ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-6537-2016Impacts of anthropogenic and natural sources on free tropospheric ozone over
the Middle EastJiangZhezhejiang@ucar.eduhttps://orcid.org/0000-0002-0086-7486MiyazakiKazuyukihttps://orcid.org/0000-0002-1466-4655WordenJohn R.LiuJane J.https://orcid.org/0000-0001-7760-2788JonesDylan B. A.HenzeDaven K.Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USAJapan Agency for Marine-Earth Science and Technology, Yokohama, JapanDepartment of Geography and Planning, University of Toronto, Toronto, ON, CanadaSchool of Atmospheric Sciences, Nanjing University, Nanjing, ChinaDepartment of Physics, University of Toronto, Toronto, ON, CanadaDepartment of Mechanical Engineering, University of Colorado, Boulder, CO, USAnow at: National Center for Atmospheric Research, Boulder, CO, USAZhe Jiang (zhejiang@ucar.edu)27May201616106537654613October201516December20154May201618May2016This 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/16/6537/2016/acp-16-6537-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/6537/2016/acp-16-6537-2016.pdf
Significant progress has been made in identifying the influence of different
processes and emissions on the summertime enhancements of free tropospheric
ozone (O3) at northern midlatitude regions. However, the exact
contribution of regional emissions, chemical and transport processes to
these summertime enhancements is still not well quantified. Here we focus on
quantifying the influence of regional emissions on the summertime O3
enhancements over the Middle East, using updated reactive nitrogen
(NOx) emissions. We then use the adjoint of the GEOS-Chem model with
these updated NOx emissions to show that the global total contribution
of lightning NOx on middle free tropospheric O3 over the Middle
East is about 2 times larger than that from global anthropogenic sources.
The summertime middle free tropospheric O3 enhancement is primarily due
to Asian NOx emissions, with approximately equivalent contributions
from Asian anthropogenic activities and lightning. In the Middle Eastern
lower free troposphere, lightning NOx from Europe and North America and
anthropogenic NOx from Middle Eastern local emissions are the primary
sources of O3. This work highlights the critical role of lightning
NOx on northern midlatitude free tropospheric O3 and the
important effect of the Asian summer monsoon on the export of Asian
pollutants.
Introduction
O3 is produced in the troposphere when volatile organic compounds (VOC)
and carbon monoxide (CO) are photochemically oxidized in the presence of
NOx. Tropospheric O3 is an important pollutant and greenhouse gas.
It also plays a critical role in determining the oxidizing capacity of the
troposphere. The O3 distribution in the troposphere is strongly
influenced by dynamical processes, as well as by the regional chemical
sources and sinks of O3. Previous studies (e.g., Park et al., 2007;
Worden et al., 2009; Vogel et al., 2014) have demonstrated that rapid
convective transport associated with the Asian monsoon anticyclone can result
in significant enhancement of O3 abundance over Asia, northern Africa
and Europe. The stratosphere–troposphere exchange of O3 also has
important effects on the distribution of tropospheric O3 (e.g., Barth et
al., 2012; Neu et al., 2014).
Tropospheric O3 peaks in the summer in broad regions of the Northern
Hemispheric middle latitudes (Zanis et al., 2007; Cristofanelli et al., 2014).
Recent studies (e.g., Liu et al., 2009; Worden et al., 2009; Zanis et al., 2014)
showed that the summertime maximum in free tropospheric O3 over the
Middle East as observed by the Tropospheric Emission Spectrometer (TES) was
consistent with the model predictions of Li et al. (2001). Liu et al. (2009)
indicated that the enhancement of free tropospheric O3 over the Middle
East is mainly due to the influence of the Arabian anticyclone in the middle
troposphere, which traps O3 that is produced locally as well as O3
and its precursors that are transported from rest of Asia. Recent studies
(Ricaud et al., 2014; Vogel et al., 2014) demonstrated that the Asian monsoon
anticyclone provides an effective pathway to redistribute Asian pollutants
globally. An improved understanding about the mechanism of the summertime
enhancement of free tropospheric O3 over the Middle East is thus
important as it will provide critical information about the sources and
variation of tropospheric O3 in the Northern Hemisphere.
