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Airborne measurements of O3, CO, CH4 and NMHCs over the Bay of Bengal during winter
Atmospheric Environment 59 (2012) 597e609
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Airborne measurements of O3, CO, CH4 and NMHCs over the Bay of Bengal during winter Shuchita Srivastava a, *, S. Lal a, S. Venkataramani a, I. Guha a, D. Bala Subrahamanyam b a b
Physical Research Laboratory, Ahmedabad 380009, India Space Physics Laboratory, VSSC, Trivandrum, India
h i g h l i g h t s < Airborne measurements of O3, CO, CH4 and NMHCs over the Bay of Bengal (BoB). < Transport of pollutants from Indo-Gangetic Plain and Southeast Asia over the BoB. < Estimation of photochemical processing and age of air masses using NMHCs. < Photo-chemically fresh pollution plume was observed over the northern BoB.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2011 Received in revised form 23 April 2012 Accepted 25 April 2012
As part of the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (WICARB), airborne measurements of ozone (O3), carbon monoxide (CO), methane (CH4), ethane (C2H6), ethene (C2H4), acetylene (C2H2), propane (C3H8) and n-butane (n-C4H10) were conducted over the Bay of Bengal (BoB) at two altitude levels, 750 m and 1500 m, from Visakhapatnam, Chennai and Port Blair during January 2009. Airborne measurements, first of its kind over the BoB, revealed distinct chemical characteristics at these two altitudes over the eastern sector of Visakhapatnam away from the coastline. This feature is attributed to the presence of marine boundary layer inversion between the two altitudes that inhibited the vertical mixing. A comparative study of trace gas distributions during all air sorties showed enhanced mixing ratios of these gases over east of Visakhapatnam and south of Port Blair in comparison to the north/northeeast and west of Port Blair and east of Chennai. The back-trajectory analysis showed the outflow from the highly polluted Indo-Gangetic Plain over eastern side of Visakhapatnam. The air masses showed lower photochemical processing (higher C2H2/CO ratio w 4e4.5 pptv ppbv1) with the age of w 3e4 days over this location in comparison to other sorties. Chemical characteristics over south of Port Blair seem to be controlled by the outflow from Southeast Asian countries. The results presented in this work would serve as valuable inputs for the atmospheric chemistry transport models and will be helpful in reducing the uncertainty in the budget estimation of trace gases over tropical marine region. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Airborne measurements Ozone Carbon monoxide NMHCs Bay of Bengal Transport
1. Introduction Ozone (O3) is an effective greenhouse gas (Gauss et al., 2003) and a major air pollutant affecting human health and crop production (Adams et al., 1989). Though it is only a trace gas, it controls the oxidizing capacity of the troposphere by producing highly reactive hydroxyl radicals (OH) which define the lifetime of
* Corresponding author. Tel.: þ91 79 26314656; fax: þ91 79 26314659. E-mail addresses: [email protected], [email protected] (S. Srivastava). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2012.04.054
several gases (Jacob, 1999). The level of O3 is controlled by local photochemical production and transport processes in the troposphere. Ozone is photo-chemically produced by carbon monoxide (CO), methane (CH4) and non-methane hydrocarbons (NMHCs) in the presence of NOx and sunlight (Seinfeld and Pandis, 1998). The sources of these precursor gases are predominantly anthropogenic (Jacob, 1999). The highly populated South Asian countries with their rapid industrialization and agricultural growth are major sources of anthropogenically emitted pollutants (Ohara et al., 2007). Emissions from South Asia are strongly influenced by inefficient combustion processes, especially biofuel burning in cook stoves,
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widespread use of two stroke engines etc. (Streets et al., 2003; Ohara et al., 2007). This region contributes a disproportionally large part of the total global CO and NMHCs emissions, approximately 1.5 times larger than its relative contribution in CO2 and NOx (Lawrence and Lelieveld, 2010). A recently developed emission inventory REAS (Regional Emission inventory in ASia) has suggested enhancement in CO emission by 11%, and NMHCs emissions by 33% from 2000 to 2010 over India (Ohara et al., 2007). These gases are precursors of O3 in the troposphere. The study of their distributions especially over the Indian subcontinent is important to understand their roles in chemistry of the tropical troposphere and global climate change. The marine regions surrounding India are impacted by the outflow of pollutants from many South Asian countries (Lelieveld et al., 2001; Ramanathan et al., 2001; de Gouw et al., 2001; Murugavel et al., 2008). The Bay of Bengal (BoB) is bordered by densely populated and anthropogenically polluted central/eastern India and Bangladesh in the north; Myanmar and Thailand, having intense biomass burning during most of the year, in the east; and relatively less polluted southern India in the west. The prevailing meteorological conditions transport continental air masses from various directions, and containing varying quantities of pollutants, over the BoB. Different transport patterns over this region make it a unique environment to study the spatial heterogeneity in the distribution of various primary and secondary pollutant gases due to outflow of continental air masses. Measurements of various trace gases were made over the BoB during several cruise campaigns namely Bay Of Bengal EXperiments (BOBEX e I in 2001 and BOBEX e II in 2003) and Bay Of Bengal Process Studies (BOBPS e 2002) to study the influence of continental outflow over this region (Sahu et al., 2006; Lal et al., 2006, 2007). The BOBEX-I was conducted over the BoB, the northern Indian Ocean and the Arabian Sea in winter and early spring of 2001 to study the surface distributions of O3, CO and CH4 (Lal et al., 2006). Dedicated cruise campaigns namely BOBPS and BOBEX-II were conducted to cover extensive areas of the BoB to study the surface distributions of O3, CO, CH4 and various NMHCs under different wind regimes (Sahu et al., 2006; Lal et al., 2007). However, airborne measurements of these trace gases were never made over this oceanic region primarily due to lack of dedicated platform. With a view to investigate the spatial variability of trace gases in the lower troposphere over the BoB, aircraft-based trace gas measurements were made for the first time over this region during the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (WICARB), January, 2009. During this period, the boundary layer over the surrounding continent often remains shallow which favours the advection of pollutants over the BoB. The objective of the present work is to characterize the continental outflow of pollutants and to study the spatial heterogeneity in the distribution of trace gases from the coastal to cleaner oceanic regions. 2. Methods The air-segment of WICARB was conducted using Beechcraft Super King Air B-200 (VT-EBB) of National Remote Sensing Centre (NRSC), Hyderabad, India during the month of January 2009. The aircraft was operated in unpressurized mode with a ceiling altitude of 3 km. A total of 6 bi-level air sorties were conducted from Visakhapatnam (17.72 N, 83.22 E), Chennai (12.99 N, 80.17 E) and Port Blair (11.64 N, 92.73 E). The spatial coverage of these sorties is shown in Fig. 1(a). The bi-level sorties were made at 750 m and 1500 m in order to study the gradients in the mixing ratios of these species from coast to the open ocean. A typical bi-level sortie path profile is shown in Fig. 1(b) and details of all the sorties are given in Table 1.
Fig. 1. (a) The flight paths of air sorties (dashed lines) conducted from Visakhapatnam (VSK), Chennai (CHN) and Port Blair (PBR) during WICARB (January, 2009). The details of each sortie are given in Table 1. (b) Typical altitude variation of aircraft path for bilevel sorties (Path shown for Visakhapatnam). The sampling locations are also shown by circles on the paths.
The ambient air was aspirated via SS inlet lines fitted under the nose of the aircraft. In-situ measurement of O3 was made with a time resolution of 10 s using UV absorption based Dual Beam 2B O3 analyzer (Model 205). The calibration of this system was performed before and after the campaign using O3 calibrator (Sonimix 6000 C2). The air samples were also collected using an oil free compressor (Metal Bellow, USA) in glass sampling bottles of 300e800 ml volume. The Hewlett Packard 5890 series II GC equipped with a flame ionization detector and a PLOT capillary column (50 m, 0.32 mm with KCl/Al2O3 as stationary phase) was used to measure the NMHCs in the air samples. The air samples were preconcentrated in the sampling loop using cryo-trapping procedure and injected into the column with helium as carrier gas. The temperature of the column was varied from 0 C to 200 C to separate different light NMHCs. The reproducibility of NMHCs measurements were found to be between 3% and 10% (Sahu et al., 2006). The Varian Vista 6000 GC, with a flame ionization detector 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 isothermal condition at 70 C during the analysis. CO is converted to CH4 by a methanizer containing Nickel catalyst at 325 C. The reproducibility of CO and CH4 measurements were found to be better than 3% and 1% respectively (Sahu et al., 2006). The key features in the distributions of trace gases are examined in the light of back-trajectories. We have calculated kinematic
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Table 1 Details of air sorties conducted during WICARB 2009. Locations
Date
Direction of sorties (bi-level)
Name of sortie
Flight take off time (IST, hh:mm)
Total flight duration (hours)
Visakhapatnam (17.72 N, 83.22 E) Port Blair (11.64 N, 92.73 E) Port Blair Port Blair Port Blair Chennai (12.99 N, 80.17 E)
05-01-2009 09-01-2009 09-01-2009 13-01-2009 14-01-2009 16-01-2009
EasteWest (Zonal) EasteNorth (Zonal-Meridional) NortheSouth (Meridional) WesteEast (Zonal) SoutheNorth (Meridional) EasteWest (Zonal)
V1EW P1EN P2NS P3WE P4SN C1EW
10:15 06:35 10:56 06:10 06:20 10:19
2.13 2.68 1.66 2.41 2.32 2.40
3. Results
1500 m and 3000 m above the mean sea level respectively) during January 1e20, 2009 over the Indian subcontinent and surrounding marine region as obtained from NCEP reanalysis. Broadly, wind patterns over this region were similar at 1000 mb, 925 mb and 850 mb. The winds were northeasterly over the BoB and wind speed is found to be higher over this region. Winds were calm over the Indian subcontinent and north-east sector of the BoB. The northwesterly winds prevailed over the northern Indian region at 700 mb. Advection of the Indian continental air is evident at the northern part of BoB which then got transported to the south India.
