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Surface distributions of O3, CO and hydrocarbons over the Bay of Bengal and the Arabian Sea during pre-monsoon season
Atmospheric Environment 47 (2012) 459e467
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Surface distributions of O3, CO and hydrocarbons over the Bay of Bengal and the Arabian Sea during pre-monsoon season S. Srivastava*, S. Lal, S. Venkataramani, S. Gupta, V. Sheel Physical Research Laboratory, Navrangpura, Ahmedabad-380009, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2011 Received in revised form 11 October 2011 Accepted 12 October 2011
Mixing ratios of ozone (O3), carbon monoxide (CO), methane (CH4) and few light non methane hydrocarbons (NMHCs) were measured on board the ocean research vessel Sagar Kanya over the Bay of Bengal and the Arabian Sea during the spring of 2006 as a part of an Integrated Campaign for Aerosol, gases and Radiation Budget (ICARB). North-westerly winds prevailing during this period transport large amount of anthropogenic pollutants from the Indo-Gangetic Plain (IGP) to the northern part of Bay of Bengal. The south-westerly and north-westerly winds carried cleaner marine air having lower abundance of pollutants over the southern Bay of Bengal and Arabian Sea. Ozone, CH4, CO, ethane and n-butane are found to be well correlated with each other over the northern Bay of Bengal indicating their common colocated sources. The latitudinal gradients of these species are found to be significant (O3 w 5.4 ppbv deg 1 , CH4 w 5.3 ppbv deg1, CO w 10 ppbv deg1, ethane w 93.2 pptv deg1 and n-butane w 59.7 pptv deg 1 ) over this region. Surprisingly, and in contrast to over the Bay of Bengal, the mixing ratios of these trace gases over the Arabian Sea are found comparatively higher over the southern region than over the northern region leading to negative latitudinal gradients. The short lived species with oceanic sources like ethene and propene show large variability and higher mixing ratios over southern parts of both the marine regions. These observations are compared with previous measurements made over these marine regions and the results obtained from the 3D MOZART chemistry transport model. The present study shows that the two marine regions adjacent to the Indian subcontinent are completely different from the perspective of surface level distributions of these species. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Ozone NMHCs Transport Bay of Bengal Arabian Sea
1. Introduction The developing regions of southern Asia are known to contribute large anthropogenic emissions of pollutants due to increasing urban and industrial growth. The emissions over this region to a large extent are characterized by inefficient combustion processes. Much of the pollutants are produced by biofuel use (cook stoves, wood, cow dungs etc.), burning of agricultural waste and widespread use of two-stroke engines resulting in greater content of carbon monoxide (CO) and non methane hydrocarbons (NMHCs) especially over the northern Indian region (Environmental Protection Agency, 1993; Venkataraman et al., 2006). The assessments of these emissions have been documented in many research publications (Streets et al., 2003a; b; Ohara et al., 2007).
* Corresponding author. Tel.: þ91 79 26314656; fax: þ91 79 26314659. E-mail address: [email protected] (S. Srivastava). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.10.023
The north-easterly and north-westerly winds transport pollutants from the Indian subcontinent to the adjoining marine regions. This process often produces urban equivalent air quality over tropical oceanic areas of the Bay of Bengal and the Arabian Sea (Lal and Lawrence, 2001; Lal et al., 2007). Strong photochemical production of ozone (O3) may occur in polluted air due to availability of intense solar radiation over this region. Photo-dissociation of O3 contributes to accelerated growth of oxidizing OH radicals particularly within the highly humid marine boundary layer. These reactive species significantly affect the chemistry of tropical marine environment through linked chemical reactions and highlights the importance of continental outflow over cleaner oceanic regions. Insitu measurements of pollutant trace gases over the Bay of Bengal and Arabian Sea are of great interest for the better understanding of the impacts of the increasing emissions of pollutants in the Indian land region on the air quality over the surrounding marine regions. The regional meteorology over south Asia plays a key role in determining the outflow characteristics of continental pollution. The meteorological conditions over the study region can be subdivided into three periods: summer or south west monsoon (June
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to August), winter or north east monsoon (December to February) and the monsoon transition periods (March to May and September to November) (Lawrence and Lelieveld, 2010). The south westerly winds bring cleaner marine air over the Indian subcontinent during the summer season. The continental outflow is particularly important during winter monsoon. During winter, north easterly winds blow from south-east Asia and India to the marine regions of the Bay of Bengal and Arabian Sea. During pre-monsoon (March to May) and post monsoon (September to November) transition periods, wind reversal takes place from north easterly to south westerly and south westerly to north easterly respectively. Realizing the importance of continental outflow during winter, INDian Ocean EXperiments (INDOEX - 1998, 1999) cruise campaigns have been conducted over the Arabian Sea and the Indian Ocean during this season (Lelieveld et al., 2001; de Gouw et al., 2001; de Laat et al., 2001; Scheeren et al., 2002; Burkert et al., 2003; Lal et al., 1998; Chand et al., 2003). During INDOEX99, the trace gas observations and meteorological analysis showed that the south Asian pollutants reached the Indian Ocean through two major wind channels: first one from the Indo-Gangetic Plain (IGP) via the Bay of Bengal and second one from south-west Asia via the Arabian Sea (Lelieveld et al., 2001). The pollution levels are found to be higher in the plume over the Bay of Bengal compared to that over the Arabian Sea. This study highlighted the importance of trace gas measurements over these two marine regions particularly over the Bay of Bengal. Later, trace gas measurements were also made over the Bay of Bengal, Arabian Sea and northern Indian Ocean during Bay Of Bengal EXperiments (BOBEX I) in winter and early spring, 2001; and over the Bay of Bengal during BOBEX II in winter 2003 (Lal et al., 2006; 2007). In order to investigate the pollutant levels during monsoon transition periods, Bay Of Bengal Process Studies (BOBPS) cruise campaign was conducted over the Bay of Bengal during post monsoon (SeptembereOctober 2002) (Sahu et al., 2006). However, such studies remained unexplored during pre-monsoon transition period until 2006. The oceanic segment of an extensive, multi-institutional, multiinstrument and multi-platform field campaign, namely, the “Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB)” was conducted over adjoining marine regions of the Indian subcontinent during MarcheMay, 2006 as a part of the Indian Space Research Organization’s Geosphere Biosphere Program (ISRO-GBP) (Moorthy et al., 2008). One of the major objectives of this campaign was to study the spatio-temporal heterogeneity and effects of continental transport on the distributions of trace gases and aerosols over the Bay of Bengal and the Arabian Sea during premonsoon season. In order to achieve these objectives, ship based measurements of precursors of O3 viz. CO, methane (CH4) and few light NMHCs were made during this campaign. In addition to these, the vertical distributions of O3 were also made (Srivastava et al., 2011). In this article, we discuss about the surface distributions of these gases over the Bay of Bengal and the Arabian Sea. This is the first study discussing about the distributions of these species over both the marine regions in the pre-monsoon season.
from 5.5 N to 20.6 N and 76 E to 93 E during its first phase over the Bay of Bengal. At the end of this phase, the ship docked at Kochi (10.0 N, 76.2 E) during 13e17 April. In the second phase of the campaign, the ship sailed off from Kochi on 18 April and covered the latitudinal span of 9 N to 22 N and longitudinal span of 58 E to 76 E over the Arabian Sea before touching Goa (15.4 N, 73.8 E) on 11 May. Twenty eight ozonesondes (EN SCI Corporation, USA) were launched almost every alternate day with the help of a 1.2 kg rubber balloon during the campaign. Each ozonesonde consists of ozone sensing electrochemical concentration cell which includes a set of teflon cathode and anode chambers with platinum electrodes immersed in KI solutions of different concentrations (Komhyr, 1969). This sensor yields a precision better than w 3e5 % and an accuracy of about w 5e10 % (Smit et al., 2007). The surface level measurements of ozonesonde (10 min average) provided the O3 values used in the present study. The surface level ozone precursors were measured from the grab air samples collected in pre-evacuated glass bottles (300e800 ml volume) everyday at 0800 h and 2000 h using an oil free compressor (Metal Bellow, USA) during the campaign. These air samples were later analyzed by gas chromatographic (GC) technique for CO, CH4 and NMHCs. A GC (Hewlett Packard 5890 series II, USA) equipped with Flame Ionization detector (FID) and PLOT capillary column (50 m, 0.32 mm, stationary phase of KCl/ Al2O3) was used to measure the NMHCs in the air samples. The air samples were pre-concentrated in the sampling loop using standard cryo-trapping procedure and injected on-line with helium as carrier gas. The temperature of the column was increased from 0 C to 200 C with specific steps to separate different light NMHCs. This system was calibrated with the standard reference gases. The reproducibility of NMHCs measurements are found between 3% and 10% (Sahu et al., 2006). Another GC (Varian Vista 6000, USA) with FID was used for the measurement of CO and CH4. The column was a 5 m long packed SS tube with a stationary phase of molecular sieves 13x. It was kept in
2. Cruise track and experimental details The measurements were made onboard the Ocean Research Vessel (ORV) Sagar Kanya over the Bay of Bengal (SK-223A) and over the Arabian Sea (SK-223B). Fig. 1 shows the cruise track of ICARB 2006 with the grab sampling (triangles) and balloon launching (circles) locations, marked with the dates of observations. This experiment was planned to cover whole of the Bay of Bengal and Arabian Sea. The ship sailed off from Chennai (13.1 N, 78.3 E) on 18 March 2006 and covered an extensive oceanic region
Fig. 1. Cruise track during ICARB 2006. Overlaid circles on cruise track depict the locations of balloon flights with dates and triangles show the grab sampling locations. ‘Mr’ denotes March, ‘Ap’ denotes April and ‘My’ denotes May.
