we apply the linear fit function a0,a1 to the convective upward mass flux between 250 and 80 hPa from
ERA-Interim and from the CESM1-CAM5 runs. Thus, we estimate observational (1979–2013), model historical
(1979–2005) and model future RCP 8.5 (2006–2100) mass-flux-derived-ODP
fields.
The ODP of such short-lived substances as CHBr3 shows a weak dependence
on the stratospheric residence time and thus on the latitude of the
injection point at the tropopause (Pisso et al., 2010). Our method of
deriving the ODP from the convective mass flux neglects the impact of
spatial variations in the stratospheric residence time on the ODP. However,
within the tropical belt, which is the main region of interest for our
analysis with high ODP values and strong convective mass fluxes, the
stratospheric residence time can be approximated by a constant as included
in the fit coefficients. Similarly, expected future changes of the
stratospheric residence time associated with an accelerating stratospheric
circulation (Butchart, 2014) are not taken into account in our calculation
of the mass-flux-derived ODP from model climate predictions. We expect that
changes in the stratospheric residence time only have a small impact on the
future ODP compared to the impacts of tropospheric transport and
stratospheric chemistry. Thus, we do not take the latter into account in our
calculation of future ODP-weighted CHBr3 emissions for the benefit of a
computationally efficient method enabling the estimation of future ODP
fields.
In addition to changing mass fluxes included in our ODP proxy, changes in
stratospheric chemistry will impact the future ODP of CHBr3. In order to
account for less effective catalytic ozone destruction, we apply a changing
α-factor to our ODP fields. The bromine α-factor describes
the chemical effectiveness of stratospheric bromine in depleting ozone relative to
that of chlorine (Daniel et al., 1999) and
is set to a global mean value of 60 (Sinnhuber et al., 2009) for the
calculation of 1999–2006 ODP fields (Sect. 2.2). As most of the
bromine-induced stratospheric ozone loss is caused by the combined
BrO / ClO catalytic cycle, the effect of bromine (and thus the α-factor) is expected to be smaller for decreasing anthropogenic chlorine. We
use idealized experiments carried out with the UM-UKCA chemistry–climate
model to derive changes in the α-factor of brominated VSLSs. The
experiments were performed under two different stratospheric chlorine
concentrations, corresponding roughly to the beginning (3 ppbv Cly)
and end (0.8 ppbv Cly) of the 21st century conditions and to 1xVSLS
versus 2xVSLS loading (see Yang et al., 2014, for details). We calculate the
difference between the 2xVSLS and 1xVSLS simulations for both chlorine
scenarios to get the overall effect of VSLSs on ozone for the beginning and
end of the 21st century conditions. From the change of this difference from
one chlorine scenario to the other, we estimate the global mean α-factor applicable for bromine from VSLSs at the end of the century to be
around 47. Compared to the current α-factor of 60 this is a reduction
of about 22 %. For simplicity, we assume the stratospheric chlorine
loading from 2000 to 2100 to be roughly linear and estimate the α-factor within this time period based on a linear interpolation between the
2000 and 2100 values. In a similar manner, we scale the ODP field before 1996
to account for the fact that during this time there was less stratospheric
chlorine and a reduced effectiveness of bromine-related ozone depletion.
Stratospheric chlorine in 1979 equals roughly the value expected for 2060
(Harris et al., 2014), thus corresponding to a 13 % reduced bromine
α-factor of 52. ODP values between 1979 and the year 1996, when the
amount of stratospheric chlorine reached a peak and started to level off
(Carpenter and Reimann, 2014), are estimated based on a linear interpolation
over this time period.
ODP-weighted CHBr3 emissions
The concept of ODP-weighted emissions combines information on the emission
strength and on the relative ozone-destroying capability of a substance. Its
application to VSLSs has been recently rendered possible by the availability
of observation-based VSLS emission maps (Ziska et al., 2013). Here, we
calculate the present-day ODP-weighted emissions of CHBr3 for data
available for 4 months (March, June, September and December) from 1999 to
2006 by multiplying the CHBr3 emissions with the trajectory-derived ODP
at each grid point. The resulting ODP-weighted emission maps are given as a
function of time (monthly averages) and location
(1∘× 1∘ grid). Global annual means are calculated
by averaging over all grid points and over the 4 given months.
In order to extend the time series of ODP-weighted CHBr3 emissions
beyond 1999 and 2006, we derive ODP fields from the ERA-Interim upward mass
flux. The method is based on the linear polynomial fit determined for the
available trajectory-derived CHBr3 ODP fields as described in Sect. 2.3.
Multiplying the mass-flux-derived ODP fields with the monthly mean emission
fields from Ziska et al. (2013) results in a long-term time series
(1979–2013) of ODP-weighted CHBr3 emissions. Similarly, we use the
CESM1-CAM5 mass-flux-derived ODP fields together with emission inventories
derived from CESM1-CAM5 meteorological data to produce historical
(1979–2005) and future (2006–2100) model-driven ODP-weighted CHBr3
emission fields.
Global CHBr3 emissions (a) and ODP (b) are
given for March 2005. The CHBr3 emissions are bottom-up estimates based
on the extrapolation of in situ measurements (Ziska et al., 2013). The ODP is
given as a function of time and location of emission and was derived based on
a Lagrangian approach (Pisso et al., 2010).