3.1. Synoptic meteorology
3.2. Spatial distribution of O3 and its precursors over the BoB
Fig. 2 shows the average wind fields at 1000 mb, 925 mb, 850 mb and 700 mb (approximately corresponding to surface, 750 m,
Spatial distribution of O3 and its precursors are examined for each bi-level sortie at 750 m and 1500 m over the BoB. The sorties
back-trajectories using METeorological data EXplorer (METEX) trajectory model developed at the National Institute of Environmental Studies, Japan and Global Environmental Forum, Japan (Zeng et al., 2010). The METEX uses the National Centres for Environmental Prediction (NCEP) reanalysis data having spatial resolution of 2.5 2.5 at 17 different pressure levels archived at every 6 h. This trajectory model is used in several research works (Naja and Akimoto, 2004; Kumar et al., 2010a).
Fig. 2. The synoptic wind fields (m/s) at 1000 mb, 925 mb, 850 mb and 700 mb pressure levels averaged for 1e20 January, 2009 over the Indian subcontinent and surrounding marine regions.
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from Port Blair, which were conducted on the same day covered the northern part of Andaman & Nicobar Islands and hence considered as a single sortie. Fig. 3 shows the spatial distributions of O3, CO, CH4, ethane (C2H6), ethene (C2H4), acetylene (C2H2), propane (C3H8) and n-butane (n-C4H10) over the BoB at 750 m altitude. Ozone mixing ratios are found to be maximum (54 2 ppbv) over east of Visakhapatnam which is about 10 ppbv higher than south of Port Blair (43 2 ppbv) and 15e20 ppbv higher than east of Chennai (35 1 ppbv), west of Port Blair (39 3 ppbv) and north/north-east of Port Blair (37 7 ppbv). Carbon monoxide and most of other hydrocarbons showed enhanced mixing ratios over south of Port Blair followed by east of Visakhapatnam away from the coast. Moderately high mixing ratios are evident over west of Port Blair. The levels of all these gases are found to be comparatively lower over east of Chennai and north/north-east of Port Blair. Ethene showed a highly variable distribution with randomly high and low values during each sortie but maximum mixing ratios are found to be present over south of Port Blair like other gases. The mixing ratio of n-butane is found below the detection limit of the instrument over several sampling locations of east of Visakhapatnam and north/north-east of Port Blair. Fig. 4 shows the spatial distributions of O3, CO, CH4 and NMHCs over each sortie location at 1500 m altitude. The trace gas distributions are found to be similar to their variations at 750 m altitude, but with relatively lower mixing ratios. The O3 mixing ratios are found similar to their 750 m values in respective sorties except over south of Port Blair where it decreased by w5 ppbv from its values at 750 m altitude. The average CH4 mixing ratios are found to be almost constant for each sortie. The average mixing ratios of these gases are given in Table 2 for each sortie at 750 m and 1500 m. These observations indicate advection of pollutants over east of Visakhapatnam and south of Port Blair having different chemical characteristics. The heterogeneity in the spatial distributions of these gases suggests distinct sources of pollutants over these two regions. Measurements of surface O3 and CO were also made during the ship segment of WICARB over the BoB (Asatar and Nair, 2010; David et al., 2011). Surface O3 mixing ratios also showed highest
levels (>60 ppbv) over the northern BoB followed by the southeast BoB (40e60 ppbv). However, surface CO mixing ratios showed 290 ppbv over the north BoB and 379 ppbv over the southeast BoB. The lowest CO values of 167 ppbv were measured over the south-west BoB. Variations in surface level O3 and CO are similar to those from aircraft measurements at 750 m and 1500 m. 3.3. Relationship between trace gases Variations of O3, CH4, ethane and acetylene with respect to CO during different sorties at 750 m and 1500 m levels (Fig. 5) show different photochemical regimes sampled in different regions (Nowak et al., 2004). Highest O3 and acetylene values are observed near Visakhapatnam whereas ethane peaks at south of Port Blair. Samples collected during Chennai and northenortheast of Port Blair showed minimum values of these species. Ethane and acetylene show systematic increase with CO. However, CH4 and O3 show relatively poor correlation with it. The lower O3 mixing ratio corresponding to high CO over south of Port Blair could be due to early hour flight (0620 h IST, 0700 h Local Time) when the photochemical production of O3 is less due to in-sufficient sunlight. The flight time for the Visakhapatnam sortie was 1015 h IST when sunlight was sufficient for photochemical O3 production. The flight times for sorties near Chennai and north of Port Blair were also about 1015e1100 h IST but the mixing ratios of precursors were lower for the in-situ O3 production. 4. Discussion 4.1. Investigation of back-trajectories The seven days back-trajectories are calculated for the observation locations at the time of flights with 0.1 spatial resolution for each sortie at both the altitude levels. The spatial and altitude variations of trajectories arriving at 750 m and 1500 m over the observation locations of the eastward sortie of Visakhapatnam and the southward sortie of Port Blair are shown in Figs. 6 and 7. The trajectories ending at the observation locations over east of
Fig. 3. The spatial distributions of O3, CO, CH4, C2H6, C2H4, C2H2, C3H8 and n-C4H10 over the Bay of Bengal at 750 m altitude during various air sorties of WICARB.
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Fig. 4. The spatial distributions of O3, CO, CH4, C2H6, C2H4, C2H2, C3H8 and n-C4H10 over the Bay of Bengal at 1500 m altitude during various air sorties of WICARB.
Visakhapatnam originated from Pakistan and central north Indian region. These trajectories passed over the states of Delhi, Uttar Pradesh, Bihar and West Bengal, which are in the Indo-Gangetic Plain, before ending at the observation points. This region is highly populated and hence source of many anthropogenic pollutants (Tripathi et al., 2006; Lal et al., 2008; Kar et al., 2010). The atmospheric boundary layer altitude remains shallow over these areas during January (Maximum convective boundary layer depth w2 km; Kumar et al., 2010b) which leads to the accumulation of pollutants within this region. The altitudes of most of the back-trajectories were within the lower 2 km in the past 120 h when they were passing through this region. Thus, these air masses brought the pollutants, accumulated within the boundary layer over the Indo-Gangetic Plain, leading to their high mixing ratios over east of Visakhapatnam. The other group of backtrajectories reaching south of Port Blair is also shown in the same figure. These trajectories passed through the regions of Southeast Asia, mainly Myanmar and Thailand, before arriving at
the observation locations. The altitudes of these trajectories were within the lower 4 km during most of their travel time. At 750 m, their altitudes were mostly within 2 km for the final few days. Higher trace gas mixing ratios at receptor location indicate that these trajectories picked up sufficient pollution from the continental locations. Seven days back-trajectories arriving at 750 m and 1500 m over east of Chennai and west of Port Blair are shown in Figs. 8 and 9. The trajectories over east of Chennai originated from eastern India and Bangladesh and arrived over observational points after spending w4 days over the cleaner marine region within the lower 2 km. This indicates the dilution of continental pollution thus justifying the lower levels of trace gases over this region. The levels of trace gases were higher over the west of Port Blair at 750 m due to extended stay of air parcel over Southeast Asia within the lower 2 km. The mixing ratios were significantly lower at 1500 m when the altitude of trajectories was higher than 2 km over the continent.
Table 2 Average mixing ratios (with 1 s variability) of trace gases at altitudes of 750 m and 1500 m during different sorties of WICARB.
Altitude 750 m O3 (ppbv) CO (ppbv) CH4 (ppmv) Ethane (pptv) Ethene (pptv) Acetylene (pptv) Propane (pptv) n-Butane (pptv) Altitude 1500 m O3 (ppbv) CO (ppbv) CH4 (ppmv) Ethane (pptv) Ethene (pptv) Acetylene (pptv) Propane (pptv) n-Butane (pptv)
Vishakhapatnam (East)
Port Blair (North/NortheEast)
Port Blair (West)
Port Blair (South)
Chennai (East)
54 2 256 19 1.88 0.03 2210 218 274 100 1159 121 187 102 77 21
37 7 161 18 1.84 0.03 1382 180 254 75 501 45 117 60 45 31
39 3 229 18 1.87 0.01 2174 270 276 99 847 130 187 98 69 17
43 2 256 9 1.88 0.01 2578 70 400 140 792 34 418 194 134 28
35 1 166 17.3 1.85 0.01 1682 229 202 86 585 90 82 55 43 19
52 2 187 37 1.84 0.03 1792 216 282 122 726 129 155 30 e
36 8 133 27 1.84 0.03 1285 229 257 133 401 167 108 96 e
40 3 158 39 1.83 0.02 1589 370 175 88 509 176 121 70 37 20
38 2 211 19 1.84 0.03 2178 221 190 64 554 77 238 150 88 26
35 2 155 23 1.84 0.01 1539 216 166 81 533 116 69 18 30 15
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Fig. 5. Variation of O3, CH4, ethane, and acetylene with respect to CO at altitudes of 750 m and 1500 m during east of Visakhapatnam (VSK-E), North/North-east of Port Blair (PBR-N-NE), West of Port Blair (PBR-W), South of Port Blair (PBR-S) and East of Chennai (CHN-E) air sorties of WICARB.