In this study, we assess the influence of anthropogenic and natural sources
of O3 precursors on free tropospheric O3 enhancement over the
Middle East. During the past decade there have been several studies using
data assimilation and inverse modeling approaches to better quantify the
emission estimates of O3 precursors (e.g., Fu et al., 2007; Lamsal et
al., 2011; Miyazaki et al., 2012; Jiang et al., 2015a). In order to better
represent the emission change in the past decade in our analysis, we adopted
the most recent updated NOx emission estimates from the assimilation
study of Miyazaki et al. (2015) for the period of 2005–2012, which employed
remote-sensing measurements from OMI (Ozone Monitoring Instrument), MLS
(Microwave Limb Sounder), TES and MOPITT (Measurement of Pollution In The
Troposphere). Miyazaki et al. (2015) obtained significant bias reductions for
O3 and nitrogen dioxide (NO2), relative to satellite and ozonesonde
measurements. Use of their updated NOx emission estimates is, therefore,
expected to provide a better simulation of tropospheric O3 than the
Global Emissions Inventory Activity (GEIA) (Benkovitz et al., 1996) used by
Liu et al. (2009). In their analysis, Liu et al. (2009) used the tagging
capability of the GEOS-Chem model to quantify the regional influence on the
Middle East O3 maximum, based on the linearized O3 production and loss
rate. However, that approach cannot track O3 sources back to emissions
of O3 precursors and only provides a coarse aggregation of the regional
contributions. Here, following Jiang et al. (2015b), we use the adjoint of
the GEOS-Chem model to carry out a more detailed sensitivity analysis, which
will allow us to better distinguish the contributions of different regions
and emission categories to free tropospheric O3 over the Middle East.
GEOS-Chem model with updated surface NOx emissions
The GEOS-Chem CTM (http://www.geos-chem.org) is driven by assimilated
meteorological data from the NASA Goddard Earth Observing System (GEOS-5) at
the Global Modeling and Assimilation Office. We used version v34 of the
GEOS-Chem adjoint model, which is based on v8-02-01 of GEOS-Chem with
relevant updates through v9-01-01. The standard GEOS-Chem chemical mechanism
includes 43 tracers, which can simulate detailed tropospheric
O3-NOx-hydrocarbon chemistry, including the radiative and
heterogeneous effects of aerosols. The global anthropogenic emission
inventory is EDGAR 3.2 FT2000 (Olivier and Berdowski, 2001), updated by the following regional emission inventories: the
INTEX-B Asia emissions inventory for 2006 (Zhang et al., 2009); the
cooperative program for monitoring and evaluation of the long-range
transmission of air pollutants, the European Monitoring and Evaluation Programme (EMEP) inventory for Europe in 2000
(Vestreng and Klein, 2002); the US Environmental
Protection Agency National Emission Inventory (NEI) for 2005 in North
America; the Criteria Air Contaminants (CAC) inventory for Canada; and the
Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study
Emissions Inventory for Mexico (Kuhns et al., 2003). Biomass burning
emissions are from the interannual GFED3 inventory (van der Werf et al.,
2010). The soil NOx emission scheme is based on Yienger and Levy (1995)
and Wang et al. (1998), as a function of vegetation type, temperature,
precipitation history and fertilizer usage. The emissions of biogenic
volatile organic compounds (VOCs) are from MEGAN 2.0 (Millet et al., 2008).
Seasonal mean NOx emission from anthropogenic, lightning, soil
and biomass burning. The unit is 1010 molec cm-2 s-1.
We adopted the updated surface NOx emission estimates from Miyazaki et
al. (2015) for the period 2005–2012. Using the combined assimilation of
remote-sensing measurements from OMI NO2, MLS and TES O3, and
MOPITT CO, Miyazaki et al. (2015) constrained NOx emissions as well as
lightning NOx sources and the chemical concentration of various species
in the troposphere with the CHASER model (Sudo and Akimoto, 2007). The analysis was conducted with a local ensemble transform
Kalman filter (LETKF) method, with 30 ensembles and a 450 km horizontal
localization scale for surface NOx emissions. A major advantage of the
multispecies data assimilation used in Miyazaki et al. (2015) is that
observations of one species (for example, O3) can provide additional
constraints on other species (for example, NOx) through the improvement
in atmospheric fields and emission fluxes influencing the NOx chemistry.
