Stable Isotopic and Chemical Characteristics of Bulk Aerosols during Winter and Summer Season at a Station in Western Coast of India ( Goa )

We measured stable isotopic and chemical characteristics of bulk aerosols collected at a coastal station in western India (Goa) between December 2009 and January 2011, to characterize lower tropospheric atmospheric conditions and their influence on particle chemistry during winter and summer seasons. Marked differences were observed in terms of sources and chemical compositions of bulk aerosols. The δNTN values of winter aerosols (10.8 ± 2.2‰, n = 10) indicate biomass burning contributions in the carbonaceous fraction, while significantly depleted δNTN values of summer aerosols (6.2 ± 2.3‰, n = 12) hints incorporation of marine N species. The δSTS showed depleted values during winter (5.0 ± 1.0‰, n = 10), which closely matched with those of typical urban polluted environments, while summer aerosols showed a systematic enrichment of δSTS (up to ~14‰ with average value 9.0 ± 2.8‰, n = 13); possibly due to incorporation of volatile dimethyl sulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) emitted from the adjacent Arabian Sea. Likewise, δCTOC values showed ~2‰ enrichment in winter aerosols (–24.8 ± 0.4‰, n = 10) with respect to those of summer values, indicating presence of bio-fuel and coal burning contributions in carbonaceous fraction of winter aerosols. We also measured major ions (Na, K, Mg, Ca, NH4, Cl, Br, NO3, SO4) in water soluble fraction of aerosols to understand winter/summer changes in the atmospheric chemistry over this coastal area. This is the first ever dataset on triple isotopic characteristics of bulk aerosols at a coastal location of India showing signatures of continental bio-mass/biofuel burning influences during winter, whereas marine inventories (e.g., sea salt, DMS and mineral dust) appear to dominate chemical composition of summer aerosols.


INTRODUCTION
Total suspended air particulate matter (TSPM) also termed as bulk aerosols generally comprise of a multi-phase complex mixture of all airborne solids and low vapor pressure liquid particles with aerodynamic particle sizes ranging from 0.01 to 100 μm or more (USEPA, 1999).To investigate chemical cycling of major biogenic elements over a coastal marine realm, a comprehensive chemical characterization of TSPM is mandatory.In this regard, stable isotopic compositions of major non-dust constituents (viz.Carbon, Nitrogen and Sulfur) of atmospheric bulk aerosols can greatly help to characterize source(s) and they can also provide clues to degree of secondary atmospheric processing (Norman et al., 1999;Agnihotri et al., 2011;Cao et al., 2013).Thus, a comprehensive chemical and stable isotopic characterization of carbonaceous (non-dust) portion of aerosols has recently been realized to be a key in providing clues to lower tropospheric chemistry and secondary processing of primary aerosols.This, in turn, can greatly help in better assessment of regional radiative forcing, air quality, human health and regional climate.Atmospheric particles are regarded as transitional geological repositories having short life span in the atmosphere (~2-5 days; Rastogi and Sarin, 2008).They integrate imprints of all the emissions from local as well as remotely located source regions.Before their eventual removal from atmosphere via dry/wet deposition, their short atmospheric existence is thought to be capable of modifying regional climatic conditions considerably through their direct and indirect effects (IPCC, 2007).In coastal areas such as Goa, atmospheric particles are of additional importance as their deposition supplements biologically important substances to coastal surface waters; hence can boost surface biological productivity during lean periods catalyzing CO 2 sequestration (Srinivas et al., 2011).In addition, anthropogenic aerosols have recently been linked to a plausible decrease in monsoonal precipitation in south Asia (Bollasinaet al., 2011).Carbon, Nitrogen and Sulfur are major constituents of aerosol mass (PÖschal, 2005), hence their isotopic signatures (δ 13 C, δ 15 N and δ 34 S) can provide important clues about primary sources and secondary processes occurring within the atmosphere during their long range transport (Cachier et al., 1986;Turekianet al., 1998;Norman et al., 1999;Martinelli et al., 2002, Kelly et al., 2005;Widory, 2007).Recent studies made on Indian aerosols also demonstrated the utility of the stable isotopes together with mass concentrations of Carbon and Nitrogen to understand complex and heterogeneous lower atmospheric chemistry that influences the nature of ambient airborne particles (e.g., Agnihotri et al., 2011;Pavuluri et al., 2010Pavuluri et al., , 2011;;Aggarwal et al., 2013).
Though coastal regions of western India, such as Goa, do not experience much seasonal changes in ambient temperatures, they do experience reversal of monsoonal winds and major shift in terms of continental and marine air inventory (http://www.goaweather.co.in/).In order to capture these winter and summer changes in winds and the associated processes that govern the composition of atmospheric particles, we measured chemical characteristic and stable isotopic anomalies of bulk aerosols collected during December 2009 to January 2011 from the roof of National Institute of Oceanography, Dona Paula Goa.Major measured parameters are mass concentrations of total carbon (TC), organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) along with their isotopic ratios (viz.δ 13 C TC , δ 13 C TOC , δ 15 N TN , δ 34 S TS ).In addition we also measured major ions (Na + , K + , Mg ++ , Ca ++ , NH 4 + , Cl -, NO 3 -, SO 4 -2 ) in water soluble fractions of bulk aerosols.This study based on total of 22 bulk aerosol samples collected mainly during summer and winter of year 2010 and was aimed to characterize influence of continental and marine air inventory on airborne particle chemistry over this coastal location and may thus serve as a template for future more focused investigations dealing with lower tropospheric ocean-atmospheric interactions.

