Tropospheric ozone variability during the East Asian summer monsoon as observed by satellite (IASI), aircraft (MOZAIC) and ground stations

. Satellite measurements from the thermal Infrared Atmospheric Sounding Interferometer (IASI), aircraft data from the MOZAIC/IAGOS project, as well as observations from ground-based stations, are used to assess the tropo-spheric ozone (O 3 ) variability during the East Asian Summer Monsoon (EASM). Six years 2008–2013 of IASI data analysis reveals the ability of the instrument to detect the onset and the progression of the monsoon seen by a decrease in the tropospheric 0–6 km O 3 column due to the EASM, and to reproduce this decrease from one year to the other. The year-to-year variability is found to be mainly dependent on meteorology. Focusing on the period of May-August 2011, taken as an example year, IASI data show clear inverse relationship between tropospheric 0–6 km O 3 on one hand and meteorological parameters such as cloud cover, relative humidity and wind speed, on the other hand. Aircraft data from the MOZAIC/IAGOS project for the EASM of 2008–2013 are used to validate the IASI data and to assess the effect of the monsoon on the vertical distribution of the tropospheric O 3 at different locations. Results show good agreement with a correlation coefﬁcient of 0.73 (12 %) between the 0–6 km O 3 column derived from IASI and aircraft data. IASI captures very well the inter-annual variation of tropospheric O 3 observed by the aircraft data over the studied domain. Analysis of vertical proﬁles of the aircraft data shows a decrease in the tropospheric O 3 that is more important in the free tro-posphere than in the boundary layer and at 10–20 ◦ N than elsewhere. Ground station data at different locations in India and China show a spatiotemporal dependence on meteorology during the monsoon, with a decrease up to 22 ppbv in Hyderabad, and up to 5 ppbv in the North China Plain.


Introduction
Over South and East Asia, tropospheric ozone (O 3 ) concentrations have significantly increased over the past few decades as a result of rapid urbanization (Cooper et al., 2014) with important implications on regional and global air quality.South and East Asian countries are experiencing increasing emissions of different pollutants, many of which are precursors of O 3 (Akimoto, 2003;Ohara et al., 2007;Richter et al., 2005).For example, China showed an increase of NO 2 reaching 29 % per year for the period 1996-2006 (van der Published by Copernicus Publications on behalf of the European Geosciences Union.A et al., 2008), and about 50 % over the industrial areas of China over the period 1996-2004(Richter et al., 2005)), though recent NO 2 observations from space are suggesting an NO 2 decrease in 2013 and 2014 (Richter et al., 2015).An increase in background O 3 concentrations is also detected in Southern China during the last decade (T.Wang et al., 2009).In Eastern China, a study by Xu et al. (2008), analyzing longterm trends at a background surface O 3 station in Linan, suggests enhanced O 3 variability linked to the increase in NO x (NO x = NO 2 + NO) concentrations.Over most of India, increasing trends in tropospheric O 3 are consistent with the observed trends in emissions from NO x and carbon monoxide (CO) as well as coal and petroleum consumption (Lal et al., 2012).
The Asian monsoon circulation dominates the regional meteorology of southern and East Asia.In summer, the East Asian Summer Monsoon (EASM) is characterized by torrential rain, strong winds carrying clean air from the ocean over the heated tropical land and deep convection processes forming cirrus clouds and further rain (Lawrence and Lelieveld, 2010).Surface observations have shown that the EASM is responsible for a decrease in surface O 3 at a rural site near Beijing (Wang et al., 2008) and a coastal site near Hong Kong (Lam et al., 2001;T. Wang et al., 2009).Yang et al. (2014) found that the largest impacts of EASM on the decrease in surface O 3 are found over central and western China, while Beijing (North East China) and Nanjing (East China) experience a summertime O 3 maxima during June and July respectively (Ding et al., 2008(Ding et al., , 2013)).Ozonesondes measurements have also detected the effect of the EASM on lower tropospheric O 3 (Chan et al., 1998;Zhou et al., 2013).Satellite measurements over South and East Asia have been used to assess the daily variability of tropospheric O 3 , notably from the Infrared Atmospheric Sounding Interferometer (IASI) (Dufour et al., 2015).The effect of the EASM on the tropospheric O 3 was previously detected with a decrease in the O 3 partial column using data from IASI over several Indian and Chinese cities (Dufour et al., 2010;Safieddine et al., 2013).Using the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) measurements, together with a regional chemistry and transport model, Zhao et al. (2010) showed that the air quality over southeastern China is affected by the EASM, leading to an influence extending to central East China from June to July.At 300 hPa, the Tropospheric Emission Spectrometer (TES) observations over India showed enhanced O 3 abundances during June and July followed by a decrease in August (Worden et al., 2009).
The main objective of this study is to document the effect of the monsoon on the regional and vertical distribution of tropospheric O 3 in East Asia during the summer using different observation data sets and relating them to one another.After this introduction, Sect. 2 will look at 6 years of tropospheric 0-6 km O 3 data and at a case study of the monsoon of 2011.We study the relationship between the tropospheric 0-6 km O 3 and CO total columns from IASI and different me-teorological parameters from the ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis (ERA) (winds, cloud cover and relative humidity) (Dee et al., 2011).Section 3 will look at a dense set of vertical O 3 airborne profiles (363 in total) used to validate the IASI-O3 columns as well assess how well IASI captures the inter-annual variability of the EASM as observed by the aircraft data.Section 4 will look at a ground stations data set at five locations: Hyderabad, Udaipur, Jabalpur, Pearl River Delta (PRD) and North China Plain (NCP), during the EASM of 2011.Conclusions are given in Sect. 5.
2 Tropospheric O 3 from IASI during the EASM