Fig. 10 shows the back-trajectories arriving at north of Port Blair at 750 m and 1500 m during P2NS sortie. These trajectories are found to be coming from Southeast Asian region at 750 m. At 1500 m altitude, trajectories originated over the northern BoB and advected via Southeast Asian region before their arrival over observational location. These trajectories are found to be descending from the height range of 4e6 km. The lower values at receptor location indicate that these trajectories do not have sufficient exposure to highly polluted continental region during their travel time. 4.2. Role of marine boundary layer inversion near Visakhapatnam The V1EW sortie conducted from Visakhapatnam towards east over the northern BoB at altitudes of 750 m and 1500 m provides longitudinal variation of different trace gases. The aircraft covered the longitudinal span from 83.2 E to 86.7 E at a constant latitude path (17.72 N) away from Visakhapatnam. The mixing ratios of O3 are averaged for every 0.1 longitude bins and are shown with 1s
variation at both the altitude levels in Fig. 11. The average mixing ratios of O3 were found to be in the range of 50 5 ppbv near the coast at 750 m which increased at a rate of 2.0 ppbv deg1 and attained the maximum O3 level (60 2 ppbv) at 86.7 E. The reverse variation is evident at 1500 m altitude, where O3 decreased with increasing longitude at a rate of 1.5 ppbv deg1 and reached its minimum mixing ratio at 86.7 E (43 4 ppbv). In a broad sense, the magnitudes of O3 mixing ratios at 750 m were found to be quite similar to that at 1500 m along the longitudinal sector from 83.2 E to 85.8 E, beyond which there is a clear-cut distinction between the two. Fig. 11 also shows longitudinal variations of CO, CH4, ethane and acetylene at 750 m and 1500 m altitudes. Similar to O3 variation, these gases also show clear increase at 750 m and decrease at 1500 m on and after 86 E. The average concentrations of these species were found to be higher at 750 m than at 1500 m. The average differences of CO, CH4, ethane and acetylene between 750 m and 1500 m levels were found to be 41 24 ppbv, 0.017 0.01 ppmv, 198 215 pptv and 312 83 pptv respectively in the
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Fig. 6. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 750 m over east of Visakhapatnam during V1EW sortie and south of Port Blair during P4SN sortie.
Fig. 7. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 1500 m over east of Visakhapatnam during V1EW sortie and south of Port Blair during P4SN sortie.
samples collected before 86 E (towards west) and 133 12 ppbv, 0.09 0.01 ppmv, 931 284 pptv and 713 250 pptv in the samples collected on and after 86 E (towards east). Overall, differences are large (2e5 times higher with respect to samples collected near coastal area) between two altitudes collected over open ocean (east of 86 E). This suggests that aircraft encountered air masses of different chemical compositions at two different altitudes at and after 86 E. These observations are investigated in detail in the light of results found by Simpson and Raman (2004) over the Arabian Sea during the INDOEX 99 cruise. They showed a schematic of marine boundary layer height variation and development of land plume over the Arabian Sea near the coast in a typical day time condition during JanuaryeMarch 1999. The marine boundary layer height grows with increasing distance away from the coast due to convection and entrainment. As per their observations, marine boundary layer height varied from 500 m near the coast to 1000 m over the open ocean. As part of the ocean segment of WICARB field experiment, balloon-borne GPS sondes were launched on a regular basis from Oceanic Research Vessel Sagar Kanya over the BoB (Subrahamanyam et al., 2012). In the present study, we used the vertical profiles of thermodynamic parameters obtained from four
marine locations 13.09 Ne80.29 E (distance from land w12 km), 13.86 Ne80.94 E (distance from land w92 km), 14.84 Ne81.47 E (distance from land w151 km) and 15.11 N-83.01 E (distance from land w320 km) during 26e28 December 2008 as collocated measurements from aircraft and ship were not available. Fig. 12 shows the vertical distributions of potential temperature and relative humidity over these locations in the increasing order of longitudinal distance from the coast. These profiles clearly show the mixed layer top at 400 m, 500 m, 600 m and 1100 m when the ship was at a distance of 12 km, 92 km, 151 km and 320 km respectively from the coast. We reproduce the schematic given by Simpson and Raman (2004) modified according to present campaign to explain the variations of trace gases observed near Visakhapatnam. The aircraft started from Visakhapatnam at an altitude of 750 m near the coast where the marine boundary layer height remained shallow (w500 m). The marine boundary layer height started increasing away from the coast as depicted in Fig. 13. It is quite possible that 85.8 E onwards off the coast of Visakhapatnam, the marine boundary layer height reached at 750 m or more. The aircraft entered into the marine boundary layer at 750 m level which is detached from the free tropospheric air due to marine boundary layer inversion present between 750 m and 1500 m.
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Fig. 8. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 750 m over east of Chennai during C1EW sortie and west of Port Blair during P3WE sortie.
Fig. 9. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 1500 m over east of Chennai during C1EW sortie and west of Port Blair during P3WE sortie.
The aircraft climbed to an altitude of 1500 m after reaching w87 E and returned back to Visakhapatnam following the same latitude path. The temperature inversion at the top of the marine boundary layer restricted the vertical mixing of marine boundary layer air mass and the lower free tropospheric air mass at and after 86 E. Thus, the two air masses having different chemical characteristics were encountered at two different altitudes after this longitude.
from source region to receptor location, on the basis of hydrocarbon ratios (de Gouw et al., 2001, 2004; Lal et al., 2007). These are commonly known as hydrocarbon clocks. This conventional hydrocarbon clock is based on the chemical loss of hydrocarbons due to OH radicals as given below.
4.3. Photochemical processing and age of air masses The major source of atmospheric CO and acetylene is the incomplete combustion of fuels. These gases are removed from the atmosphere by reaction with OH radicals, with mean atmospheric lifetimes of two months and two weeks respectively. The level of C2H2 decreases faster compared to that of CO with time in an air parcel after emission, leading to lower C2H2/CO ratio. The C2H2/CO ratio will be higher in the air parcel exposed in recent emission than the aged air parcel. Thus, this ratio can be used as an indicator of photochemical processing of air mass. In the present study, this ratio is used for the qualitative estimation of age. Several studies have estimated the quantitative “Age” of the pollution in an air mass, or transport time of the pollutant gases
½At ¼ ½A0 expð kA ½OH tÞ
(1)
Where [A]0 is the concentration of hydrocarbon A at the time of emission, [A]t is the concentration at time t after the emission and kA is the reaction rate coefficient for the reaction of hydrocarbon and OH radical. Using chemical reactions of two hydrocarbons, the photochemical age “t” of an air mass can be calculated from simultaneous measurements of hydrocarbons A and B, having different reactivities towards OH radical.
t ¼
1 ln ½A0 =½B0 ln ½At =½Bt ½OHðkA kB Þ
(2)
This method is based on the assumption that dilution during transport is negligible or proportional to the concentrations within the air parcel. In the present study, the age of the air mass has been estimated using measured mixing ratios of acetylene (A) and ethane (B). These two NMHCs are selected due to their different chemical lifetimes in the atmosphere. The average
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Fig. 10. The spatial and altitude distribution of seven days back-trajectories calculated at altitudes of 750 m and 1500 m over north of Port Blair during P2NS sortie.
lifetime of acetylene is about 14 days and ethane is about 45 days due to photochemical loss by second order reaction with hydroxyl radical (OH). The emissions from the South and Southeast Asia are strongly influenced by biofuel use (wood, crop residue, dung, charcoal etc.) and biomass burning (Streets et al., 2003; Ohara et al., 2007). The initial emission ratio of acetylene to ethane (C2H2/C2H6) is taken as 0.85, which are mainly emitted from biofuel and biomass burning (Lal et al., 2007). The reaction rate coefficients of acetylene and ethane with OH radicals are 9 1013 cm3 s1 and 2.5 1013 cm3 s1 respectively at 298 K (Atkinson et al., 1997). The 24 h averaged concentration value of OH radical used in the calculation is 3 106 molecules cm3, which was calculated using quasi-Lagrangian experiment and model results during INDOEX (de Gouw et al., 2001). The age has been estimated for each air sample using above information. However, C2H2/C2H6 emission ratio from biofuel combustion can vary between 0.52 and 0.90 (Andreae and Merlet, 2001), and OH number density can vary between 2.6 and 4.5 106 molecules cm3 (de Gouw et al., 2001). This can cause an uncertainty of 1.4e1.8 days in the estimation of age of air mass. The age and C2H2/CO ratios of the air parcels sampled during different air sorties at 750 m and 1500 m are shown in Fig. 14 with corresponding latitudes and longitudes. As expected, the age shows opposite variation with C2H2/CO ratio. For Visakhapatnam sortie, the estimated age is shortest (2.9 0.3 days at 750 m and 4.4 0.5 days at 1500 m) and C2H2/CO ratio is highest (4.5 0.3 pptv ppbv1 at 750 m and 3.9 0.8 pptv ppbv1 at 1500 m). This range of age
and C2H2/CO ratio suggest the rapid transport of air parcels exposed to recently emitted pollutants. Except few sampling locations, age was between 4 and 8 days with average level of photochemical processing of air masses sampled during other sorties. 5. Comparison with previous measurements Measurements of surface O3 and its various precursor gases were made over the BoB during the BOBEX II e 2003 (Lal et al., 2007) and ICARB-2006 (Srivastava et al., 2012). The BOBEX campaign was conducted in the months of FebruaryeMarch and ICARB was conducted during MarcheApril months. Due to the nonavailability of airborne measurements over this region, surface level data is used for comparison. However, airborne measurements of these gases were made during INDOEX in 1999 over the northern Indian Ocean. A detailed comparison of the main trace species is made for these campaigns by investigating their correlations with CO. Fig. 15 shows the O3, CH4, ethane and acetylene vs CO for WICARB, ICARB, BOBEX and INDOEX. During each campaign, O3 is observed to be correlated with CO. The correlation is slightly poor for WICARB (r2 ¼ 0.26) with a slope of 0.07 0.01 ppbv ppbv1. They were well correlated during ICARB (r2 ¼ 0.88) and BOBEX (r2 ¼ 0.77) with slopes of 0.31 0.03 ppbv ppbv1 and 0.10 0.01 ppbv ppbv1 respectively. The O3eCO correlation was moderately good during INDOEX within the lower 3 km (r2 ¼ 0.52). The correlation is particularly strong for surface measurements probably due to local
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Fig. 13. Schematic depiction of path of aircraft during bi-level sortie over east of Visakhapatnam (solid blue line). Variation of atmospheric boundary layer and marine boundary layer from coast to open ocean is shown by dashed green line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. The longitudinal variation of O3, CO, CH4, ethane and acetylene over the Bay of Bengal at altitudes of 750 m and 1500 m during V1EW sortie of Visakhapatnam.
photochemical production of O3 in polluted air whereas for the airborne measurements, the correlation was affected probably by the transport and mixing of different air masses. The CH4eCO correlation is indicative of common emission in case of good correlation and represents mixing of air having different chemical characteristics in case of moderate to poor correlations. The degree of correlation was lower during WICARB (r2 ¼ 0.30), moderate during ICARB (r2 ¼ 0.42) and higher during BOBEX (r2 ¼ 0.89). Ethane and acetylene are found to be well correlated with CO during each campaign. These three species have sufficiently long lifetimes (COw 2 months, C2H6 w 45 days and C2H2 w 14 days) to maintain their good correlation in relatively aged air masses also.