STUDY AREA, SAMPLING DETAILS AND LOCAL CLIMATOLOGY
Goa is the smallest state in terms of area and the fourth smallest in terms of population.It is located on the west coast of India and is situated on the slopes of the Western Ghats.Goa is bounded by the state of Maharashtra in the north, Karnataka in the east and south, and by the Arabian Sea in the west.Goa is mainly a tourist place with ~100 km long coastline.
A total of 22 bulk atmospheric particles were collected using high volume sampler (Environtech, APM 430) installed on 7 th floor roof (15.46°N, 73.8°E; at a height ~55.8 m MASL) of CSIR-National Institute of Oceanography (NIO) (Fig. 1) between December 2009 and January 2011.Aerosols were collected on pre-desiccated and pre-weighed Tissuequartz filters (Pall Scientific ®) of 10 × 8 inch size.The filters containing aerosols particles were carefully stored in a desiccator under controlled humidity conditions.The samples were collected for 6 to 24 hour periods with flow rates ranging from 1 to 1.4 m 3 /min with an average of 1.3 m 3 /min.Filters were weighed after the sample collection and total airborne material collected was thus estimated gravimetrically.
Contemporaneous meteorological data were obtained from an automatic weather station (AWS) MASL (installed on top of the National Institute of Oceanography building at ~48 m; Mehra et al., 2005).AWS records the meteorological data at 10 s intervals over a window of 10 min, which are averaged to generate daily time series data.Average meteorological conditions of the present study period (December 2009-January 2011) are shown in Fig. 2. Relative humidity in Goa varied in a narrow range i.e., ~74 ± 4% during summer/premonsoon seasons but ~62 ± 11% in winter.As mentioned earlier Goa being a coastal location does not experience wide variations in the ambient temperature as reflected by ~29.6 ± 1°C and ~25.7 ± 1.5°C for summer and winter months respectively.In contrast, wind speeds may be significantly variable between monsoon and non-monsoon season.However, as far as sampling days are concerned wind speeds at the location during summer and winter period varied in narrow ranges ~1.02 ± 1 and 1.34 ± 0.6 m/s respectively though wind direction showed remarkable seasonal differences (Fig. 2).In winter, winds blew mainly from continental regions of India i.e., north-northwest, north, east-northeast, and east-southeast directions.In contrast, summer (pre-monsoon and monsoon) months experienced winds laden with marine air mainly from west-northwest, west, west-southwest, and southwest directions.Meteorological parameters besides those shown in the Fig. 2, average atmospheric pressure was found to vary in a narrow range 1004.6 ± 2.3 mb and average incident solar radiation during summer and winter periods varied in ranges of ~3976.6 ± 500 and 4176.8 ± 1110 mW/cm 2 respectively.Aerosol sampling was mainly conducted on dry days except for three days (21 st , 27 th May and 12 th July 2010) when some rain showers occurred.

