Measurement of ambient NH3 over Bay of Bengal during W ICARB Campaign

Concentrations of ambient NH 3, NO, NO2 and SO2 were measured over Bay of Bengal (BoB) during 28 December 2008 to 25 January 2009 to study their diurnal variation and relationship of NH 3 with other trace gases over BoB. The measurements were done under the winter phase of Integrated Campaign on Aerosols and Radiation Budget (WICARB). For the first time, ambient NH 3 was monitored precisely over BoB based on chemiluminescence method, having estimation efficiency more precise than the chemical trap method. The average concentration of ambient NH3, NO, NO2 and SO2 were recorded as 4.78 ± 1.68, 1.89± 1.26, 0.31± 0.14 and 0.80± 0.30 μg m−3, respectively, over BoB. The prominent latitudinal and longitudinal variations of the trace gases were observed over BoB, whereas NH3 and NO showed the non-significant diurnal variation. Results reveal that the concentration of ambient NH3 negatively correlated with ambient NO 2 (r2 =−0.56), SO2 (r2 =−0.58) and ambient temperature ( 2 =−0.27) during the study.


Introduction
In the atmosphere and ocean, NH 3 and its ionized form NH + 4 are ubiquitous.Naturally and anthropogenically produced NH x (NH 3 + NH + 4 ) are transported through the atmosphere and generally their concentrations in air decrease as the distance from land increases.It has been suggested that in preindustrial times, the oceans were probably a net source of NH x of the continents (Duce et al., 1991), but this is not the case today (Sutton et al., 1995(Sutton et al., , 2000)).NH x is produced in surface water by the biological reduction of nitrate (ei-ther directly or via the degradation of biologically synthesized organic nitrogenous material/agricultural run-off).In a solution, NH x is partitioned between NH + 4 and NH 3 according to equilibrium thermodynamics: the proportion of NH x that occurs as NH 3 (depending on pH, temperature and ionic strength of the medium) is available for emission to the atmosphere (Aneja et al., 2001).NH 3 is also emitted to the atmosphere by plants, animals and its environments, by soil micro-organisms and by various industrial and agricultural processes, including the direct volatilization of solid NH 4 NO 3 salts and fertilizers (Sutton et al., 2000;Li et al., 2006;Sharma et al., 2010a, b).There is also evidence of volcanic source of NH x to the atmosphere (Uematsu et al., 2004) and of substantial NH 3 emissions from seabird colonies (Blackall et al., 2007;Theobald et al., 2006) Norman and Leck have reported NH 3 of the order of 0.05-0.2nmol m −3 (0.0085-0.0034 µg m −3 ), whereas, Gibb and Mantoura (1999) and Gibb et al. (1999) have reported 10-20 nmol m −3 (0.17-0.34 µg m −3 ) in the central Indian Ocean and 2.5-5.6 nmol m −3 (0.043-0.095 µg m −3 ) over coastal Arabian Sea and 0.4-1.8nmol m −3 (0.007-0.031 µg m −3 ) over remote Arabian Sea.Schafer et al. (1993) have reported the concentration of NH 3 gas over BoB of the order of 14.29-29.29 nmol m −3 (0.243-0.500 µg m −3 ).Johnson et al. (2008a, b) has reviewed the measurement of ambient NH 3 gases along with sea water NH x (NH 3 + NH + 4 ) over Pacific, Atlantic and Indian Ocean.Table 1 summaries the results on concentrations of ambient NH 3 over various oceans reported by different researchers.
Ambient NH 3 plays an important role not only in the formation of secondary aerosols while combining with atmospheric acid gases (sulfuric acid, nitric acid and hydrochloric acid) but also contributes to adverse health effects, mainly respiratory diseases (Warneck, 1988) and climate change.
Published by Copernicus Publications on behalf of the European Geosciences Union.This study is the first time ambient NH 3 has been monitored precisely over BoB based on chemiluminescence method along with the other trace gases NO, NO 2 and SO 2 onboard Sagar Kanya (a research vessel).The meteorological parameters (temperature, sea surface temperature (SST), relative humidity (RH), wind direction and wind speed) were also recorded over BoB during the campaign to correlate with trace gases.The main objective of this campaign was to study the diurnal variation of ambient NH 3 and other trace gases over BoB and its interaction.The cruise stationed at 3 places for 18 h to take time series/diurnal variation measurements on 5, 8 and 23 January 2009.Sampling inlets of all analyzers were connected to the sampling system, which was placed in the opposite direction of the cruise ship plume to reduce the self-contamination; the portable weather station was also mounted at same height (about 11 m from sea surface) and same location on the deck of the ship.These instruments operated continuously for the entire period of the campaign (28 December 2008 to 25 January 2009).±0.050 ppb of all the ranges).The response time of these analyzers are <1 s with a signal noise of 1 % of measured value and the estimation efficiency is >90 %.The zero air calibrations of these analyzers were done using a pure air generator (model: PAG-003, M/s.ECO Physics AG, Switzerland) having air pollutant elimination capacity <0.010 ppb.The analyzers were calibrated and validated using NIST-USA traceable certified NO gas (250 ppb ± 2.5 %, M/s Spectra Gases Inc., USA).The analyzers showed less than ±1 % error rate during calibration.Zero and span calibrations (before and after the measurement) of these analyzers were performed for a week to obtain reproducible values.Ambient SO 2 was measured continuously using a calibrated SO 2 -analyzer (model: APSA 360A, M/s.Horiba Ltd, Japan) at the same location.
The instrument was calibrated daily using in-built calibrator for zero and span.Meteorological parameters such as temperature (accuracy:  The gas analysers were used to record the ambient NH 3 , NO, NO 2 and SO 2 concentration at 1 min intervals throughout the study period.The concentrations of all the above trace gases were converted into µg m −3 from ppb by respective factors for uniform representation of the results.Statistical analysis of all the data sets collected during the study period was performed using standard recommended methods.