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isothermal condition at 70 C during sample processing. CO was converted to CH4 by a methanizer containing nickel catalyst at 325 C in the presence of hydrogen for the detection. The calibrations of CO and CH4 were performed with standard calibration gases. The reproducibility of CO and CH4 measurements are found to be better than 3% and 1% respectively (Sahu et al., 2006). 3. Description of MOZART model The Model for OZone And Related Tracers (MOZART, version 2) is a global chemical transport model which simulates the chemistry and transport processes in the troposphere. This model is based on the detailed atmospheric chemistry of 63 chemical species (Ox, NOx, HOx, ethane, propane, ethene, propene, n-butane, isoprene, apinene etc.) with 133 gas phase, 2 heterogeneous and 33 photolytic reactions. The model run is made with horizontal resolution of 2.8 2.8 (128 longitude and 64 latitude grid points). The global POET (Precursors of Ozone and their Effect on the Troposphere, available for the period of 1990e2000) 1 1 gridded emission inventory is used as input for anthropogenic, biogenic and biomass burning emissions in the model (Granier et al., 2005). A recently developed REAS (Regional Emission inventory in ASia) anthropogenic emission inventory is used over Asia (Ohara et al., 2007). This inventory includes detailed 0.5 0.5 gridded anthropogenic emissions of CO, CH4, NOx, SO2, NMHCs, black carbon and organic carbon from fuel combustion and industrial sources over 24 countries in east, south-east and southern Asia for the period of 1980e2020. For the present work, MOZART was used with anthropogenic emissions from REAS for the study period and biogenic and biomass burning emissions from POET. Meteorological input data are taken from NCEP reanalysis at 28 sigma pressure levels from 995 hPa to 2.7 hPa (Sheel et al., 2010). MOZART incorporates the dynamical processes like advection, convection, boundary layer exchange and surface depositions. The complete description of MOZART, version 2 is given by Horowitz et al., (2003). The surface distributions of ozone and CO are computed from this MOZART Model and are compared with the observations. 4. Meteorology during ICARB The average wind patterns during March, April and May months of year 2006 at 925 mb are presented in Fig. 2. The measurements were made over the northern part of Bay of Bengal during March. During this period, the winds were mostly northwesterly over this region. Southwesterly wind prevailed over the southern Bay of
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Bengal during the observational period of April. A cyclonic circulation was present over the central Arabian Sea resulting in north easterly and north westerly wind flow over the southern Arabian Sea during this period. Strong south-westerly wind flow was observed over the north Arabian Sea during the investigation period of May. The individual day observations are also investigated with seven day back-trajectories obtained from METeorological data EXplorer (METEX) trajectory model (Zeng et al., 2008) at 0.5 km and discussed to interpret the observed variations of trace gases. 5. Results and discussions 5.1. Distributions of O3, CH4, CO and NMHCs The spatio-temporal variations of surface O3, CO, CH4 and NMHCs are shown along with the cruise track data (latitude and longitude) during the campaign in Fig. 3. Most of these trace gases showed very high mixing ratios over the northern Bay of Bengal. The mixing ratios of O3, CH4, CO, ethane (C2H6) and n-butane (n-C4H10) decreased from 40e54 ppbv to 17e25 ppbv, 1.91 ppmv to 1.75e1.79 ppmv, 234 ppbv to 88 ppbv, 1269 pptv to 300e400 pptv and 637 pptv to 50e100 pptv respectively from the northern to southern Bay of Bengal. In contrast, these species do not show large variations over the Arabian Sea. Their mixing ratios are found slightly higher over the southern Arabian Sea than over the northern Arabian Sea. The time series variations of alkenes namely ethene (C2H4) and propene (C3H6) showed very distinct variations from other trace species. These showed large variability during the entire cruise period. Their distributions showed higher mixing ratios over the southern parts of both the Bay of Bengal and Arabian Sea. This is analogous to the observations reported by Sahu et al. (2006) showing higher levels of alkenes over the oceanic region away from the continent over the Bay of Bengal during BOBPS 2002. The seven days back-trajectories at each sampling location at an altitude of 500 m above mean sea level were used to identify possible path of air parcel. These different sampling locations were subdivided into six different groups namely northern, central and southern, Bay of Bengal and Arabian Sea according to back-trajectories (Fig. 4). The trajectories passing over the highly polluted IGP region correspond to the measurements made over the northern Bay of Bengal. Consequently, the levels of O3, CH4, CO, ethane and n-butane were highest in these air masses. Over
Fig. 2. Synoptic wind fields at 925 mb over the Indian subcontinent during March (left panel), April (mid panel) and May (right panel) months of 2006.
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Fig. 3. Variations of surface ozone and its precursor gases observed over the Bay of Bengal (left panels) and Arabian Sea (right panels) during the ICARB 2006 ship based campaign. The bottom panel shows the latitude and longitude of the measurements shown in the top five panels.
the central Bay of Bengal, few trajectories originated from the Indian mainland and the remaining originated from the marine region. The mixing ratios of anthropogenic species are found to be comparatively lower in these air masses due to the mixing of polluted air with pristine marine air. The observations over the southern Bay of Bengal are influenced mainly by the air masses arriving from the Indian Ocean. This is the cleanest part of the Bay of Bengal having lower levels of pollutants. The left three panels of the figure show the back-trajectories arriving at the observational locations over northern, central and southern parts of the Arabian Sea respectively. The maximum number of trajectories is found to be originating and circulating over the Arabian Sea itself in all the three regions. The transport of continental pollution is mostly absent in this region. Thus, the mixing ratios of species are comparatively lower over the Arabian Sea.
The inter species correlations between trace gases can provide important information of their common sources. Strong intercorrelations between species show that they are emitted from common and co-located sources and had undergone dilution on their way. Strong inter-correlations between various trace species are discernable over the Bay of Bengal whereas these correlations are rather poor over the Arabian Sea. The correlations of CO with O3, ethane and n-butane are very strong with correlation coefficient (r2) values of 0.88, 0.77 and 0.78 respectively and moderate with CH4 with 0.42. Strong correlations among CO, CH4, ethane and n-butane indicate their common and co-located sources over the Indian region. A good correlation between O3 and CO indicates the photochemical production of O3 in the polluted air mass. However, over the Arabian Sea, lower levels of all these species and their poor inter-correlations indicate their background levels over cleaner marine regions of the Arabian Sea. Moderate to good
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Fig. 4. Seven days back-trajectories arriving over observational locations at 500 m amsl during the campaign over the northern, central and southern parts of the Arabian Sea (left panels) and Bay of Bengal (right panels).
correlations are observed between ethene and propene over both the Bay of Bengal and Arabian Sea with r2 values of 0.63 and 0.59 respectively. These are the only species, which showed a significant correlation over the Arabian Sea. This suggests the source being the local biological activities in the ocean. According to several studies, part of dissolved organic material released from phytoplankton/algae over local oceanic regions transform into alkenes (Plass et al., 1992; Ratte et al., 1993; 1998; Riemer et al., 2000). Sahu et al. (2010) have also showed significant transfer of alkenes from ocean surface to marine boundary layer over the Bay of Bengal during BOBEX II 2003.
5.2. Latitudinal variations of trace gases Fig. 5 shows the latitudinal dependence of O3 and its few precursors having anthropogenic sources over the northern part of the Bay of Bengal (latitude > 12 N). The wind regime over this region was mostly northwesterly during the study period. The back-trajectories show that the pollutants were mainly transported from the IGP. The north to south gradient, in such a case, provides information on the combined effects of chemical transformation and dispersion of these gases over this region. The latitudinal gradients of O3, CO, CH4, ethane and
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Fig. 5. Latitudinal variations of ozone, methane, ethane, carbon monoxide and n-butane over the Northern Bay of Bengal (latitude > 12 N) during ICARB 2006. The straight lines are best linear fits.
n-butane are found to be 5.4 0.9 ppbv deg1, 9.9 1.9 ppbv deg1, 5.3 2.7 ppbv deg1, 93.2 9.2 pptv deg1 and 59.7 7.9 pptv deg1 respectively. The decrease in the mixing ratios of species are expected, as their chemical loss combined with dilution in the absence of local sources lead to continuously decreasing mixing ratios while moving farther south over the ocean. The north-south gradients are due to injection of highly polluted air mass in the MBL from northern coast of the Bay of Bengal and the absence of any other source in between. Ethene and propene do not show north-south gradient over the northern Bay of Bengal. This also indicates that the oceanic emissions play dominant role in their distributions over this marine region. Negative, though small, latitudinal gradients of O3 (0.39 0.22 ppbv deg1), CO (1.68 0.45 ppbv deg1), CH4 (4.5 0.7 ppbv deg1), ethane (15.0 4.0 pptv deg1) and nbutane (2.7 1.0 pptv deg1) are found over the Arabian Sea (8e22 N). The negative gradients show the absence of polluted air advection from northern region. The negative trends in these gases also indicate slight influence of continental transport over the southern Arabian Sea. However, the air over the northern Arabian Sea is found to be transported mostly from the southern Arabian Sea.