EXPERIMENTAL METHODOLOGY
Detailed methodology for sample analyses by using used stable isotope mass-spectrometer has been presented in Agnihotri et al. (2014).Briefly, sub-samples of filter samples were cut using circular Hole-puncher of ~1 cm diameter.For measurements of TC, TN and TS contents and their isotopic ratios (δ 13 C, δ 15 N, δ 34 S) usually two aliquots were placed in pre-clean Tin cups and made into round pellets.The pellets thus made were combusted in an Elemental analyzer (Pyrocube) coupled with stable Isotope ratio mass spectrometer (Isoprime 100) in combustion column (containing WO 3 as catalyst) at 1150°C.Evolved gases were then passed via a reduction column containing activated (reduced) Cu at 850°C for converting all NO x to N 2 .
Dry helium (5.5 grade) was used as carrier gas for sample introduction.Gas streams were then passed through a watertrap filled with sicapent or magnesium perchlorate (MgClO 4 ) ensuring complete removal of moisture.Analyte gas mixture comprising N 2 , CO 2 , and SO 2 is separated using Purge & Trap separation controlled by thermal programmed desorption (see Agnihotri et al., 2014 for deatils).In this method, the first analyte gas is N 2 that is directly analyzed after its purification in reduction column and moisture trap, while CO 2 and SO 2 are held in two separate columns in-built in the Elemental analyzer.By providing suitable thermal desorptions CO 2 and SO 2 are then eluted respectively in a quantitative manner from the purge columns and analyzed for their isotopic composition measurement on IRMS in the order of their elution.The purge columns for CO 2 and SO 2 are normally kept at temperatures 20-25°C (room temperature) and 55°C respectively for adsorption.When N 2 analysis is over, CO 2 is allowed to be desorbed by raising the temperature of the purge column to 240°C.Similarly after CO 2 analysis is over, SO 2 is allowed to be desorbed from purge column by raising the temperature to 220°C.All the gas streams were passed through a moisture trap containing Sicapent (or Mg per-chrolorate).Detailed methodology for C, N, S isotopic analyses have been discussed in Agnihotri et al. (2014).For TOC measurements one or two aliquots were placed in pre-cleaned tin cups and made wet with 25 μL of ultra pure water followed by once or twice with 25 μL of 10% (by volume) supra-pure HCL till effervescence disappeared, dried overnight and made into pellets for analysis.The pellets were combusted in the same way and analyzed for C and δ 13 C TOC .
Isotopic anomalies are expressed in terms of ratio of less abundant (heavier) atoms with respect to naturally more abundant (lighter) atoms and they are expressed using Delta (δ) notation.For example, isotopes anomalies of carbon, nitrogen and sulfur are expressed as δ 13 C, δ 15 N and δ 34 S respectively and defined as - (1) R = 13 C/ 12 C, 15 N/ 14 N and 34 S/ 32 S Hence δ values are simply ratios; permil notation (‰) is just often used for isotopic composition for that, δ is multiplied with 1000.i.e., deviation expressed in per thousand.δ 13 C values of all the carbon containing substances are expressed with respect to V-PDB (Vienna-Peedee Belemnite).Likewise, δ 15 N values are expressed with respect to atmospheric N 2 , which is assumed to be of 0‰ value.The international standard for Sulfur isotope measurements is Vienna-Cation Diablo Troilite (V-CDT) (Norman et al., 1999).
Calibrations of the elemental and isotopic data were accomplished using several laboratory and international IAEA standards such as: 3-Amino-n-Caproic acid (C 6 H 15 NO 2 ; ACA; Agnihotri et al., 2011), L-Glutamic acid and Sulfanilamide, IAEA-S1 and IAEA-S2.Reported δ 13 C and δ 15 N values of ACA and Glutamic acid are 4.6‰ and -25.3‰, and -5.98 and -14.62, respectively.TC, TN and TS contents were quantified using calibrations made from known amounts of ACA, Glutamic acid and Sulfanilamide standards.Sample blanks (empty tin cups, unexposed quartz filter punches) were run along with samples.N isotopic data as well as concentrations were corrected for blank contribution as described in Agnihotri et al. (2014).TC and TS concentrations were also corrected for blanks while no blanks corrections were made on isotopic values of δ 13 C and δ 34 S. Overall estimated uncertainties of C, N and S isotopic measurements are better than 0.2, 0.2 and 0.3‰, respectively (as described in detail in Agnihotri et al., 2014).
Circular punches of ~10 cm 2 area of each aerosol filters were cut into fine pieces and dissolved in 50 mL of deionized (Milli-Q) water using an ultrasonicator for 30 minutes, for extracting water soluble fraction of airborne particulate matter.Around 30 mL of extracted (filtered) solution was transferred to clean Teflon tubes for Ion chromatographic analyses.The target ions (F -, Br -, Cl -, SO4 2-, NO 3 -, PO 4 3-, NH 4 + , Na + , K + , Ca 2+ and Mg 2+ ) were analyzed by Ion Chromotragraph, 850 Professional IC with a cation exchange column, Metrosep C4-150/4.0and an anion exchange column, Metrosep A Supp 5 250/4.0 with a conductivity detector.The eluent used for anion was 3.2 mM/L Na 2 CO 3 / 1 mM/L NaHCO 3 and 100 mM/L H 2 SO 4 at a flow rate of 0.7 mL/min and for cation 2 mM/L HNO 3 with the regenerate flow rate of 0.9 mL/min was used (Kumar et al., 2010).Two blank filter punches were also treated in same as sample filter punches and concentration for each ionic species was corrected for average blank concentration.Six samples were taken as duplicates i.e., two circular punches of same sample filter were cut and processed in same way as samples and duplicate waters extracts were used for measurement of all ions to check the reproducibility of the measured ionic concentrations.Reproducibities of all the reported ions viz.cations (Na + , NH 4 + , K + , Mg +2 , Ca +2 ) and anions (Cl -, NO 3 -, SO 4 -2 ) were found to be better than 10%.

