We perform a source attribution for tropospheric and ground-level ozone using a novel technique
that accounts separately for the contributions of the two chemically distinct
emitted precursors (reactive carbon and oxides of nitrogen) to the
chemical production of ozone in the troposphere.
By tagging anthropogenic emissions of these precursors according to the geographical region
from which they are emitted, we determine source–receptor relationships for ground-level ozone.
Our methodology reproduces earlier results obtained via other techniques for ozone
source attribution, and it also delivers additional information about the modelled processes
responsible for the intercontinental transport of ozone, which is especially strong during
the spring months.
The current generation of chemical transport models used to support international negotiations
aimed at reducing the intercontinental transport of ozone shows especially strong inter-model
differences in simulated springtime ozone.
Current models also simulate a large range of different responses of surface ozone to methane, which is one of the
major precursors of ground-level ozone.
Using our novel source attribution technique, we show that emissions of NOx (oxides of nitrogen)
from international shipping over the high seas
play a disproportionately strong role in our model system regarding the hemispheric-scale response of surface ozone
to changes in methane, as well as to the springtime maximum in intercontinental transport of ozone
and its precursors.
We recommend a renewed focus on the improvement of the representation of the chemistry
of ship NOx emissions in current-generation models.
We demonstrate the utility of ozone source attribution as a powerful model diagnostic tool and
recommend that similar source attribution techniques become a standard part of future
model intercomparison studies.
Introduction
Tropospheric ozone plays a central role in the chemistry and self-cleansing capacity
of the troposphere ;
however, at high concentrations close to the ground,
it is harmful to human health and vegetation .
As well as being transported into the troposphere through exchange with the
stratosphere, ozone can be formed via chemical reactions in the troposphere
involving two chemically distinct precursors: oxides of nitrogen (collectively NOx)
and reactive carbon species, including carbon monoxide, methane, and volatile organic
compounds .
Increases in tropospheric ozone since preindustrial times have been attributed primarily
to increases in anthropogenic emissions of NOx and methane – the latter of which is the most abundant
reactive carbon species in the atmosphere
.
Ozone is long-lived enough in the troposphere to circumnavigate the Northern Hemisphere along
the prevailing westerly winds .
Thus, emissions of NOx or reactive carbon in any Northern Hemisphere source region can
contribute to the ozone mixing ratio in any other region of the Northern Hemisphere.
This long-range contribution to the ozone mixing ratio is often referred to as “baseline” ozone
.
Due to seasonal variation in the lifetime of ozone, this effect is strongest in spring
and weakest in summer .
The ambient ozone mixing ratio at any location is a combination of ozone transported from the
hemispheric background and in-situ photochemical production.
Recent analyses of long-term trends in baseline ozone in western regions of North America
and Europe
have shown increasing trends since reliable measurements began in the 1980s until approximately 2000–2010,
and they indicate that these trends may be beginning to reverse.
Seasonal cycle of monthly mean surface ozone (ppb) in Hemispheric Transport of Air Pollution (HTAP) Tier 2 regions from
our base model run (blue line), compared with observations from the Tropospheric Ozone Assessment Report (TOAR; black line),
and other models from the HTAP ensemble of global models: ensemble mean (red line), ensemble
standard deviation (green shaded area), and ensemble range (grey shaded area).
Only grid cells containing TOAR observations have been used.
Chemical transport models (CTMs) are commonly used to interpret observations of ozone and to
synthesize an understanding of the fundamental processes controlling its origin and fate
in the atmosphere in order to project future trends e.g..
The range in values of the surface ozone mixing ratio over the Northern Hemisphere
simulated by contemporary CTMs is extremely high (see, for example, our Fig.
in Sect. ), requiring the use of a large ensemble
of models e.g..
When compared with available measurements of ozone for the Northern Hemisphere e.g.,
ensembles of global CTMs are generally able to simulate the spatial distribution and seasonal cycles of
surface ozone; however, they are consistently biased high in the Northern Hemisphere and have
difficulty simulating long-term trends .
Potential sources of uncertainty in CTMs include uncertainties in their chemical mechanisms
(the representations of the relevant chemical reactions and their rates);
their representation of atmospheric transport processes; and their representation of exchange processes between
the atmosphere and the surface of the Earth, including emissions of the ozone
precursors NOx and reactive carbon.
The most important class of reactions for the formation of ozone in the troposphere is the reaction of
NO (nitric oxide) with a peroxy radical, which is itself formed during the oxidation
of reactive carbon .
During this process, the NO is converted to NO2 (nitrogen dioxide), which can be rapidly photolysed,
ultimately forming ozone and recycling NO.
The ozone production efficiency of NOx (the combined concentration of NO and NO2)
can vary significantly depending on the location and timing of the NOx emissions.
In the polluted boundary layer,
NOx is rapidly removed from the atmosphere through the reaction of NO2 with OH, forming
HNO3, which is subsequently lost via dry or wet deposition.
Under less polluted conditions, NO2 photolysis competes more effectively with HNO3
production, allowing each unit of NOx to react with a higher number of peroxy radicals before eventually
being scavenged by OH, thereby
leading to a higher ozone production efficiency per unit of NOx.
When NOx is lofted into the free troposphere, its ozone productivity
increases substantially .
Thus, emissions of NOx in the tropics are especially effective at producing tropospheric ozone
due the fact that they are transported aloft by deep convection .
NOx emissions from both aircraft and lightning are also highly efficient at producing
tropospheric ozone .
Combustion of fossil fuels is the largest source of NOx in the atmosphere .
first pointed out that international
shipping can have a disproportionately high influence on tropospheric ozone due to the disperse nature of
NOx emissions from this source.
quantified the sensitivity of tropospheric ozone to NOx emissions from different
modes of transport (land, sea, and air), finding that aircraft emissions were most efficient at
producing ozone (per molecule of NOx emitted), followed by ships and then land transport.
For near-surface ozone, however,
NOx emissions from ships were shown to have a higher influence than NOx emissions
from aircraft. Nevertheless, ozone production from ship NOx is highly uncertain in current CTMs.
and showed that global CTMs, due to their coarse
resolution (usually in the hundreds of kilometres), do not resolve the chemistry of ship exhaust
plumes; instead, there is a tendency for NOx to be removed from the atmosphere more quickly than simulated
by the global CTMs, which effectively instantly dilute these emissions into very large volumes.
also showed that this effect applies more generally to other concentrated
emission sources such as urban areas.
introduced a method for parameterizing ship exhaust plume chemistry using lookup tables
in their global CTM, but this method has not been widely adopted by the modelling community.
Modelling of ship NOx and the effects of NOx on the atmosphere remains a challenge
for global CTMs.
The term “reactive carbon” encompasses a very wide range of atmospheric
constituents e.g..
In contrast to NOx, most of the reactive carbon emitted to the atmosphere is
not of anthropogenic origin but rather emitted from the biosphere.
In this study, we restrict our definition to molecules that yield peroxy radicals
(either hydroperoxy radicals HO2 or organic peroxy radicals RO2)
during their gas-phase oxidation and, thus, contribute to ozone formation by potentially
converting NO to NO2.
Therefore, this definition includes carbon monoxide (CO) and the large family of
molecules known as volatile organic compounds (VOCs).
The simplest VOC is methane, which is often considered separately from non-methane volatile organic compounds
(NMVOCs) due to its very long lifetime in the troposphere.
The ozone production potential of reactive carbon depends on the rate at which it is oxidized
in the atmosphere, usually through reaction with the OH radical ,
as well as the subsequent chemistry of its oxidation products .
Most reactive carbon species have relatively short lifetimes in the troposphere due to
their reaction with OH radicals.
Methane, due to its exceptionally low reactivity, is well mixed in the troposphere.
In contrast to other forms of reactive carbon, emissions of methane can contribute to
ozone formation at any location in the troposphere where photochemical conditions
are favourable .
Despite its low reactivity in comparison to other types of reactive carbon, methane
is highly abundant and has been shown to make a large contribution to tropospheric ozone
.
PAN (peroxyacetyl nitrate) is an important reservoir species for both peroxy radicals and for NOx.
Peroxyacetyl radicals are formed during the oxidation of a wide range of different types of NMVOCs
from a wide range of different sources. PAN is formed through the reaction of peroxyacetyl radicals with NO2,
primarily in the polluted boundary layer where both are abundant .