Fig. 12. The vertical distributions of potential temperature (K) and relative humidity (%) over different locations near west coast of the Bay of Bengal during 26e28 December 2008. These figures are placed in the increasing order of distance from land (DFL). The arrows indicate the top of marine boundary layer.
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Fig. 14. Variation of C2H2/CO ratio and age of air parcels during east of Visakhapatnam (VSK-E), north-east of Port Blair (PBR-NE), north of Port Blair (PBR-N), west of Port Blair (PBR-W), south of Port Blair (PBR-S) and east of Chennai (CHN-E) air sorties of WICARB at 750 m and 1500 m.
607
The ethane vs CO slope is highest during WICARB (8.5 0.4 pptv ppbv1, r2 ¼ 0.80) followed by BOBEX (7.6 0.3 pptv ppbv1, r2 ¼ 0.97). This slope is 6.5 0.5 pptv ppbv1 (r2 ¼ 0.77) for ICARB and 4.6 0.5 pptv ppbv1 (r2 ¼ 0.75) for INDOEX. Acetylene vs CO shows maximum slope during BOBEX (6.7 0.4 pptv ppbv1, r2 ¼ 0.94) followed by WICARB (4.2 0.2 pptv ppbv1, r2 ¼ 0.71), ICARB (3.6 0.3 pptv ppbv1, r2 ¼ 0.78) and INDOEX (3.0 0.3 pptv ppbv1, r2 ¼ 0.77) respectively. Very good correlations between these species during various campaigns conducted during the past decade indicate their common or collocated sources over the Indian subcontinent. The mixing ratios of these gases are found to be significantly higher during WICARB than values reported for previous surface measurements, even though these measurements were made at higher altitudes. This may be due to boundary layer dynamics and prevailing wind regime during winter season. The atmospheric boundary layer remains shallow during winter over the land resulting in the accumulation of anthropogenic emission in the lower atmospheric volume. This increases the mixing ratios of pollutant gases. During the spring, the boundary layer mixing takes place in relatively larger volume resulting in lower levels of pollutants. As discussed earlier, the BoB region is being loaded by pollutants coming from both Indo-Gangetic Plain and Southeast Asia. All studies based on surface level measurements show the impact of Indian outflow without any influence from Southeast Asia. The airborne study during INDOEX shows lowest trace gas mixing ratios among all the campaigns. The Indian continental outflow had travelled long distance over the BoB before arriving at observational location thus leading to lover values during INDOEX. This data also includes the free troposphere data (up to 3 km).
Fig. 15. Correlation plots of ozone, methane, ethane and acetylene with respect to CO during WICARB (January 2009), ICARB (MarcheApril 2006), BOBEX (February 2003) and INDOEX (FebruaryeMarch 1999). WICARB, ICARB and BOBEX were conducted over the Bay of Bengal and INDOEX was conducted over southern Arabian Sea/northern Indian Ocean.
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6. Summary and conclusions Airborne measurements of O3, CO, CH4 and NMHCs were made for the first time over the BoB at 750 m and 1500 m from Visakhapatnam, Chennai and Port Blair during WICARB in January 2009. The observations indicated the presence of air parcels having different chemical characteristics at two altitudes away from the coastline of Visakhapatnam. This is attributed to the presence of mixed layer inversion between these two altitudes restricting the vertical mixing of marine boundary layer air mass with free tropospheric air. High levels of these gases are observed over east of Visakhapatnam and south of Port Blair. These are attributed to the effects of continental outflow from the Indo-Gangetic Plain and Southeast Asia, respectively. Mixing ratios of these gases are found to be significantly lower over north/north-east/west of Port Blair and east of Chennai. The C2H2/CO ratio and age of air samples are calculated for each sortie. In accordance with back-trajectory analysis, the air masses over east of Visakhapatnam are less chemically processed (high C2H2/CO ratio, lower age of pollution) in comparison to air samples of other sorties. The comparison of mixing ratios of several trace gases between present and previous campaigns shows that outflow of continental air during winter time contains higher concentration of pollutants due to their accumulation in the shallower boundary layer over land. The central BoB region is mainly affected by transport from Southeast Asian countries. This continental region is often influenced by intense biomass burning and produce enormous amount of trace gases. The present measurements show elevated levels of pollutants over the marine region in the global perspective. For example, the cleaner marine regions of southern hemisphere show CO mixing ratio less than 100 ppbv (Novelli et al., 1998). In the northern hemisphere, CO mixing ratios over the marine regions are found to show their maximum (w200e220 ppbv) during winter mainly over the regions affected by continental outflow (Novelli et al., 1998). The mixing ratio of CO more than 150 ppbv over marine region is considered as polluted continental air (Nowak et al., 2004). Present measurements show CO mixing ratios higher than 150 ppbv for almost all samples at 750 m and for about two-third samples at 1500 m. These measurements clearly indicate that outflow from northern India and Southeast Asia pollutes the BoB significantly in the winter season. These observations enhance our understanding of trace gas distributions over a data sparse region in the tropics. These gases collectively affect the atmospheric lifetime of OH radical and its atmospheric abundance. The present observations will be very useful as the input for the atmospheric chemistry transport models and will contribute to reduce the uncertainty in the budget estimation of these gases in this fast developing region. Acknowledgement The work was supported by Physical Research Laboratory (PRL), Ahmedabad and Indian Space Research Organization-Geosphere Biosphere Program (ISRO-GBP). We thank team members of WICARB for their help in successfully conducting this experiment on board the aircraft. 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 making available air parcel back-trajectory model. We are thankful to anonymous reviewers for their constructive comments which improved the quality of this manuscript significantly. References Adams, R.M., Glyer, J.D., Johnson, S.L., McCarl, B.A.,1989. A reassessment of the economic effects of ozone on U. S. agriculture. J. Air Pollut. Contr. Assoc. 39, 960e968.