In 2005, the assimilation resulted in a 25 % increase in NOx
emissions for Asia, relative to the GEOS-Chem a priori emissions. The
adjustments for NOx emissions from Europe and North America were much
smaller. The inversion result was evaluated with independent data from
satellite, aircraft, ozonesonde and surface in situ measurements, which
demonstrated large bias reductions after assimilation. For free tropospheric
O3, the mean model bias relative to ozonesonde measurements was reduced
from -2.3 to 0.4 ppb in the tropics and -1.4 to 0.9 ppb in the Northern
Hemisphere after assimilation, in which the surface NOx emission
optimization played a crucial role in reducing the model bias in the lower
and middle troposphere (Miyazaki et al., 2015). It should be noted that we
did not use the updated lightning NOx emissions in this work, because of
the larger uncertainties for those emission estimates (e.g., spurious
variations were introduced because of the lack of constraints from the TES
measurements after 2010). Because of the limitation of short horizontal
localization length (with the cut-off radius of 1643 km) and the short data
assimilation window (i.e., 2 h), the influence of long-range transport
processes cannot be sufficiently considered in the data assimilation
framework of Miyazaki et al. (2015), and thus, the estimated CO emissions may
have large uncertainty. Therefore, we did not use the optimized CO emissions
in this work. In 2005, the global total lightning NOx source in the
GEOS-Chem simulation is 6.0 Tg N; the value is within the range of recent
best estimates (e.g., 5 ± 3 Tg N yr-1 in Schumann and
Huntrieser, 2007, and 6.3 ± 1.4 Tg N yr-1 in Miyazaki et al.,
2014).
Figure 1 shows NOx emissions from anthropogenic activities, lightning,
soil and biomass burning emissions in the model. There are strong
anthropogenic emissions from eastern Asia, eastern North America and Europe,
and the emission strengths are nearly constant between summer and winter.
The seasonality of lightning and soil NOx are similar: more NOx
emissions in the summer hemisphere, but the emission strength is lower than
that for the anthropogenic sources. The emissions from biomass burning have
strong seasonality, generally peaking in the biomass burning seasons.
Seasonal mean middle free tropospheric O3 (464 hPa) in the
period of March 2005–February 2006. (a–d) GEOS-Chem simulation.
(e–h) GEOS-Chem simulation smoothed with TES averaging kernel and a
priori. (i–l) TES O3 retrievals.
Summertime enhancement of free tropospheric ozone over the Middle
East
The TES instrument was launched on NASA's Aura spacecraft on 15 July 2004.
The satellite is in a sun-synchronous polar orbit of 705 km and crosses the
equator at 01:45 and 13:45 LT. With a footprint of 8 km × 5 km,
TES measures radiances between 3.3 and 15.4 µm with global coverage
of 16 days (Beer et al., 2001) of observations. In the troposphere, TES
O3 profile retrievals have 1–2 degrees of freedom for signal (DOFS). We
use data from the “lite” product (http://tes.jpl.nasa.gov/data/),
which reports volume mixing ratios (VMR) on 26 pressure levels for O3.
The TES retrievals use a monthly mean profile of the trace gas from the
MOZART-4 CTM (chemical transport model), averaged over 10∘
latitude × 60∘ longitude, as the a priori information. We
refer the reader to Jiang et al. (2015b) for more details about the
application and evaluation of TES O3 data.
Figure 2a–d present the modeled middle free tropospheric (464 hPa) O3
distribution for March 2005–February 2006. An obvious feature is the low O3
concentrations over the maritime continent and the Amazon, which is
consistent with previous studies using measurements from satellite,
ozonesonde and aircraft (Rex et al., 2014; Bela et al., 2015). Over the
northern middle latitudes, O3 concentrations are highest in summer. The
tropospheric O3 concentrations in the middle troposphere start to
increase in spring and then decrease dramatically in fall, which is
consistent with seasonal cycle observed at European mountain sites
(Zanis et al., 2007; Cristofanelli et al., 2014). Figure 2e–h show the modeled
middle tropospheric (464 hPa) O3 smoothed with the TES averaging
kernels and a priori. The unsmoothed (Fig. 2a–d) and smoothed (Fig. 2e–h)
O3 distributions are highly consistent, although there is a
small difference in the magnitude. Figure 2i–l present the TES O3
retrievals at 464 hPa, which demonstrates good agreement globally with
respect to the model.