RESULTS
Location of aerosol collection from the roof of NIO is shown in Fig. 1.Contemporaneous meteorological data obtained from AWS installed at NIO and averaged for the day of sampling are presented in the Fig. 2. We also made wind rose plot depicting prevalent wind directions on days of sampling (Fig. 2; upper panel) which indicates winds at the sampling location are blowing mainly from sea side (south, southwest) during summer while from the continental side during winter (north, northeast and east).Chemical composition of bulk aerosols and their stable isotopic characteristics are discussed in terms of changes observed during winter and summer months.Table 1 presents summary of winter and summer statistics of measured chemical and isotopic parameters of bulk aerosols.Observed variabilites in mass concentrations of TC, TOC, TN and TS are discussed in section.4.1; while variations in stable isotopic characteristics are discussed in section 4.2.

Variations in TC, TOC, TN and TS Mass Concentrations
Average TC, TOC, TN and TS contents of bulk aerosols over Goa were found to be 18.80 ± 12.05, 9.87 ± 8.46, 3.35 ± 2.56, 5.70 ± 2.53 μg/m 3 , respectively.Significantly higher contents of all these carbonaceous and sulfurous (non-dust) components were found in winter aerosols as compared to those in summer aerosols (Table 1; Fig. 3).This observation is consistent with expected climatology of the north India where organic carbonaceous components tend to dominate aerosol chemistry during winter (Rengarajan et al., 2007;Sudheer and Sarin, 2008;Gustafssonet al., 2009;Ram and Sarin, 2010).TSPM were also measured gravimetrically in 15 samples (out of 22) as in few cases some filter side portions got little torn and damaged during recovery from air-sampler.The average TSPM for the 15 samples was 80.78 ± 29.46 μg/m 3 .Figs. 3(a)-3(c) shows variations in TC, TOC, TN and TS contents.The TC and TOC concentrations can be observed to be almost overlapping during winter months, while they were significantly different during summer (TOC being lower than TC).This most likely indicate present of inorganic carbon from mineral dust, which is well supported by significantly higher concentrations of Ca and Mg in water soluble portions (Fig. 4(c)).Average TC/TN ratios were found to be 6.0 ± 1.5 where TC/TS ratios varied with an average 3.4 ± 1.9.