5.3. Distribution of CO over the study region The distribution of CO at 900 mb is obtained from MOPITT (Measurements Of Pollution In The Troposphere) (Warner et al., 2001; Deeter et al., 2004) with a spatial resolution of 1 1. The monthly averaged gridded CO data (level 3, version 4) is used for tracking the changes from March to May, 2006. CO with its higher life time (w 2 months) is an ideal indicator of transport of anthropogenic emissions. The IGP and the eastern Indian region are highly polluted with CO levels more than 200 ppbv during March and April (Bay of Bengal segment) resulting in the outflow from IGP towards the northern Bay of Bengal during these months. The pre-monsoon season is known as the transition period and the height of atmospheric boundary layer increases and wind pattern changes after winter season resulting into dilution of pollutants. This feature is clearly discernable from Fig. 6 as March is found most polluted month, followed by April and May. Thus, the temporal variation is also contributing to strong north-south gradient over the Bay of Bengal. Over the Arabian Sea, measurements were made over the southern region (April 19e30, 2006) prior to that over the northern region (May 1e10, 2006). The MOPITT observations suggest that temporal dilution of pollutants resulted into negative gradient over the Arabian Sea.
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Fig. 6. Distribution of monthly mean CO at 900 mb obtained from MOPITT over the Indian subcontinent and adjacent marine locations during March, April and May months of year 2006.
5.4. Comparison with previous measurements
5.5. Comparison with MOZART model
The trace gas observations are compared with previous measurements made over the study regions during nearly similar period (FebruaryeMarch) namely INDOEX-99 and BOBEXs (Muhle et al., 2002; Chand et al., 2001; 2003; Lal et al., 2006; 2007). Their average values with 1s variations are given in Table 1. Measurements of surface levels of various trace species including O3, CO and CH4 were made over the Arabian Sea during INDOEX 1999. The mixing ratios of O3 and CO are found higher during INDOEX than ICARB. The winds were mostly northeasterly during INDOEX blowing the pollutants from the Indian subcontinent and southwest Asian countries over this region (de Gouw et al., 2001). However, during ICARB, air was circulating over the cleaner marine region of the Arabian Sea. The mixing ratios of most of the trace gases are found to be higher over the Bay of Bengal during BOBEX I and BOBEX II than ICARB. This difference may be due to prevailing wind regimes from different directions over the study region. During FebruaryeMarch, the northeasterly winds dominate over the Bay of Bengal carrying continental pollutants. The higher level of pollution also depends on the dynamics of continental boundary layer and the level of anthropogenic emission. Accumulation of pollutants takes place in the shallow boundary layer resulting in higher abundance of pollution. The atmospheric boundary layer remains shallow during FebruaryeMarch (BOBEXs) with respect to late March and April months (ICARB period over the Bay of Bengal) and prevailing winds transport them over the ocean.
Fig. 7 shows the surface distributions of O3 and CO using observational data and simulated by MOZART. The model results show good agreement with high levels of O3 over the northern Bay of Bengal till Day of Year (DOY) # 88 when the ship was located near 13 N, 83.2 E. Thereafter, the model overestimated the mixing ratios of O3 over the Bay of Bengal with average mixing ratios of 38.4 6.0 ppbv whereas the average observed mixing ratio is found about 18.8 4.2 ppbv. The overestimation of O3 increased over the Arabian Sea with average model mixing ratio of 49.5 6.5 ppbv and average observed mixing ratio of 19.8 4.1 ppbv. The mixing ratios of CO are found to be well matched in observational and modelling studies during this campaign. The model clearly reproduces the high levels of CO in the IGP outflow over the northern Bay of Bengal and also captures few small scale features like sharp peak on DOY # 80, then decrease till DOY # 82, further increase till DOY # 84 and decrease afterwards. However, the mixing ratios of CO are slightly underestimated over the Bay of Bengal with average difference of about 32 ppbv (22%) and over the Arabian Sea by about 16 ppbv (19%). The CO distribution in the marine boundary layer over the Bay of Bengal, Arabian Sea and Indian Ocean were also found to be underestimated by ECHAM 5 global circulation model during BOBEX I 2001 (Lal et al., 2006). Overall the MOZART results fairly reproduce the broad features present in the distribution of CO both over the Bay of Bengal and Arabian Sea. The linear equation fit shows the following relation in these two data sets.
Table 1 Comparison of mixing ratios of ozone and its various precursor gases over the Bay of Bengal (BoB) and Arabian Sea (AS) during various cruise campaigns. Species
INDOEX
February eMarch 1999 ASa O3 (ppbv) 43.9 8.0 CO (ppbv) 217 42.0 1.73 0.05 CH4 (ppmv) Ethane e (pptv) Ethene e (pptv) Propene e (pptv) n-Butane e (pptv) a b
Lal et al. (2006). Lal et al. (2007).
ICARB
BOBEX I
BOBEX II
ICARB
AprileMay 2006 AS 19.8 4.1 86.7 14.1 1.74 0.02
February eMarch 2001 BoBa 42.2 12.0 217 31 1.92 0.05
February 2003 BoBb 34 6.4 193 40 1.72 0.04
March eApril 2006 BoB 28.3 14.4 145 38 1.81 0.03
338 125 e
1181 362 589 280
165 106 e
198 60
196 154
132 88
e
106 50
112 92
49 23
e
98 123
187 157
½COMOZART ¼ 0:83*½COobs 5:74 The fairly good value of correlation coefficient (r2 ¼ 0.66) and low value of offset (5.74 ppbv) between the two data sets show that model strongly follows the CO observations. The production of O3 depends on complex chain reactions of CO, CH4 and various non-methane hydrocarbons in the presence of sunlight and NOx. Any discrepancy in the emission estimates of these species will affect the O3 mixing ratio obtained from MOZART. Higher mixing ratios of O3 may be attributed to uncertainty in the emission estimated in the used inventories. Sheel et al. (2010) showed that the MOZART overestimates the columnar NO2 compared to GOME (Global Ozone Monitoring Experiment) observations over the Indian region. Their study highlights the need to develop an emission inventory particularly over the Indian region with higher spatial resolution. In addition to that, reaction of O3 with biogenic halogens can be an important loss process within the marine boundary layer (Vogt et al., 1996; Dickerson et al., 1999; McFiggans et al., 2000). These reactions are not included in the chemical mechanisms of MOZART. Thus, the uncertainty in emission estimates together with loss of O3 because of halogen chemistry may contribute to overestimation of ozone mixing ratio by MOZART.
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Fig. 7. Comparison of observational and MOZART model estimated surface distributions of O3 and CO over the Bay of Bengal and Arabian Sea during ICARB 2006. The bottom panel shows the latitude and longitude of the measurements shown in the top two panels.
6. Summary and conclusion The ICARB cruise campaign was conducted over large areas of the Bay of Bengal and Arabian Sea during the pre-monsoon season of 2006. For the first time, the surface distribution of O3, CO, CH4 and light NMHCs were investigated near simultaneously over these two geographically segregated marine regions using ozonesonde and grab sampling. Higher mixing ratios of these trace gases are observed over the northern Bay of Bengal due to transport of polluted air from the IGP. The levels of these gases are found to be much lower over the southern Bay of Bengal and over the entire Arabian Sea due to the absence of transport of polluted air parcel over this region. The ozonesonde observations also showed a thick layer of comparatively higher O3 mixing ratios (60e90 ppbv) in the altitude range of 0.5e3 km over the northern Bay of Bengal (Srivastava et al., 2011). The inter-correlation studies showed strong correlations among CO, CH4, ethane and n-butane over the Bay of Bengal indicating common and co-located continental sources. A good correlation between CO and O3 indicate photochemical production of O3 in the polluted air mass. Lower levels of trace gases and their poor inter-correlations over the Arabian Sea point their background levels over cleaner marine region. Moderate to good correlation between ethene and propene over both the Bay of
Bengal and Arabian Sea shows their pre-dominant oceanic sources. O3, CO, ethane and n-butane show very strong positive latitudinal gradients over the northern Bay of Bengal (latitude > 12 N) and slightly negative gradient over the Arabian Sea. The positive latitudinal gradient over the northern Bay of Bengal is found to be due to injection of pollutants from the northern coastal areas of Bay of Bengal. The negative gradients over the Arabian Sea are attributed to transport of polluted air from southern India over the southern Arabian Sea and continuous dilution of pollution with time over this region. These results are confirmed by CO distribution over the Indian region obtained from MOPITT. Alkenes (ethene and propene) did not show any latitudinal gradient due to dominant oceanic emission. The distributions of O3 and CO are compared with the results of MOZART and broadly found to match over the northern Bay of Bengal. However, O3 is over estimated and CO is slightly underestimated over the southern Bay of Bengal and Arabian Sea. These results further demand for the improvement of emission inventories over the southern Asian region. The present observations are compared with the observations made during previous campaigns. Mixing ratios of most of the trace gases are found to be lower during present campaign than during previous campaigns over this region.