ODP-weighted CHBr3 emissions for present-day conditions
We will introduce the concept of the ODP-weighted emissions of CHBr3
exemplarily for March 2005 and discuss how the ODP-weighted emissions of this
very short-lived compound compare to those of long-lived halogens. The
CHBr3 emissions (Ziska et al., 2013) for March 2005 are shown in Fig. 1a
with highest emissions in coastal regions, in the upwelling equatorial waters
and the Northern Hemisphere (NH) midlatitude Atlantic. The emissions show
large variations and reach values higher than 1500 pmol m-2 h-1
in coastal regions characterized by high concentrations due to biological
productivity and anthropogenic activities. In the tropical open ocean,
emissions are often below 100 pmol m-2 h-1, while in the
subtropical gyre regions, ocean and atmosphere are nearly in equilibrium and
fluxes are around 0. Globally, the coastal and shelf regions account for
about 80 % of all CHBr3 emissions (Ziska et al., 2013). Apart from
the gradients between coastal, shelf and open ocean waters the emissions show
no pronounced longitudinal variations. Negative emissions occur in parts of
the Southern Ocean, northern Pacific and North Atlantic and indicate a
CHBr3 sink given by a flux from the atmosphere into the ocean. The
evaluation of various CHBr3 emission inventories from Hossaini et
al. (2013) shows that in the tropics the best agreement between model and
observations is achieved using the bottom-up emissions from Ziska et al.
(2013). In the extratropics, however, the CHBr3 emissions from Ziska are
found to result in too-low atmospheric model concentrations diverging from
observations by 40 to 60 %.
The potential impact of CHBr3 on the stratospheric ozone layer is
displayed in Fig. 1b in the form of the ODP of CHBr3 given as a function of
time and location of the emissions but independent of its strength. Overall,
the ODP of CHBr3 is largest in the tropics (tropical ODP belt) and has
low values (mostly below 0.1) north and south of 20∘. The ODP depends
strongly on the efficiency of rapid transport from the ocean surface to the
stratosphere which is in turn determined by the intensity of high reaching
convection. In the NH winter/spring of most years, the strongest convection
and therefore the highest ODP values of up to 0.85 are found over the
equatorial western Pacific (Pisso et al., 2010). In contrast to the CHBr3
emission estimates, the ODP shows pronounced longitudinal variations linked
to the distribution of convection and low-level flow patterns.
The ODP-weighted CHBr3 emissions for March 2005 are displayed in Fig. 2.
While the emissions themselves describe the strength of the CHBr3
sea-to-air flux, the ODP-weighted emissions cannot be interpreted directly as
a physical quantity but only relative to ODP-weighted emissions of long-lived
halocarbons. The spatial distribution of the ODP-weighted emissions combines
information on where large amounts of CHBr3 are emitted from the ocean
and where strong vertical transport enables CHBr3 to reach the
stratosphere. Only for regions where both quantities are large, strong
ODP-weighted emissions will be found. Regions where one of the quantities is
close to 0 will not be important, such as the midlatitude North Atlantic
where large CHBr3 emissions occur but the ODP is very low. Negative
ODP-weighted emissions occur in regions where the flux is from the atmosphere
into the ocean. Since negative ODP-weighted emissions are not a meaningful
quantity and occur in regions where the ODP is small, they will not be displayed in the following figures and are
not taken into account for the calculations of the global mean values. The
ODP-weighted emissions are in general largest between 20∘ S and
20∘ N (72 % of the overall global amount) as a result of the
tropical ODP belt and peak at the Equator and tropical coast lines as a
result of the emission distribution. The distribution of the ODP-weighted
emissions demonstrates clearly that CHBr3 emissions from the NH and
Southern Hemisphere (SH) extratropics have negligible impact on stratospheric
ozone chemistry. Thus, the fact that the emissions from Ziska et al. (2013)
might be too low in the extratropics (Hossaini et al., 2013) does not impact
our results. Of particular importance for the stratosphere, on the other
hand, are emissions from the Maritime Continent (Southeast Asia), the
tropical Pacific and the Indian Ocean.
Global ODP-weighted CHBr3 emissions are given for March 2005.
The ODP-weighted emissions have been calculated by multiplying the CHBr3
emissions with the ODP at each grid point.
The global annual mean ODP-weighted emissions of CHBr3 are about
40 Gg year-1 for 2005 (Fig. 3) based on the March, June, September and
December values of this year. The concept of ODP-weighted emissions becomes
particularly useful when comparing this quantity for CHBr3 with the ones
of manmade halocarbons. For the year 2005, ODP-weighted emissions of
CHBr3 amount up to 50 % of the ODP-weighted emissions of methyl
bromide (CH3Br, natural and anthropogenic), CFC-11 or CFC-12
(CCl2F2) and are of similar magnitude as the ODP-weighted emissions
of CCl4 and the individual halons. While the ODP of CHBr3 exceeds
the value of 0.5 only in less than 10 % of the regions over the globe,
the relatively large CHBr3 emissions make up for the overall relatively
small ODPs. Current estimates of global CHBr3 emissions range between
249 and 864 Gg year-1 (Ziska et al., 2013, and references therein),
with the higher global emission estimates coming from top-down methods while
the lower boundary is given by the bottom-up study from Ziska et al. (2013).