Variations in Stable Isotopic Ratios
Stable isotopic characteristics of C, N and S components are shown in Figs.3(d)-3(f).As observed in case of TN concentrations, δ 15 N TN of bulk aerosols also showed significantly lower values during summer (pre-monsoon) period with an average value of 6.2 ± 2.3‰ (Table 1).This can be interpreted in terms of mixing of N species with lighter N isotopic values emanating from the adjacent Arabian Sea.The emission of various isotopically lighter N species is well known during the water column denitrification processes (Naqvi et al., 2006 and references therein).During winter, average δ 15 N TN of bulk aerosols was 10.8 ± 2.2‰, which is close to the values expected from emissions from biomass burning of C3 type of vegetation (Fig. 6).
Overall δ 13 C TC and δ 13 C TOC of aerosol particles over Goa exhibited variations in narrow ranges: -24.8 ± 1.1‰ and -25.7 ± 0.9‰, respectively.However, significant differences (as high as ~2.3‰) between δ 13 C TC and δ 13 C TOC were observed during the pre-monsoon period (Fig. 3(e)).The average δ 13 C TC and δ 13 C TOC values of Goa aerosols can be seen as an integrated emission signature of carbon emissions from biomass-biofuel-and coal-burning activities (see Fig. 6).In fact, enrichment of ~2‰ found in δ 13 C TOC values during winter (Table 1) can be interpreted in terms of mixing of effluents of coal burning activities (typical end-member δ 13 C value of coal burning as ~22‰; Agnihotri et al. (2011)).
δ 34 S TS of bulk aerosols over Goa show an almost reverse trend as observed in δ 15 N TN (Fig. 3), i.e., significantly heavier Date of collection (mm/dd/yyyy) values during summer (pre-monsoon) period (9 ± 2.8‰; Table 1) compare winter counterparts (5 ± 1‰; Table 1).Very little δ 34 S data of atmospheric aerosols exist worldwide and for Indian region.To the best of our knowledge, this is the first report containing S isotopic information on Indian aerosols.Nonetheless, from available literature containing S isotopic composition data from other regions and oceanic realms (Norman et al., 1999;Oduro et al., 2012;Amrani et al., 2013), we showed typical end member values in the Fig. 6 and attempted to provide interpretation for observed δ 34 S values of bulk aerosols over Goa.
δ 15 N TN showed positive correlation with TN mass concentration (Fig. 5(a)), indicating that δ 15 N TN values of bulk aerosols are mainly controlled by primary sources contributing to total nitrogen in aerosols (biomass burning emissions during winter and marine emissions of N species containing lighter N during summer).In contrast, δ 13 C TC did not show any such relationship with TC mass concentrations, however δ 13 C TOC -TOC plot (Fig. 5(d)) showed two distinct groups enclosed in ellipsoid and rectangular boxes.Data in the ellipsoid represent a group of aerosols with low TOC mass concentrations (mainly during summer (pre-monsoon) period) for which δ 13 C TOC was ~-26.5‰,whereas that in rectangular box identify the group with a δ 13 C TOC of -25.0 ± 0.5‰ and widely varied TOC mass concentrations (Fig. 5(d)).δ 34 S values show an inverse relationship with TS concentrations (Fig. 5(g)) with higher TS concentration and lower δ 34 S values during winter and lower concentration of  TS but higher δ 34 S values during summer (Table 1).

Variations in Ionic Concentrations
In addition to CNS contents and their isotopic values, we also measured major ions (Na + , K + , Mg ++ , Ca ++ , NH 4 + , Cl -, NO 3 -, SO 4 -2 ) in water extracts of bulk aerosol samples.Using concentration of Na + and Cl -we estimated salt content of aerosol samples by using formulae as given below (Kumar et al., 2008). (2) where [ ] represent concentrations in μg/m 3 .Variations in estimated salt content is shown in Fig. 4(a), which shows sea salt in bulk aerosols of this coastal location acts as diluent for major non-dust components i.e., sum of C, N and S contents.[NO 3 -] were found to be relatively higher during summer (pre-monsoon) compared to those abundant in winter aerosols, whereas [SO 4 -2 ] in aerosols were found to be only marginally higher in winter months (Fig. 4(b)).The expected decrease in [SO 4  -2 ] in bulk aerosols during summer (pre-monsoon) period appears to be compensated by prevailing marine winds bringing SO 4 -2 derived from sea salt and DMS/H 2 S from the adjacent Arabian Sea.Isotopic enrichment of S isotopes (Fig. 3(f)) supports this contention.