The lifetime of PAN is strongly temperature dependent. At colder temperatures higher in the troposphere,
PAN can be transported over long distances, and it can act as a source of NO2 and peroxyacetyl
radicals in remote regions upon subsidence and thermal decomposition .
The chemical mechanisms and reaction rate constants involved in the formation and decomposition
of PAN vary widely between CTMs , leading to
large inter-model differences in simulated PAN .
suggested that
measurements of PAN at northern midlatitude mountaintop sites in spring could provide a useful constraint on
CTMs, although the number of observations available is limited.
With careful interpretation, the results of ensembles of CTMs can be used to diagnose long-range
transboundary transport of ozone and to develop intercontinental source–receptor relationships, which relate
the effects of precursor emissions from different regions of the Northern Hemisphere to mixing ratios
of ground-level ozone in other regions of the Northern Hemisphere.
An example of this is the activity of the Task Force on Hemispheric Transport of Air Pollution
(TF-HTAP; ), which reports to the Convention on Long-Range Transboundary Air Pollution (CLRTAP)
and, thus, informs international policymaking for the mitigation of air pollution.
The TF-HTAP studies used a “perturbation” approach that compared a control simulation with sensitivity simulations in which emissions of particular ozone precursors
were reduced by 20 %.
Combined 20 % reductions of global average methane and remote anthropogenic emissions of NOx, CO, and NMVOCs
were shown to have an approximately equal effect on annual average ozone as a 20 % reduction
of local precursor emissions, indicating the strong role of long-range transport in
influencing surface ozone in the Northern Hemisphere.
Results derived from Phase 1 of the TF-HTAP exercise
and the Phase 2 exercise described by
are discussed in more detail by , and .
An alternative approach to the perturbation technique for source attribution is “tagging”
e.g..
When applied to ozone source attribution in a CTM, this technique involves labelling (or “tagging”)
modelled ozone with the identity of either the geographical region in which it is chemically produced or with the
identity of the emitted precursor(s) that ultimately led to its production.
A common challenge faced by all tagging approaches is that the production of one molecule
of ozone in the troposphere requires two precursors:
one molecule of NO
and one peroxy radical (produced during reactive carbon oxidation).
This then poses the question of whether the ozone molecule should inherit its tag from the emitted NOx, from the emitted reactive carbon,
or in some other way?
provides
a detailed review of the different approaches to answering this question, including the trade-offs made
in each case.
also describe a novel and unique tagging methodology that allows for the separate
attribution of tropospheric ozone to both its NOx and its reactive carbon precursor,
at the cost of extra computational expense compared with other tagging methodologies.
Recent work from takes the approach of defining an extended odd oxygen
family including peroxy radicals, which effectively shifts the production of odd oxygen purely
to photolysis reactions. Further comparison of different tagging approaches is beyond the scope of
this paper, but it remains an interesting topic for future work.
Tagging and perturbation are complementary approaches .
While tagging delivers information about the contribution of different emission sources
to a pollutant of interest, perturbation studies deliver information about the sensitivity
of pollutants to changes in emissions, including changes in the chemical
lifetimes of pollutants in response to variations in emissions.
In the absence of non-linear chemical interactions,
these two different approaches ultimately yield the same results; however, for tropospheric ozone,
which can show highly non-linear interactions between its NOx
and reactive carbon precursors under some circumstances, these approaches can sometimes yield very different results
.
As air pollution mitigation strategies must involve some change in emissions, perturbation
studies will always be necessary for policy-relevant modelling of atmospheric chemistry.
However, tagging studies on their own can play a role in helping to identify which emissions to mitigate
.
When combined with perturbation studies, tagging can reveal how the contribution of
unmitigated sources to ozone changes in response to mitigation measures .
also noted that tagging studies provide useful diagnostic information
about model processes and argued for their inclusion in model intercomparison exercises.
The method described by is currently the only available approach that
provides the separate attribution of tropospheric ozone to its NOx and reactive carbon
precursors. Other schemes take different approaches to the attribution of ozone to these two chemically
distinct precursors. A thorough review of several different approaches is presented in .
In this study, we use the ozone tagging methodology previously described by
to perform a source attribution for ground-level ozone to both NOx
and reactive carbon.
This work builds on the work of by tagging anthropogenic
emissions of NOx and reactive carbon by their geographical source region as well as examining
the seasonal cycle of the surface ozone attribution in receptor regions as defined in the
HTAP Phase 2 exercise.
By performing the separate attribution of ground-level ozone to both NOx and reactive
carbon (including methane), we hope to provide more useful information to inform emission
mitigation scenarios.
We also show how our tagging methodology can be used as a model
diagnostic tool to understand the atmospheric budgets of ozone and PAN in more detail than previously
possible, potentially informing efforts to reduce the currently high level of inter-model
uncertainty.
Furthermore, we examine the changing contributions of the different sources
of NOx and reactive carbon to a perturbation of the global methane burden,
showing how the contribution of emissions from unmitigated sectors would respond to the mitigation of methane
emissions.
The tagging approach and model set-up is described in Sect. .
In Sect. , we evaluate our simulations against observations from the Tropospheric Ozone Assessment Report (TOAR) and
the ensemble of simulations from HTAP Phase 2, show the intercontinental source attribution
for ozone and its precursors, and examine the response of this source attribution to a 20 %
perturbation in the global methane burden.
Conclusions are drawn in Sect. .
Experimental design
Simulations are performed with CAM4-chem (Community Atmosphere Model version 4 with chemistry), which is
a component of the
CESM (Community Earth System Model) version 1.2.2 , using the same
model configuration as .
The model is run at a horizontal resolution of 1.9∘×2.5∘, with 56 vertical levels using
specified dynamics for the year 2010 from the MERRA reanalysis .
As in , we have replaced the default chemical mechanism with a tagged mechanism based
on an earlier version of the MOZART-4 mechanism .
Our tagging system allows for the attribution of tropospheric ozone to chemical production by either NOx
or reactive carbon precursors (as well as transport from the stratosphere). A complete attribution of
tropospheric ozone to both kinds of precursors requires two model runs: one with NOx emissions tagged
and another with reactive carbon emissions tagged.
Chemical production of ozone in the stratosphere (primarily through the photolysis of molecular oxygen)
and
other minor production pathways for tropospheric ozone are also tagged, as described in .
For both NOx tagging and VOC tagging, the sum of the tagged ozone tracers
is equal to the total ozone as simulated by the model.
As in , anthropogenic emissions of NOx, CO, and NMVOCs for 2010 are taken from the
EDGAR-HTAPv2 emission inventory ,
biomass burning emissions are from GFEDv3 ,
and methane is held fixed at the surface to a global average value of 1760 ppb,
as in .
Two simulations (base runs)
are performed with this model set-up: one in which all sources of NOx are tagged
as described below
(the “NOx-tagged” run) and one in which all sources of reactive carbon are tagged
as described below (the “VOC-tagged” run).
As in , the length of the spin-up period was 1 year for the NOx-tagged run, and it was 2 years
for the VOC-tagged run.
The model was deemed to be spun up when the maximum difference between the simulated December mean surface ozone
attributable to any tagged source
was less than 1 % in any 2 subsequent years of simulation.
With the exception of surface-based anthropogenic emissions of NOx, CO, and NMVOCs, the tag identities
used in this study are identical to those used in . In this study, all
surface-based anthropogenic emissions are tagged with a label representing the geographical location
at which the emissions occur. This approach allows for the attribution of simulated ozone to
anthropogenic precursor emissions from specific locations.
Specifically, anthropogenic emissions of NOx and reactive carbon
are tagged according to their Tier 1 source region as defined for the
HTAP Phase 2 multi-model ensemble experiment, which is described in more detail in .
Due to computational constraints, not all of the HTAP Tier 1 regions are tagged in this study.
As the primary focus of this study is on the attribution of ground-level ozone in the Northern Hemisphere,
only the major anthropogenic Northern Hemisphere source regions are tagged, whereas other anthropogenic sources
are tagged with the label “Rest of the world”.
A full list of the tags used in the NOx- and VOC-tagged runs is given in Table .
The explicitly tagged source regions differ between the NOx-tagged and VOC-tagged runs because
VOC tagging is computationally more expensive than the NOx tagging .
One important difference between this study and is that
anthropogenic emissions of CO for each source region are tagged together with
emissions of NMVOCs in this study in order to save computational resources.
For the emissions tagged as “Oceanic emissions” (Table ), we note that the only source of
NOx from this region in our simulations is from shipping and that the major source of reactive
carbon is biogenic emissions of dimethyl sulfide (DMS).