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The wind patterns significantly affect the degree of pollution over the oceanic region adjacent to continental areas. Over the southern Asia, wind changes from season to season. The ship based trace gas study during a particular season can provide only limited information of continental transport with large degree of uncertainty due to changing source strengths over the densely populated continental regions. Frequent cruise campaigns during different seasons will provide detailed information on the monthly/seasonal variation of continental outflow and their influence over these marine regions. Acknowledgement The campaign was supported by Indian Space Research Organization-Geosphere Biosphere Program. We thank team members of ICARB and crew of Sagar-Kanya (SK 223) for their help in successfully conducting these experiments on board the ship. We are thankful to Mr. T. K. Sunil Kumar for his help in the analysis of grab samples. We are grateful to METEX team members for providing air back trajectory model. We thank the anonymous reviewers for their comments and suggestions. References Burkert, J., Andrés-Hernández, M.D., Reichert, L., Meyer-Arnek, J., Doddridge, B., Dickerson, R.R., Mühle, J., Zahn, A., Carsey, T., Burrows, J.P., 2003. Trace gas and radical diurnal behavior in the marine boundary layer during INDOEX 1999. J. Geophys. Res. 108 (D8), 8000. doi:10.1029/2002JD002790. Chand, D., Modh, K.S., Naja, M., Venkataramani, S., Lal, S., 2001. Latitudinal trends in O3, CO, CH4 and SF6 over the Indian Ocean during the INDOEX IFP-1999 cruise. Curr. Sci. 80, 100e104. Chand, D., Lal, S., Naja, M., 2003. Variations of ozone in the marine boundary layer over the Arabian Sea and the Indian Ocean during the 1998 and 1999 INDOEX campaigns. J. Geophys. Res. 108 (D6), 4190. doi:10.1029/2001JD001589. de Gouw, J.A., et al., 2001. Overview of the trace gas measurements on board the citation aircraft during the intensive field phase of INDOEX. J. Geophys. Res. 106, 28453e28467. de Laat, A.T.J., Lelieveld, J., Roelofs, G.J., 2001. Source analysis of carbon monoxide pollution during INDOEX 1999. J. Geophys. Res. 106, 28481e28495. Deeter, M.N., et al., 2004. Evaluation of operational radiances for the measurements of pollution in the troposphere (MOPITT) instrument CO thermal-band channels. J. Geophys. Res. 109 (D3), D03308. doi:10.1029/2003JD003970. Dickerson, R.R., Rhoads, K.P., Carsey, T.P., Oltmans, S.J., Crutzen, P.J., 1999. Ozone in the remote boundary layer: a possible role for halogens. J. Geophys. Res. 104, 21385e21395. Environmental Protection Agency, 1993. Alternative Control Techniques Document: NOx Emissions from Stationary Reciprocating Internal Combustion Engines, EPAe453/R-93/032. Off. of Air Qual. Planning and Stand., Environ. Prot. Agency, Research Triangle Park, N. C. Granier, C., Guenther, A., Lamarque, J.F., Mieville, A., Muller, J.F., Olivier, J., Orlando, J., Peters, J., Petron, G., Tyndall, G., Wallens, S., 2005. POET, a Database of Surface Emissions of Ozone Precursors. http://www.aero.jussieu.fr/projet/ACCENT/ POET.php. Horowitz, L.W., et al., 2003. A global simulation of tropospheric ozone and related tracers: description and evaluation of MOZART, version 2. J. Geophys. Res. 108, 4784. doi:10.1029/2002JD002853. Komhyr, W.D., 1969. Electrochemical concentration cells for gas analysis. Ann. Geoph. 25, 203e210. Lal, S., Lawrence, M.G., 2001. Elevated mixing ratios of surface ozone over the Arabian Sea. Geophys. Res. Lett. 28, 1487e1490. Lal, S., Naja, M., Jayaraman, A., 1998. Ozone in the marine boundary layer over the tropical Indian Ocean. J. Geophys. Res. 103 (D15), 18907e18917. Lal, S., Chand, D., Sahu, L.K., Venkataramani, S., Brasseur, G., Schultz, M.G., 2006. High levels of ozone and related gases over the Bay of Bengal during winter and early spring of 2001. Atmos. Environ. 40, 1633e1644.
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Lal, S., Sahu, L.K., Venkataramani, S., 2007. Impact of transport from the surrounding continental regions on the distributions of ozone and related trace gases over the Bay of Bengal during February 2003. J. Geophys. Res. 112, D14302. doi:10.1029/2006JD008023. Lawrence, M.G., Lelieveld, J., 2010. Atmospheric pollutants outflow from southern Asia: a review. Atmos. Chem. Phys. 10, 11017e11096. Lelieveld, J., et al., 2001. The Indian Ocean experiment: widespread air pollution from south and Southeast Asia. Science 291, 1031e1036. McFiggans, G., Plane, J.M.C., Allan, B.J., Carpenter, L.J., Coe, H., O’Dowd, C., 2000. A modeling study of iodine chemistry in the marine boundary layer. J. Geophys. Res. 105, 14371e14385. Moorthy, K.K., Satheesh, S.K., Babu, S.S., Dutt, C.B.S., 2008. Integrated campaign for aerosols, gases and radiation budget (ICARB): an overview. J. Earth Syst. Sci. 117 (S1), 243e262. Muhle, J., Zahn, A., Brenninkmeijer, C.A.M., Gros, V., Crutzen, P.J., 2002. Air mass classification during the INDOEX Ronald Brown cruise using measurements of non-methane hydrocarbons, CH4, CO2, CO, 14CO and d18O(CO). J. Geophys. Res. 107 (D19), 8021. doi:10.1029/2001JD000730. Ohara, T., Akimoto, H., Kurokawa, J., Horii, N., Yamaji, K., Yan, X., Hayasaka, T., 2007. An Asian emission inventory of anthropogenic emission sources for the period 1980e2020. Atmos. Chem. Phys. 7, 4419e4444. doi:10.5194/acp-7-4419-2007. Plass, C., Koppmann, R., Rudolph, J., 1992. Light hydrocarbons in the surface water of the mid-Atlantic. J. Atmos. Chem. 15, 235e251. Ratte, M., Plass-Dülmer, C., Koppmann, R., Rudolph, J., Denga, J., 1993. Production mechanism of C2eC4 hydrocarbons in seawater: field measurements and experiments. Glob. Biogeochem. Cycles 7 (2), 369e378. doi:10.1029/ 93GB00054. Ratte, M., Bujok, O., Spitzy, A., Rudolph, J., 1998. Photochemical alkene formation in seawater from dissolved organic carbon: results from laboratory experiments. J. Geophys. Res. 103 (D5), 5707e5717. doi:10.1029/97JD03473. Riemer, D.D., et al., 2000. Photoproduction of nonmethane hydrocarbons (NMHCs) in seawater. Mar. Chem. 71, 177e198. Sahu, L.K., Lal, S., Venkataramani, S., 2006. Distributions of O3, CO and hydrocarbons over the Bay of Bengal: a study to assess the role of transport from southern India and marine regions during SeptembereOctober 2002. Atmos. Environ. 40, 4633e4645. Sahu, L.K., Lal, S., Venkataramani, S., 2010. Impact of monsoon circulations on oceanic emissions of light alkenes over Bay of Bengal. Glob. Biogeochem. Cycles. doi:10.1029/2009GB003766. Scheeren, H.A., Lelieveld, J., de Gouw, J.A., Van der Veen, C., Fischer, H., 2002. Methyl chloride and other chlorocarbons in polluted air during INDOEX. J. Geophys. Res. 107 (D19), 8015. doi:10.1029/2001JD001121. Sheel, V., Lal, S., Richter, A., Burrows, J.P., 2010. Comparison of satellite observed tropospheric NO2 over India with model simulations. Atmos. Environ. 44, 3314e3321. Smit, H.G.J., et al., 2007. Assessment of the performance of ECC-ozonesondes under quasi-flight conditions in the environmental simulation chamber: insights from the Juelich ozone sonde intercomparison experiment (JOSIE). J. Geophys. Res. 112, D19306. doi:10.1029/2006JD007308. Srivastava, S., Lal, S., Venkataramani, S., Gupta, S., Acharya, Y.B., 2011. Vertical distribution of ozone in the lower troposphere over the Bay of Bengal and the Arabian Sea during ICARB-2006: effects of continental outflow. J. Geophys. Res. doi:10.1029/2010JD015298. Streets, D.G., Yarber, K.F., Woo, J.H., Carmichael, G.R., 2003a. Biomass burning in Asia: annual and seasonal estimates and atmospheric emissions. Glob. Biogeochem. Cycles 17 (4), 1099. doi:10.1029/2003GB002040. Streets, D.G., et al., 2003b. An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. J. Geophys. Res. 108 (D21), 8809. doi:10.1029/ 2002JD003093. Venkataraman, C., Habib, G., Kadamba, D., Shrivastava, M., Leon, J.F., Crouzille, B., Boucher, O., Streets, D.G., 2006. Emissions from open biomass burning in India: integrating the inventory approach with high resolution moderate resolution imaging spectroradiometer (MODIS) active fire and land cover data. Glob. Biogeochem. Cycles 20, GB2013. doi:10.1029/2005GB002547. Vogt, R., Crutzen, P.J., Sander, R., 1996. A mechanism for halogen release from seasalt aerosol in the remote marine boundary layer. Nature 383, 327e329. Warner, J.X., Gille, J.C., Edwards, D.P., Ziskin, D.C., Smith, M.W., Bailey, P.L., Rokke, L., 2001. Cloud detection and clearing for the Earth observing system Terra satellite measurements of pollution in the troposphere (MOPITT) experiment. Appl. Opt. 40, 1269e1284. Zeng, J., Matsunaga, T., Mukai, H., 2008. METEX e A Flexible Tool for Air Trajectory Calculation, Environmental Modelling & Software. doi:10.1016/ j.envsoft.2008.10.015.