The IASI instrument
The IASI instruments launched onboard the MetOp platforms in October 2006 (IASI-A) and September 2012 (IASI-B) are nadir looking Fourier transform spectrometers that probe the Earth's atmosphere in the thermal infrared spectral range between 645 and 2760 cm −1 , with a spectral resolution of 0.5 cm −1 (apodized) and 0.25 cm −1 spectral sampling.In this study, and to have a consistent O 3 product over the period 2008-2013, only IASI-A data have been used.The IASI footprint is a matrix of 2 × 2 pixels, each with 12 km diameter at nadir.IASI monitors the atmospheric composition at any location two times per day (the satellite's ground track is at about 09:30 local time in the morning and 21:30 in the evening).Each IASI measures many of the chemical components that play a key role in the climate system and in several aspects of atmospheric pollution.Global distributions of O 3 vertical profiles are retrieved in near real time using a dedicated radiative transfer and retrieval software for the IASI O 3 product, the Fast Optimal Retrievals on Layers for IASI (FORLI-O 3 ) (Hurtmans et al., 2012).Data are selected using a filter for scenes with no or low cloud coverage (below 13 %), and by rejecting all observations with root mean square (RMS) of the spectral fit residual larger than 3.5 × 10 −8 W cm −2 sr cm −1 .Details about the chemical components that can be measured by IASI can be found in Clerbaux et al. (2009), Coheur et al. (2009) and Clarisse et al. (2011).IASI tropospheric O 3 retrieved from FORLI has been extensively validated against ozonesondes during the period 2008-2014 and results show that IASI underestimates tropospheric O 3 by 12 to 14 % in the mid-latitudes and tropics (Boynard et al., 2016).