Results and discussion
The concentration of trace gases, i.e.NH 3 , NO, NO and SO 2 , were measured over BoB during W ICARB campaign.The average values of trace gases with day and night average values are summarized in Table 2.The average concentration of ambient NH 3 was recorded as 4.78 ± 1.68 µg m −3 with a day/night ratio of 1.03, whereas average concentration of NO was recorded 0.31 ± 0.14 µg m −3 with a day/night ratio of 0.79.
Figure 2 shows the average diurnal variation of NH , NO, NO 2 and SO 2 along with ambient temperature and RH over  BoB.In the present observations, non-significant diurnal varitions of ambient NH 3 and NO (Fig. 2) were observed, whereas NO 2 and SO 2 showed significant diurnal variations.The average daytime concentration of NO 2 was recorded as 0.27±0.12µg m −3 , whereas average nighttime NO 2 concentration recorded was as 0.34 ± 0.16 µg m −3 .Nighttime incerase in NO 2 concentration may be attributed to conversion of NO to NO 2 with the reaction of O 3 as well as lowering of the boundary layer during winter.Figure 3   the present study is almost one order higher than the earlier reported values over the central Indian Ocean as well as BoB (Norman and Leck, 2005;Schafer et al., 1993) (Table 1).
A strong positive west-east (zonal) gradient (0.184) was observed in ambient NH 3 concentration (6.0-9.0 µg m −3 ) over the area close to the west coast of BoB as compared to other parts of the BoB (Fig. 3).It is to be noted that agricultural activities (Sharma et al., 2010a, b), livestock, biomass burning and transport may contribute to the emission of large amounts of NH 3 (Sutton et al., 1995(Sutton et al., , 2000)).Khemani et al. (1987) reported the concentration of NH 3 in coastal region of BoB of the order of 1.4-2.9µg m −3 .Carmichael et al. (2003) also reported high NH 3 concentrations (6.4-7.1 µg m −3 ) at two sites (Bhubneswar and Berhampur) of the western coast of BoB.West-east positive gradient observed in the western coast of BoB could be due to transport of NH 3 locally, since we do not have any record of dissolved NH 3 in seawater and NH + 4 in the sea water.It is difficult to comment on comparative quantification of biogenic oceanic source of NH 3 due to phytoplankton and sea birds as compared to anthropogenic activities.Norman and Leck (2005) reported distribution of NH 3 with the range of 1.1-3.2nmol m −3 (0.019-0.054 µg m −3 ) in the marine boundary layer over Atlantic and South Indian Ocean during Cruise99.They also connected peak value of NH 3 to biomass combustion and dust sources on the African continent.Compared to these data, it may be concluded that NH 3 concentrations over BoB are quite high.Since the lifetime of ambient NH 3 gas is short (1-5 d), particularly in the humid oceanic atmosphere, conversion to particulate NH + 4 is supposed to be very fast (30 % h −1 ).In the middle region of BoB and southern part of BoB, ambient NH 3 is of the order of 1-2 µg m −3 .Observation of nitric oxide (NO) over BoB is not reported so far except over the Indian Ocean (Naja et al., 1999;Rhoads et al., 1997).The reported value over the Arabian Sea and North Indian Ocean varies in the range of 0.06-0.19µg m −3 .In the present study, average concentration of NO was of the order of 1.89 µg m −3 with day/night ratio 0.98, one order higher than the reported value over the Arabian Sea.Spatial distribution of NO shows two peaks in the southern part of BoB with low values in the coastal region and middle of BoB.The peak value of NO resembles the peak of SST and RH.Due to its short lifetime, source of NO in the southern BoB is obviously marine rather than continental transport.The conversion from gaseous NO to particulate NO − 3 is unlikely in the southern BoB.Observation of in-situ NO 2 over the Indian Ocean, particularly BoB, is very limited and only supported by satellite observations (Kunhikrishnan et al., 2004;Franke et al., 2009).They showed that the central Indian Ocean in the Southern Hemisphere is not always as pristine as observed earlier during the winter monsoon period, but is polluted during the monsoon transition periods by pollution plumes from Africa and Southeast Asia.Generally, the most polluted region is the BoB, which is influenced by Indian and south-east Asian outflow during most of the year and China during part of the year.In the present study, average concentration of NO 2 was recorded as 0.31 µg m −3 with a day/night ratio 0.79.Concentration of NO 2 is very low and very often it is observed to be below the lower detection limit (LDL ± 0.04 µg m −3 ) of the instrument (NO x -analyzer; model: CLD88p; M/s.ECO Physics AG) at few locations over BoB.Correlations of these trace gases with meteorological parameters are summarized in Table 3.
Similarly, measurement of SO 2 over the Indian Ocean is also very limited (Reiner et al., 2001;Shon et al., 2001;  They have reported an elevated layer with concentration >1.9 µg m −3 just above 2000 m altitude.In the present study, average concentration of SO 2 was recorded as 0.80 µg m −3 with a day/night ratio of 1.15.Since its lifetime is a few days, transport of continental SO 2 to BoB, which is surrounded by densely polluted areas, changes the concentration, depending on the wind pattern.In the eastern coastal region, gaseous SO 2 shows low value and it seems that the gas-phase of SO 2 has been immediately converted to SO 2− 4 in the region.Spatial distribution of sea surface temperature (SST), ambient temperature and relative humidity (RH) are plotted over BoB (Fig. 4).North-to-south positive gradient in ambient temperature and SST is noticed with peak value in the southern of BoB, whereas spatial distribution of RH shows zonal gradient.Influences of ambient temperature and RH on aerosol could be identified in nitrate formation (Seinfeld and Pandis, 2006).During daytime with increasing UV radiation, NO rapidly converts into NO 2 in the presence of O 1 D and atmospheric NO 2 reacts with hydroxyl radical (OH) to form the nitric acid (HNO 3 ).However, SO 2 also reacts with the hydroxyl radical (OH) to form sulphuric acid (H 2 SO 4 ).During nighttime, NO − 3 is the source of the HNO 3 in the atmosphere.NO − 3 reacts either with the NO 2 to form N 2 O 5 , which reacts with OH to form HNO 3 , or NO − 3 directly reacts with water vapor to form HNO 3 during nighttime.The reaction of HNO 3 or H 2 SO 4 with NH 3 is reversible and forms NH 4 NO 3 and (NH 4 ) 2 SO 4 , respectively.