List of tags used for the attribution of tropospheric ozone
in the NOx- and VOC-tagged runs.
Anthropogenic emissions of NOx and reactive carbon
are tagged based on their HTAP Tier 1 region.
Other tags are as in . RoW refers to “Rest of the world”.
Tag nameNOx taggingVOC taggingHTAP Tier 1 regions Oceans1ExplicitExplicitNorth AmericaExplicitExplicitEuropeExplicitExplicitSouth AsiaExplicitExplicitEast AsiaExplicitExplicitRussia, Belarus, and the UkraineExplicitExplicitSouth East AsiaExplicitRoWNorthern AfricaExplicitRoWMiddle EastExplicitRoWMiddle AmericaExplicitRoWCentral AsiaExplicitRoWPacific, Australia, and New ZealandRoWRoWSouthern AfricaRoWRoWSouth AmericaRoWRoWArcticRoWRoWAntarcticRoWRoWOther tags StratosphereGlobal2GlobalAircraftGlobalGlobalBiogenicGlobalGlobalBiomass burningGlobalGlobalLightningGlobalN/AMethaneN/AGlobalExtra productionGlobal3Global4
1NOx from “Oceans” is exclusively from shipping, whereas reactive carbon from this
region is predominantly biogenic.
2 For NOx tagging, the stratosphere tag is applied directly to ozone
produced in the stratosphere (as for VOC tagging) and also to NO produced from the dissociation of
N2O.
3 For NOx tagging, “Extra production” of ozone is due to
the self reaction of OH radicals and
reactions between HO2 and organic peroxy radicals.
4 For VOC tagging, “Extra production” of ozone is due to
the self reaction of OH radicals and
reaction of OH with H2O2.
In addition to the NOx- and VOC-tagged base runs described above, we also perform two additional
runs in order to investigate the response of tropospheric ozone to a perturbation in the
tropospheric burden of methane: one with NOx tagging and another with VOC tagging.
In each of these methane perturbation runs, the initial atmospheric methane burden and
the methane mixing ratio
imposed at the surface as a boundary condition are reduced by 20 %.
This translates to a surface methane mixing ratio of 1410 ppb in these methane perturbation runs.
In the above-mentioned perturbation runs,
all other sources of NOx and reactive carbon are left unchanged.
The methane perturbation runs also require 2 years of spin-up for the model to arrive at steady state.
CAM4-chem in version 1.2.2 of the CESM has previously been evaluated by ,
and the modified version used in this study has also been discussed thoroughly by .
In Sect. , we describe the key differences in methane and tropospheric ozone
between our base simulation and the
CAM4-chem simulation reported by , and we compare our simulated surface ozone
with observations from TOAR as well as with the ensemble of CTM
simulations from the HTAP Phase 2 multi-model study .
The full set of CTMs that participated in the HTAP Phase 2 multi-model ensemble is given in Table 3 of
. In this study we compare surface ozone from
our base simulation with results from a subset of 12 CTMs:
CAM-chem (simulations performed by the National Center for Atmospheric Research, NCAR),
CHASER_re1,
CHASER_t106,
C-IFS,
C-IFS_v2,
EMEP_rv4.5,
EMEP_rv48,
GEM-MACH,
GEOS-Chem Adjoint,
GEOS-Chem,
OsloCTM3.v2, and
RAQMS.
Details of the configurations used by each of these models in the HTAP Phase 2 ensemble can be found
in and references therein.
Results and discussion
All results presented in this study are based on the definition of the troposphere as the model grid cells
below the level of 150 ppb of ozone.
By design,
the ozone simulated in our base model runs is identical to the simulation reported in
. Our simulation for 2010 produces a tropospheric ozone burden of 319 Tg(O3),
which is within 1 standard deviation of the multi-model mean reported by for
the year 2000 (337±23 Tg(O3)).
Our simulated tropospheric ozone burden is slightly higher than the burden reported by
using a similar model set-up
(309 Tg(O3)), which could be due to the use of different emissions datasets.
Our simulated tropospheric methane burden (4150 Tg(CH4)) is the same as that reported by ,
but our methane lifetime (due to oxidation in the troposphere by OH),
at 7.59 years, is shorter than the 8.82 years reported by , which is also likely due to the
use of different emission datasets.
Our methane lifetime is towards the lower end of the range (7.1–10.6 years)
simulated in CTMs, as reported by .
In Fig. , we compare our simulated monthly mean surface ozone mixing ratio for 2010
with data from TOAR and with the other models in the
HTAP Phase 2 CTM ensemble.
Results are shown averaged over HTAP Tier 2 receptor regions, and they only include grid cells for which
TOAR observations are available.
In general, most of the HTAP models overestimate the monthly mean surface ozone mixing ratio
in regions where observations are available, which is consistent with the high model bias reported by
.
Also apparent from Fig. is the large range in simulated surface ozone between
members of the HTAP model ensemble, which is especially
high in the northern spring, approaching a spread of approximately
30 ppb between the lowest and highest ensemble members, which is of a similar order to the Northern
Hemisphere annual mean surface ozone itself.
Our modelled monthly average surface ozone mixing ratio in the HTAP Tier 2 receptor regions
is generally close to the HTAP ensemble mean
and is usually within 1 standard deviation of the ensemble mean.
Source attribution of tropospheric ozone
The attribution of annual average tropospheric ozone to emissions of NOx and reactive carbon precursors
based on the source tags from Table is shown in
Figs. and
and is quantified in
Tables and .
Figures showing the attribution of monthly mean ozone are available in the Supplement.
For each tagged source, Tables and
include the respective emissions of NOx and reactive carbon and, where applicable,
the contribution of each source to
the 2010 average tropospheric ozone burden, the contribution to the Northern Hemisphere 2010 annual average
surface mixing ratio, and the ozone production efficiency of each emission source
(defined here as the contribution of each emission source
to the tropospheric ozone burden, in units of moles of ozone per mole of N or C emitted).
Figures and show the spatial distribution of the annual average
surface ozone as attributed to each source of NOx and reactive carbon, respectively.
In Fig. , ozone attributable to anthropogenic NOx
emissions in some source regions
(specifically South East Asia, northern Africa, the Middle East, Middle America, and Central Asia)
has been added to the “Rest of the world” total in order to unify the definition of
this source region with the definition of this region in the VOC-tagged run.
The difference in the stratospheric contribution between the
NOx- and VOC-tagged runs is due to the role of NOx produced
in the stratosphere from the dissociation of N2O.
Ozone produced in reactions involving this
stratospheric source of NOx are counted in our source attribution as stratospheric ozone,
as described in .
Annual mean surface ozone (ppb) from the NOx-tagged base run.
Total ozone is shown in the top left panel. Tagged ozone tracers are shown in the other panels. “RUS/BEL/UKR” refers to Russia, Belarus, and the Ukraine.
Annual mean surface ozone (ppb) from the VOC-tagged base run.
Total ozone is shown in the top left panel. Tagged ozone tracers are shown in the other panels. “RUS/BEL/UKR” refers to Russia, Belarus, and the Ukraine.
The attribution of ozone to tagged sources of NOx.
Also shown are the contribution of the stratosphere and the contribution
of minor chemical production pathways in the troposphere. Contributions of each
tagged source to both the 2010 annual average tropospheric burden
and to the annual average Northern Hemisphere (NH) surface mixing ratio are shown.
Where applicable, the
ozone production efficiency (OPE) of NOx emissions from each tagged source
is also given.
NOx SourceEmissionsOzone burdenOPENH surface(Tg(N) yr-1)(Tg)(mol mol-1)(ppb)Lightning3.4380.56.853.14Stratosphere–75.5–3.17Biogenic5.0426.01.502.57Oceanic sources4.2819.91.355.33East Asia9.9716.90.4953.01South East Asia1.6215.32.760.755Aircraft0.64612.25.491.13Biomass burning5.0312.10.7041.45South Asia3.4910.80.9071.27North America4.7910.40.6322.88Middle America1.278.812.021.00Europe3.164.810.4441.76Middle East1.824.110.6591.02Russia, Belarus, and the Ukraine1.372.150.4570.852North Africa0.5311.720.9470.513Central Asia0.2870.6270.6380.238Rest of the world2.5415.71.800.500Extra production–1.47–0.132Total tropospheric ozone–319–30.7Attribution to NOx emissions
Anthropogenic NOx emissions from the three major high-latitude
source regions (Europe, East Asia, and North America)
contribute to high modelled ozone concentrations both locally and in the Northern Hemisphere background.