Tropospheric O 3 during the EASM for 2008-2013
To look at the inter-seasonal and inter-annual variation of tropospheric O 3 during the monsoon, we show in Fig. 1 the monthly distribution (May-September) of the IASI 0-6 km O 3 column over the period 2008-2013.Only IASI daytime observations are used, since better thermal contrast, and hence better sensitivity to the lower troposphere, is usually obtained during the day (Clerbaux et al., 2009).Earlier studies have shown that the information content in the satellite measurement varies, and is generally maximal in the midto upper troposphere, and lower at the surface (Barret et al., 2011;Dufour et al., 2012;Safieddine et al., 2014).The 0-6 km O 3 column is used to eliminate any possible stratospheric intrusions and previous studies have shown that with the 0-6 km column, at least one piece of information is available for the IASI retrieval (Barret et al., 2011;Dufour et al., 2012).Limitations in the sensitivity provide sources of error that can influence our conclusions about the observed O 3 distribution, especially close to the surface.Another source of error emerge from the fact that the IASI observations used here are for scenes (pixels) with no or low cloud contamination, and therefore reflect mostly the state of the atmosphere before or after the rain/high cloud episodes that the EASM will generate.
The monthly average tropospheric O 3 columns from IASI shown here have different observation counts at each grid point, which may correspond to the average of one to more than 200 observations.Despite these limitations, this is the best-known data set of remote infrared retrieved O 3 columns covering the entire monsoon region, and we assume that the average effect of the monsoon on the tropospheric O 3 column from one month to the other can be reflected in one or more observations.The seasonal variation of O 3 as detected by IASI is highly dependent on photochemical activity and is generally higher in summer and lower in winter (Safieddine et al., 2013).However, within the EASM region, IASI shows in Fig. 1 that this typical seasonality is broken and on average, the 0-6 km O 3 columns are lower in June-July-August (JJA) in comparison with May.The Asian monsoon onset date is around the mid-May and June (Parthasarathy et al., 1994;B. Wang et al., 2009;Yang and Lau, 1998).Figure 1 shows that the largest decrease is recorded in southern India, where clean air masses from the Pacific starting typically in May (generating the Indian Summer Monsoon, a subsystem of the EASM) will be responsible of a decrease reaching 15-20 Dobson unit (DU) in JJA.With the march of the monsoon northeastward, the decrease becomes most prominent in July and August at higher latitudes.To understand the year-to-year variability in tropospheric ozone, we looked at possible stratospheric intrusions into the 0-6 km O 3 column.Potential vorticity (PV) and the water vapor mixing ratio (Qvap) measurements were used as markers of transport from the upper troposphere-lower stratosphere (UTLS) to the troposphere, and extracted at different altitudes.We used data from the ERA-interim reanalysis (more info about this data set is provided in Sect.2.3), and it was seen that downward transport from the UTLS can have an effect at altitude www.atmos-chem-phys.net/16/10489/2016/Atmos.Chem.Phys., 16, 10489-10500, 2016 > 10 km and latitudes > 30 • N (results not shown here).The analysis therefore suggests that the 0-6 km O 3 monthly averaged columns studied here are not affected by transport from the stratosphere.We also looked at the year-to-year variations associated with changes in regional emissions to study their effect on local production of ozone.We use emission estimates of NO  (Chou et al., 2011;Wang et al., 2010).Studies suggested O 3 transport by winds from nearby polluted areas such as the North China Plain (Y.Wang et al., 2009;Wang et al., 2010), or O 3 formation in the boundary layer since the photostationary state of the nitrogen cycle was perturbed (Wang and Xie, 2009).Over the rest of the domain, the increase in emissions of O 3 precursors, at least between 2008 and 2010, suggests that tropospheric ozone values also increased between these 2 years during the different months, but the IASI-O 3 columns do not show the same pattern.The analysis presented here suggests therefore that the meteorology associated with the EASM is the main driver of the O 3 regional distribution and year-to-year variability.Over the different years, the seasonal variation is well reproduced but the decrease in the tropospheric O 3 column will therefore change in magnitude depending on the monsoon strength.For example, in June 2010, around 30 • N and 120-130 • E, the Western North Pacific region shows void area of IASI retrievals (with white pixels) due to large cloud cover.The Western North Pacific Monsoon Index (WNPI, Wang and Fan, 1999) is the highest in 2010 over this region in comparison with the rest of the years shown here.On the other hand, 2011 is rather a typical monsoon season year, and will be used as a case study hereafter.