Figure 3 shows the monthly variation of mean O3 over the Middle East at
different levels. In the lower and middle troposphere, the relative
difference between the model and data is generally less than 10 %, whereas
the bias is a little larger in the upper troposphere. Figure 3 shows
significant and moderate O3 enhancement during the summer in the middle
and lower troposphere, respectively, over the Middle East. In contrast,
O3 concentrations in the upper troposphere are at a minimum in summer,
implying altitude-dependent mechanisms for the O3 variations. Obtaining
a better understanding of these mechanisms is important because it provides
critical insights about the sources and variations of tropospheric O3
in the Northern Hemisphere.
Monthly mean O3 concentration for lower free troposphere
(681 hPa), middle free troposphere (464 hPa) and upper free troposphere
(215 hPa) in the period of March 2005–February 2006 over Middle Eastern
Asia (blue box in Fig. 4) for TES O3
retrievals and GEOS-Chem simulation (smoothed with TES averaging kernel and a
priori). There is no TES data available in June 2005.
Response of middle free tropospheric (450–350 hPa) and lower free
tropospheric (700–600 hPa) O3 over Middle Eastern Asia (blue box) to
precursor emission perturbation from anthropogenic NOx, lightning
NOx, other NOx sources (biomass burning, biofuel and soil
NOx), anthropogenic CO and biogenic isoprene, for June–August 2005. The
response can be explained as the mean change (unit of ppbv) of regional mean
O3 due to 10 % increase of precursor emissions in a particular grid
assuming unchanged chemical environment.
Impact of anthropogenic and natural sources on the Middle East
ozone
Liu et al. (2009) indicated that O3 production over the Middle East and
rest of Asia both contribute about 30 % of free tropospheric O3 over
the Middle East in July 2005. However, due to the limitation of the tagging
approach that they employed, they were not able to obtain a detailed
description of the sensitivity of Middle East O3 to the precursor
emissions. In this section, we will use the adjoint of the full-chemistry
GEOS-Chem model (Henze et al., 2007) to quantify O3 source
contributions, similar to previous studies (Lapina et al., 2014; Jiang et al., 2015b). The adjoint model, which includes both chemistry and transport, is
run backwards to computationally efficiently provide sensitivities with
respect to each of the model's emissions from each species, sector and grid
cell.
Figure 4 shows the response of O3 in the lower free troposphere (700–600 hPa) and middle troposphere (450–350 hPa) over the Middle East
(30–60∘ E, 20–40∘ N) to O3 precursor emission
perturbation for June–August 2005. The response can be explained as the mean
change (unit of ppbv) of regional mean O3 due to 10 % increase of
O3 precursor emissions in a particular grid assuming unchanged chemical
environment. For example, one particular grid with response 0.02 ppb implies
mean free tropospheric O3 over the Middle East will be increased by
0.02 ppb, if the O3 precursor emission in this grid is increased by
10 % under current chemical regime.
Response of middle free tropospheric (450–350 hPa) O3 over
Middle Eastern Asia (30–60∘ E, 20–40∘ N) to NOx emission perturbation in
the period of March 2005–February 2006. The value can be explained as the
mean change (unit of ppbv) of regional mean O3 due to 10 % increase
of NOx emission in a particular region (Asia, North America (NA) and
Europe (EU), Middle Eastern Asia, and the rest of the world) assuming unchanged chemical
environment. The last column shows the multiyear mean value for
June–August 2005–2012.