Influence of Meteorological Conditions on Chemical and Isotopic Characteristics of Bulk Aerosols over Goa
Goa is typically a tropical coastal state where air temperatures vary marginally (maximum up to 4-5°C between summer and winter months), however relative humidity is always on relatively higher side except during peak winter months.Average ambient meteorological conditions as measured by AWS during the days of sampling are presented in the Fig. 2. Higher wind speeds were experienced only during summer (pre-monsoon) period; with higher relative humidity of ~75 ± 5 and ambient temperatures of ~29 ± 1°C (Fig. 2).Upper panel of Fig. 2 shows prevalent wind direction at the sampling site during days of sampling in the form of wind-rose plot, which clearly reveals that wind directions appear to play a larger role in determining chemical and isotopic characteristics of ambient aerosols over Goa.During winter when winds are predominantly from north-northeastern parts of continental India, higher amounts of carbonaceous fraction with higher proportion of TOC contents were observed in ambient aerosols.In contrast, during summer (pre-monsoon) period winds are mainly from south-southeast (marine air), higher proportion inorganic carbon (most likely in form of mineral dust containing higher Ca and Mg; Fig. 4(c)) can be inferred from the differences between TC & TOC contents and δ 13 C TC & δ 13 C TOC isotopic values (Figs. 3(b) and 3(e)).The influence of seasonally changing winds (from continental and marine realms) is clearly captured in N and S isotopic compositions of bulk aerosols over Goa (Figs. 3(d) and 3(f)).