Case study of 2011
In order to look at the O 3 response to change in meteorology during the monsoon, we show in Fig. 2 the monsoon period May-August of 2011 taken as an example year of a typical monsoon.Carbon monoxide (CO) total columns from IASI are also shown.CO is often used as an anthropogenic pollution and biomass-burning tracer (e.g.Edwards et al., 2004;McMillan et al., 2010).Note that the seasonal variation of CO is such that it is lower in summer, because of the destruction of CO by the OH radical in the presence of sunlight.We consider different meteorological parameters in order to highlight the relationship between change in meteorology and the 0-6 km O 3 column.These are the following: (i) the total cloud cover that gives an insight on the photochemical activity in the troposphere, (ii) relative humidity, since increasing water vapor increases O 3 loss as the production rate of the reactions ) is the product of the photo-dissociation of O 3 in the presence of ultraviolet light), and (iii) horizontal and vertical wind fields.Horizontal wind speed and direction are used to assess monsoon strength and possible transport.Vertical velocity is used to investigate possible ascending motion of air masses from the boundary layer towards the free troposphere.All of the meteorological parameters are extracted from the ECMWF re-analysis (Dee et al., 2011).The data assimilation produces four analyses per day at 00:00, 06:00, 12:00 and 18:00 UTC at 37 pressure levels from 1000 to 1 hPa.Monthly means of total cloud cover, relative humidity at 850 hPa and u, v and w horizontal and vertical wind directions at 850 hPa are extracted over a grid size of 0.75 • × 0.75 • .During the EASM, the southwesterly monsoon flow brings warm, wet and clean air masses from the Indian Ocean to South, Southeast and East Asia.The winds at 850 hPa in Fig. 2 show a typical monsoon flow where in May and June, it mostly impacts South and Southeast Asia while the coastal region of East Asia is impacted by south-easterlies from the Pacific.In June, in particular, the monsoon becomes stronger as the wind force and the cloud cover (and therefore lower photochemical activity) increase over the regions < 20 • N. A decrease in tropospheric O 3 and total CO columns is recorded over India, and over all the countries around the Bay of Bengal and South East China.In Southern India, negative (ascending winds, over the Arabian Sea) and positive (descending winds, over land) vertical velocities are present from the surface up to 700 hPa (we show here an example at 850 hPa) suggesting exchanges of air masses vertically.Since CO's chemistry does not depend on cloud cover, the decrease in CO between May and June suggests that transport might be the main driver of decrease in pollutants.During July and August, the monsoon reaches its maximal strength.Due to high cloud cover and strong horizontal winds, the tropospheric O 3 columns show a large decrease.For latitudes <30 • N, the drop in O 3 is more notable, particularly over the Indo-Gangetic Plain where the decline in O 3 is driven by decreasing photochemistry since CO values over this region do not follow the same trend.
Looking at specific regions, tropospheric 0-6 km O 3 columns in Korea show a decrease in particular in July for Atmos.Chem.Phys., 16, 10489-10500, 2016 www.atmos-chem-phys.net/16/10489/2016/High CO total columns, used as pollution tracer, indicate the anthropogenic origin of the observed ozone enhancements.In fact, the persistence of O 3 during the monsoon season in Beijing was previously documented using aircraft data from the MOZAIC program which suggested a summertime O 3 maximum attributed to strong photochemical production (Ding et al., 2008).The other interesting region in China that shows little or no change is the area between the Chongqing and Sichuan provinces (and designated with a black circle in Fig. 2).This region does not exhibit any monsoon characteristics with low cloud cover and weak winds.This region is also between two mountains, making the persistence of O 3 and CO during summer favorable.The vertical velocity plots show that the monsoonal convection responsible of uplift of pollutants from the boundary layer to the free and upper troposphere, is more prominent for latitudes < 30 • N, except for Southern India, in accordance with previous studies (e.g.Randel et al., 2010;Fadnavis et al., 2013Fadnavis et al., , 2015)).(Nedelec et al., 2015), has provided in situ observations of ozone, water vapor, carbon monoxide and other trace gases made from multiple commercial aircraft since 1994 (Marenco et al., 1998;Thouret et al., 1998; http://www.iagos.org).In this study, we start by performing a validation analysis of the IASI data with 363 profiles from aircraft take-off and landing; we then check how well IASI captures the inter-annual variation of O 3 seen by the aircraft data and finally we look at the monthly averaged vertical profiles of tropospheric O 3 over our study domain.