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Surface distributions of O3, CO and hydrocarbons over the Bay of Bengal and the Arabian Sea during pre-monsoon season S. Srivastava*, S. Lal, S. Venkataramani, S. Gupta, V. Sheel Physical Research Laboratory, Navrangpura, Ahmedabad-380009, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 January 2011 Received in revised form 11 October 2011 Accepted 12 October 2011
Mixing ratios of ozone (O3), carbon monoxide (CO), methane (CH4) and few light non methane hydrocarbons (NMHCs) were measured on board the ocean research vessel Sagar Kanya over the Bay of Bengal and the Arabian Sea during the spring of 2006 as a part of an Integrated Campaign for Aerosol, gases and Radiation Budget (ICARB). North-westerly winds prevailing during this period transport large amount of anthropogenic pollutants from the Indo-Gangetic Plain (IGP) to the northern part of Bay of Bengal. The south-westerly and north-westerly winds carried cleaner marine air having lower abundance of pollutants over the southern Bay of Bengal and Arabian Sea. Ozone, CH4, CO, ethane and n-butane are found to be well correlated with each other over the northern Bay of Bengal indicating their common colocated sources. The latitudinal gradients of these species are found to be significant (O3 w 5.4 ppbv deg 1 , CH4 w 5.3 ppbv deg1, CO w 10 ppbv deg1, ethane w 93.2 pptv deg1 and n-butane w 59.7 pptv deg 1 ) over this region. Surprisingly, and in contrast to over the Bay of Bengal, the mixing ratios of these trace gases over the Arabian Sea are found comparatively higher over the southern region than over the northern region leading to negative latitudinal gradients. The short lived species with oceanic sources like ethene and propene show large variability and higher mixing ratios over southern parts of both the marine regions. These observations are compared with previous measurements made over these marine regions and the results obtained from the 3D MOZART chemistry transport model. The present study shows that the two marine regions adjacent to the Indian subcontinent are completely different from the perspective of surface level distributions of these species. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Ozone NMHCs Transport Bay of Bengal Arabian Sea
1. Introduction The developing regions of southern Asia are known to contribute large anthropogenic emissions of pollutants due to increasing urban and industrial growth. The emissions over this region to a large extent are characterized by inefficient combustion processes. Much of the pollutants are produced by biofuel use (cook stoves, wood, cow dungs etc.), burning of agricultural waste and widespread use of two-stroke engines resulting in greater content of carbon monoxide (CO) and non methane hydrocarbons (NMHCs) especially over the northern Indian region (Environmental Protection Agency, 1993; Venkataraman et al., 2006). The assessments of these emissions have been documented in many research publications (Streets et al., 2003a; b; Ohara et al., 2007).
* Corresponding author. Tel.: þ91 79 26314656; fax: þ91 79 26314659. E-mail address: [email protected] (S. Srivastava). 1352-2310/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2011.10.023
The north-easterly and north-westerly winds transport pollutants from the Indian subcontinent to the adjoining marine regions. This process often produces urban equivalent air quality over tropical oceanic areas of the Bay of Bengal and the Arabian Sea (Lal and Lawrence, 2001; Lal et al., 2007). Strong photochemical production of ozone (O3) may occur in polluted air due to availability of intense solar radiation over this region. Photo-dissociation of O3 contributes to accelerated growth of oxidizing OH radicals particularly within the highly humid marine boundary layer. These reactive species significantly affect the chemistry of tropical marine environment through linked chemical reactions and highlights the importance of continental outflow over cleaner oceanic regions. Insitu measurements of pollutant trace gases over the Bay of Bengal and Arabian Sea are of great interest for the better understanding of the impacts of the increasing emissions of pollutants in the Indian land region on the air quality over the surrounding marine regions. The regional meteorology over south Asia plays a key role in determining the outflow characteristics of continental pollution. The meteorological conditions over the study region can be subdivided into three periods: summer or south west monsoon (June
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to August), winter or north east monsoon (December to February) and the monsoon transition periods (March to May and September to November) (Lawrence and Lelieveld, 2010). The south westerly winds bring cleaner marine air over the Indian subcontinent during the summer season. The continental outflow is particularly important during winter monsoon. During winter, north easterly winds blow from south-east Asia and India to the marine regions of the Bay of Bengal and Arabian Sea. During pre-monsoon (March to May) and post monsoon (September to November) transition periods, wind reversal takes place from north easterly to south westerly and south westerly to north easterly respectively. Realizing the importance of continental outflow during winter, INDian Ocean EXperiments (INDOEX - 1998, 1999) cruise campaigns have been conducted over the Arabian Sea and the Indian Ocean during this season (Lelieveld et al., 2001; de Gouw et al., 2001; de Laat et al., 2001; Scheeren et al., 2002; Burkert et al., 2003; Lal et al., 1998; Chand et al., 2003). During INDOEX99, the trace gas observations and meteorological analysis showed that the south Asian pollutants reached the Indian Ocean through two major wind channels: first one from the Indo-Gangetic Plain (IGP) via the Bay of Bengal and second one from south-west Asia via the Arabian Sea (Lelieveld et al., 2001). The pollution levels are found to be higher in the plume over the Bay of Bengal compared to that over the Arabian Sea. This study highlighted the importance of trace gas measurements over these two marine regions particularly over the Bay of Bengal. Later, trace gas measurements were also made over the Bay of Bengal, Arabian Sea and northern Indian Ocean during Bay Of Bengal EXperiments (BOBEX I) in winter and early spring, 2001; and over the Bay of Bengal during BOBEX II in winter 2003 (Lal et al., 2006; 2007). In order to investigate the pollutant levels during monsoon transition periods, Bay Of Bengal Process Studies (BOBPS) cruise campaign was conducted over the Bay of Bengal during post monsoon (SeptembereOctober 2002) (Sahu et al., 2006). However, such studies remained unexplored during pre-monsoon transition period until 2006. The oceanic segment of an extensive, multi-institutional, multiinstrument and multi-platform field campaign, namely, the “Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB)” was conducted over adjoining marine regions of the Indian subcontinent during MarcheMay, 2006 as a part of the Indian Space Research Organization’s Geosphere Biosphere Program (ISRO-GBP) (Moorthy et al., 2008). One of the major objectives of this campaign was to study the spatio-temporal heterogeneity and effects of continental transport on the distributions of trace gases and aerosols over the Bay of Bengal and the Arabian Sea during premonsoon season. In order to achieve these objectives, ship based measurements of precursors of O3 viz. CO, methane (CH4) and few light NMHCs were made during this campaign. In addition to these, the vertical distributions of O3 were also made (Srivastava et al., 2011). In this article, we discuss about the surface distributions of these gases over the Bay of Bengal and the Arabian Sea. This is the first study discussing about the distributions of these species over both the marine regions in the pre-monsoon season.
from 5.5 N to 20.6 N and 76 E to 93 E during its first phase over the Bay of Bengal. At the end of this phase, the ship docked at Kochi (10.0 N, 76.2 E) during 13e17 April. In the second phase of the campaign, the ship sailed off from Kochi on 18 April and covered the latitudinal span of 9 N to 22 N and longitudinal span of 58 E to 76 E over the Arabian Sea before touching Goa (15.4 N, 73.8 E) on 11 May. Twenty eight ozonesondes (EN SCI Corporation, USA) were launched almost every alternate day with the help of a 1.2 kg rubber balloon during the campaign. Each ozonesonde consists of ozone sensing electrochemical concentration cell which includes a set of teflon cathode and anode chambers with platinum electrodes immersed in KI solutions of different concentrations (Komhyr, 1969). This sensor yields a precision better than w 3e5 % and an accuracy of about w 5e10 % (Smit et al., 2007). The surface level measurements of ozonesonde (10 min average) provided the O3 values used in the present study. The surface level ozone precursors were measured from the grab air samples collected in pre-evacuated glass bottles (300e800 ml volume) everyday at 0800 h and 2000 h using an oil free compressor (Metal Bellow, USA) during the campaign. These air samples were later analyzed by gas chromatographic (GC) technique for CO, CH4 and NMHCs. A GC (Hewlett Packard 5890 series II, USA) equipped with Flame Ionization detector (FID) and PLOT capillary column (50 m, 0.32 mm, stationary phase of KCl/ Al2O3) was used to measure the NMHCs in the air samples. The air samples were pre-concentrated in the sampling loop using standard cryo-trapping procedure and injected on-line with helium as carrier gas. The temperature of the column was increased from 0 C to 200 C with specific steps to separate different light NMHCs. This system was calibrated with the standard reference gases. The reproducibility of NMHCs measurements are found between 3% and 10% (Sahu et al., 2006). Another GC (Varian Vista 6000, USA) with FID was used for the measurement of CO and CH4. The column was a 5 m long packed SS tube with a stationary phase of molecular sieves 13x. It was kept in
2. Cruise track and experimental details The measurements were made onboard the Ocean Research Vessel (ORV) Sagar Kanya over the Bay of Bengal (SK-223A) and over the Arabian Sea (SK-223B). Fig. 1 shows the cruise track of ICARB 2006 with the grab sampling (triangles) and balloon launching (circles) locations, marked with the dates of observations. This experiment was planned to cover whole of the Bay of Bengal and Arabian Sea. The ship sailed off from Chennai (13.1 N, 78.3 E) on 18 March 2006 and covered an extensive oceanic region
Fig. 1. Cruise track during ICARB 2006. Overlaid circles on cruise track depict the locations of balloon flights with dates and triangles show the grab sampling locations. ‘Mr’ denotes March, ‘Ap’ denotes April and ‘My’ denotes May.