For our study, even the choice of the lowest emission inventory leads to
relatively large ODP-weighted emissions of the very short-lived CHBr3 as
discussed above. Choosing a different emission inventory than Ziska et
al. (2013) would result in larger ODP-weighted CHBr3 emissions. Still
more important than the overall CHBr3 emission strength is the fact that
emissions and ODP show similar latitudinal gradients with both fields having
higher values at the low latitudes. This spatial coincidence of large sources
and efficient transport leads to the relatively large global mean value of
ODP-weighted CHBr3 emissions.
A comparison of the global annual mean ODP-weighted emissions of
CHBr3 and long-lived halocarbons is shown for 2005. Emissions of
long-lived halocarbons have been derived
from NOAA and AGAGE global sampling network measurements (Montzka et al.,
2011).
ODP-weighted emissions calculated as the product of the emissions
maps (Fig. S1 in the Supplement) and the trajectory-based ODP fields
(Fig. 5a) are displayed for June and December 2001.
It is important to keep in mind that the long-lived halocarbons are to a
large degree of anthropogenic origin, while CHBr3 is believed to have
mostly natural sources. However, CHBr3 in coastal regions also results
from anthropogenic activities such as aquafarming in Southeast Asia (Leedham
et al., 2013) and oxidative water treatment (Quack and Wallace, 2003). While
these sources accounted for only a small fraction of the global budget in
2003 (Quack and Wallace, 2003), their impact is increasing. In particular,
aquafarming used, among other things, for food production and CO2
sequestering has started to increase as an anthropogenic VSLS source. Leedham
et al. (2013) estimated tropical halocarbon production from macroalgae in the
Malaysian costal region and suggest that only 2 % of the local CHBr3
emissions originate from farmed seaweeds. However, based on recent production
growth rates, the Malaysian seaweed aquaculture has been projected to
experience a 6–11-fold increase over the next years (Phang et al., 2010).
More importantly, other countries such as Indonesia, Philippines and China
are known to produce considerably more farmed seaweed than Malaysia (e.g.,
Tang et al., 2011), but their contribution to the total anthropogenic VSLS
emissions has not yet been assessed. The ODP of CHBr3 demonstrates the
high sensitivity of the Southeast Asia region to growing emissions. Globally
the highest ODP values (Fig. 1b) are found in the same region where we expect
future anthropogenic CHBr3 emissions to increase substantially. An
assessment of current and future seaweed farming activities including
information on farmed species, fresh or dry-weight macroalgal biomass and
incubation-derived halocarbon production values is required to estimates the
net oceanic aquaculture VSLS production. Since the general ODP concept has
been originally defined for anthropogenic halogens, the ODP-weighted
CHBr3 emissions should be calculated for the anthropogenic component of
the emissions. However, since no such estimates are available at the moment,
the method is applied to the combined emission field. Given that the natural
oceanic production and emissions of halogenated VSLSs are expected to change
in the future due to increasing ocean acidification, changing primary
production and ocean surface meteorology (Hepach et al., 2014), it will
remain a huge challenge to properly separate natural and anthropogenic
emissions of these gases.
ODP proxy
It is necessary to understand the short- and long-term changes of the
ODP-weighted CHBr3 emissions in order to predict their future
development. On the seasonal timescales, the ODP-weighted CHBr3
emissions show large variations as demonstrated in Fig. 4 for June and
December 2001. In the NH summer, 57 % of the ODP-weighted emissions stem
from the NH tropical belt (30–0∘ N) with the largest contributions from
the Maritime Continent and Asian coastal areas. In the NH winter, the
ODP-weighted emissions shift to the SH tropical belt (48 %) with the
strongest contributions from the western Pacific. While the Maritime Continent
is an important source region year-round, emissions from the southern
coast line of Asia during NH winter are not very important for stratospheric
ozone depletion. The emissions reveal some seasonal variations which are most
apparent in the Indian Ocean with peak values during NH summer along the
Equator and along the NH coast lines (see Fig. S1 in the Supplement). Note
that CHBr3 concentrations maps represent climatological fields and the
seasonal variations in the emission fields stem from varying surface winds
and sea surface temperature (see Sect. 2.1). Global average CHBr3
emissions show a seasonal cycle of about 25 % with a maximum in July and
a minimum in April (Ziska et al., 2013). The seasonality of the ODP (Fig. 5a)
driven by the seasonality of deep convection amplifies the seasonal
variations in the emissions and thus causes the pronounced shift of the
ODP-weighted emissions from one hemisphere to the other.
In order to analyze the long-term changes of ODP-weighted CHBr3
emissions, we need to extend the time series beyond the 1999–2006 time
period. While CHBr3 emissions are available for 1979–2013, the ODP
itself, based on costly trajectory calculations, is restricted to 1999–2006.