Middle free troposphere Jun–AugSep–NovDec–FebMar–MayJun–Aug2005–2012AsiaAnthro0.400.170.090.130.38Lightning0.530.210.080.220.47Total0.930.370.170.350.85NA and EUAnthro0.110.130.060.140.11Lightning0.340.280.100.310.35Total0.450.410.160.450.46Middle EastAnthro0.040.030.010.030.07Lightning0.080.030.010.050.08Total0.120.050.020.080.15Rest of worldAnthro0.010.040.040.030.02Lightning0.330.550.460.400.33Total0.340.580.510.430.35GlobalAnthro0.570.360.200.340.58Lightning1.281.060.650.981.23Total1.851.420.851.321.81Others 0.360.400.320.350.38
In the middle troposphere, anthropogenic and natural NOx emissions from
Asia, particularly from India, are the primary sources of O3 precursors
and subsequent O3 production (Fig. 4a). In contrast, O3 and
O3 production in the lower
free troposphere depends primarily on NOx emissions in the Middle East,
but with significant contributions from natural and anthropogenic sources
elsewhere in the Northern Hemisphere. This distinct difference in source
regions for O3, between the middle and lower free troposphere,
highlights the complex transport pathways that bring air from other parts of
the world to this region (e.g., Liu et al., 2011; Safieddine et al., 2014).
For both the lower and middle free troposphere, the contributions from other
source types, primarily NOx from biofuel and soil emissions, or biomass
burning, are less significant in this season. The contributions from
anthropogenic CO and biogenic isoprene are small in this season, indicating
that O3 production is primarily NOx limited, and thus, we will
focus on the contributions of NOx to O3 in the following
discussions.
Table 1 provides the seasonal mean value of the response of Middle Eastern
O3 in the middle troposphere (450–350 hPa) to NOx perturbations
between March 2005 and February 2006. The analysis shows a maximum total global
response (1.85 ppb) in summer, corresponding to the summertime O3
maximum. The total global contribution from lightning NOx is about 2
times larger than that from anthropogenic emissions in all seasons, implying
that lightning NOx is the dominant source for middle free tropospheric
O3 over the Middle East, which is consistent with Liu et al. (2009), who
indicated that most free tropospheric O3 (about 75 %) over the
Middle East is produced in the free troposphere (700 hPa – tropopause).
During June–August 2005, the region that makes the largest contributions to
O3 in the middle troposphere over the Middle East is Asia (0.93 ppb),
followed by Europe and North America (0.45 ppb). The contribution from Middle
Eastern local emissions is much smaller (0.12 ppb), representing only 13 %
of Asian contributions. In contrast, Liu et al. (2009) found that O3
production (as opposed to emissions) over the Middle East and O3
production over Asia make contributions to free tropospheric O3 of
similar magnitude, and the contribution from North America and Europe is
negligible. The large discrepancy between these two studies implies that
most O3 produced over the Middle East is due to imported O3
precursors from long-range transport, which would not be accounted for with
the method employed by Liu et al. (2009), underscoring the significant role
of long-range transport of O3 precursors on free tropospheric O3
production.
(a–c) Distribution of CO-like tracer (30-day lifetime) in
June–August 2005 in lower free troposphere (700–600 hPa), middle free
tropospheric (450–350 hPa) and upper free tropospheric (300–100 hPa). The
blue box defines the Middle East domain. The black box defines the region
where CO was released from combustion sources only (fossil fuel, biofuel and
biomass burning); (d–f) difference of CO-like tracer (30-day
lifetime) concentration between June–August and March–May 2005. The CO
emission in March–May 2005 is set as the same as that in June–August 2005;
(g–i) difference of CO-like tracer (1-day lifetime) concentration
between June–August and March–May 2005; (j–l) difference of
CO-like tracer (7-day lifetime) concentration between June–August and
March–May 2005; the combustion sources are released in middle free
troposphere.
There are pronounced discrepancies between the seasonality of the regional
contributions. For Asia, the total contribution to O3 in the Middle
Eastern middle troposphere is 0.93 ppb in summer, which is about 3 times
larger than in spring (0.35 ppb) and fall (0.37 ppb). In contrast, the
total contribution of Europe and North America is 0.45 ppb in summer,
similar as that in spring (0.45 ppb) and fall (0.41 ppb); the total
contribution from the rest of the world is minimum in summer. The discrepancy
in the seasonal variations suggests that Asian emissions are the main sources
driving the summertime O3 maximum over the Middle East. Asian
anthropogenic and lightning NOx emissions have similar impacts, 0.40 and
0.53 ppb, respectively, on the Middle Eastern summertime O3. It should
be noted that the influence from stratosphere–troposphere exchange is not
assessed in this work, as previous studies with GEOS-Chem model (Li et al.,
2001; Liu et al., 2009) showed that the contribution from stratospheric
O3 to the summertime O3 enhancement is small. More efforts are
needed in the future to sufficiently evaluate the contribution of
stratosphere–troposphere exchange, as suggested by some recent model studies
(e.g., Lelieveld et al., 2009; Spohn and Rappenglück, 2014; Zanis et al.,
2014).