DISCUSSION
The average TSP concentration of 80.8 ± 29.5 μg/m 3 (n = 15) in the bulk aerosol samples are within the range of annual average TSP concentration (109.2 ± 36.8) measured by the GSPCB at the nearby Mormugoa Port during April 2010 to March 2011 (source: http://goaspcb.gov.in/wpcontent/uploads/2012/06/Ambient-Air-Quality.pdf).The significant difference between the TC and TOC mass concentrations occurring during pre-monsoon period is due to the contribution of inorganic carbon supplied by incoming mineral dust (well supported by enhanced concentrations of Ca and Mg; Fig. 4(c)).In winter, TOC is clearly dominating the TC contents in carbonaceous aerosols (Fig. 3(b)).
Despite experiencing almost a complete reversal of winds between winter and summer at the coastal station at Goa, δ 13 C TC varied in a very narrow range of -24.8 ± 1.1‰.This value is very close to the average δ 13 C TC (-25.0 ± 0.6‰) observed in aerosols of Chennai, a coastal metropolitan city in south India (Pavuluriet al., 2011).The average δ 13 C TOC in aerosols of winter (-24.8 ± 0.35‰) is enriched over that of summer value (average: -26.4 ± 0.4‰).These observations are consistent with those of Aggarwal et al. (2013), who reported δ 13 C of TC in the range of (-27.0 to -25.4‰) with a slightly lower value in summer (-26.5 ± 0.3‰) than in late-winter (-25.9 ± 0.3‰) for aerosols over Mumbai, a coastal metropolitan city situated at ~700 km to the north of Goa along the west coast.Average δ 13 C TOC in bulk aerosols of winter in Goa agreed closely with that of δ 13 C TOC in fine mode (PM 2.5 ) aerosols over Pudong, China (-24.5 ± 0.8‰; Cao et al., 2013).The δ 13 C TC and δ 13 C TOC values of bulk aerosols over Goa are also comparable to those found in urban locations of Tokyo (-25.0 ± 0.8‰) and Mexico City (-25.1‰),where fossil fuel combustion is a predominant source of carbonaceous material.
As δ 13 C of TC is mainly controlled by source composition in the tropics (Turekianet al., 1998) and its particle formation and transport (Cachieret al., 1985), we attempted to understand dominant sources contributing to aerosol carbon.Cachier et al. (1986) found the δ 13 C associated with seasalt droplets is -21 ± 2‰.The typical C-3 and C-4 type biomass combustion generally emits particles with mean δ 13 C values ~-26 and -12‰, respectively (Martinelliet al., 2002).The δ 13 C TOC measured in this study for winter months (-24.8 ± 0.4; Table 1) appear to be an intermediate values of particles emitted from burning of (i) C-3 type biomass (with δ 13 C ~-27‰) (ii) fossil fuel (diesel) (~-26‰) and coal (average ~-22‰) (see Fig. 6 for end-member source isotopic compositions).The δ 13 C TOC values of bulk aerosols over Goa during summer (pre-monsoon) tending towards ~-26.4‰,they can be interpreted mainly owing to Carbon emissions from local fossil fuel burning activities as particles emitted during diesel and petrol combustion tend towards values ~-26.5‰ and -26.0‰ respectively.Despite high sea salt content and presence of inorganic carbon, total carbon or organic carbon concentrations are much lower compared to winter months.
As far as δ 34 S of air particulate matter is concerned, we anticipate S isotopic anomaly (i.e., δ 34 S) also represents source of sulfur, just like the case for δ 13 C for carbon.With available literature we attempted to provide δ 34 S variability in different types of source emissions (Fig. 6).Using this template for interpretation, it can be seen that winter δ 34 S values (~5 ± 1; Table 1) closely match with those typically dominated with sulfur emissions from urban pollution (Norman et al., 1999).However, in pre-monsoon (summer) season (especially since month of May) δ 34 S show a clear increasing trend from ~6 to ~14‰.As Goa is a typical costal locale experiencing strong biogeochemical in the coastal waters just before arrival of monsoon, enhancement of δ 34 S shows most likely tracks changes in the sulfur inventory.Recently Amrani et al. (2013) and Oduro et al. (2012) have shown oceanic emissions of volatile dimethyl sulfide (DMS) and its precursor, dimethylsulfoniopropionate (DMSP) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics and may, in turn, affect climate.Amrani et al. (2013) reported δ 34 S of DMS and DMSP ranging between +18.9 and +20.3‰, remarkable consistent across the globe.Earlier δ 34 S values of SO 4 -2 have been reported between +15 and +19‰; marine biogenic ~+17.5‰ and sea salt SO 4 -2 as +21‰ (Normal et al., 1999;Fig. 6).Hence oceanic DMS appear to major source of sulfur in atmospheric particulate matter over Goa during pre-monsoon season.In the Arabian Sea, Shenoy and Kumar (2007) have found significantly higher concentrations of DMS in the upwelling regime of the west coast of India during southwest monsoon and fall inter-monsoon seasons.Shenoy and Patil (2003) also noticed high emission of DMS from the Zuari estuary of Goa, during SW monsoon (peak discharge period) associated with river discharge and dinoflagellate blooms.All these studies collectively indicate high efflux of DMS from the marine realm appear to be source of biogenic sulfur of coastal aerosols during pre-monsoon season.Future studies involving stable isotopic characterization of coastal aerosols can exploit the fact that DMS flux to the atmosphere is isotopically distinct from anthropogenic sources of atmospheric sulfate, thereby enabling quantified source apportionment or estimation of the DMS contribution to aerosols.
The atmospheric aerosol contains both inorganic (mainly NH 4 + and NO 3 -) and organic N substances produced by a variety of natural and anthropogenic processes.Aerosol N can originate through primary (combustions) and secondary (gas to particle conversions) sources, and δ 15 N of TN carry information about both the processes.The δ 15 N has been, therefore, used as a potential tool in understating aerosol chemistry (Heaton et al., 1997;Russell et al., 1998;Turekian et al., 1998;Yeatman et al., 2001a, b;Martinelli et al., 2002;Kawamura et al., 2004;Kelly et al., 2005;Widory, 2007;Pavuluri et al., 2010).In this study, winter aerosols are characterized by significantly higher (10.8 ± 2.2‰) δ 15 N values compared to those in summer (6.2 ± 2.3‰) (Table 1).More than the statistical difference between winter and summer, systematic decrease in δ 15 N values of aerosols from winter to pre-monsoon period is noteworthy as it closely matches with systematically enhancing influence of marine winds over the sampling site.Higher δ 15 N values of aerosols over coastal locations of India during winter have been reported in metropolitan cites like Mumbai and Chennai (Pavuluriet al., 2010;Aggarwal et al., 2013).δ 15 N TN approaching to their lowest values during pre-monsoon to monsoon period can be interpreted in terms of mixing of marine N species from the adjacent Arabian Sea.Water column denitrification is known to produce isotopically lighter N species which emanate from the surface of the ocean (Naqvi et al., 2006 and reference therein).
As far as winter δ 15 N values of aerosols are concerned we can interpret them in terms of plausible source signatures.Fig. 6 has been made using reported δ 15 N values of N from various kinds of emissions.For instance, Widory (2007) reported δ 15 N compositions of different combustion sources of aerosols over Paris (France) as: 4.6 ± 0.8‰ for aerosols from diesel, 7.7 ± 5.9‰ from natural gas, -7.5 ± 8.3‰ from for fuel oil, -5.3‰ from for coal and ~4.6‰ from unleaded gasoline.For the aerosols generated from waste incineration a range of 5.5-8.0‰has been found (Widory, 2007).Martinelli et al. (2002) found no great difference between δ 15 N of aerosols produced by the burning of C-3 and C-4 plants biomass and are within ~11 ± 3‰.In agreement with this, observations by Agnihotri et al. (2011) revealed an average value of δ 15 N ~13.2 ± 4.2‰ for typical C-3 type biomasses normally burnt in India.Aerosols formed from incomplete burning of cow dung cake, used as a cooking fuel in rural northern India, have been found to have δ 15 N of 13.4-15.5‰ (Pavuluriet al., 2010).Comparing average δ 15 N of winter aerosols over Goa (10.8 ± 2.2‰; Table 1) closely matches with those of emissions of biomass burning.This inference is also in consistence higher K + concentrations observed in winter aerosols.Also, as the prevailing winds blow from north -northeastern continental India that carry pollutant aerosols (of biomass-biofuel burning) towards Indian Ocean in winter, signatures of biomass burning over ambient air of Goa can be anticipated.
Thus, ambient aerosols over Goa appear to be significantly influenced by seasonal reversal of winds i.e. inventory of mineral dust and marine aerosols during pre-monsoon and monsoon seasons and mixing of carbonaceous aerosols emitted from biomass -biofuel burning, mainly occurring in northern parts of India, with atmospheric processing occurring during their long range transport of aerosols during winter, respectively.To intra-seasonal changes in aerosol chemistry in response to meteorological changes and secondary aerosol formations, size-segregated coarse and fine mode air borne particles have to be collected (at a high temporal resolution) and investigated.Nonetheless, the present study offers a good working template for future in-depth chemical and isotopic investigations of Indian aerosols, especially involving triple isotopic tracers for better source characterization.