In order to develop an ODP proxy, we first analyze the variations of the
trajectory-derived ODP fields and their relation to meteorological
parameters. The ODP fields for the months June and December 2001 (Fig. 5a)
have their maxima between 0 and 20∘ N for the NH summer and
5∘ N and 15∘ S for the NH winter. In the NH summer, the
dominant source region for stratospheric CHBr3 is located in the
equatorial western Pacific region including Southeast Asia. In the NH winter,
the source region is shifted westward and southward with its center now over
the western Pacific. These seasonal variations agree with results from previous
trajectory studies (e.g., Fueglistaler et al., 2005; Krüger et al., 2008)
and are consistent with the main patterns of tropical convection (Gettelman
et al., 2002).
A detailed picture of the high-reaching convective activities for June and
December is given in Fig. 5b in form of the ERA-Interim monthly mean
updraught mass flux between 250 and 80 hPa. The rapid updraughts
transporting air masses from the boundary layer into the tropical tropopause
layer (TTL) are part of the ascending branch of the tropospheric
circulation constituted by the position of the intertropical convergence zone
(ITCZ). The updraught convective mass fluxes are largest in and near the
summer monsoon driven circulations close to the Equator. Over the western
Pacific and Maritime Continent the region of intense convection is quite
broad compared to the other ocean basins due to the large oceanic warm pool
and strong monsoon flow. In addition to the overall annual north–south
migration pattern, large seasonal changes of the updraught mass flux are
visible over South America and the Maritime Continent consistent with the
climatological distribution of the ITCZ. The southeastward pointing extension
in the Pacific is strongest in the NH winter and indicates a double ITCZ.
Trajectory-based CHBr3 ODP fields (a), monthly mean
ERA-Interim updraught mass flux between 250 and 80 hPa (b) and the
mass-flux-derived ODP (c) are displayed for June and December 2001.
We derive a CHBr3 ODP proxy from the ERA-Interim updraught mass fluxes
(referred to as mass-flux-derived ODP, see Sect. 2.3 for details). While the
downdraught mass fluxes can also impact (5–15 %) the composition in the
upper troposphere/lower stratosphere (Frey et al., 2015), they are not
included in our proxy since their importance for the contribution of
CHBr3 to stratospheric bromine is less clear and cannot be prescribed by
a fit relation. The strong correlation between CHBr3 ODP and
high-reaching convection justifies our method by indicating that we capture
the most important process for explaining the ODP variability. The mass-flux-derived ODP fields are shown in Fig. 5c and explain 76 and 81 % of
the variance of the original trajectory-derived ODP fields (Fig. 5a).
Differences between the trajectory-derived ODP fields and the mass-flux-derived proxy may be caused by the fact that not only the location of
the most active convective region will determine the ODP distribution but
also patterns of low-level flow into these regions. Additionally, spatial and
seasonal variations in the expected stratospheric residence time may have a
small impact on the trajectory-derived ODP and cause deviations to the mass-flux-derived proxy. Largest disagreement between the trajectory-derived and
mass-flux-derived ODP is found over South America and Africa. However, the
ODP values over the continents are not important for the ODP-weighted
CHBr3 emissions due to the very low to non-existent emissions over land
(Quack and Wallace, 2003) and are not used in our study.
Our analysis confirms that the ODP of species with short lifetimes, such as
CHBr3, is to a large degree determined by the high-reaching convective
activity (Pisso et al., 2010). As a result, updraught mass-flux fields can be
used to derive a proxy of the ODP fields. Such a proxy can also be derived
from related meteorological parameters such as the ERA-Interim detrainment
rates (not shown here). The ODP proxies identified here provide a
cost-efficient method to calculate ODP fields for past (ERA-Interim) and
future (climate model output) meteorological conditions. Long-term changes in
stratospheric chemistry due to declining chlorine background levels are taken
into account by variations of the bromine α-factor (see Sect. 2.3 for
details). Our method enables us to analyze long-term changes of the ODP and
the ODP-weighted emissions, which would otherwise require very large
computational efforts.
ODP-weighted CHBr3 emissions for 1979–2013
Based on the ODP proxy and the correction of the α-factor introduced
in Sect. 4, we calculate ODP-weighted CHB3 emission fields for the
ERA-Interim time period from 1979 to 2013. As a test for our method, we
compare the global mean ODP-weighted emissions based on the trajectory- and
mass-flux-derived ODP fields for the years 1999–2006. The two time series of
ODP-weighted emissions are displayed in Fig. 6 and show a very good agreement
with slightly lower mass-flux-derived values (green line) than
trajectory-derived values (black line). Individual months can show stronger
deviations; e.g., for December 1999 the mass-flux-derived ODP-weighted
emissions are about 30 % smaller than the trajectory-derived ones. The
pronounced seasonal cycle with maximum values in the NH summer and autumn is
captured by both methods. The seasonal cycle of the global mean values is
mostly caused by the very high ODP-weighted emissions along the Southeast
Asian coast line which are present during the NH summer/autumn but not
during the NH winter. The same signal is evident from the CHBr3
emissions itself (see Fig. S1 in the Supplement) and is amplified by the
shift of high ODP values to the NH tropics during NH summer (Fig. 5a and c).