Lightning NOx, soil NOx, and ozone input from the stratosphere all contribute additionally
to modelled global background ozone.
Emissions of NOx from shipping contribute significantly
to ozone over the major Northern Hemisphere ocean basins, which is also transported over continental regions.
South Asia stands out in comparison with the other major Northern Hemisphere source regions in that ozone
produced from NOx emitted in South Asia is relatively localized to the South Asian region itself
and is not transported into the hemispheric background to the same extent as ozone produced from NOx
emissions in the other major Northern Hemisphere source regions.
Table shows that
NOx emissions from lightning and aircraft are especially efficient at producing
ozone in the free troposphere, consistent with previous work (e.g. ; ).
Similarly, surface emissions
of NOx from regions closer to the tropics (e.g. South East Asia and Middle America)
produce ozone more effectively due to the rapid convective transport of emitted NOx into the free troposphere,
as shown by .
Of the major Northern Hemisphere source regions, NOx emissions from South Asia are the most
efficient at producing ozone, consistent with a stronger role of vertical transport over this region.
In contrast, NOx emissions from the major
anthropogenic source regions in the high northern latitudes (Europe, East Asia, and North America)
are among the least productive of all global NOx
emissions, consistent with a
relatively small amount of convective transport, leading to higher rates of NOx removal.
Despite their low ozone production efficiency, emissions of NOx in the high northern
latitudes contribute significantly to surface ozone across the Northern Hemisphere
(Fig. , Table ).
Table also shows that NOx emissions from shipping are also relatively efficient
at producing ozone, which is also consistent with previous work e.g..
The high ozone production efficiency of ship emissions is due to their location in relatively
pristine regions with few other sources of NOx.
Owing to the high ozone productivity of ship emissions and the fact that they are emitted at relatively high
latitudes, they contribute significantly to the Northern
Hemispheric background (Fig. , Table ).
As noted above, the ozone production from ship NOx is likely to be overestimated due to the
artificial dilution of emissions into relatively coarse model grid cells.
reported a contribution of shipping to the tropospheric ozone burden of 18 Tg(O3)
using their tagging technique and based on a model simulation with
ship NOx emissions of 6 Tg(N) yr-1.
In our study, we calculate a contribution of ship NOx to tropospheric ozone of
19.9 Tg(O3) based on ship NOx emissions of 4.28 Tg(N) yr-1,
implying a much higher ozone production efficiency for ship NOx.
As the tagging technique used by is based on the method described by
, which combines the effects of tagged NOx and reactive carbon
precursors into a single tagged ozone molecule during ozone production, we do not expect our results
to be directly comparable.
As shipping emits significantly more NOx than reactive carbon,
we would expect the combinatorial tagging approach of to attribute less ozone
to shipping than our method, because the ozone produced from ship NOx would also be partially
attributed to the reactive carbon precursor involved in the ozone production.
Indeed, reported maximum contributions of shipping to surface ozone of about 10 ppb in summer
over major Northern Hemisphere ocean basins. In our study, surface ozone attributable to shipping
over these regions can exceed 20 ppb (see the Supplement).
Attribution to reactive carbon emissions
Methane and biogenic emissions clearly stand out as major reactive carbon precursors to tropospheric
ozone, contributing 35 % and 24 % to the tropospheric ozone burden in our simulation, respectively.
Anthropogenic emissions of reactive carbon (excluding biomass burning) together contribute about 14 %
to the tropospheric ozone burden.
The relatively low influence of anthropogenic reactive carbon emissions on ground-level
ozone has been noted elsewhere e.g.; however,
despite this low overall ozone productivity, anthropogenic reactive carbon emissions from source
regions in higher northern latitudes still have disproportionately
high contributions to surface ozone in the Northern Hemisphere (Table ).
The attribution of ozone to tagged sources of reactive carbon.
Also shown are the contribution of the stratosphere and the contribution
of minor chemical production pathways in the troposphere. Contributions of each
tagged source to both the 2010 annual average tropospheric burden
and to the annual average Northern Hemisphere (NH) surface mixing ratio are shown.
Where applicable, the
ozone production efficiency (OPE) of reactive carbon emissions from each tagged source
is also given.
Emissions Reactive carbon sourceVOCCOOzone burdenOPENH surface(Tg(C) yr-1)(Tg(C) yr-1)(Tg)(mol mol (C)-1)(ppb)Methane410–1130.068912.4Biogenic66842.276.80.02707.20Stratosphere––66.8–2.91Biomass burning29.816213.40.01761.25East Asia20.080.410.30.02571.96South Asia16.236.56.670.03160.715North America12.023.84.390.03071.18Europe6.1211.62.040.02880.695Russia, Belarus, and the Ukraine4.044.921.220.03410.433Oceanic sources11.00.5870.09570.002060.0162Rest of the world55.382.819.50.03521.43Extra production––4.58–0.546Total tropospheric ozone––319–30.7
Due to the emissions of CO being tagged together with emitted VOCs in this
study, the contribution of each tagged source to the tropospheric ozone burden (and, therefore,
also the ozone production efficiency of each tagged source)
is a mixture of ozone production due to emitted CO and emitted NMVOCs.
The ozone attributed to methane oxidation in Table
is due to all stages of methane oxidation in the MOZART-4 chemical mechanism, including the final step
in which CO from earlier stages of methane oxidation is itself oxidized to CO2.
The oxidation of CO can produce a maximum of one peroxy radical (HO2). Therefore, the maximum ozone
production potential of CO is 1 mol of ozone per mole of emitted CO.
VOCs (including methane) can produce significantly more ozone per mole of emitted carbon,
when the subsequent oxidation of the initial oxidation products is taken into account
.
Future studies using this tagging methodology should consider tagging CO emissions separately from
NMVOC emissions if they aim to determine the ozone production efficiency of anthropogenic
NMVOC emissions from different world regions.
tagged NMVOC emissions separately from CO emissions, but they did not tag anthropogenic
emissions separately according to their geographical region.
We re-examined the output of the otherwise identical
VOC-tagged run described by in order
to determine the ozone production efficiency of NMVOC emissions from anthropogenic,
biomass burning, and biogenic sources:
they are 0.0580, 0.0354, and 0.0268 mol(O3) mol (C)-1 for anthropogenic,
biomass burning, and biogenic sources, respectively.
The ozone production efficiency of biogenic NMVOCs recalculated from is not significantly
different from the value reported here in Table , reflecting the relatively minor contribution
of CO to the total amount of emitted biogenic reactive carbon.
For biomass burning and anthropogenic sources, however,
the ozone production efficiency of NMVOCs emitted from these sources is greater than the
corresponding value from Table , reflecting the fact that the numbers from
Table also include emissions of CO.
We note that methane has a higher ozone production efficiency per unit of reactive carbon
(0.0689 mol mol (C)-1; Table )
than any of the NMVOCs in our runs.
The low ozone production efficiency of biogenic NMVOCs is consistent
with large amounts of isoprene being emitted in remote regions under low-NOx conditions, where
loss of peroxy radicals through reaction with other peroxy radicals could be expected to dominate
.
It might, however, be expected that anthropogenic NMVOCs would have a higher ozone production efficiency,
due to the fact that they are co-emitted with anthropogenic NOx, favouring the conversion of NO to NO2
via reaction with peroxy radicals and, thus, the production of ozone.
The relatively low production efficiency of anthropogenic NMVOCs in our model runs
could be due to the fact that the relatively simple
chemistry of methane oxidation is well described in the version of the MOZART-4 chemical mechanism
used here, in which the relatively complex chemistry of the higher NMVOCs has been simplified.
noted that the ozone production potential of NMVOCs in simplified chemical mechanisms
tended to be lower than the more comprehensive Master Chemical Mechanism .
previously noted increased tropospheric ozone in a CTM
when using a more explicit oxidation mechanism for NMVOCs.
The extremely low ozone production efficiency of reactive carbon from oceanic sources in
Table is due to the lack of any ozone-forming pathways in the oxidation
of dimethyl sulfide (DMS) in the MOZART-4 chemical mechanism as used in
this study. DMS is the dominant source of reactive carbon over the oceans in our model simulations.
To our knowledge, the only other study to perform source attribution of global
tropospheric ozone specifically
to reactive carbon precursors is , on which the present study builds.