Validation of IASI data
The IASI-retrieved tropospheric O 3 product is not a real concentration profile, but an estimation of the true profile considering the ability of the instrument to discriminate different atmospheric layers.Therefore one cannot directly compare satellite-retrieved profiles with high-resolution in situ observations such as the aircraft data.Instead, each high-resolution O 3 profile measured by the aircraft needs to be convolved by the low-resolution IASI averaging kernel matrix with the a priori profile, following Rodgers (2000) formulation: where x smoothed is the smoothed profile that uses lowresolution measurement characteristics.A IASI is the lowresolution averaging kernel matrix.x aircraft is the highresolution profile given by the aircraft, and x a,IASI is the low-resolution IASI a priori profile constructed from the McPeters/Labow/Logan climatology of O 3 vertical distribution, which combines long-term satellite limb measurements and measurements from ozonesondes (see McPeters et al., 2007;Hurtmans et al., 2012).Before the smoothing, the validation profile has to cover the whole retrieval altitude range, which is from the ground up to 41 km.The aircraft data profiles were therefore completed by the same a priori used for IASI so that the matrix calculation of Eq. ( 1) is valid.
We have also used an IASI spatial coincidence criterion of ±200 km around the position of the aircraft ascent or descent.In this section, both IASI day and nighttime observations are used for the validation of the aircraft data, and the temporal coincidence is ±10 h. Figure 4 shows the correlation of 0-6 km O 3 column retrieved from spatio-temporal coincidence of 363 IASI and MOZAIC/IAGOS smoothed profiles during May-August of 2008-2013 (except for 2010 where no aircraft profiles were available), and over the airports located in the study domain (see Fig. 3 for the location).Over the 5 years, a good agreement between the two data sets is found with correlation of 0.73 and absolute relative bias of 12 ± 9 %.Analysis of each year data leads to correlations ranging between 0.7 and 0.8 and bias ranging between 11 and 19 %.Our results suggest a good ability of IASI to reproduce O 3 variability in the troposphere over this region.Discrepancies arise from the spa-  tial resolution of the IASI footprint resulting in an observation averaged over tens of kilometers around the airport and therefore may include other surface O 3 contributions.Moreover, the aircraft observation takes place at different times during the day whereas IASI observation is at around 09:30 and 21:30 local time.With its limited sampling time during the day and its lower sensitivity towards the surface, IASI observation is not able to capture the diurnal variation of O 3 like an aircraft profile.Our results show a declined correlation between IASI and aircraft products as compared to Barret et al. (2011) where they reported a correlation coefficient of 0.87 (12 ± 6 %).This could be due to the different retrieval algorithm used: SOFRID (Barret et al., 2011) vs. FORLI (Hurtmans et al., 2012) ferences between the two algorithms can be found in Dufour et al. (2012).Another source of difference may arise from the different season and time period studied (Barret et al., 2011 uses a 6-month profiles over the period July-December 2008).Over the monsoon period, and in particular in May and June, the diurnal variability of tropospheric ozone is much more pronounced and highly dependent on the local meteorology.Therefore, discrepancies between IASI and the aircraft profile will carry larger discrepancies given the ±10 h coincidence criteria we used.Moreover, our study takes only the column from 0 to 6 km O 3 column from IASI whereas the lower tropospheric column used by Barret et al., 2011 is based on the column from the surface up to 250 hPa (∼ 10 km) and IASI is known to have a better sensitivity in the upper middle troposphere (Boynard et al., 2009;Safieddine et al., 2013).
Figure 5 shows the time series of the columns plotted in Fig. 4

Monsoon effect on the vertical profiles
We show in Fig. 6 the monthly averaged (with the horizontal bars as standard deviation) raw MOZAIC profiles during the EASM of 2011 taken as an example year.
At 10-20 • N, and from May to June, a clear decrease of around 20 ppbv in O 3 volume mixing ratios (VMR) is recorded at 1 and 2 km.At higher altitudes up to around 7 km, an important decrease is also detected from May to June and then from June to both July and August.With a small standard deviation around the O 3 profiles, the observations at 10-20 • N, suggest consistent averaged monthly behavior and show a decrease in the O 3 profile at different altitudes from 0 to 7 km, which was also seen over the whole 0-6 km O 3 column from IASI in Figs. 1 and 2. At 20-30 • N, the O 3 VMR in the lower troposphere (1 to 5 km, no monsoon signature at the surface) decreases from May to June of 10-20 ppbv but then increases back in July and/or August.At 5-8 km, the different months averaged O 3 VMR becomes comparable.Looking at this latitude band in Fig. 2, we can see how the decrease in the lower tropospheric O 3 in June can be explained by the increase in the cloud cover and in particular an increase in the wind speed at 850 hPa coming from the west Pacific.The profiles located at 30-40 • N, except at the surface, show a decrease in the O 3 VMR is detected from June to July and August (till 6 km).All three averaged profiles show no monsoon signature at the surface, probably due to enhanced O 3 precursor emissions near the ascent and descent of the aircraft.