Andreae, M.O., Merlet, P., 2001. Emission of trace gases and aerosols from biomass burning. Glob. Biogeochem. Cycles 15, 955e966. Asatar, G.I., Nair, P.R., 2010. Spatial distribution of near-surface CO over Bay of Bengal during winter: role of transport. J. Atmos. Solar-Terrestrial Phys. 72, 1241e1250. doi:10.1016/j.jastp.2010.07.025. Atkinson, R., Baulch, D.L., Cox, R.A., Hampson Jr., R.F., Kerr, J.A., Rossi, M.J., Troe, J., 1997. Evaluated kinetic and photochemical data for atmospheric chemistry Supplement VI. J. Phys. Chem. Ref. Data 26, 1329e1499. David, L.M., Girach, I.A., Nair, P.R., 2011. Distribution of ozone and its precursors over Bay of Bengal during winter 2009: role of meteorology. Ann. Geophys. 29, 1613e1627. de Gouw, J.A., Warneke, C., Scheeren, H.A., van der Veen, C., Bolder, M., Scheele, M.P., Williams, J., Wong, S., Lange, L., Fischer, H., Lelieveld, J., 2001. Overview of the trace gas measurements onboard the Citation aircraft during the intensive field phase of INDOEX. J. Geophys. Res. 106, 28453e28467. de Gouw, J.A., et al., 2004. Chemical composition of air masses transported from Asia to the U.S. West Coast during ITCT 2K2: fossil fuel combustion versus biomass-burning signatures. J. Geophys. Res. 109, D23S20. doi:10.1029/ 2003JD004202. Gauss, M., et al., 2003. Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere. J. Geophys. Res. 108, 4292. doi:10.1029/2002JD002624. Jacob, Daniel J., 1999. Introduction to Atmospheric Chemistry. Princeton University Press. Kar, J., Deeter, M.N., Fishman, J., Liu, Z., Omar, A., Creilson, J.K., Trepte, C.R., Vaughan, M.A., Winker, D.M., 2010. Wintertime pollution over the Eastern IndoGangetic Plains as observed from MOPITT, CALIPSO and tropospheric ozoneresidual data. Atmos. Chem. Phys. 10, 12273e12283. Kumar, R., Naja, M., Venkataramani, S., Wild, O., 2010a. Variations in surface ozone at Nainital: a high-altitude site in the central Himalayas. J. Geophys. Res. 115, D16302. doi:10.1029/2009JD013715. Kumar, M., Mallik, C., Kumar, A., Mahanti, N.C., Shekh, A.M., 2010b. Evaluation of the boundary layer depth in semi-arid region of India. Dynam. Atmos. Oceans 49, 96e107. 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. 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. Lal, S., Sahu, L.K., Gupta, S., Srivastava, S., Modh, K.S., Venkataramani, S., Rajesh, T.A., 2008. Emission characteristic of ozone related trace gases at a semi-urban site in the Indo-Gangetic plain using inter-correlations. J. Atmos. Chem. 60, 189e204. Lawrence, M.G., Lelieveld, J., 2010. Atmospheric pollutant 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. Murugavel, P., Gopalakrishnan, V., Pant, V., Kamra, A.K., 2008. Airborne measurements of submicron aerosols acrossthe coastline at Bhubaneswar during ICARB. J. Earth Syst. Sci. 117, 273e280. Naja, M., Akimoto, H., 2004. Contribution of regional pollution and long-range transport to the AsiaePacific region: analysis of long-term ozonesonde data over Japan. J. Geophys. Res. 109, D21306. doi:10.1029/2004JD004687. Novelli, P.C., Masarie, K.A., Lang, P.M., 1998. Distributions and recent changes of carbon monoxide in the lower troposphere. J. Geophys. Res. 103 (D15), 19015e19033. Nowak, J.B., et al., 2004. Gas-phase chemical characteristics of Asian emission plumes observed during ITCT 2K2 over the eastern North Pacific Ocean. J. Geophys. Res. 109, D23S19. doi:10.1029/2003JD004488. Ohara, T., Akimoto, H., Kurokawa, J., Horri, 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. Ramanathan, V., et al., 2001. Indian Ocean experiment: an integrated analysis of the climate forcing and effects of the great IndioeAsian haze. J. Geophys. Res. 106, 28371e28398. 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. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. John Wiley & Sons Inc. Simpson, M.D., Raman, S., 2004. Role of the land plume in the transport of ozone over the ocean during INDOEX (1999). Boundary Layer Meteorology 111, 133e152. Srivastava, S., Lal, S., Venkataramani, S., Gupta, S., Sheel, V., 2012. Surface distributions of O3, CO and hydrocarbons over the Bay of Bengal and the Arabian Sea during pre-monsoon season. Atmos. Environ. 47, 459e467. Streets, D.G., Bond, T.C., Carmichael, G.R., Fernandes, S.D., Fu, Q., He, D., Klimont, Z., Nelson, S.M., Tsai, N.Y., Wang, M.Q., Woo, J., Yarber, K.F., 2003. An inventory of gaseous and primary aerosol emissions in Asia in the year 2000. J. Geophys. Res. 108, 8809. doi:10.1029/2002JD003093. Subrahamanyam, D.B., Anurose, T.J., Kiran Kumar, N.V.P., Mohan, Mannil, Kunhikrishnan, P.K., John, S.R., Prijith, S.S., Dutt, C.B.S., 2012. Spatial and
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Contents lists available at SciVerse ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Airborne measurements of O3, CO, CH4 and NMHCs over the Bay of Bengal during winter Shuchita Srivastava a, *, S. Lal a, S. Venkataramani a, I. Guha a, D. Bala Subrahamanyam b a b
Physical Research Laboratory, Ahmedabad 380009, India Space Physics Laboratory, VSSC, Trivandrum, India
h i g h l i g h t s < Airborne measurements of O3, CO, CH4 and NMHCs over the Bay of Bengal (BoB). < Transport of pollutants from Indo-Gangetic Plain and Southeast Asia over the BoB. < Estimation of photochemical processing and age of air masses using NMHCs. < Photo-chemically fresh pollution plume was observed over the northern BoB.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2011 Received in revised form 23 April 2012 Accepted 25 April 2012
As part of the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (WICARB), airborne measurements of ozone (O3), carbon monoxide (CO), methane (CH4), ethane (C2H6), ethene (C2H4), acetylene (C2H2), propane (C3H8) and n-butane (n-C4H10) were conducted over the Bay of Bengal (BoB) at two altitude levels, 750 m and 1500 m, from Visakhapatnam, Chennai and Port Blair during January 2009. Airborne measurements, first of its kind over the BoB, revealed distinct chemical characteristics at these two altitudes over the eastern sector of Visakhapatnam away from the coastline. This feature is attributed to the presence of marine boundary layer inversion between the two altitudes that inhibited the vertical mixing. A comparative study of trace gas distributions during all air sorties showed enhanced mixing ratios of these gases over east of Visakhapatnam and south of Port Blair in comparison to the north/northeeast and west of Port Blair and east of Chennai. The back-trajectory analysis showed the outflow from the highly polluted Indo-Gangetic Plain over eastern side of Visakhapatnam. The air masses showed lower photochemical processing (higher C2H2/CO ratio w 4e4.5 pptv ppbv1) with the age of w 3e4 days over this location in comparison to other sorties. Chemical characteristics over south of Port Blair seem to be controlled by the outflow from Southeast Asian countries. The results presented in this work would serve as valuable inputs for the atmospheric chemistry transport models and will be helpful in reducing the uncertainty in the budget estimation of trace gases over tropical marine region. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Airborne measurements Ozone Carbon monoxide NMHCs Bay of Bengal Transport
1. Introduction Ozone (O3) is an effective greenhouse gas (Gauss et al., 2003) and a major air pollutant affecting human health and crop production (Adams et al., 1989). Though it is only a trace gas, it controls the oxidizing capacity of the troposphere by producing highly reactive hydroxyl radicals (OH) which define the lifetime of
* Corresponding author. Tel.: þ91 79 26314656; fax: þ91 79 26314659. E-mail addresses: [email protected], [email protected] (S. Srivastava). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2012.04.054
several gases (Jacob, 1999). The level of O3 is controlled by local photochemical production and transport processes in the troposphere. Ozone is photo-chemically produced by carbon monoxide (CO), methane (CH4) and non-methane hydrocarbons (NMHCs) in the presence of NOx and sunlight (Seinfeld and Pandis, 1998). The sources of these precursor gases are predominantly anthropogenic (Jacob, 1999). The highly populated South Asian countries with their rapid industrialization and agricultural growth are major sources of anthropogenically emitted pollutants (Ohara et al., 2007). Emissions from South Asia are strongly influenced by inefficient combustion processes, especially biofuel burning in cook stoves,
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widespread use of two stroke engines etc. (Streets et al., 2003; Ohara et al., 2007). This region contributes a disproportionally large part of the total global CO and NMHCs emissions, approximately 1.5 times larger than its relative contribution in CO2 and NOx (Lawrence and Lelieveld, 2010). A recently developed emission inventory REAS (Regional Emission inventory in ASia) has suggested enhancement in CO emission by 11%, and NMHCs emissions by 33% from 2000 to 2010 over India (Ohara et al., 2007). These gases are precursors of O3 in the troposphere. The study of their distributions especially over the Indian subcontinent is important to understand their roles in chemistry of the tropical troposphere and global climate change. The marine regions surrounding India are impacted by the outflow of pollutants from many South Asian countries (Lelieveld et al., 2001; Ramanathan et al., 2001; de Gouw et al., 2001; Murugavel et al., 2008). The Bay of Bengal (BoB) is bordered by densely populated and anthropogenically polluted central/eastern India and Bangladesh in the north; Myanmar and Thailand, having intense biomass burning during most of the year, in the east; and relatively less polluted southern India in the west. The prevailing meteorological conditions transport continental air masses from various directions, and containing varying quantities of pollutants, over the BoB. Different transport patterns over this region make it a unique environment to study the spatial heterogeneity in the distribution of various primary and secondary pollutant gases due to outflow of continental air masses. Measurements of various trace gases were made over the BoB during several cruise campaigns namely Bay Of Bengal EXperiments (BOBEX e I in 2001 and BOBEX e II in 2003) and Bay Of Bengal Process Studies (BOBPS e 2002) to study the influence of continental outflow over this region (Sahu et al., 2006; Lal et al., 2006, 2007). The BOBEX-I was conducted over the BoB, the northern Indian Ocean and the Arabian Sea in winter and early spring of 2001 to study the surface distributions of O3, CO and CH4 (Lal et al., 2006). Dedicated cruise campaigns namely BOBPS and BOBEX-II were conducted to cover extensive areas of the BoB to study the surface distributions of O3, CO, CH4 and various NMHCs under different wind regimes (Sahu et al., 2006; Lal et al., 2007). However, airborne measurements of these trace gases were never made over this oceanic region primarily due to lack of dedicated platform. With a view to investigate the spatial variability of trace gases in the lower troposphere over the BoB, aircraft-based trace gas measurements were made for the first time over this region during the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (WICARB), January, 2009. During this period, the boundary layer over the surrounding continent often remains shallow which favours the advection of pollutants over the BoB. The objective of the present work is to characterize the continental outflow of pollutants and to study the spatial heterogeneity in the distribution of trace gases from the coastal to cleaner oceanic regions. 2. Methods The air-segment of WICARB was conducted using Beechcraft Super King Air B-200 (VT-EBB) of National Remote Sensing Centre (NRSC), Hyderabad, India during the month of January 2009. The aircraft was operated in unpressurized mode with a ceiling altitude of 3 km. A total of 6 bi-level air sorties were conducted from Visakhapatnam (17.72 N, 83.22 E), Chennai (12.99 N, 80.17 E) and Port Blair (11.64 N, 92.73 E). The spatial coverage of these sorties is shown in Fig. 1(a). The bi-level sorties were made at 750 m and 1500 m in order to study the gradients in the mixing ratios of these species from coast to the open ocean. A typical bi-level sortie path profile is shown in Fig. 1(b) and details of all the sorties are given in Table 1.
Fig. 1. (a) The flight paths of air sorties (dashed lines) conducted from Visakhapatnam (VSK), Chennai (CHN) and Port Blair (PBR) during WICARB (January, 2009). The details of each sortie are given in Table 1. (b) Typical altitude variation of aircraft path for bilevel sorties (Path shown for Visakhapatnam). The sampling locations are also shown by circles on the paths.