Here, we attribute 113 Tg(O3) to methane oxidation (Table ).
attribute 45 Tg(O3) to methane using their
tagging approach, which combines the effects of tagged NOx and reactive carbon
precursors into a single tagged ozone molecule during ozone production.
Ozone production due to methane oxidation under their combinatorial
tagging approach would be expected to
also include the attribution to the source of NOx involved in the ozone production.
Thus, we would expect to attribute approximately half of the amount of ozone
to methane that we would. Doubling the value reported by yields 90 Tg(O3)
attributable to methane oxidation, which is much closer to our value of 113 Tg.
Widespread implementation of tagging techniques for the separate tagging of NOx
and reactive carbon emissions in other CTMs as well as systematic intercomparisons of their results
could help to understand differences in
the simulated budgets of tropospheric ozone.
Due to the variety of approaches taken by different tagging techniques summarized in ,
an intercomparison of a range of different tagging techniques with other methods for ozone source attribution
could be informative.
Source–receptor relationships for ozoneAnnual average surface ozone
Figure shows the modelled
annual average surface ozone concentration in the five major
HTAP Tier 1 regions in the Northern Hemisphere (Europe; Russia, Belarus, and the Ukraine; South Asia; East Asia; and
North America), including a full attribution of ozone in each of these regions to all sources, such as
transport from the stratosphere and emitted precursors
of both NOx and reactive carbon.
Annual average ozone in most of the regions shown in Fig. is close to the
Northern Hemisphere annual average of 30 ppb (Table ), except in South Asia and East Asia
where the annual average surface ozone mixing ratio is closer to 40 ppb.
The difference in each case is primarily due to a larger source of ozone produced from locally emitted precursors.
Transport from the stratosphere contributes approximately 2–4 ppb to annual
average surface ozone depending on the receptor region, which is consistent with the 2.91 ppb contribution of
stratospheric ozone to the annual average surface ozone in the Northern Hemisphere
average surface ozone (Table ).
As already shown in the previous section, anthropogenic sources of NOx dominate
other NOx sources as ozone precursors, while the major reactive carbon precursors are methane and biogenic volatile organic compounds
(BVOCs).
Source–receptor relationships for annual average surface ozone (ppb)
in major Northern Hemisphere Tier 1 regions:
EUR (Europe), RBU (Russia, Belarus, and the Ukraine), SAS (South Asia), EAS (East Asia),
and NAM (North America).
The attribution relates the annual average surface ozone modelled in each region to
the emitted precursors – NOx(a)
and reactive carbon (b) – from all HTAP Tier 1 regions.
In each of the five regions shown in Fig. ,
natural sources and long-range transport of ozone produced from extra-regional anthropogenic precursors
together
contribute more to the annual average surface ozone than anthropogenic emissions
within the region itself.
In each region, the local anthropogenic NOx emissions produce more ozone than
can be attributed to anthropogenic NOx emissions in any other Tier 1 regions; moreover, with the exception of
(South Asia), the combined
contribution of external anthropogenic NOx emissions to annual average surface ozone is greater
than the local contribution.
The importance of long-range transboundary transport of ozone has also been noted elsewhere
.
While anthropogenic precursor emissions from South Asia contribute significantly
to surface ozone within the South Asian region,
they contribute relatively little to surface ozone in the other
four regions shown in Fig. .
This is also consistent with the surface ozone maps in Fig.
and the higher ozone productivity of NOx emissions from South Asia
when compared with the other major Northern Hemisphere source regions
(Table ).
Emissions of ozone precursors (particularly NOx) from South Asia
are transported efficiently into the free troposphere, where they contribute disproportionately
to the global tropospheric ozone burden as also noted by,
but the contribution of South Asian emissions to surface
ozone in other parts of the Northern Hemisphere is disproportionately smaller than emissions from the
other HTAP Tier 1 regions.
Seasonal cycles of surface ozone
Figures , , and show
the seasonal cycles of surface ozone in the three selected Tier 2 regions
“North West Europe”, ”North East China”, and “North West United States” respectively.
These regions are selected in order to compare two regions on the western side of their respective
continents (where long-range transport is expected to be important) with a region on the eastern side
of its continent that is also a major source region.
A set of figures for other HTAP Tier 2 regions
with a complete attribution of surface ozone to all tagged HTAP Tier 1 source regions is
available in the Supplement.
In Figs. , , and ,
results are shown for both NOx and VOC tagging.
In each receptor region, the contribution of long-range transport due to extra-regional anthropogenic
emissions from HTAP Tier 1 regions is shown both in aggregate (panels a and c) and by individual Tier 1
source region (panels b and d).
The definition of the “Rest of the world” ozone tracer has been harmonized
between the NOx- and VOC-tagged runs
in these figures.
Consistent with the annual averages from
Fig. , anthropogenic NOx sources also dominate the seasonal
cycle of modelled
ozone, while the major reactive carbon precursors of ozone are methane and BVOCs.
Seasonal cycle of surface ozone (ppb) in the HTAP Tier 2 receptor region “North West Europe”.
NOx tagging is shown in panels (a) and (b),
and reactive carbon tagging is shown in panels (c) and (d).
Panels (a) and (c) show the total monthly mean ozone (black line) as well as the local anthropogenic
component, the long-range transported anthropogenic component, and the natural components.
Panels (b) and (d) show the individual Tier 1
source regions responsible for the long-range transported component of ozone.
All three receptor regions show a seasonal cycle of ozone
with a spring–summer ozone maximum superimposed on a year-round ozone baseline.
The summertime maximum in ozone is clearly due to local photochemical production
from the combination of locally emitted anthropogenic NOx and BVOCs.
The strong role of locally emitted precursors in the production of ozone in summer
is consistent with earlier work e.g.,
while the importance of BVOC emissions, especially isoprene, for ozone production in summer
has also been noted elsewhere e.g..
Biogenic emissions of NOx (from soils) also contribute to this summertime maximum in the local
photochemical ozone production in all three of the regions shown in
Figs. , , and ,
although to a much smaller extent than anthropogenic NOx emissions.
Seasonal cycle of surface ozone (ppb) in the HTAP Tier 2 receptor region “North East China”.
NOx tagging is shown in panels (a) and (b),
and reactive carbon tagging is shown in panels (c) and (d).
Panels (a) and (c) show the total monthly mean ozone (black line) as well as the local anthropogenic
component, the long-range transported anthropogenic component, the and natural components.
Panels (b) and (d) show the individual Tier 1
source regions responsible for the long-range transported component of ozone.
Seasonal cycle of surface ozone (ppb) in the HTAP Tier 2 receptor region “North West United States”.
NOx tagging is shown in panels (a) and (b),
and reactive carbon tagging is shown in panels (c) and (d).
Panels (a) and (c) show the total monthly mean ozone (black line) as well as the local anthropogenic
component, the long-range transported anthropogenic component, and the natural components.
Panels (b) and (d) show the individual Tier 1
source regions responsible for the long-range transported component of ozone.
The year-round baseline ozone in our model
simulations in all three receptor regions
can be primarily explained by slower photochemistry involving methane as the reactive carbon precursor,
in combination with extra-regional anthropogenic NOx
(Figs. , , and ).
The contribution of methane to surface ozone is slightly higher in summer,
coinciding with the peak in local anthropogenic NOx emissions, which is
consistent with local photochemical ozone production from enhanced local methane oxidation.
The contribution of extra-regional anthropogenic NOx to surface ozone is largest in spring,
coinciding with the peak in the contribution of extra-regional anthropogenic reactive carbon, which is
consistent with long-range transboundary transport of ozone produced elsewhere.
Maxima in springtime ozone have previously been linked to long-range transboundary
transport in all major receptor regions .
This transported ozone can be attributed to input from the
stratosphere as well as extra-regional
anthropogenic emissions of NOx and reactive carbon.
In our simulations,
the contribution of stratospheric ozone peaks around March, whereas the contribution of extra-regional anthropogenic
emissions tends to peak around April, when it contributes
more strongly to the monthly average surface ozone in each region than local anthropogenic NOx
(Figs. , , and ).
In all Northern Hemisphere regions,
the springtime peak in the contribution of extra-regional anthropogenic
reactive carbon is smaller
than the corresponding springtime peak in the contribution of extra-regional anthropogenic NOx.
Previous work has identified uncertainties in the treatment of ozone production from NMVOC oxidation
as a potential source of inter-model differences e.g..
The relatively large influence of anthropogenic NMVOCs on springtime ozone (compared with
their influence during other times of the year)
could be a contributing
factor to the large spread in springtime ozone simulated by current-generation CTMs (Fig. ).