Ground-based measurements
The MOZAIC profiles illustrated in Fig. 6 have shown that the effect of the EASM on surface and/or boundary layer O 3 largely depends on the location.In this section, we investigate ground station data during the monsoon of 2011 over five different locations (see Fig. 3, "+" sign).We distinguish between the Indian and Chinese stations by the sampling  Hyderabad shows continuous decreasing O 3 values from May to August, of total magnitude of 22 ppbv in accordance with Fig. 6 at the same latitude band.Jabalpur is located in a region where the monsoon effect is strong as seen in Figs. 1  and 2, and the O 3 at the surface behaves similarly, with a decrease of 20 ppbv.Udaipur lies in a region where the monsoon is milder, leading to a small decrease in the summertime O 3 values that decrease by 5 ppbv between May and July and then increase back in August to 32 ppbv.Figure 7b station data are the 24 h running average (and the associated standard deviation in shaded blue) of 12 stations in the PRD region and 7 stations in the NCP region.Since we are interested in the regional EASM effect on O 3 , we show the average of the stations here (the stations are between 25 to 300 km away).For details and more timely resolved observations for each of the stations, please check the Supplement.The ground observations of the PRD stations, represented also by the 20-30 • N MOZAIC profiles in Fig. 6, detect a decrease of > 35 ppbv from June to July coinciding with when the northwesterly winds from the Pacific become stronger (see Fig. 2).The O 3 VMR increase slightly afterwards dur- ing July and August due to the decrease in monsoon strength over this region (also seen in Fig. 2). Figure 7b for the NCP stations shows a weak monthly decrease in O 3 concentrations from June to July and August of 5-10 ppbv.The IASI and meteorological data presented in Fig. 2 also suggest the same decreasing pattern in O 3 concentrations driven by the slight increase in cloud cover.

Conclusions
The East Asian Summer Monsoon plays an important role in changing the pollutants concentration as well as the weather and climate system over the monsoon regions leading to effects on the global air quality and climate system (Rodwell and Hoskins, 2001).The study of the dynamics and variability of the East Asian monsoon provides useful information to analyze the distribution and losses of pollutants such as tropospheric O 3 .The latter is shown to have a particular seasonal variation over south and East Asia due to the monsoon.This study shows that the monsoon variability is recorded and well captured over the different years by the infrared remote sensor IASI during 2008-2013.The IASI 0-6 km O 3 columns decrease starting in the months of May and June of each year, and reach a minimum in July-August.This decrease is most prominent in south Asia where the monsoon is stronger.In order to assess the monsoon meteorological signature on tropospheric O 3 , we compare tropospheric O 3 to cloud cover, relative humidity and wind fields from the ERA-interim archive during the monsoon of 2011 taken as an example year.We add CO total columns from IASI to check if the regions showing persistence of ozone are stimulated by the presence of anthropogenic precursors.Over most of the domain, clear inverse relationship is seen between the Atmos.Chem.Phys., 16,[10489][10490][10491][10492][10493][10494][10495][10496][10497][10498][10499][10500]2016 www.atmos-chem-phys.net/16/10489/2016/IASI 0-6 km tropospheric O 3 on one hand and cloud cover and winds on the other hand.This is explained by the fact that the high cloud cover that the monsoon generates, accompanied with high relative humidity in the troposphere lead to a lower production rate since the photochemical activity will be much lower and relative humidity is a sink of O 3 in particular in the background troposphere.On the other hand, the winds are also strong during the monsoon and O 3 during this period can be transported either vertically, or horizontally to or from the Pacific and to the globe.Validation of the IASI-O 3 columns with aircraft data shows a good correlation (r = 0.73, 12 %) between the 0-6 km columns from IASI to that derived from 363 aircraft profiles.IASI is shown to reproduce very well the year-to-year variability in O 3 seen by the aircraft data.A monsoon signature is detected on the O 3 profiles over the different latitude bands of our study domain that is in agreement with the IASI 0-6 km O 3 column spatial distribution.Ground station measurements of O 3 also show spatial dependence and are anti-correlated to the EASM strength.The stations in Central and North India as well as in Pearl River Delta show a signature of the monsoon on ground O 3 that can be explained by the change in meteorology over these regions.In northwest India, and the North China plain, winds are weaker, and the cloud cover is smaller and thus the ground O 3 shows a smaller decrease during the EASM.
More generally, this study shows that in case of no or scarce ground and aircraft measurements, satellite observations, such as the one provided in this work using IASI, provide valuable information that could help investigate the regional and global monsoon impact on the distribution of pollutants in the troposphere and in particular tropospheric O 3 .With IASI-C to be launched in 2018 onboard MetOp-C and a new series of three IASI-NG instruments on MetOp Second Generation Satellites around 2022 (Crevoisier et al., 2014), long-term IASI O 3 time series obtained at high spatial resolution will be available to characterize and detect the onset and the progression of the monsoon.
The Supplement related to this article is available online at doi:10.5194/acp-16-10489-2016-supplement.