To better understand the transport of Asian emissions to the Middle East, we
conducted an analysis using an idealized CO-like tracer. We performed a
tagged-CO simulation for the periods March–May and June–August 2005. Combustion CO
emissions (fossil fuel, biofuel and biomass burning) are released over India
and southeast Asia either from surface (Fig. 5a–i) or from middle free
troposphere (Fig. 5j–l). The CO emission in March–May 2005 is set as the
same as that in June–August 2005. Following Jiang et al. (2015a), we assume a
constant and uniform timescale for loss (lifetime). The simulations were
initialized with a uniformly low abundance of 1 pptv for the tracer.
With 30-day lifetime, our analysis shows significant influence from
transport of Asian emissions to the upper free troposphere (Fig. 5c) in
June–August 2005, associated with the Tibetan anticyclone. Figure 5d–f
show the difference of CO-like tracer concentrations between June–August and March–May 2005. Because the imposed emissions and sink for the tracer are constant,
these differences are completely driven by seasonal variations in transport.
Compared to spring, the transport of Asian emissions in summer has a
moderate impact in the middle troposphere (Fig. 5e), but a significant
influence in the upper troposphere (Fig. 5d). This shows the transport
pathway during the Asian summer monsoon season is as follows: pollutants are
lifted into upper troposphere through convection (e.g., Park et al., 2007;
Worden et al., 2009) and trapped within the Tibetan anticyclone (e.g., Li et al., 2001). On the other hand, the enhancement of summertime O3 over
the Middle East is at a maximum in the middle free troposphere (Fig. 3).
This altitude discrepancy suggests the existence of other processes besides
the Asian summer monsoon. As mentioned in the introduction, Liu et al. (2009)
indicated that the Arabian anticyclone in the middle troposphere plays an
important role in trapping the subsided O3 and its precursors in the
Middle East, which is consistent with our results.
Response of lower free tropospheric (700–600 hPa) O3 over
Middle Eastern Asia (30–60∘ E, 20–40∘ N) to NOx emission perturbation in
the period of March 2005–February 2006. The last column shows the multiyear
mean value for June–August 2005–2012.
Lower free troposphere Jun–AugSep–NovDec–FebMar–MayJun–Aug2005–2012AsiaAnthro0.120.100.070.090.11Lightning0.180.100.040.100.15Total0.290.200.110.200.26NA and EUAnthro0.160.140.080.160.15Lightning0.470.280.090.230.46Total0.630.420.160.380.61Middle EastAnthro0.290.170.030.160.38Lightning0.070.040.020.060.07Total0.370.210.050.220.44Rest of worldAnthro0.030.030.040.040.03Lightning0.210.350.240.170.19Total0.230.380.270.210.22GlobalAnthro0.590.440.210.450.67Lightning0.930.770.380.560.87Total1.521.220.601.001.54Others 0.380.360.300.320.41
Over India and southeast Asia, the intensity of NOx emissions from
anthropogenic sources (Fig. 1a) is much larger than that from lightning
(Fig. 1c), however, the contributions of anthropogenic and lightning
emissions to middle free tropospheric O3 over the Middle East are similar
(Fig. 4a, b). This discrepancy suggests that free tropospheric NOx
sources have larger impacts than surface sources on free tropospheric
O3, associated with faster transport and longer lifetime in free
troposphere. In order to evaluate the influence of source level, we
conducted a model analysis by releasing (simulated) combustion CO emissions
from the surface (1-day lifetime, Fig. 5g–i) and middle free troposphere
(7-day lifetime, Fig. 5j–l). The results confirmed the significant
contribution from free tropospheric sources.