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isothermal condition at 70 C during sample processing. CO was converted to CH4 by a methanizer containing nickel catalyst at 325 C in the presence of hydrogen for the detection. The calibrations of CO and CH4 were performed with standard calibration gases. The reproducibility of CO and CH4 measurements are found to be better than 3% and 1% respectively (Sahu et al., 2006). 3. Description of MOZART model The Model for OZone And Related Tracers (MOZART, version 2) is a global chemical transport model which simulates the chemistry and transport processes in the troposphere. This model is based on the detailed atmospheric chemistry of 63 chemical species (Ox, NOx, HOx, ethane, propane, ethene, propene, n-butane, isoprene, apinene etc.) with 133 gas phase, 2 heterogeneous and 33 photolytic reactions. The model run is made with horizontal resolution of 2.8 2.8 (128 longitude and 64 latitude grid points). The global POET (Precursors of Ozone and their Effect on the Troposphere, available for the period of 1990e2000) 1 1 gridded emission inventory is used as input for anthropogenic, biogenic and biomass burning emissions in the model (Granier et al., 2005). A recently developed REAS (Regional Emission inventory in ASia) anthropogenic emission inventory is used over Asia (Ohara et al., 2007). This inventory includes detailed 0.5 0.5 gridded anthropogenic emissions of CO, CH4, NOx, SO2, NMHCs, black carbon and organic carbon from fuel combustion and industrial sources over 24 countries in east, south-east and southern Asia for the period of 1980e2020. For the present work, MOZART was used with anthropogenic emissions from REAS for the study period and biogenic and biomass burning emissions from POET. Meteorological input data are taken from NCEP reanalysis at 28 sigma pressure levels from 995 hPa to 2.7 hPa (Sheel et al., 2010). MOZART incorporates the dynamical processes like advection, convection, boundary layer exchange and surface depositions. The complete description of MOZART, version 2 is given by Horowitz et al., (2003). The surface distributions of ozone and CO are computed from this MOZART Model and are compared with the observations. 4. Meteorology during ICARB The average wind patterns during March, April and May months of year 2006 at 925 mb are presented in Fig. 2. The measurements were made over the northern part of Bay of Bengal during March. During this period, the winds were mostly northwesterly over this region. Southwesterly wind prevailed over the southern Bay of
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Bengal during the observational period of April. A cyclonic circulation was present over the central Arabian Sea resulting in north easterly and north westerly wind flow over the southern Arabian Sea during this period. Strong south-westerly wind flow was observed over the north Arabian Sea during the investigation period of May. The individual day observations are also investigated with seven day back-trajectories obtained from METeorological data EXplorer (METEX) trajectory model (Zeng et al., 2008) at 0.5 km and discussed to interpret the observed variations of trace gases. 5. Results and discussions 5.1. Distributions of O3, CH4, CO and NMHCs The spatio-temporal variations of surface O3, CO, CH4 and NMHCs are shown along with the cruise track data (latitude and longitude) during the campaign in Fig. 3. Most of these trace gases showed very high mixing ratios over the northern Bay of Bengal. The mixing ratios of O3, CH4, CO, ethane (C2H6) and n-butane (n-C4H10) decreased from 40e54 ppbv to 17e25 ppbv, 1.91 ppmv to 1.75e1.79 ppmv, 234 ppbv to 88 ppbv, 1269 pptv to 300e400 pptv and 637 pptv to 50e100 pptv respectively from the northern to southern Bay of Bengal. In contrast, these species do not show large variations over the Arabian Sea. Their mixing ratios are found slightly higher over the southern Arabian Sea than over the northern Arabian Sea. The time series variations of alkenes namely ethene (C2H4) and propene (C3H6) showed very distinct variations from other trace species. These showed large variability during the entire cruise period. Their distributions showed higher mixing ratios over the southern parts of both the Bay of Bengal and Arabian Sea. This is analogous to the observations reported by Sahu et al. (2006) showing higher levels of alkenes over the oceanic region away from the continent over the Bay of Bengal during BOBPS 2002. The seven days back-trajectories at each sampling location at an altitude of 500 m above mean sea level were used to identify possible path of air parcel. These different sampling locations were subdivided into six different groups namely northern, central and southern, Bay of Bengal and Arabian Sea according to back-trajectories (Fig. 4). The trajectories passing over the highly polluted IGP region correspond to the measurements made over the northern Bay of Bengal. Consequently, the levels of O3, CH4, CO, ethane and n-butane were highest in these air masses. Over
Fig. 2. Synoptic wind fields at 925 mb over the Indian subcontinent during March (left panel), April (mid panel) and May (right panel) months of 2006.
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Fig. 3. Variations of surface ozone and its precursor gases observed over the Bay of Bengal (left panels) and Arabian Sea (right panels) during the ICARB 2006 ship based campaign. The bottom panel shows the latitude and longitude of the measurements shown in the top five panels.
the central Bay of Bengal, few trajectories originated from the Indian mainland and the remaining originated from the marine region. The mixing ratios of anthropogenic species are found to be comparatively lower in these air masses due to the mixing of polluted air with pristine marine air. The observations over the southern Bay of Bengal are influenced mainly by the air masses arriving from the Indian Ocean. This is the cleanest part of the Bay of Bengal having lower levels of pollutants. The left three panels of the figure show the back-trajectories arriving at the observational locations over northern, central and southern parts of the Arabian Sea respectively. The maximum number of trajectories is found to be originating and circulating over the Arabian Sea itself in all the three regions. The transport of continental pollution is mostly absent in this region. Thus, the mixing ratios of species are comparatively lower over the Arabian Sea.
The inter species correlations between trace gases can provide important information of their common sources. Strong intercorrelations between species show that they are emitted from common and co-located sources and had undergone dilution on their way. Strong inter-correlations between various trace species are discernable over the Bay of Bengal whereas these correlations are rather poor over the Arabian Sea. The correlations of CO with O3, ethane and n-butane are very strong with correlation coefficient (r2) values of 0.88, 0.77 and 0.78 respectively and moderate with CH4 with 0.42. Strong correlations among CO, CH4, ethane and n-butane indicate their common and co-located sources over the Indian region. A good correlation between O3 and CO indicates the photochemical production of O3 in the polluted air mass. However, over the Arabian Sea, lower levels of all these species and their poor inter-correlations indicate their background levels over cleaner marine regions of the Arabian Sea. Moderate to good
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Fig. 4. Seven days back-trajectories arriving over observational locations at 500 m amsl during the campaign over the northern, central and southern parts of the Arabian Sea (left panels) and Bay of Bengal (right panels).
correlations are observed between ethene and propene over both the Bay of Bengal and Arabian Sea with r2 values of 0.63 and 0.59 respectively. These are the only species, which showed a significant correlation over the Arabian Sea. This suggests the source being the local biological activities in the ocean. According to several studies, part of dissolved organic material released from phytoplankton/algae over local oceanic regions transform into alkenes (Plass et al., 1992; Ratte et al., 1993; 1998; Riemer et al., 2000). Sahu et al. (2010) have also showed significant transfer of alkenes from ocean surface to marine boundary layer over the Bay of Bengal during BOBEX II 2003.
5.2. Latitudinal variations of trace gases Fig. 5 shows the latitudinal dependence of O3 and its few precursors having anthropogenic sources over the northern part of the Bay of Bengal (latitude > 12 N). The wind regime over this region was mostly northwesterly during the study period. The back-trajectories show that the pollutants were mainly transported from the IGP. The north to south gradient, in such a case, provides information on the combined effects of chemical transformation and dispersion of these gases over this region. The latitudinal gradients of O3, CO, CH4, ethane and
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Fig. 5. Latitudinal variations of ozone, methane, ethane, carbon monoxide and n-butane over the Northern Bay of Bengal (latitude > 12 N) during ICARB 2006. The straight lines are best linear fits.
n-butane are found to be 5.4 0.9 ppbv deg1, 9.9 1.9 ppbv deg1, 5.3 2.7 ppbv deg1, 93.2 9.2 pptv deg1 and 59.7 7.9 pptv deg1 respectively. The decrease in the mixing ratios of species are expected, as their chemical loss combined with dilution in the absence of local sources lead to continuously decreasing mixing ratios while moving farther south over the ocean. The north-south gradients are due to injection of highly polluted air mass in the MBL from northern coast of the Bay of Bengal and the absence of any other source in between. Ethene and propene do not show north-south gradient over the northern Bay of Bengal. This also indicates that the oceanic emissions play dominant role in their distributions over this marine region. Negative, though small, latitudinal gradients of O3 (0.39 0.22 ppbv deg1), CO (1.68 0.45 ppbv deg1), CH4 (4.5 0.7 ppbv deg1), ethane (15.0 4.0 pptv deg1) and nbutane (2.7 1.0 pptv deg1) are found over the Arabian Sea (8e22 N). The negative gradients show the absence of polluted air advection from northern region. The negative trends in these gases also indicate slight influence of continental transport over the southern Arabian Sea. However, the air over the northern Arabian Sea is found to be transported mostly from the southern Arabian Sea.