The ambient air was aspirated via SS inlet lines fitted under the nose of the aircraft. In-situ measurement of O3 was made with a time resolution of 10 s using UV absorption based Dual Beam 2B O3 analyzer (Model 205). The calibration of this system was performed before and after the campaign using O3 calibrator (Sonimix 6000 C2). The air samples were also collected using an oil free compressor (Metal Bellow, USA) in glass sampling bottles of 300e800 ml volume. The Hewlett Packard 5890 series II GC equipped with a flame ionization detector and a PLOT capillary column (50 m, 0.32 mm with KCl/Al2O3 as stationary phase) was used to measure the NMHCs in the air samples. The air samples were preconcentrated in the sampling loop using cryo-trapping procedure and injected into the column with helium as carrier gas. The temperature of the column was varied from 0 C to 200 C to separate different light NMHCs. The reproducibility of NMHCs measurements were found to be between 3% and 10% (Sahu et al., 2006). The Varian Vista 6000 GC, with a flame ionization detector 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 isothermal condition at 70 C during the analysis. CO is converted to CH4 by a methanizer containing Nickel catalyst at 325 C. The reproducibility of CO and CH4 measurements were found to be better than 3% and 1% respectively (Sahu et al., 2006). The key features in the distributions of trace gases are examined in the light of back-trajectories. We have calculated kinematic
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Table 1 Details of air sorties conducted during WICARB 2009. Locations
Date
Direction of sorties (bi-level)
Name of sortie
Flight take off time (IST, hh:mm)
Total flight duration (hours)
Visakhapatnam (17.72 N, 83.22 E) Port Blair (11.64 N, 92.73 E) Port Blair Port Blair Port Blair Chennai (12.99 N, 80.17 E)
05-01-2009 09-01-2009 09-01-2009 13-01-2009 14-01-2009 16-01-2009
EasteWest (Zonal) EasteNorth (Zonal-Meridional) NortheSouth (Meridional) WesteEast (Zonal) SoutheNorth (Meridional) EasteWest (Zonal)
V1EW P1EN P2NS P3WE P4SN C1EW
10:15 06:35 10:56 06:10 06:20 10:19
2.13 2.68 1.66 2.41 2.32 2.40
3. Results
1500 m and 3000 m above the mean sea level respectively) during January 1e20, 2009 over the Indian subcontinent and surrounding marine region as obtained from NCEP reanalysis. Broadly, wind patterns over this region were similar at 1000 mb, 925 mb and 850 mb. The winds were northeasterly over the BoB and wind speed is found to be higher over this region. Winds were calm over the Indian subcontinent and north-east sector of the BoB. The northwesterly winds prevailed over the northern Indian region at 700 mb. Advection of the Indian continental air is evident at the northern part of BoB which then got transported to the south India.
3.1. Synoptic meteorology
3.2. Spatial distribution of O3 and its precursors over the BoB
Fig. 2 shows the average wind fields at 1000 mb, 925 mb, 850 mb and 700 mb (approximately corresponding to surface, 750 m,
Spatial distribution of O3 and its precursors are examined for each bi-level sortie at 750 m and 1500 m over the BoB. The sorties
back-trajectories using METeorological data EXplorer (METEX) trajectory model developed at the National Institute of Environmental Studies, Japan and Global Environmental Forum, Japan (Zeng et al., 2010). The METEX uses the National Centres for Environmental Prediction (NCEP) reanalysis data having spatial resolution of 2.5 2.5 at 17 different pressure levels archived at every 6 h. This trajectory model is used in several research works (Naja and Akimoto, 2004; Kumar et al., 2010a).
Fig. 2. The synoptic wind fields (m/s) at 1000 mb, 925 mb, 850 mb and 700 mb pressure levels averaged for 1e20 January, 2009 over the Indian subcontinent and surrounding marine regions.
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from Port Blair, which were conducted on the same day covered the northern part of Andaman & Nicobar Islands and hence considered as a single sortie. Fig. 3 shows the spatial distributions of O3, CO, CH4, ethane (C2H6), ethene (C2H4), acetylene (C2H2), propane (C3H8) and n-butane (n-C4H10) over the BoB at 750 m altitude. Ozone mixing ratios are found to be maximum (54 2 ppbv) over east of Visakhapatnam which is about 10 ppbv higher than south of Port Blair (43 2 ppbv) and 15e20 ppbv higher than east of Chennai (35 1 ppbv), west of Port Blair (39 3 ppbv) and north/north-east of Port Blair (37 7 ppbv). Carbon monoxide and most of other hydrocarbons showed enhanced mixing ratios over south of Port Blair followed by east of Visakhapatnam away from the coast. Moderately high mixing ratios are evident over west of Port Blair. The levels of all these gases are found to be comparatively lower over east of Chennai and north/north-east of Port Blair. Ethene showed a highly variable distribution with randomly high and low values during each sortie but maximum mixing ratios are found to be present over south of Port Blair like other gases. The mixing ratio of n-butane is found below the detection limit of the instrument over several sampling locations of east of Visakhapatnam and north/north-east of Port Blair. Fig. 4 shows the spatial distributions of O3, CO, CH4 and NMHCs over each sortie location at 1500 m altitude. The trace gas distributions are found to be similar to their variations at 750 m altitude, but with relatively lower mixing ratios. The O3 mixing ratios are found similar to their 750 m values in respective sorties except over south of Port Blair where it decreased by w5 ppbv from its values at 750 m altitude. The average CH4 mixing ratios are found to be almost constant for each sortie. The average mixing ratios of these gases are given in Table 2 for each sortie at 750 m and 1500 m. These observations indicate advection of pollutants over east of Visakhapatnam and south of Port Blair having different chemical characteristics. The heterogeneity in the spatial distributions of these gases suggests distinct sources of pollutants over these two regions. Measurements of surface O3 and CO were also made during the ship segment of WICARB over the BoB (Asatar and Nair, 2010; David et al., 2011). Surface O3 mixing ratios also showed highest
levels (>60 ppbv) over the northern BoB followed by the southeast BoB (40e60 ppbv). However, surface CO mixing ratios showed 290 ppbv over the north BoB and 379 ppbv over the southeast BoB. The lowest CO values of 167 ppbv were measured over the south-west BoB. Variations in surface level O3 and CO are similar to those from aircraft measurements at 750 m and 1500 m. 3.3. Relationship between trace gases Variations of O3, CH4, ethane and acetylene with respect to CO during different sorties at 750 m and 1500 m levels (Fig. 5) show different photochemical regimes sampled in different regions (Nowak et al., 2004). Highest O3 and acetylene values are observed near Visakhapatnam whereas ethane peaks at south of Port Blair. Samples collected during Chennai and northenortheast of Port Blair showed minimum values of these species. Ethane and acetylene show systematic increase with CO. However, CH4 and O3 show relatively poor correlation with it. The lower O3 mixing ratio corresponding to high CO over south of Port Blair could be due to early hour flight (0620 h IST, 0700 h Local Time) when the photochemical production of O3 is less due to in-sufficient sunlight. The flight time for the Visakhapatnam sortie was 1015 h IST when sunlight was sufficient for photochemical O3 production. The flight times for sorties near Chennai and north of Port Blair were also about 1015e1100 h IST but the mixing ratios of precursors were lower for the in-situ O3 production. 4. Discussion 4.1. Investigation of back-trajectories The seven days back-trajectories are calculated for the observation locations at the time of flights with 0.1 spatial resolution for each sortie at both the altitude levels. The spatial and altitude variations of trajectories arriving at 750 m and 1500 m over the observation locations of the eastward sortie of Visakhapatnam and the southward sortie of Port Blair are shown in Figs. 6 and 7. The trajectories ending at the observation locations over east of
Fig. 3. The spatial distributions of O3, CO, CH4, C2H6, C2H4, C2H2, C3H8 and n-C4H10 over the Bay of Bengal at 750 m altitude during various air sorties of WICARB.
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Fig. 4. The spatial distributions of O3, CO, CH4, C2H6, C2H4, C2H2, C3H8 and n-C4H10 over the Bay of Bengal at 1500 m altitude during various air sorties of WICARB.
Visakhapatnam originated from Pakistan and central north Indian region. These trajectories passed over the states of Delhi, Uttar Pradesh, Bihar and West Bengal, which are in the Indo-Gangetic Plain, before ending at the observation points. This region is highly populated and hence source of many anthropogenic pollutants (Tripathi et al., 2006; Lal et al., 2008; Kar et al., 2010). The atmospheric boundary layer altitude remains shallow over these areas during January (Maximum convective boundary layer depth w2 km; Kumar et al., 2010b) which leads to the accumulation of pollutants within this region. The altitudes of most of the back-trajectories were within the lower 2 km in the past 120 h when they were passing through this region. Thus, these air masses brought the pollutants, accumulated within the boundary layer over the Indo-Gangetic Plain, leading to their high mixing ratios over east of Visakhapatnam. The other group of backtrajectories reaching south of Port Blair is also shown in the same figure. These trajectories passed through the regions of Southeast Asia, mainly Myanmar and Thailand, before arriving at
the observation locations. The altitudes of these trajectories were within the lower 4 km during most of their travel time. At 750 m, their altitudes were mostly within 2 km for the final few days. Higher trace gas mixing ratios at receptor location indicate that these trajectories picked up sufficient pollution from the continental locations. Seven days back-trajectories arriving at 750 m and 1500 m over east of Chennai and west of Port Blair are shown in Figs. 8 and 9. The trajectories over east of Chennai originated from eastern India and Bangladesh and arrived over observational points after spending w4 days over the cleaner marine region within the lower 2 km. This indicates the dilution of continental pollution thus justifying the lower levels of trace gases over this region. The levels of trace gases were higher over the west of Port Blair at 750 m due to extended stay of air parcel over Southeast Asia within the lower 2 km. The mixing ratios were significantly lower at 1500 m when the altitude of trajectories was higher than 2 km over the continent.