The pronounced seasonal cycle of the ODP-weighted emissions indicates a
seasonality of the CHBr3 concentrations in the TTL, which needs to be
verified by observations. Note that the ODP-weighted emissions of long-lived
halocarbons discussed in Sect. 3 show no strong seasonal variations. The good
agreement between the trajectory-derived and the mass-flux-derived
ODP-weighted CHBr3 emissions encourages the use of the latter for the
analysis of longer time series.
Time series of ODP-weighted CHBr3 emissions based on
ERA-Interim trajectory-derived ODP (black line) and mass-flux-derived ODP
(green line) for March, June, September and December 1999 to 2006.
The 35-year-long time series (1979–2013) of ODP-weighted CHBr3
emissions is based on the ERA-Interim surface parameters, TTL convective mass
flux and a changing bromine α-factor (Fig. 7a). The time series is
relatively flat over the first 27 years ranging from 34 to
39 Gg year-1. Over the last years from 2006 to 2013, a steep increase
occurred and ODP-weighted CHBr3 emissions of more than
41 Gg year-1 are reached. In order to analyze which component, the
mass-flux-derived ODP fields, the oceanic emissions or the stratospheric
chemistry, causes this steep increase, three sensitivity studies are
performed. In the first study, the emissions vary over the whole time period
(1979–2013), while the ODP field and the bromine α-factor are held
fixed at their 35-year mean values. Changes in the resulting global mean
ODP-weighted emission time series (Fig. 7b) are driven by changes in the
emissions alone and show a steady increase over the whole time period of
about 2.2 % per decade. This is in agreement with the linear trend of the
global mean CHBr3 emissions estimated to be 7.9 % over the whole
time period caused by increasing surface winds and sea surface temperatures
(Ziska et al., 2015). We do not expect the two trends to be identical, since
the ODP-weighted emissions only include emissions in convective active
regions, while the global mean emissions correspond to non-weighted mean
values including CHBr3 emissions from mid- and high latitudes.
Time series of ODP-weighted CHBr3 emissions for 1979–2013
based on ERA-Interim mass-flux-derived ODP is shown (a).
Additionally, sensitivity studies are displayed where two factors are kept
constant at their respective 1979–2013 mean values, while the other factor
varies with time. The sensitivity studies include ODP-weighted CHBr3
emissions driven by time-varying emissions (b), time-varying mass-flux-derived ODP (c) and time-varying stratospheric
chemistry (d).
CHBr3 emissions (a), mass-flux-derived
ODP (b) and ODP-weighted CHBr3 emissions (c) are shown
for ERA-Interim and for CESM1-CAM5 for March 2000.
For the second study, the emission fields and the α-factor are kept
constant at the 35-year mean values and the mass-flux-derived ODP is allowed
to vary with time. Changes in the resulting, global mean ODP-weighted
emission time series (Fig. 7c) are mainly driven by changes in the tropical
high-reaching convection and show a negative trend from 1979 to 2005 of
-3.4 % per decade. Over the years 2006–2013, however, changes in
convective activity lead to a steep increase of the ODP-weighted emissions.
These changes can either result from a general strengthening of the tropical
convective activity or from changing patterns of convective activity,
shifting regions of high activity so that they coincide with regions of
strong CHBr3 emissions. For the third sensitivity study, the emissions
and mass-flux-derived ODP are kept constant at the 35-year mean values, while
the α-factor varies with time according to the stratospheric chlorine
loading. ODP-weighted CHBr3 emissions increase by 13 % from 1979 to
1999 and peak during the time of the highest stratospheric chlorine loading
from 1999 to 2006. Overall, variations of the ODP-weighted CHBr3
emissions induced by the stratospheric chorine-related chemistry are in the
same range as the variations induced by changes in convective transport and
oceanic emissions.
Combining the conclusions of all three sensitivity studies reveals that for
the time period 1979 to 2005, the positive trend of the emissions and the
α-factor on the one hand and the negative trend of the mass-flux-derived ODP on the other hand mostly cancel out leading to a flat time
series of ODP-weighted CHBr3 emissions (Fig. 7a) with no long-term
changes. From 2005 to 2013, however, a strong increase in ODP and
continuously increasing emissions lead to a step-like increase of the
ODP-weighted CHBr3 emissions from 35 to 41 Gg year-1.
Model-derived ODP-weighted CHBr3 emissions
We aim to estimate ODP-weighted CHBr3 emissions from earth system model
runs. Therefore, we use CHBr3 emissions and the CHBr3 ODP proxy
calculated with CESM1-CAM5 sea surface temperature, surface wind and upward
mass flux (see Sect. 2 for details). In a first step, we
evaluate how well the results of our analysis based on the earth system model
compare to the results based on ERA-Interim. Figure 8a shows the distribution
of the three quantities, CHBr3 emissions, mass-flux-derived ODP and
ODP-weighted emissions, for ERA-Interim and CESM1-CAM5 exemplary for
March 2000. The distribution of the emission field is very similar between
ERA-Interim and CESM1-CAM5. Largest deviations are found in the Indian Ocean
along the Equator, where higher surface winds and temperatures in the model
force a stronger sea-to-air flux. Note that in this region, very limited
observational data were available for the construction of the emission
inventories and future updates will reveal whether these isolated data points are
representative of the equatorial Indian Ocean.