NOx from shipping is the largest single contributor to springtime
transboundary ozone transport in all three receptor regions shown here.
We note, however, that the coarse resolution of our model (2∘) would be expected to exaggerate the
effects of ship NOx on ozone production due to rapid dilution of the emissions
and
exaggerate the transport of NOx and ozone near coastlines due to
unrealistically high diffusion between adjacent land and ocean grid cells.
Thus, the contribution of shipping emissions to surface ozone
in our simulations should be considered an upper
bound, especially in coastal regions.
The high contribution of ship NOx to summertime ozone in Europe (Fig. )
may be an artefact of the coarse model resolution combined with the
high shipping volume near coasts in the eastern North Atlantic Ocean as well as the North Sea.
Early work by showed a stronger influence of ship
NOx on surface ozone in northwestern Europe than on any other continental region.
showed that the only other CTM in the HTAP Phase 2 ensemble to report the results
of a perturbation of shipping emissions (the EMEP_rv48 CTM with a resolution of
0.5∘×0.5∘) shows a similar magnitude
for the influence of ship NOx on summertime ozone in Europe as for springtime ozone.
Using a regional model at 50 km ×50 km resolution
and a similar ozone tagging system as that used in the present study,
showed that the contribution of ship NOx to ozone in coastal regions of Europe
reaches a maximum level in summer.
used a global model with a resolution of 0.5∘×0.5∘ to show
that shipping near coastal areas strongly influences ozone over northwestern Europe in
both spring and summer,
while NOx emissions from
shipping on the high seas have a stronger influence on European ozone
in spring than in summer.
In contrast, using a higher-resolution
regional model (20 km ×20 km) for Europe,
did not indicate that ship NOx played such a strong role in summertime ozone
over Europe.
In other receptor regions, the influence of ship NOx emissions on surface ozone
is largest in spring (Figs. , ), suggesting
a stronger influence of NOx emissions over the high seas on springtime ozone in
our simulations in these regions.
Model-dependent inconsistencies in the treatment of ship NOx emissions
may play a role in the large spread of simulated ozone between models in springtime (Fig. ).
Future work should examine the contribution of ship NOx emissions to ozone in both spring and summer,
using model systems that include the better representations of plume dilution over the major shipping
routes, more refined attribution to shipping emissions from coastal regions and the high seas,
and higher resolution over receptor regions.
Previous work has indicated a strong influence of anthropogenic
emissions from both North America and East Asia on
springtime ozone in Europe , a strong influence of East Asian emissions on springtime ozone
in North America , and a diverse range of intercontinental influences on springtime
ozone in East Asia .
The direct numerical comparison of our results with these previous studies is difficult due
to the different methodologies used.
These previous studies all employed the perturbation technique to determine the influence
of all anthropogenic emissions (both NOx and reactive carbon) from each source region,
whereas this work uses a tagging approach that separately attributes ozone to emitted NOx
and reactive carbon.
However, qualitatively, our results (as shown in
Figs. , , and )
do appear to be consistent with this earlier work.
Comparison of the NOx-tagged and VOC-tagged results in these figures shows
that anthropogenic NOx emissions from most source regions have a stronger influence
on springtime ozone in any given receptor region than anthropogenic emissions of reactive carbon.
The only exception to this is East Asia, where reactive carbon emissions
are substantially higher than in other HTAP Tier 1 source regions (Table ).
Reactive carbon emissions from East Asia contribute approximately
equally to springtime ozone in North America when compared to the East Asian NOx emissions, whereas East Asian
reactive carbon contributes more to springtime ozone than East Asian NOx in Europe.
Long-range transport of ozone precursors
suggested that measurements of the abundance of
PAN at mountaintop sites in spring may be useful as an
indicator of the intercontinental transport of ozone and its precursors,
as well as being a diagnostic
for uncertainties in CTM simulations, which show large inter-model
differences in simulated PAN .
The column integrated density of PAN in the lower troposphere (defined here as the model layers between 500
and 800 hPa) in the three HTAP Tier 2 receptor regions
“North West Europe”, ”North East China”, and “North West United States”
are shown in
Figs. , , respectively.
A set of figures for other HTAP Tier 2 regions,
with the complete attribution of surface ozone to all tagged HTAP Tier 1 source regions is
available in the Supplement.
Simulated PAN is highest in late winter to early spring, which is consistent with earlier work
.
The extra-regional contribution to PAN is also highest in spring;
this is primarily due to anthropogenic NMVOCs, which is also consistent with .
Seasonal cycle of column-integrated
lower tropospheric PAN (10-15 molec. cm-2) in the HTAP Tier 2 receptor region “North West Europe”.
The lower troposphere is defined here as all model levels between 800 and 500 hPa.
NOx tagging is shown in panels (a) and (b),
and reactive carbon tagging is shown in panels (c) and (d).
Panels (a) and (c) show the total monthly mean PAN (black line) as well as the local anthropogenic
component, the long-range transported anthropogenic component, and the natural components.
Panels (b) and (d) show the individual Tier 1
source regions responsible for the long-range transported component of PAN.
Seasonal cycle of column-integrated
lower tropospheric PAN (10-15 molec. cm-2) in the HTAP Tier 2 receptor region “North East China”.
The lower troposphere is defined here as all model levels between 800 and 500 hPa.
NOx tagging is shown in panels (a) and (b),
and reactive carbon tagging is shown in panels
(c) and (d).
Panels (a) and (c) show total monthly mean PAN (black line) as well as the local anthropogenic
component, the long-range transported anthropogenic component, and the natural components.
Panels (b) and (d) show the individual Tier 1
source regions responsible for the long-range transported component of PAN.
Seasonal cycle of column-integrated
lower tropospheric PAN (10-15 molec. cm-2) in the HTAP Tier 2 receptor region “North West United States”.
The lower troposphere is defined here as all model levels between 800 and 500 hPa.
NOx tagging is shown in panels
(a) and (b),
and reactive carbon tagging is shown in panels (c) and (d).
Panels (a) and (c)
show the total monthly mean PAN (black line) as well as the local anthropogenic
component, the long-range transported anthropogenic component, and the natural components.
Panels (b) and (d) show the individual Tier 1
source regions responsible for the long-range transported component of PAN.
Our model simulations with NOx and VOC tagging
provide a unique opportunity to examine the origin and fate
of PAN as simulated in our model, as this allows the simultaneous attribution of simulated
PAN to both its NOx precursor and its reactive carbon precursor.
Comparison of panels b and d in
Figs. , , and
consistently shows that, for any given land-based HTAP Tier 1 source region, the
anthropogenic NMVOC emissions contribute more to PAN formation than the anthropogenic
NOx emissions from that region to the PAN modelled in all HTAP Tier 2
receptor regions.
In all cases, the balance of extra-regional PAN is due to NOx emissions from shipping.
In our simulations, significant amounts of PAN are formed downwind of the regions in which
the anthropogenic NMVOC precursors are emitted, often via reaction with NOx
emitted from shipping.
A strong influence of anthropogenic NOx emissions on PAN in the northern midlatitudes
is consistent with the results shown by Fischer et al. (2014; their Fig. 7, which does not
distinguish between different sources of anthropogenic NOx).
Figures , , and also show that
the reactive carbon component of PAN
is generally more persistent than the NOx component.
For example, the contribution of anthropogenic
NMVOCs from North America to springtime PAN over East Asia
is only slightly lower than its contribution to springtime PAN over Europe,
which is much closer to North America considering the prevailing westerly winds
(panel d of Figs. and ).
In contrast, the contribution of anthropogenic NOx from North America
to springtime PAN in East Asia is substantially less than its contribution to springtime PAN over Europe
(panels b of Figs. and ).
Attribution of Northern Hemisphere total organic reactivity
We examine the Northern Hemisphere budget of reactive carbon in more detail in Fig. .
This figure shows the seasonal cycle of the Northern Hemisphere column-integrated
total reactivity with respect to the OH radical of all reactive
carbon-containing species in our simulation attributed to their emission source.
The total OH reactivity of reactive carbon species of an air mass
is often linked to its ozone production potential .
In each case, the OH reactivities shown in Fig. include the OH reactivity of
the primary emitted species as well as the OH reactivity of each carbon-containing
oxidation product.