Table 2 Average mixing ratios (with 1 s variability) of trace gases at altitudes of 750 m and 1500 m during different sorties of WICARB.
Altitude 750 m O3 (ppbv) CO (ppbv) CH4 (ppmv) Ethane (pptv) Ethene (pptv) Acetylene (pptv) Propane (pptv) n-Butane (pptv) Altitude 1500 m O3 (ppbv) CO (ppbv) CH4 (ppmv) Ethane (pptv) Ethene (pptv) Acetylene (pptv) Propane (pptv) n-Butane (pptv)
Vishakhapatnam (East)
Port Blair (North/NortheEast)
Port Blair (West)
Port Blair (South)
Chennai (East)
54 2 256 19 1.88 0.03 2210 218 274 100 1159 121 187 102 77 21
37 7 161 18 1.84 0.03 1382 180 254 75 501 45 117 60 45 31
39 3 229 18 1.87 0.01 2174 270 276 99 847 130 187 98 69 17
43 2 256 9 1.88 0.01 2578 70 400 140 792 34 418 194 134 28
35 1 166 17.3 1.85 0.01 1682 229 202 86 585 90 82 55 43 19
52 2 187 37 1.84 0.03 1792 216 282 122 726 129 155 30 e
36 8 133 27 1.84 0.03 1285 229 257 133 401 167 108 96 e
40 3 158 39 1.83 0.02 1589 370 175 88 509 176 121 70 37 20
38 2 211 19 1.84 0.03 2178 221 190 64 554 77 238 150 88 26
35 2 155 23 1.84 0.01 1539 216 166 81 533 116 69 18 30 15
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Fig. 5. Variation of O3, CH4, ethane, and acetylene with respect to CO at altitudes of 750 m and 1500 m during east of Visakhapatnam (VSK-E), North/North-east of Port Blair (PBR-N-NE), West of Port Blair (PBR-W), South of Port Blair (PBR-S) and East of Chennai (CHN-E) air sorties of WICARB.
Fig. 10 shows the back-trajectories arriving at north of Port Blair at 750 m and 1500 m during P2NS sortie. These trajectories are found to be coming from Southeast Asian region at 750 m. At 1500 m altitude, trajectories originated over the northern BoB and advected via Southeast Asian region before their arrival over observational location. These trajectories are found to be descending from the height range of 4e6 km. The lower values at receptor location indicate that these trajectories do not have sufficient exposure to highly polluted continental region during their travel time. 4.2. Role of marine boundary layer inversion near Visakhapatnam The V1EW sortie conducted from Visakhapatnam towards east over the northern BoB at altitudes of 750 m and 1500 m provides longitudinal variation of different trace gases. The aircraft covered the longitudinal span from 83.2 E to 86.7 E at a constant latitude path (17.72 N) away from Visakhapatnam. The mixing ratios of O3 are averaged for every 0.1 longitude bins and are shown with 1s
variation at both the altitude levels in Fig. 11. The average mixing ratios of O3 were found to be in the range of 50 5 ppbv near the coast at 750 m which increased at a rate of 2.0 ppbv deg1 and attained the maximum O3 level (60 2 ppbv) at 86.7 E. The reverse variation is evident at 1500 m altitude, where O3 decreased with increasing longitude at a rate of 1.5 ppbv deg1 and reached its minimum mixing ratio at 86.7 E (43 4 ppbv). In a broad sense, the magnitudes of O3 mixing ratios at 750 m were found to be quite similar to that at 1500 m along the longitudinal sector from 83.2 E to 85.8 E, beyond which there is a clear-cut distinction between the two. Fig. 11 also shows longitudinal variations of CO, CH4, ethane and acetylene at 750 m and 1500 m altitudes. Similar to O3 variation, these gases also show clear increase at 750 m and decrease at 1500 m on and after 86 E. The average concentrations of these species were found to be higher at 750 m than at 1500 m. The average differences of CO, CH4, ethane and acetylene between 750 m and 1500 m levels were found to be 41 24 ppbv, 0.017 0.01 ppmv, 198 215 pptv and 312 83 pptv respectively in the
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Fig. 6. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 750 m over east of Visakhapatnam during V1EW sortie and south of Port Blair during P4SN sortie.
Fig. 7. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 1500 m over east of Visakhapatnam during V1EW sortie and south of Port Blair during P4SN sortie.
samples collected before 86 E (towards west) and 133 12 ppbv, 0.09 0.01 ppmv, 931 284 pptv and 713 250 pptv in the samples collected on and after 86 E (towards east). Overall, differences are large (2e5 times higher with respect to samples collected near coastal area) between two altitudes collected over open ocean (east of 86 E). This suggests that aircraft encountered air masses of different chemical compositions at two different altitudes at and after 86 E. These observations are investigated in detail in the light of results found by Simpson and Raman (2004) over the Arabian Sea during the INDOEX 99 cruise. They showed a schematic of marine boundary layer height variation and development of land plume over the Arabian Sea near the coast in a typical day time condition during JanuaryeMarch 1999. The marine boundary layer height grows with increasing distance away from the coast due to convection and entrainment. As per their observations, marine boundary layer height varied from 500 m near the coast to 1000 m over the open ocean. As part of the ocean segment of WICARB field experiment, balloon-borne GPS sondes were launched on a regular basis from Oceanic Research Vessel Sagar Kanya over the BoB (Subrahamanyam et al., 2012). In the present study, we used the vertical profiles of thermodynamic parameters obtained from four
marine locations 13.09 Ne80.29 E (distance from land w12 km), 13.86 Ne80.94 E (distance from land w92 km), 14.84 Ne81.47 E (distance from land w151 km) and 15.11 N-83.01 E (distance from land w320 km) during 26e28 December 2008 as collocated measurements from aircraft and ship were not available. Fig. 12 shows the vertical distributions of potential temperature and relative humidity over these locations in the increasing order of longitudinal distance from the coast. These profiles clearly show the mixed layer top at 400 m, 500 m, 600 m and 1100 m when the ship was at a distance of 12 km, 92 km, 151 km and 320 km respectively from the coast. We reproduce the schematic given by Simpson and Raman (2004) modified according to present campaign to explain the variations of trace gases observed near Visakhapatnam. The aircraft started from Visakhapatnam at an altitude of 750 m near the coast where the marine boundary layer height remained shallow (w500 m). The marine boundary layer height started increasing away from the coast as depicted in Fig. 13. It is quite possible that 85.8 E onwards off the coast of Visakhapatnam, the marine boundary layer height reached at 750 m or more. The aircraft entered into the marine boundary layer at 750 m level which is detached from the free tropospheric air due to marine boundary layer inversion present between 750 m and 1500 m.
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Fig. 8. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 750 m over east of Chennai during C1EW sortie and west of Port Blair during P3WE sortie.
Fig. 9. The spatial and altitude distribution of seven days back-trajectories calculated at altitude of 1500 m over east of Chennai during C1EW sortie and west of Port Blair during P3WE sortie.
The aircraft climbed to an altitude of 1500 m after reaching w87 E and returned back to Visakhapatnam following the same latitude path. The temperature inversion at the top of the marine boundary layer restricted the vertical mixing of marine boundary layer air mass and the lower free tropospheric air mass at and after 86 E. Thus, the two air masses having different chemical characteristics were encountered at two different altitudes after this longitude.
from source region to receptor location, on the basis of hydrocarbon ratios (de Gouw et al., 2001, 2004; Lal et al., 2007). These are commonly known as hydrocarbon clocks. This conventional hydrocarbon clock is based on the chemical loss of hydrocarbons due to OH radicals as given below.
4.3. Photochemical processing and age of air masses The major source of atmospheric CO and acetylene is the incomplete combustion of fuels. These gases are removed from the atmosphere by reaction with OH radicals, with mean atmospheric lifetimes of two months and two weeks respectively. The level of C2H2 decreases faster compared to that of CO with time in an air parcel after emission, leading to lower C2H2/CO ratio. The C2H2/CO ratio will be higher in the air parcel exposed in recent emission than the aged air parcel. Thus, this ratio can be used as an indicator of photochemical processing of air mass. In the present study, this ratio is used for the qualitative estimation of age. Several studies have estimated the quantitative “Age” of the pollution in an air mass, or transport time of the pollutant gases
½At ¼ ½A0 expð kA ½OH tÞ
(1)
Where [A]0 is the concentration of hydrocarbon A at the time of emission, [A]t is the concentration at time t after the emission and kA is the reaction rate coefficient for the reaction of hydrocarbon and OH radical. Using chemical reactions of two hydrocarbons, the photochemical age “t” of an air mass can be calculated from simultaneous measurements of hydrocarbons A and B, having different reactivities towards OH radical.
t ¼
1 ln ½A0 =½B0 ln ½At =½Bt ½OHðkA kB Þ
(2)
This method is based on the assumption that dilution during transport is negligible or proportional to the concentrations within the air parcel. In the present study, the age of the air mass has been estimated using measured mixing ratios of acetylene (A) and ethane (B). These two NMHCs are selected due to their different chemical lifetimes in the atmosphere. The average
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Fig. 10. The spatial and altitude distribution of seven days back-trajectories calculated at altitudes of 750 m and 1500 m over north of Port Blair during P2NS sortie.