The ERA-Interim mass-flux-derived CHBr3 ODP (Fig. 8b) shows an almost
zonally uniform region of higher ODP values (around 0.4) extending south of
the Equator down to 20∘ S. In contrast, the CESM1-CAM5 mass-flux-derived ODP shows only three regions in the deep tropics (the Maritime
continent, Africa, South America) with values exceeding 0.3. While the ODP
from CESM1-CAM5 show higher local maxima than the ODP from ERA-Interim, the
globally averaged ODP field is larger for the reanalysis data than for the
model. As a result, the ODP-weighted CHBr3 emissions (Fig. 8c) based on
reanalysis data are higher in most of the tropics. Particularly, in the
eastern
Pacific and Indian Ocean large-scale features of enhanced ODP-weighted
CHBr3 emissions exist for ERA-Interim but not for the earth system
model. However, enhanced ODP-weighted emissions along some coast
lines are present in the model results (e.g., Indonesia) but are not as
pronounced in ERA-Interim. Overall, the ODP-weighted CHBr3 emissions for
March 2000 based on ERA-Interim and CESM1-CAM5 show similar distribution and
similar magnitude. The model-derived values are slightly smaller than the
observation-derived values mostly as a result of less high-reaching
convective activity in the model.
We compare the global mean ODP-weighted CHBr3 emissions based on the
ERA-Interim reanalysis data (observation derived) to the same quantity from the CESM1-CAM5 historical model run
for the 1999–2006 time period (Fig. 9). The historical ODP-weighted
emissions from CESM1-CAM5 show larger variations than the observation-derived
time series. The stronger variability is caused by a stronger variability in
the ODP time series possibly related to larger meteorological fluctuations in
the earth system model during this short time period. The overall magnitude
as well as the phase and amplitude of the seasonal cycle are captured
reasonably well by CESM1-CAM5, lending confidence in the use of the model to
estimate ODP-weighted CHBr3 emissions for future climate scenarios.
Recent improvements have been reported in the regional cloud representation
in the deep convective tropical Pacific (Kay et al., 2012) and in the
parameterization of deep convection and ENSO simulation (Neale et al., 2008).
Overall, our analysis demonstrates that the spatial and seasonal variability
of the model fields allow us to derive realistic ODP-weighted CHBr3
emission estimates.
Time series of CHBr3 ODP-weighted emissions based on
ERA-Interim (green line) and on historical CESM1-CAM5 runs (red line) are
shown. The ODP is calculated from the updraught mass-flux fields.
ODP-weighted CHBr3 emissions for 2006–2100
Future ODP-weighted CHBr3 emissions shown in Fig. 10a are based on
future model estimates of the CHBr3 emissions and the CHBr3 ODP
proxy. Both quantities are calculated based on the meteorological and marine
surface variables and convective mass flux from the CESM1-CAM5 RCP 8.5 runs.
In addition, we have applied a correction factor to the ODP fields to account
for a changing α-factor based on less effective ozone loss cycles in
the stratosphere due to the decrease of anthropogenic chlorine (Sect. 2.3).
The future estimates of the ODP-weighted CHBr3 emissions show pronounced
interannual variations of up to 20 %. Overall, the ODP-weighted emissions
increase steadily until 2100 by about 31 % of the 2006–2015 mean value
corresponding to a linear trend of 2.6 % per decade.
Time series of CHBr3 ODP-weighted emissions for 2006–2100
based on future (RCP 8.5 scenario) CESM1-CAM5 runs are shown (a).
Additionally, the future time series are displayed with two factors kept
constant at their respective 2006–2015 mean value while the other factor
varies with time. The sensitivity studies include ODP-weighted CHBr3
emissions driven by time-varying emissions (b), time-varying mass-flux-derived ODP (c) and time-varying stratospheric
chemistry (d).
Future projections of annual mean ODP-weighted emissions of
CHBr3 and other long-lived halocarbons are shown for 2000–2100. Future
ODP-weighted emission estimates for long-lived halocarbons (halons: halon
1211, 1301, 2402; HCFCs: HCFC-22, -141, -142) are shown.
In order to analyze what causes the strong interannual variability and the
long-term trend, we conduct sensitivity studies where only one factor
(emissions, mass-flux-derived ODP, stratospheric chemistry) changes while the
other two are kept constant. Figure 10b displays the time series of
ODP-weighted CHBr3 emissions for varying oceanic emission fields. The
emission-driven time series for 2006–2100 shows a positive trend of
2.2 % per decade which is very similar to the trend observed for the emission-driven time series for
1979–2013 based on ERA-Interim (Fig. 7b). However, the model-based
ODP-weighted emissions show no long-term change over the first 15 years and
the positive, emission-driven trend only starts after 2020. The second
sensitivity study (Fig. 10c) highlights changes in the ODP-weighted emissions
attributable to high-reaching convection (via the mass-flux-derived ODP),
while emission fields and α-factor are kept constant. Clearly, the
strong interannual variations in the combined time series (Fig. 10a) are
caused by the same fluctuations in the mass-flux-driven time series. In
comparison, the interannual variability of the emission-driven time series is
less pronounced. The projected changes in atmospheric transport cause a
positive trend of the ODP-weighted emissions of about 3.1 % per decade.