5.3. Distribution of CO over the study region The distribution of CO at 900 mb is obtained from MOPITT (Measurements Of Pollution In The Troposphere) (Warner et al., 2001; Deeter et al., 2004) with a spatial resolution of 1 1. The monthly averaged gridded CO data (level 3, version 4) is used for tracking the changes from March to May, 2006. CO with its higher life time (w 2 months) is an ideal indicator of transport of anthropogenic emissions. The IGP and the eastern Indian region are highly polluted with CO levels more than 200 ppbv during March and April (Bay of Bengal segment) resulting in the outflow from IGP towards the northern Bay of Bengal during these months. The pre-monsoon season is known as the transition period and the height of atmospheric boundary layer increases and wind pattern changes after winter season resulting into dilution of pollutants. This feature is clearly discernable from Fig. 6 as March is found most polluted month, followed by April and May. Thus, the temporal variation is also contributing to strong north-south gradient over the Bay of Bengal. Over the Arabian Sea, measurements were made over the southern region (April 19e30, 2006) prior to that over the northern region (May 1e10, 2006). The MOPITT observations suggest that temporal dilution of pollutants resulted into negative gradient over the Arabian Sea.
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465
Fig. 6. Distribution of monthly mean CO at 900 mb obtained from MOPITT over the Indian subcontinent and adjacent marine locations during March, April and May months of year 2006.
5.4. Comparison with previous measurements
5.5. Comparison with MOZART model
The trace gas observations are compared with previous measurements made over the study regions during nearly similar period (FebruaryeMarch) namely INDOEX-99 and BOBEXs (Muhle et al., 2002; Chand et al., 2001; 2003; Lal et al., 2006; 2007). Their average values with 1s variations are given in Table 1. Measurements of surface levels of various trace species including O3, CO and CH4 were made over the Arabian Sea during INDOEX 1999. The mixing ratios of O3 and CO are found higher during INDOEX than ICARB. The winds were mostly northeasterly during INDOEX blowing the pollutants from the Indian subcontinent and southwest Asian countries over this region (de Gouw et al., 2001). However, during ICARB, air was circulating over the cleaner marine region of the Arabian Sea. The mixing ratios of most of the trace gases are found to be higher over the Bay of Bengal during BOBEX I and BOBEX II than ICARB. This difference may be due to prevailing wind regimes from different directions over the study region. During FebruaryeMarch, the northeasterly winds dominate over the Bay of Bengal carrying continental pollutants. The higher level of pollution also depends on the dynamics of continental boundary layer and the level of anthropogenic emission. Accumulation of pollutants takes place in the shallow boundary layer resulting in higher abundance of pollution. The atmospheric boundary layer remains shallow during FebruaryeMarch (BOBEXs) with respect to late March and April months (ICARB period over the Bay of Bengal) and prevailing winds transport them over the ocean.
Fig. 7 shows the surface distributions of O3 and CO using observational data and simulated by MOZART. The model results show good agreement with high levels of O3 over the northern Bay of Bengal till Day of Year (DOY) # 88 when the ship was located near 13 N, 83.2 E. Thereafter, the model overestimated the mixing ratios of O3 over the Bay of Bengal with average mixing ratios of 38.4 6.0 ppbv whereas the average observed mixing ratio is found about 18.8 4.2 ppbv. The overestimation of O3 increased over the Arabian Sea with average model mixing ratio of 49.5 6.5 ppbv and average observed mixing ratio of 19.8 4.1 ppbv. The mixing ratios of CO are found to be well matched in observational and modelling studies during this campaign. The model clearly reproduces the high levels of CO in the IGP outflow over the northern Bay of Bengal and also captures few small scale features like sharp peak on DOY # 80, then decrease till DOY # 82, further increase till DOY # 84 and decrease afterwards. However, the mixing ratios of CO are slightly underestimated over the Bay of Bengal with average difference of about 32 ppbv (22%) and over the Arabian Sea by about 16 ppbv (19%). The CO distribution in the marine boundary layer over the Bay of Bengal, Arabian Sea and Indian Ocean were also found to be underestimated by ECHAM 5 global circulation model during BOBEX I 2001 (Lal et al., 2006). Overall the MOZART results fairly reproduce the broad features present in the distribution of CO both over the Bay of Bengal and Arabian Sea. The linear equation fit shows the following relation in these two data sets.
Table 1 Comparison of mixing ratios of ozone and its various precursor gases over the Bay of Bengal (BoB) and Arabian Sea (AS) during various cruise campaigns. Species
INDOEX
February eMarch 1999 ASa O3 (ppbv) 43.9 8.0 CO (ppbv) 217 42.0 1.73 0.05 CH4 (ppmv) Ethane e (pptv) Ethene e (pptv) Propene e (pptv) n-Butane e (pptv) a b
Lal et al. (2006). Lal et al. (2007).
ICARB
BOBEX I
BOBEX II
ICARB
AprileMay 2006 AS 19.8 4.1 86.7 14.1 1.74 0.02
February eMarch 2001 BoBa 42.2 12.0 217 31 1.92 0.05
February 2003 BoBb 34 6.4 193 40 1.72 0.04
March eApril 2006 BoB 28.3 14.4 145 38 1.81 0.03
338 125 e
1181 362 589 280
165 106 e
198 60
196 154
132 88
e
106 50
112 92
49 23
e
98 123
187 157
½COMOZART ¼ 0:83*½COobs 5:74 The fairly good value of correlation coefficient (r2 ¼ 0.66) and low value of offset (5.74 ppbv) between the two data sets show that model strongly follows the CO observations. The production of O3 depends on complex chain reactions of CO, CH4 and various non-methane hydrocarbons in the presence of sunlight and NOx. Any discrepancy in the emission estimates of these species will affect the O3 mixing ratio obtained from MOZART. Higher mixing ratios of O3 may be attributed to uncertainty in the emission estimated in the used inventories. Sheel et al. (2010) showed that the MOZART overestimates the columnar NO2 compared to GOME (Global Ozone Monitoring Experiment) observations over the Indian region. Their study highlights the need to develop an emission inventory particularly over the Indian region with higher spatial resolution. In addition to that, reaction of O3 with biogenic halogens can be an important loss process within the marine boundary layer (Vogt et al., 1996; Dickerson et al., 1999; McFiggans et al., 2000). These reactions are not included in the chemical mechanisms of MOZART. Thus, the uncertainty in emission estimates together with loss of O3 because of halogen chemistry may contribute to overestimation of ozone mixing ratio by MOZART.
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Fig. 7. Comparison of observational and MOZART model estimated surface distributions of O3 and CO over the Bay of Bengal and Arabian Sea during ICARB 2006. The bottom panel shows the latitude and longitude of the measurements shown in the top two panels.
6. Summary and conclusion The ICARB cruise campaign was conducted over large areas of the Bay of Bengal and Arabian Sea during the pre-monsoon season of 2006. For the first time, the surface distribution of O3, CO, CH4 and light NMHCs were investigated near simultaneously over these two geographically segregated marine regions using ozonesonde and grab sampling. Higher mixing ratios of these trace gases are observed over the northern Bay of Bengal due to transport of polluted air from the IGP. The levels of these gases are found to be much lower over the southern Bay of Bengal and over the entire Arabian Sea due to the absence of transport of polluted air parcel over this region. The ozonesonde observations also showed a thick layer of comparatively higher O3 mixing ratios (60e90 ppbv) in the altitude range of 0.5e3 km over the northern Bay of Bengal (Srivastava et al., 2011). The inter-correlation studies showed strong correlations among CO, CH4, ethane and n-butane over the Bay of Bengal indicating common and co-located continental sources. A good correlation between CO and O3 indicate photochemical production of O3 in the polluted air mass. Lower levels of trace gases and their poor inter-correlations over the Arabian Sea point their background levels over cleaner marine region. Moderate to good correlation between ethene and propene over both the Bay of
Bengal and Arabian Sea shows their pre-dominant oceanic sources. O3, CO, ethane and n-butane show very strong positive latitudinal gradients over the northern Bay of Bengal (latitude > 12 N) and slightly negative gradient over the Arabian Sea. The positive latitudinal gradient over the northern Bay of Bengal is found to be due to injection of pollutants from the northern coastal areas of Bay of Bengal. The negative gradients over the Arabian Sea are attributed to transport of polluted air from southern India over the southern Arabian Sea and continuous dilution of pollution with time over this region. These results are confirmed by CO distribution over the Indian region obtained from MOPITT. Alkenes (ethene and propene) did not show any latitudinal gradient due to dominant oceanic emission. The distributions of O3 and CO are compared with the results of MOZART and broadly found to match over the northern Bay of Bengal. However, O3 is over estimated and CO is slightly underestimated over the southern Bay of Bengal and Arabian Sea. These results further demand for the improvement of emission inventories over the southern Asian region. The present observations are compared with the observations made during previous campaigns. Mixing ratios of most of the trace gases are found to be lower during present campaign than during previous campaigns over this region.