These were calculated using the monthly averaged output of the modelled
concentration of each carbon-containing species (including its associated tags) and the temperature-
and pressure-dependent rate coefficients for their reaction with the OH radical; they were then
averaged over all Northern Hemisphere grid cells and weighted by air density.
Seasonal cycle of Northern hemispheric tropospheric column-integrated OH reactivity (s-1)
due to reactive carbon from the VOC-tagged run.
The complete attribution is shown in panel (a), and the detailed attribution to
anthropogenic emissions from HTAP Tier 1 source
regions is shown in panel (b).
The total Northern Hemisphere OH reactivity of reactive carbon remains fairly constant year-round
at about 0.6–0.7 s-1, but the seasonal cycles of the OH reactivity attributable to
different reactive carbon sources show more variability.
Methane (and its oxidation products) contribute about 0.2–0.3 s-1 (almost half of
the total hemispheric reactivity), with a slight maximum
in the summer, which is consistent with enhanced oxidation (and, thus, the enhanced availability of more
reactive methane oxidation products) due to higher OH in summer.
The contributions of anthropogenic and biogenic reactive carbon sources to total hemispheric
reactivity are similar, ranging between about 0.1 and 0.3 s-1, but with distinct seasonal cycles.
The reactivity of biogenic carbon is highest in summer–autumn
(which is consistent with the Northern Hemisphere growing season),
whereas the reactivity of anthropogenic carbon is highest in winter–spring
(which is consistent with constant year-round anthropogenic
emissions and a build-up of reactive carbon over winter due to lower hemispheric OH).
The build-up of anthropogenic reactive carbon throughout the Northern Hemisphere
over winter combined
with the resumption of OH chemistry in spring is consistent with
the disproportionate effect of extra-regional
anthropogenic reactive carbon on springtime ozone seen in
Figs. , , and .
Thus, uncertainties in the model chemical mechanisms associated with the oxidation of anthropogenic
NMVOCs e.g.
may also contribute to the large spread in simulated ozone seen in the
HTAP ensemble during spring (Fig. ).
Figure also shows the geographical origin of Northern Hemisphere
anthropogenic carbon reactivity.
Emissions of reactive carbon from East Asia stand out as the single major source
of enhanced anthropogenic carbon reactivity in winter and spring in our simulations.
This is consistent with the high emissions of reactive carbon from this region in 2010 noted earlier
(Table ).
Growth in NMVOC emissions from East Asia may have continued since this time ,
while NOx emissions have been decreasing .
Increasing trends in the local production of ozone during summer over East Asia e.g.
should be associated with the
increased oxidation of reactive carbon and, thus, potentially less export of reactive carbon into the
Northern Hemisphere background during summer.
We expect, however, that
increasing emissions of reactive carbon in East Asia could lead to
an increased build-up of East Asian reactive carbon in the Northern Hemisphere over winter
and, in turn, to an increased East Asian contribution to extra-regional springtime ozone
in other parts of the Northern Hemisphere.
Our tagging technique is currently the only one we know of that is capable of examining the
budget of reactive carbon in the level of detail presented in this study.
The separate tracking of the carbon-containing
and nitrogen-containing components of PAN is particularly informative,
suggesting that significant amounts of PAN are formed downwind of source regions in our model,
especially during winter and spring, due to a build-up of anthropogenic reactive carbon
over winter when photochemistry is relatively slow.
Given the large variety in model representations of NMVOC chemistry, including
PAN formation and decomposition processes and the large
inter-model differences in simulated PAN , the widespread implementation
of similar tagging diagnostics in other CTMs may help to provide additional
information about the origin and fate of simulated PAN and, more generally, about the influence
of reactive carbon on atmospheric composition.
In combination with routine mountaintop observations of springtime PAN, this may aid understanding
of the global PAN budget and other processes responsible for the intercontinental
transport of air pollution.
Better constraints on these chemical and transport
processes should also help to reduce inter-model differences
in simulated springtime ozone (Fig. ).
Tropospheric ozone sensitivity to methane
We performed an additional set of model runs with both NOx and VOC tagging
with the methane surface boundary condition reduced
from 1760 to 1410 ppb, which amounts to a reduction of 350 ppb (or 20 %).
This perturbation can also be expressed as an increase of 25 %.
Here we interpret the methane perturbation run in terms of the atmospheric response to a 25 %
increase in the methane surface mixing ratio at steady state.
In response to the 25 % increase in the imposed surface mixing ratio of methane, the total tropospheric
burden increased by 776 Tg(CH4), which amounts to an increase of 23 %.
The strength of the annual tropospheric chemical
sink of methane due to OH increased by 72.5 Tg(CH4) (or 15.2 %).
The corresponding increase in the methane lifetime was 0.48 years (or 6.75 %).
The relatively small growth in the chemical methane sink
compared with the magnitude of the perturbation in methane itself is consistent with the feedback of
methane on its own lifetime due to the depletion of OH .
Table shows the response of the tropospheric ozone burden (and the contributions of
different reactive carbon precursors) to the 25 % increase in the imposed
surface mixing ratio of methane.
The 1 ppb simulated
increase in Northern Hemisphere surface ozone in response to a 25 % increase in methane
burden is consistent with previous work .
The 9.22 Tg increase in tropospheric ozone burden is also consistent with the review of
, who derived a sensitivity of 0.11–0.16 Tg(O3) per Tg(CH4) yr-1 emitted
based on an analysis of the literature.
We calculate 0.13 Tg(O3) per Tg(CH4) yr-1 based on our results.
Change in the contribution of reactive carbon sources to tropospheric and Northern Hemisphere (NH) surface
ozone in response to a 25 % increase in the imposed surface
mixing ratio of methane. Absolute changes and percentage changes are both shown.
Tropospheric burden NH surface mixing ratio Ozone sourceChange in ozonePercentageChange in mixingPercentageburden (Tg)changeratio (ppb)changeMethane13.013.01.4713.5Biogenic-1.88-2.40-0.168-2.28Stratosphere-0.683-1.01-0.0226-0.770Rest of the world-0.379-1.91-0.0315-2.15Biomass burning-0.243-1.78-0.0316-2.47East Asia-0.238-2.26-0.0583-2.90South Asia-0.131-1.93-0.0155-2.13Extra production-0.0351-0.761-0.00890-1.60North America-0.0975-2.17-0.0351-2.88Europe-0.0479-2.29-0.0222-3.09Russia, Belarus, and the Ukraine-0.0286-2.29-0.0132-2.96Oceanic sources-0.00287-2.91-0-4.72Aircraft-0.00158-2.59-0-3.81Total tropospheric ozone9.222.981.073.59
The relative increase in tropospheric ozone attributed to methane (13.0 %) by our tagging scheme is comparable
to, but slightly smaller than, the increase in the magnitude of the chemical methane sink due to OH
(15.2 %), which is consistent
with the troposphere as a whole becoming slightly more NOx limited with increasing methane.
The absolute increase in the total ozone burden (9.22 Tg(O3))
is, however, significantly lower
than the increase in the burden of ozone
attributed to methane by our tagging scheme (13.0 Tg(O3)).
When the methane burden is increased, the contribution of every other reactive carbon source
to the tropospheric ozone burden decreases
(each by approximately 1 %–2 %) to partially offset the increased ozone production from methane oxidation.
This is also consistent with a slightly more NOx-limited atmosphere with increasing methane.
In a future with an increased methane burden, control of NMVOC emissions could be expected to be less
effective at the large-scale reduction of annual average ground-level ozone.
Table shows the change in the contributions of different NOx sources
to tropospheric ozone in response to the 25 % increase in the methane burden.
As expected, all NOx sources become more productive when the total atmospheric burden of
reactive carbon is increased (which is consistent with the troposphere as a whole becoming more NOx limited).
The increase in the productivity of the different NOx sources under an increased burden of methane
is, however, not uniform.
Ozone production due to NOx from shipping stands out as highly sensitive
to the global methane burden in our simulations.
Ship NOx accounts for almost 30 % of the
1 ppb increase in Northern Hemisphere average surface ozone
when the methane burden is increased by 25 % (Table ),
despite being a much smaller percentage of total global NOx emissions (Table ).
Change in the contribution of NOx sources to tropospheric and Northern Hemisphere surface
ozone in response to a 25 % increase in the imposed surface
mixing ratio of methane. Absolute changes and percentage changes are both shown.