Figure 1 .
Figure 1.Monthly averaged daytime tropospheric 0-6 km O 3 column from IASI over the EASM region and period (May-September) for the years 2008-2013.The decrease due to the monsoon is more prominent to the south of the domain.

Figure 2 .
Figure 2. Monthly averaged tropospheric 0-6 km O 3 column from IASI, CO total columns from IASI, along with ECMWF total cloud cover, relative humidity, horizontal and vertical winds at 850 hPa for each of the months of May to August 2011.The black square and circle (upper right plot) are the regions the least affected by the EASM and discussed more in the text.

Figure 3 .
Figure 3.In red and scaled to number of observations: location of the MOZAIC/IAGOS flight data at the different airports in our study domain and period.The "+" sign locations corresponds to the ground stations used in Sect. 4 for the year 2011.

Figure 4 .
Figure 4.The 0-6 km O 3 columns retrieved from IASI correlation with 363 coincident MOZAIC/IAGOS profiles convolved with IASI averaging kernels for the period May-August of 2008-2013.No aircraft data are available for 2010.Grey dots correspond to raw aircraft O 3 columns before smoothing.

Figure 5 .
Figure 5.Time series of 0-6 km available O 3 columns from co-located IASI (black) and aircraft (smoothed in red and raw in grey) data at the airports (Fig. 3) averaged between 10-20, 20-30 and 30-40 • N from May till August of 2008 to 2013.No aircraft data were recorded in 2010.
averaged over three latitude bands (the vertical bars are the standard deviation around the mean).The IASI-O 3 product captures very well the inter-annual variability of the EASM as observed by the aircraft data over the different latitude bands.The decrease in the O 3 columns is most important at 10-20 • N, and over the different years, a result that can also be seen in Fig. 1.Fewer data are available at 20-30 • N (no co-located observations are recorded for 2008, 2009 and 2013).For 2011 for example, a decrease -though smaller in magnitude than the one in the 10-20 • N -is recorded both by IASI and the aircraft observations between May and June.Between July and August, a decrease is recorded in 2011, and an increase is recorded in 2012, and both events are well captured by IASI.At 30-40 • N, the effect of the EASM on the tropospheric O 3 column, as Figs. 1 and 2 showed, is weak or not clear.IASI captures very well the variability during both 2011 (the consistent slight decrease between May to August 2011), and 2013 (various increasing and/or decreasing behavior).

Figure 6 .
Figure 6.Monthly averaged vertical profiles of tropospheric O 3 from MOZAIC during the period May-August 2011 at 10-20, 20-30 and 30-40 • N. Error bars correspond to the standard deviation.

Figure 7 .
Figure 7. Ground station data during the EASM of 2011 over India: Hyderabad, Jabalpur and Udaipur in panel (a); and over China, Pearl River Delta (PRD), and North China Plain (NCP) in panel (b).The shaded region in panel (b) corresponds to the standard deviation of the 24-hour running average of the different stations in the PRD and NCP sites respectively.The location of the stations is plotted in Fig. 3, and more information is provided in the Supplement.
(Li et al., 2009)09)drocarbons, which are O 3 precursors, from EDGAR-HTAP project (http://edgar.jrc.ec.europa.eu/htap_v2/index.php)available for only 2008 and 2010.An increase in both NO x and non-methane hydrocarbons is observed between 2008 and 2010 over the studied domain and months, except for Japan (see Fig.S1in the Supplement).Note that strict controls on vehicles and industries emissions were implemented for the summer Olympic and Paralympic Games in Beijing between July and September 2008 (United Nations Environment Programme, 2009).Traffic was reduced by 22 % during the Olympics(Wang and Xie, 2009)and restrictions were applied on polluting industries in Beijing and surrounding provinces(Li et al., 2009).While these standards did not include surface O 3 limitations, O 3 values were expected to be indirectly affected.When comparing 2008 to the rest of the years, O 3 distributions do not show a significant decrease in Beijing and/or the surrounding regions (in fact August 2010 and 2012 have lower O 3 values over Beijing than those recorded in 2008), a result also detected in previous studies looking at O 3 surface measurements www.atmos-chem-phys.net/16/10489/2016/Atmos.Chem.Phys., 16, 10489-10500, 2016