This positive trend projection in the mass-flux-derived ODP reveals a future
change in the tropical circulation with significant consequences for trace
gas transport from the troposphere into the stratosphere. More detailed
evaluations demonstrate that the CESM1-CAM5 tropical convective upward mass
flux is projected to decrease in the lower and middle troposphere (not shown
here) in agreement with results from UKCA chemistry–climate model
simulations (Hossaini et al., 2012). Contrary to the changes in the middle
troposphere, the convective mass flux in the upper troposphere (above the
250 hPa level) is projected to increase in the future, again in agreement
with Hossaini et al. (2012). A higher extension of tropical deep convection
has also been found in other model projections, and
increasing greenhouse-gas-induced
tropospheric warming leading to an uplift of the tropopause has been
suggested as the possible cause (Chou and Chen, 2010; Rybka and Tost, 2014).
Overall, an increasing upward mass flux in the upper troposphere/lower
stratosphere would lead to enhanced entrainment of CHBr3 into the
stratosphere, consistent with results from Hossaini et al. (2012) and Dessens
et al. (2009), and thus to increasing ODP-weighted emissions. Finally, for
the last sensitivity study, the chemistry-driven time series of the
ODP-weighted emissions shows no interannual variability and a negative trend
of -2.6 % per decade. Decreasing anthropogenic chlorine emissions and
thus a less efficient BrO / ClO ozone loss cycle leads to a reduction of
bromine-related ozone depletion of 22 % as prescribed by the results of
the idealized chemistry–climate model experiments from Yang et al. (2014).
In summary, changing emissions and changing convection
would lead to a projected increase of
5.4 % per decade of the ODP-weighted emissions over the 21st century for
the RCP 8.5 scenario. However, due to declining anthropogenic chlorine,
stratospheric ozone chemistry will become less effective and the
corresponding decreasing α-factor reduces the ODP-weighted CHBr3
emissions, resulting in an overall projected trend of about 2.6 % per
decade.
A comparison of the model-derived CHBr3 ODP-weighted emissions with those of other long-lived substances is shown in Fig. 11.
For the other ozone-depleting substances included in the comparison, changing emissions are taken
into account by applying their potential emission scenarios (Velders et al.,
2007; Ravishankara et al., 2009). The ODP of CFC-11 is nearly independent of the
stratospheric chlorine levels (Ravishankara et al., 2009) and is thus kept
constant for the whole time period. The same is assumed for all other
long-lived halocarbons included in the comparison. Our comparison shows that
emissions of the short-lived CHBr3 can be expected to have a larger
impact on stratospheric ozone than the other anthropogenic halocarbons after
approximately 2025 (Fig. 11). Two exceptions to this are ODP-weighted
emissions of CH3Br and anthropogenic N2O (Ravishankara et al.,
2009), both of which are not shown in our plot.
CH3Br, with partially anthropogenic and partially natural sources, is
not included in the comparison, since neither an potential emission scenario nor
an
estimate on how changes in atmospheric transport will impact its ODP is
available at the moment. If we assume a CH3Br scenario with
constant emissions from natural and anthropogenic sources and a constant
α-factor, its ODP-weighted emissions would be around
70 Gg year-1 over the 21st century. However, we know this to be
unrealistic and expect changes in anthropogenic CH3Br emissions and a
decreasing α-factor, both of which would lead to smaller projections of
its ODP-weighted emissions. N2O emissions have been projected to be the
most important ozone-depleting emissions in the future with ODP-weighted
emissions between 100 and 300 Gg year-1 expected for the end of the
century (Ravishankara et al., 2009).
Discussion and summary
The ODP-weighted emissions of CHBr3 give a detailed picture of where
and when oceanic CHBr3 emissions take place that will later impact
stratospheric ozone. Furthermore, they provide a useful tool of comparing
the emission strength of CHBr3 with those of long-lived
anthropogenic gases in an ozone depletion framework. Since currently no
information is available on the strength of anthropogenic CHBr3
emissions, the ODP concept is applied to the complete emission budget
including the natural oceanic contribution. While we focus our analysis on
one VSLS and introduce the method and application within a case-study
framework for CHBr3, the concept can be applied to all VSLSs where
emissions and ODP are available at a spatial resolution necessary to
describe their variability.
While the ODP-weighted emissions are an important step towards assessing the
current and future effects of VSLSs on the ozone layer, one needs to keep in
mind that the absolute values are subject to large uncertainties arising from
uncertainties in the emission inventories and in the parameterization of the
convective transport. Existing global CHBr3 emission inventories show
large discrepancies due to sparse observational data sets and
have particularly high
uncertainties in coastal
regions due to differing types and
amounts of macroalgae (Carpenter and Reimann, 2014). We have used the Ziska
et al. (2013) emission inventory, which suggests a lower flux of CHBr3
from the tropical oceans to the atmosphere than other inventories. Based on comparison of the emission
inventories in Hossaini et al. (2013) we would expect that the application of
a different emission scenario in our approach could lead to a 2- to 3-fold
increase in ODP-weighted emissions. However, for the tropics, the relatively
low emissions from Ziska et al. (2013) provide the best fit with the limited
available atmospheric data (Hossaini et al., 2013). The sensitivity of our
results to uncertainties in transport becomes apparent when we apply the ODP
fields calculated from FLEXPART trajectories without taking into account
convective parameterization (Pisso et al., 2010). The ODP calculated without
convective parameterization results in roughly 50 % lower global mean
ODP-weighted CHBr3 emissions. Additionally, uncertainties may arise from
the simplified tropospheric and stratospheric chemistry schemes with an
altitude-independent α-factor and a prescribed tropospheric lifetime.