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The wind patterns significantly affect the degree of pollution over the oceanic region adjacent to continental areas. Over the southern Asia, wind changes from season to season. The ship based trace gas study during a particular season can provide only limited information of continental transport with large degree of uncertainty due to changing source strengths over the densely populated continental regions. Frequent cruise campaigns during different seasons will provide detailed information on the monthly/seasonal variation of continental outflow and their influence over these marine regions. Acknowledgement The campaign was supported by Indian Space Research Organization-Geosphere Biosphere Program. We thank team members of ICARB and crew of Sagar-Kanya (SK 223) for their help in successfully conducting these experiments on board the ship. We are thankful to Mr. T. K. Sunil Kumar for his help in the analysis of grab samples. We are grateful to METEX team members for providing air back trajectory model. We thank the anonymous reviewers for their comments and suggestions. References Burkert, J., Andrés-Hernández, M.D., Reichert, L., Meyer-Arnek, J., Doddridge, B., Dickerson, R.R., Mühle, J., Zahn, A., Carsey, T., Burrows, J.P., 2003. Trace gas and radical diurnal behavior in the marine boundary layer during INDOEX 1999. J. Geophys. Res. 108 (D8), 8000. doi:10.1029/2002JD002790. Chand, D., Modh, K.S., Naja, M., Venkataramani, S., Lal, S., 2001. Latitudinal trends in O3, CO, CH4 and SF6 over the Indian Ocean during the INDOEX IFP-1999 cruise. Curr. Sci. 80, 100e104. Chand, D., Lal, S., Naja, M., 2003. Variations of ozone in the marine boundary layer over the Arabian Sea and the Indian Ocean during the 1998 and 1999 INDOEX campaigns. J. Geophys. Res. 108 (D6), 4190. doi:10.1029/2001JD001589. de Gouw, J.A., et al., 2001. Overview of the trace gas measurements on board the citation aircraft during the intensive field phase of INDOEX. J. Geophys. Res. 106, 28453e28467. de Laat, A.T.J., Lelieveld, J., Roelofs, G.J., 2001. Source analysis of carbon monoxide pollution during INDOEX 1999. J. Geophys. Res. 106, 28481e28495. Deeter, M.N., et al., 2004. Evaluation of operational radiances for the measurements of pollution in the troposphere (MOPITT) instrument CO thermal-band channels. J. Geophys. Res. 109 (D3), D03308. doi:10.1029/2003JD003970. Dickerson, R.R., Rhoads, K.P., Carsey, T.P., Oltmans, S.J., Crutzen, P.J., 1999. Ozone in the remote boundary layer: a possible role for halogens. J. Geophys. Res. 104, 21385e21395. Environmental Protection Agency, 1993. Alternative Control Techniques Document: NOx Emissions from Stationary Reciprocating Internal Combustion Engines, EPAe453/R-93/032. Off. of Air Qual. Planning and Stand., Environ. Prot. Agency, Research Triangle Park, N. C. Granier, C., Guenther, A., Lamarque, J.F., Mieville, A., Muller, J.F., Olivier, J., Orlando, J., Peters, J., Petron, G., Tyndall, G., Wallens, S., 2005. POET, a Database of Surface Emissions of Ozone Precursors. http://www.aero.jussieu.fr/projet/ACCENT/ POET.php. Horowitz, L.W., et al., 2003. A global simulation of tropospheric ozone and related tracers: description and evaluation of MOZART, version 2. J. Geophys. Res. 108, 4784. doi:10.1029/2002JD002853. Komhyr, W.D., 1969. Electrochemical concentration cells for gas analysis. Ann. Geoph. 25, 203e210. Lal, S., Lawrence, M.G., 2001. Elevated mixing ratios of surface ozone over the Arabian Sea. Geophys. Res. Lett. 28, 1487e1490. Lal, S., Naja, M., Jayaraman, A., 1998. Ozone in the marine boundary layer over the tropical Indian Ocean. J. Geophys. Res. 103 (D15), 18907e18917. Lal, S., Chand, D., Sahu, L.K., Venkataramani, S., Brasseur, G., Schultz, M.G., 2006. High levels of ozone and related gases over the Bay of Bengal during winter and early spring of 2001. Atmos. Environ. 40, 1633e1644.
467
Lal, S., Sahu, L.K., Venkataramani, S., 2007. Impact of transport from the surrounding continental regions on the distributions of ozone and related trace gases over the Bay of Bengal during February 2003. J. Geophys. Res. 112, D14302. doi:10.1029/2006JD008023. Lawrence, M.G., Lelieveld, J., 2010. Atmospheric pollutants outflow from southern Asia: a review. Atmos. Chem. Phys. 10, 11017e11096. Lelieveld, J., et al., 2001. The Indian Ocean experiment: widespread air pollution from south and Southeast Asia. Science 291, 1031e1036. McFiggans, G., Plane, J.M.C., Allan, B.J., Carpenter, L.J., Coe, H., O’Dowd, C., 2000. A modeling study of iodine chemistry in the marine boundary layer. J. Geophys. Res. 105, 14371e14385. Moorthy, K.K., Satheesh, S.K., Babu, S.S., Dutt, C.B.S., 2008. Integrated campaign for aerosols, gases and radiation budget (ICARB): an overview. J. Earth Syst. Sci. 117 (S1), 243e262. Muhle, J., Zahn, A., Brenninkmeijer, C.A.M., Gros, V., Crutzen, P.J., 2002. Air mass classification during the INDOEX Ronald Brown cruise using measurements of non-methane hydrocarbons, CH4, CO2, CO, 14CO and d18O(CO). J. Geophys. Res. 107 (D19), 8021. doi:10.1029/2001JD000730. Ohara, T., Akimoto, H., Kurokawa, J., Horii, N., Yamaji, K., Yan, X., Hayasaka, T., 2007. An Asian emission inventory of anthropogenic emission sources for the period 1980e2020. Atmos. Chem. Phys. 7, 4419e4444. doi:10.5194/acp-7-4419-2007. Plass, C., Koppmann, R., Rudolph, J., 1992. Light hydrocarbons in the surface water of the mid-Atlantic. J. Atmos. Chem. 15, 235e251. Ratte, M., Plass-Dülmer, C., Koppmann, R., Rudolph, J., Denga, J., 1993. Production mechanism of C2eC4 hydrocarbons in seawater: field measurements and experiments. Glob. Biogeochem. Cycles 7 (2), 369e378. doi:10.1029/ 93GB00054. Ratte, M., Bujok, O., Spitzy, A., Rudolph, J., 1998. Photochemical alkene formation in seawater from dissolved organic carbon: results from laboratory experiments. J. Geophys. Res. 103 (D5), 5707e5717. doi:10.1029/97JD03473. Riemer, D.D., et al., 2000. Photoproduction of nonmethane hydrocarbons (NMHCs) in seawater. Mar. Chem. 71, 177e198. Sahu, L.K., Lal, S., Venkataramani, S., 2006. Distributions of O3, CO and hydrocarbons over the Bay of Bengal: a study to assess the role of transport from southern India and marine regions during SeptembereOctober 2002. Atmos. Environ. 40, 4633e4645. Sahu, L.K., Lal, S., Venkataramani, S., 2010. Impact of monsoon circulations on oceanic emissions of light alkenes over Bay of Bengal. Glob. Biogeochem. Cycles. doi:10.1029/2009GB003766. Scheeren, H.A., Lelieveld, J., de Gouw, J.A., Van der Veen, C., Fischer, H., 2002. Methyl chloride and other chlorocarbons in polluted air during INDOEX. J. Geophys. Res. 107 (D19), 8015. doi:10.1029/2001JD001121. Sheel, V., Lal, S., Richter, A., Burrows, J.P., 2010. Comparison of satellite observed tropospheric NO2 over India with model simulations. Atmos. Environ. 44, 3314e3321. Smit, H.G.J., et al., 2007. Assessment of the performance of ECC-ozonesondes under quasi-flight conditions in the environmental simulation chamber: insights from the Juelich ozone sonde intercomparison experiment (JOSIE). J. Geophys. Res. 112, D19306. doi:10.1029/2006JD007308. Srivastava, S., Lal, S., Venkataramani, S., Gupta, S., Acharya, Y.B., 2011. Vertical distribution of ozone in the lower troposphere over the Bay of Bengal and the Arabian Sea during ICARB-2006: effects of continental outflow. J. Geophys. Res. doi:10.1029/2010JD015298. Streets, D.G., Yarber, K.F., Woo, J.H., Carmichael, G.R., 2003a. Biomass burning in Asia: annual and seasonal estimates and atmospheric emissions. Glob. Biogeochem. Cycles 17 (4), 1099. doi:10.1029/2003GB002040. Streets, D.G., et al., 2003b. An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. J. Geophys. Res. 108 (D21), 8809. doi:10.1029/ 2002JD003093. Venkataraman, C., Habib, G., Kadamba, D., Shrivastava, M., Leon, J.F., Crouzille, B., Boucher, O., Streets, D.G., 2006. Emissions from open biomass burning in India: integrating the inventory approach with high resolution moderate resolution imaging spectroradiometer (MODIS) active fire and land cover data. Glob. Biogeochem. Cycles 20, GB2013. doi:10.1029/2005GB002547. Vogt, R., Crutzen, P.J., Sander, R., 1996. A mechanism for halogen release from seasalt aerosol in the remote marine boundary layer. Nature 383, 327e329. Warner, J.X., Gille, J.C., Edwards, D.P., Ziskin, D.C., Smith, M.W., Bailey, P.L., Rokke, L., 2001. Cloud detection and clearing for the Earth observing system Terra satellite measurements of pollution in the troposphere (MOPITT) experiment. Appl. Opt. 40, 1269e1284. Zeng, J., Matsunaga, T., Mukai, H., 2008. METEX e A Flexible Tool for Air Trajectory Calculation, Environmental Modelling & Software. doi:10.1016/ j.envsoft.2008.10.015.