Tropospheric burden NH surface mixing ratio Ozone sourceChange in ozonePercentageChange in mixingPercentageburden (Tg)changeratio (ppb)changeLightning3.374.370.1494.97Stratosphere-0.0598-0.07910.009800.310Biogenic0.9153.650.08613.47Oceanic sources1.045.530.2875.70East Asia0.5323.250.08843.02Rest of the world0.5573.680.01673.45South East Asia0.5093.430.02142.92Aircraft0.5584.810.05575.20Biomass burning0.2772.340.03612.55South Asia0.3953.780.03703.00North America0.3403.380.09283.33Middle America0.3133.680.03093.19Europe0.1543.310.05593.29Middle East0.1463.670.03893.96Russia, Belarus, and the Ukraine0.07033.380.02693.26North Africa0.07364.460.02314.72Extra production0.002290.156-0-0.00590Central Asia0.02353.890.009364.09Total tropospheric ozone9.222.981.073.59
The spatial distribution of the increase in annual average surface ozone from ship NOx
in response to the 25 % increase in methane is similar to the spatial distribution of surface
ozone due to ship NOx in our base run (Fig. ).
Figures showing the response of the attributed surface ozone are available in the Supplement.
The response is largest over the major Northern Hemisphere ocean basins, but it also extends over
continental regions.
The seasonal cycle of the increase in annual average surface ozone from ship NOx
in the three HTAP Tier 2 regions examined here
in response to the 25 % increase in methane is similar to the seasonal cycle of surface
ozone due to ship NOx in our base run
(Figs. , , and ).
The maximum response of surface ozone from ship NOx to rising methane is simulated over
the major Northern Hemisphere ocean basins in summer
(which influences surface ozone in northwestern Europe in our simulations; Fig. ),
while the influence of this response
over most Northern Hemisphere continental regions is generally higher in winter–spring
(as seen in northeastern China; Fig. ).
Previous work
has noted the disproportionate influence of ship NOx on tropospheric
ozone due to the diffuse and widespread nature of this source over regions that would
otherwise have very low NOx mixing ratios.
noted that the response of surface ozone to increased methane was especially
strong in ship tracks.
also showed that ship NOx emissions reduce the global methane lifetime
much more than terrestrial NOx emissions.
We note again that the contribution of ship NOx to ozone
in our simulations (as in most current-generation CTMs)
is likely to be an overestimate due to the unrealistic dilution of these emissions
into coarse model grid cells
and the lack of explicit plume chemistry .
However, we do expect that the interaction between ship NOx and methane for ozone
production would persist in our model even with a more realistic treatment of ship emissions, as this
interaction is likely due to the location rather than the magnitude of ship emissions.
We are not aware of any previous work linking the combined influence
of these two sources to a potentially disproportionate influence on background ozone
in the Northern Hemisphere or on modelled
surface ozone air quality in inhabited regions of the Northern Hemisphere, especially in spring.
Given the current uncertainty in the attribution of recent trends in methane
and the potential for future increases in methane emissions, combined with slower reductions
of NOx emissions from international shipping than from other sectors
e.g. the SSP5 future emission scenario, we expect that model
simulations of future background ozone in the Northern
Hemisphere, especially during spring, may come to be increasingly influenced
by ozone produced via the interaction of methane and ship NOx.
Future work should investigate the ozone production via the interaction of these two sources in
more detail.
Conclusions
We have performed a source attribution for tropospheric ozone
in a chemical transport model using a novel technique that
separately accounts for the influence of both the emitted NOx and the emitted
reactive carbon precursors on simulated tropospheric ozone.
By tagging anthropogenic emissions of NOx and reactive carbon
according to their geographical region, we have calculated
source–receptor relationships for the Northern Hemisphere.
The results of our study are consistent with previous work, and they provide
a number of important new insights that are relevant to both the mitigation of
intercontinental transboundary air pollution and ongoing efforts to reduce the
uncertainty in the current generation of chemical transport models.
Consistent with previous work,
the annual average ground-level ozone in all major Northern Hemisphere regions is primarily
influenced by extra-regional emissions of both NOx and reactive carbon.
In all cases, local anthropogenic emissions of ozone precursors have a smaller influence on annual average
ozone than the combined effect of precursor emissions from the rest of the world.
As a reactive carbon precursor, methane contributes 35 % of the tropospheric ozone burden
and 41 % of the Northern Hemisphere annual average surface mixing ratio, which is
more than any other source of reactive carbon.
Our novel tagging methodology also reproduces the well-known dependence of
summer ozone maxima on local emissions of anthropogenic NOx and biogenic
reactive carbon as well as the enhanced importance of the intercontinental transport
of ozone from remote anthropogenic sources in spring.
Consistent with previous work, we find that emissions of NOx
at low latitudes produce free-tropospheric ozone more effectively due to more efficient
vertical transport.
We show, however, that NOx sources at higher northern latitudes
have a stronger influence on ground-level ozone, which is known to have a lower radiative forcing
but a higher influence on human health and ecosystems.
The current generation of chemical transport models has particular difficulty simulating
the intercontinental transport of ozone, as shown by the
large spread in ensemble simulations of ground-level ozone during the spring months.
We show that our tagging methodology can deliver detailed diagnostic information
about the origin and budget of springtime ozone in our model, along with information about
the springtime budget of peroxyacetyl nitrate (PAN), which is also associated with
springtime long-range transport and ozone production.
We show that a substantial proportion of the free-tropospheric PAN simulated by our
model in spring is not produced in the polluted boundary layer over the major
anthropogenic source regions; instead, it is produced downwind of these regions in our model
via the interaction of transported anthropogenic reactive carbon and NOx
emitted from international shipping.
Reactive carbon of anthropogenic origin (and its oxidation products, including PAN) builds up
in our model across the entire Northern Hemisphere during the winter months
and then contributes
to a short burst of hemispheric-scale ozone production during spring in our simulations.
In all but the most polluted source regions, anthropogenic NMVOCs do not make a significant
contribution to simulated ground level ozone in any other season but spring.
In this study, we showed that the export of anthropogenic reactive carbon from East Asia may be playing a dominant
role in contributing to the build-up of reactive carbon in the Northern Hemisphere over winter and, in turn,
to the hemispheric-scale production of ground-level ozone in spring.
Given the likely lack of recent mitigation in reactive carbon emissions from East Asia, we expect
this effect to be ongoing, and we recommend that future work continue to
investigate this possibility using updated emission inventories.
In addition to a contribution from the stratosphere,
the springtime peak in transported
ozone in our model is influenced by the interaction of two processes
known to be especially poorly represented in current models: the chemistry of the intermediate
oxidation products of NMVOCs and the emissions of NOx from international shipping.
Furthermore, the response of ground-level ozone to changes in methane also appears highly
sensitive to the treatment of ship NOx, especially in spring.
We believe that our tagging technique could
deliver useful information about the large differences in simulated springtime ozone
between current-generation models
if it were to be implemented in a larger number of models and used systematically
in model intercomparison exercises.
This could potentially point the way to improved representations
of the processes responsible for the intercontinental transport of ozone.
Improved global CTMs are required to inform effective policies aimed at reducing the
intercontinental transport of ground-level ozone – a problem which is most urgent in the springtime.
In particular, we recommend that
developers of emission inventories and CTMs revisit their representations of anthropogenic
NMVOC emissions and the associated oxidation chemistry in order to reduce the uncertainties in the
modelled springtime ozone.
Additionally, more explicit representations of the NOx chemistry of ship exhaust plumes
should be prioritized in order to
improve the suitability of the current models for simulating both the intercontinental transport of ozone
and the response of ozone to changing atmospheric methane.
Code availability
The CESM is maintained by NCAR and is provided free to the community.
The specific modifications made to the model to enable the tagging of ozone production
have been described by and are available in the online supplement
to that work, which is archived and accessible via the following DOI: 10.5194/gmd-11-2825-2018.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-10707-2020-supplement.
Author contributions
TB designed the study. Model runs were performed by AL.
The analysis of model runs was performed by AL, AN, and TB.
TB wrote the paper with input from AL and AN.
Competing interests
The authors declare that they have no competing interests.
Acknowledgements
The authors would like to thank Mark Lawrence,
Louisa Emmons, Simone Tilmes, and Terry Keating
for numerous helpful discussions during the preparation of this paper.
Financial support
This work was hosted by IASS Potsdam, and financial support was provided by the Federal Ministry of Education and
Research of Germany (BMBF) and the Ministry for Science, Research and Culture of the State of Brandenburg (MWFK).
Review statement
This paper was edited by Frank Dentener and reviewed by Volker Grewe and one anonymous referee.
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