Conclusion
In the present study, the concentration of ambient NH 3 , NO, NO 2 and SO 2 were measured over BoB.The prominent latitudinal and longitudinal variations of the trace gases were observed over BoB, whereas NH 3 and NO showed the nonsignificant diurnal variation.The average concentration of S. K. Sharma et al.: Measurement of ambient NH 3 over BoB during W ICARB Campaign ambient NH 3 , NO, NO 2 and SO 2 were recorded as 4.78 ± 1.78, 1.89 ± 1.26, 0.31 ± 0.14 and 0.80 ± 0.30 µg m −3 , respectively, with a range of 0.23-14.34,0.21-9.80,0.10-1.00and 0.35-3.53µg m −3 , respectively.Coastal region of BoB reported large concentrations of NH 3 , NO, NO 2 and SO 2 , suggesting the role of continental influence in addition to biogenic sources, particularly near Kolkata and Chennai regions.Presence of higher concentrations of SO 2 than expected in the south of BoB suggest biogenic sources, particularly the role of DMS (dimethyl sulfide).Since we do not have measurement of DMS or its precursor MSA (methanesulfonic acid), it is difficult to conclude for certain.
track of W ICARB Measurement of trace gases, i.e.NH 3 , NO, NO 2 , and SO 2 , were made over BoB under the W ICARB campaign from 28 December 2008 to 25 January 2009 onboard Sagar Kanya (SK-254) cruise (a research vessel) as per its scheduled track given in Fig. 1.The cruise tracks covered the region ranging from 21 • N to 3.5 • N latitude and 76.3 • E to 98 • E longitudes over BoB.The cruise embarked on 27 December 2008 from Chennai and ended at Kochi (9.96 • N, 76.3 • E), India, on 31 January 2009.The ship halt times are also indicated in the figure by open circles.

Table 3 .
Correlation matrix of ambient NH 3 , NO, NO 2 and SO 2 with meteorological parameters over BoB.