CONCLUSIONS
Major inferences from this triple isotopic investigation of aerosol over Goa can be listed as following-(a) The atmosphere over Goa comes under the influence of changing large scale seasonal wind circulation, and chemical and isotopic properties of major non-dust components appear to be able trace out these effects.(b) Atmospheric aerosols during pre-monsoon period, a period when ambient temperatures and relative humidity are higher, are characterized by lower mass concentrations of TC, TOC, TN, significantly lower δ 15 N, slightly lighter δ 13 C and higher δ 34 S values, compared to those of winter aerosols.(c) Influence of mixing of marine air on bulk aerosol chemical composition can be traced out by observing both δ 15 N and δ 34 S values, which showed significantly depleted and enriched values respectively.While lowering of δ 15 N is most likely due to mixing of marine N species (with lighter δ 15 N), enhancement of δ 34 S values can be interpreted in terms of mixing marine DMS (with enriched δ 34 S values).(d) The present study provides measured estimates of TC, TOC, TN and TS mass concentrations of atmospheric aerosols over a relatively less urban coastal region of Goa (compared to Mumbai and Chennai).(e) The present study highlights the importance and utility of triple stable isotopic investigation of aerosols which can greatly help source apportionment and provide important clues about secondary processes (aging effect) in atmospheric particles.

Fig. 1 .
Fig. 1.Location of bulk aerosol sampling at the roof of National Institute of Oceanography Goa (at a height ~55.8 meter above sea level; 15.46°N, 73.8°E).

Fig. 2 .
Fig. 2. Upper panel shows changes in wind-directions (as depicted by different symbols); and the lower panel shows observed variability in other meteorological parameters during the study period.

Fig. 3 .
Fig. 3.Variability observed in TC, TOC, TN, TS concentrations (a,b and c panels) and δ 13 C TC , δ 13 C TOC , δ 15 N TN , δ 34 S TS (d, e and f panels) of bulk aerosols over Goa collected during December 2009 to January 2011.

Fig. 5 .
Fig. 5. Noticeable inter-relationships between isotopic and chemical properties of aerosols sampled at NIO-Goa during December 2009 to January 2011.

Fig. 6 .
Fig. 6.End-member isotopic values (δ 15 N TN , d 13 C TC and δ 34 S TS ) of major typical sources.(Data adapted from various references mentioned in the discussion).

Table 1 .
Winter and summer statistics of measured chemical and isotopic parameters of bulk aerosols over coastal site (Goa) collected during December 2009 to January 2011.All concentrations [M] are in μg/m 3 .All the isotopic values are expressed in per mil (‰).