lifetime of acetylene is about 14 days and ethane is about 45 days due to photochemical loss by second order reaction with hydroxyl radical (OH). The emissions from the South and Southeast Asia are strongly influenced by biofuel use (wood, crop residue, dung, charcoal etc.) and biomass burning (Streets et al., 2003; Ohara et al., 2007). The initial emission ratio of acetylene to ethane (C2H2/C2H6) is taken as 0.85, which are mainly emitted from biofuel and biomass burning (Lal et al., 2007). The reaction rate coefficients of acetylene and ethane with OH radicals are 9 1013 cm3 s1 and 2.5 1013 cm3 s1 respectively at 298 K (Atkinson et al., 1997). The 24 h averaged concentration value of OH radical used in the calculation is 3 106 molecules cm3, which was calculated using quasi-Lagrangian experiment and model results during INDOEX (de Gouw et al., 2001). The age has been estimated for each air sample using above information. However, C2H2/C2H6 emission ratio from biofuel combustion can vary between 0.52 and 0.90 (Andreae and Merlet, 2001), and OH number density can vary between 2.6 and 4.5 106 molecules cm3 (de Gouw et al., 2001). This can cause an uncertainty of 1.4e1.8 days in the estimation of age of air mass. The age and C2H2/CO ratios of the air parcels sampled during different air sorties at 750 m and 1500 m are shown in Fig. 14 with corresponding latitudes and longitudes. As expected, the age shows opposite variation with C2H2/CO ratio. For Visakhapatnam sortie, the estimated age is shortest (2.9 0.3 days at 750 m and 4.4 0.5 days at 1500 m) and C2H2/CO ratio is highest (4.5 0.3 pptv ppbv1 at 750 m and 3.9 0.8 pptv ppbv1 at 1500 m). This range of age
and C2H2/CO ratio suggest the rapid transport of air parcels exposed to recently emitted pollutants. Except few sampling locations, age was between 4 and 8 days with average level of photochemical processing of air masses sampled during other sorties. 5. Comparison with previous measurements Measurements of surface O3 and its various precursor gases were made over the BoB during the BOBEX II e 2003 (Lal et al., 2007) and ICARB-2006 (Srivastava et al., 2012). The BOBEX campaign was conducted in the months of FebruaryeMarch and ICARB was conducted during MarcheApril months. Due to the nonavailability of airborne measurements over this region, surface level data is used for comparison. However, airborne measurements of these gases were made during INDOEX in 1999 over the northern Indian Ocean. A detailed comparison of the main trace species is made for these campaigns by investigating their correlations with CO. Fig. 15 shows the O3, CH4, ethane and acetylene vs CO for WICARB, ICARB, BOBEX and INDOEX. During each campaign, O3 is observed to be correlated with CO. The correlation is slightly poor for WICARB (r2 ¼ 0.26) with a slope of 0.07 0.01 ppbv ppbv1. They were well correlated during ICARB (r2 ¼ 0.88) and BOBEX (r2 ¼ 0.77) with slopes of 0.31 0.03 ppbv ppbv1 and 0.10 0.01 ppbv ppbv1 respectively. The O3eCO correlation was moderately good during INDOEX within the lower 3 km (r2 ¼ 0.52). The correlation is particularly strong for surface measurements probably due to local
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Fig. 13. Schematic depiction of path of aircraft during bi-level sortie over east of Visakhapatnam (solid blue line). Variation of atmospheric boundary layer and marine boundary layer from coast to open ocean is shown by dashed green line. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 11. The longitudinal variation of O3, CO, CH4, ethane and acetylene over the Bay of Bengal at altitudes of 750 m and 1500 m during V1EW sortie of Visakhapatnam.
photochemical production of O3 in polluted air whereas for the airborne measurements, the correlation was affected probably by the transport and mixing of different air masses. The CH4eCO correlation is indicative of common emission in case of good correlation and represents mixing of air having different chemical characteristics in case of moderate to poor correlations. The degree of correlation was lower during WICARB (r2 ¼ 0.30), moderate during ICARB (r2 ¼ 0.42) and higher during BOBEX (r2 ¼ 0.89). Ethane and acetylene are found to be well correlated with CO during each campaign. These three species have sufficiently long lifetimes (COw 2 months, C2H6 w 45 days and C2H2 w 14 days) to maintain their good correlation in relatively aged air masses also.
Fig. 12. The vertical distributions of potential temperature (K) and relative humidity (%) over different locations near west coast of the Bay of Bengal during 26e28 December 2008. These figures are placed in the increasing order of distance from land (DFL). The arrows indicate the top of marine boundary layer.
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Fig. 14. Variation of C2H2/CO ratio and age of air parcels during east of Visakhapatnam (VSK-E), north-east of Port Blair (PBR-NE), north of Port Blair (PBR-N), west of Port Blair (PBR-W), south of Port Blair (PBR-S) and east of Chennai (CHN-E) air sorties of WICARB at 750 m and 1500 m.
607
The ethane vs CO slope is highest during WICARB (8.5 0.4 pptv ppbv1, r2 ¼ 0.80) followed by BOBEX (7.6 0.3 pptv ppbv1, r2 ¼ 0.97). This slope is 6.5 0.5 pptv ppbv1 (r2 ¼ 0.77) for ICARB and 4.6 0.5 pptv ppbv1 (r2 ¼ 0.75) for INDOEX. Acetylene vs CO shows maximum slope during BOBEX (6.7 0.4 pptv ppbv1, r2 ¼ 0.94) followed by WICARB (4.2 0.2 pptv ppbv1, r2 ¼ 0.71), ICARB (3.6 0.3 pptv ppbv1, r2 ¼ 0.78) and INDOEX (3.0 0.3 pptv ppbv1, r2 ¼ 0.77) respectively. Very good correlations between these species during various campaigns conducted during the past decade indicate their common or collocated sources over the Indian subcontinent. The mixing ratios of these gases are found to be significantly higher during WICARB than values reported for previous surface measurements, even though these measurements were made at higher altitudes. This may be due to boundary layer dynamics and prevailing wind regime during winter season. The atmospheric boundary layer remains shallow during winter over the land resulting in the accumulation of anthropogenic emission in the lower atmospheric volume. This increases the mixing ratios of pollutant gases. During the spring, the boundary layer mixing takes place in relatively larger volume resulting in lower levels of pollutants. As discussed earlier, the BoB region is being loaded by pollutants coming from both Indo-Gangetic Plain and Southeast Asia. All studies based on surface level measurements show the impact of Indian outflow without any influence from Southeast Asia. The airborne study during INDOEX shows lowest trace gas mixing ratios among all the campaigns. The Indian continental outflow had travelled long distance over the BoB before arriving at observational location thus leading to lover values during INDOEX. This data also includes the free troposphere data (up to 3 km).
Fig. 15. Correlation plots of ozone, methane, ethane and acetylene with respect to CO during WICARB (January 2009), ICARB (MarcheApril 2006), BOBEX (February 2003) and INDOEX (FebruaryeMarch 1999). WICARB, ICARB and BOBEX were conducted over the Bay of Bengal and INDOEX was conducted over southern Arabian Sea/northern Indian Ocean.
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6. Summary and conclusions Airborne measurements of O3, CO, CH4 and NMHCs were made for the first time over the BoB at 750 m and 1500 m from Visakhapatnam, Chennai and Port Blair during WICARB in January 2009. The observations indicated the presence of air parcels having different chemical characteristics at two altitudes away from the coastline of Visakhapatnam. This is attributed to the presence of mixed layer inversion between these two altitudes restricting the vertical mixing of marine boundary layer air mass with free tropospheric air. High levels of these gases are observed over east of Visakhapatnam and south of Port Blair. These are attributed to the effects of continental outflow from the Indo-Gangetic Plain and Southeast Asia, respectively. Mixing ratios of these gases are found to be significantly lower over north/north-east/west of Port Blair and east of Chennai. The C2H2/CO ratio and age of air samples are calculated for each sortie. In accordance with back-trajectory analysis, the air masses over east of Visakhapatnam are less chemically processed (high C2H2/CO ratio, lower age of pollution) in comparison to air samples of other sorties. The comparison of mixing ratios of several trace gases between present and previous campaigns shows that outflow of continental air during winter time contains higher concentration of pollutants due to their accumulation in the shallower boundary layer over land. The central BoB region is mainly affected by transport from Southeast Asian countries. This continental region is often influenced by intense biomass burning and produce enormous amount of trace gases. The present measurements show elevated levels of pollutants over the marine region in the global perspective. For example, the cleaner marine regions of southern hemisphere show CO mixing ratio less than 100 ppbv (Novelli et al., 1998). In the northern hemisphere, CO mixing ratios over the marine regions are found to show their maximum (w200e220 ppbv) during winter mainly over the regions affected by continental outflow (Novelli et al., 1998). The mixing ratio of CO more than 150 ppbv over marine region is considered as polluted continental air (Nowak et al., 2004). Present measurements show CO mixing ratios higher than 150 ppbv for almost all samples at 750 m and for about two-third samples at 1500 m. These measurements clearly indicate that outflow from northern India and Southeast Asia pollutes the BoB significantly in the winter season. These observations enhance our understanding of trace gas distributions over a data sparse region in the tropics. These gases collectively affect the atmospheric lifetime of OH radical and its atmospheric abundance. The present observations will be very useful as the input for the atmospheric chemistry transport models and will contribute to reduce the uncertainty in the budget estimation of these gases in this fast developing region. Acknowledgement The work was supported by Physical Research Laboratory (PRL), Ahmedabad and Indian Space Research Organization-Geosphere Biosphere Program (ISRO-GBP). We thank team members of WICARB for their help in successfully conducting this experiment on board the aircraft. 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 making available air parcel back-trajectory model. We are thankful to anonymous reviewers for their constructive comments which improved the quality of this manuscript significantly. References Adams, R.M., Glyer, J.D., Johnson, S.L., McCarl, B.A.,1989. A reassessment of the economic effects of ozone on U. S. agriculture. J. Air Pollut. Contr. Assoc. 39, 960e968.
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