Table 2 provides the seasonal mean value of the O3 response in the
Middle Eastern lower free troposphere (700–600 hPa) to regional NOx
emissions. During June–August 2005, the largest contribution (0.63 ppb) to lower
tropospheric O3 over the Middle East is from European and North American
emissions, followed by Middle Eastern local emissions (0.37 ppb). The large
differences in regional contributions with altitude demonstrate the
significant influence of dynamics on the distribution of free tropospheric
O3. The global contribution from lightning NOx is about
25–75 % larger than that from anthropogenic emissions, implying
lightning still plays an important role at these lower altitudes. Note that
the lightning NOx parameterization used in GEOS-Chem may have large
uncertainties (e.g., Schumann and Huntrieser, 2007) and have influenced our
estimates. For instance, the C-shape assumption, with a first maximum in the
upper troposphere and a second maximum in the boundary layer as proposed by
Pickering et al. (1998), may place too much NOx near the surface (Ott
et al., 2010) and overestimate the peak source height over land and the
tropical oceans (Miyazaki et al., 2014).
In order to isolate the potential influence of interannual variations in
factors such as dynamics and biomass burning, we conducted a sensitivity
analysis for the period June–August 2005–2012, using the updated surface NOx
emission estimates from Miyazaki et al. (2015). For the global total response
in the middle free troposphere (Table 1), there is good consistency between
the 2005 analysis (1.85 ppb) and 8-year mean value (1.81 ppb). Small
discrepancies are obtained for regional contributions; for example, the Asian
contribution is 0.93 ppb in 2005, and 0.85 ppb in the 8-year mean value.
Despite the small discrepancies, the consistency between the 2005 analysis
and the 8-year mean values suggests that our conclusions based on the 2005
analysis provide a good representation for the free tropospheric O3
variation over the Middle East.
Conclusions
Remote sensing measurements from TES show a maximum in summertime free
tropospheric O3 over the Middle East (Worden et al., 2009; Liu et al., 2009).
Using updated NOx emission estimates from
Miyazaki et al. (2015), we conducted an adjoint sensitivity analysis to study the impact of
anthropogenic and natural sources on free tropospheric O3 over the
Middle East.
Our results reveal that the global total contribution of lightning NOx
on middle free tropospheric O3 over the Middle East is about 2 times
larger than that from global anthropogenic sources. We find that emissions
from Asia contribute the most to middle tropospheric O3 over the Middle
East in summer, followed by European and North American emissions. The middle
tropospheric O3 maximum in summer is driven by Asian emissions, with
Asian anthropogenic and lightning NOx emissions having similar
contributions to the enhanced O3. Dynamics play a critical role on the
buildup of middle free tropospheric O3 over the Middle East: O3
and its precursors are lifted into the upper troposphere through convection,
trapped within the Tibetan anticyclone, and descend over the Middle East
and are subsequently trapped within the Arabian anticyclone. In contrast,
O3 in the lower free troposphere is influenced primarily by O3
precursor emissions in the Middle East, with significant contributions from
natural and anthropogenic sources elsewhere in the Northern Hemisphere. This
distinct difference in source regions for O3 and its precursors, and
the altitude variations of the regional influences, highlights the complex
transport pathways that bring air from other parts of the world to this
region.
Although our conclusions are based on an analysis in 2005, the consistency
between our 2005 analysis and an 8-year (2005–2012) climatology suggests
that our analysis provides a good representation for the free tropospheric
O3 variations over the Middle East. However, noticeable discrepancies
were obtained for some regional contributions; for example, the 8-year mean
Asian contribution is 10 % lower than that in 2005. In future studies, we
will investigate the influences of changes in emissions and interannual
variations in the meteorological conditions on free tropospheric O3 over
the Middle East and across the Northern Hemisphere to provide critical
information for enhanced understanding of the processes contributing to
variations in tropospheric O3.
Data availability
The TES lite product is available at
http://avdc.gsfc.nasa.gov/index.php?site=635564035&id=10.
Acknowledgements
This research was carried out at the Jet Propulsion Laboratory, California
Institute of Technology, under a contract with the National Aeronautics and
Space Administration.
Edited by: M. Van Roozendael
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