Further detailed studies including different convective parameterization
schemes, more detailed representation of tropospheric chemistry, product gas
impacts, various emission inventories and multi-model mean scenarios are
required in order to obtain reliable uncertainty ranges which need to be
included in any communication of ODPs to policy makers.
Our analysis reveals that the spatial variability of trajectory-derived ODP
fields of species with short lifetimes, such as CHBr3, is to a large
degree determined by deep tropical convection. As a result, a cost-efficient
method to calculate ODP field proxies from updraught mass flux fields has
been developed and applied. Past ODP-weighted CHBr3 emission estimates
have been derived based on ERA-Interim meteorological fields. For the time
period 1979 to 2005, a positive trend in the CHBr3 emissions and a
negative trend in mass-flux-derived ODP mostly cancel out, leading to a flat
time series of ODP-weighted emissions with no long-term changes. From 2006
to 2013, however, a strong increase in both quantities leads to a step-like
increase of the ODP-weighted CHBr3 emissions.
Future ODP-weighted CHBr3 emission estimates have been derived from
CESM1-CAM5 RCP 8.5 runs taking into account changing meteorological and
marine surface parameters, convective activity and stratospheric chemistry.
Changes in tropospheric chemistry and stratospheric residence time are not
taken into account for the calculation of the future ODP-weighted emissions.
While our methodology is somewhat limited by these simplifications,
CHBr3 delivery from the surface to the tropopause in a future changing
climate is expected to be mostly related to changes in tropospheric transport
rather than changes in tropospheric chemistry (Hossaini et al., 2013),
suggesting that we include the most important processes here. Furthermore, we
do not account for changing biogeochemistry in the ocean and anthropogenic
activities that can lead to increasing CHBr3 emissions and further
amplify the importance of VSLSs for stratospheric ozone chemistry. Such
changes in the oceanic sources are important for estimating the future impact
of VSLSs on atmospheric processes, but they are not understood well enough
yet to derive reliable future projections. Finally, we do not consider
potential future changes in stratospheric aerosol which could impact the
contribution of VSLSs to stratospheric ozone depletion (Salawitch et al.,
2005; Sinnhuber et al., 2009). Variations in the background stratospheric
aerosol loading (e.g., Vernier et al., 2011) are mostly attributed to minor
volcanic eruptions (Neely et al., 2013). Since future volcanic eruptions are
not accounted for in the simulations scenarios used here, we do not include
the impact of natural aerosol variations. Suggested future geoengineering
would intentionally enhance the stratospheric aerosol loading and is
projected to increase the impact of VSLSs on stratospheric ozone by as much
as 2 % at high latitudes (Tilmes et al., 2012). Such a scenario is not
included in our simulations but could effectively enhance the ODP of CHBr3
due to an enhanced BrO/ClO ozone loss cycle in the lower stratosphere (Tilmes
et al., 2012). Overall, some discrepancies between the observation- and
model-derived ODP-weighted CHBr3 emissions exist, very likely related to
out-of-phase tropical meteorology in the model. However, there is general
good agreement between the spatial and seasonal variability of the
observation- and model-derived fields, encouraging the use of this model to derive realistic ODP-weighted
CHBr3 emission estimates.
Variability of the ODP-weighted CHBr3 emissions on different timescales
are driven by different processes. Spatial and seasonal variations are caused
by variations in the surface to tropopause transport via deep convection.
Interannual variability is mostly driven by transport variations but also by the variability of the oceanic emissions. Both processes are weakly correlated
on interannual timescales (with a Pearson correlation coefficient between the
interannual anomalies of r=0.3), suggesting that in years with stronger
emissions (driven by stronger surface winds and higher temperatures) stronger
troposphere-to-stratosphere transport exist. The long-term trend, finally,
can be attributed in equal parts to changes in emissions,
troposphere-to-stratosphere transport and stratospheric chemistry. While
growing oceanic emissions and changing convective activity lead to increasing
ODP-weighted CHBr3 emissions, the expected decline in stratospheric
chlorine background levels has the opposite effect and leads to a decrease.
Taking all three processes into account, the future model projections suggest
a 31 % increase of the 2006 ODP-weighted CHBr3 emissions until 2100
for the RCP 8.5 scenario. This anthropogenically driven increase will further
enhance the importance of CHBr3 for stratospheric ozone chemistry.
The Supplement related to this article is available online at doi:10.5194/acp-15-13647-2015-supplement.
Acknowledgements
The authors are grateful to the ECMWF for making the reanalysis product
ERA-Interim available. This study was carried out within the EU project SHIVA
(FP7-ENV-2007-1-226224) and the BMBF project ROMIC THREAT (01LG1217A). We
thank Steve Montzka for helpful discussions.
Edited by: P. Haynes
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