Marine Chemistry 121 (2010) 167–175

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Marine Chemistry

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Aerosol iron solubility over Bay of Bengal: Role of anthropogenic sources andchemical processing

Ashwini Kumar, M.M. Sarin ⁎, Bikkina SrinivasPhysical Research Laboratory, Ahmedabad-380 009, India

⁎ Corresponding author. Tel.: +91 79 26314306; fax:E-mail address: sarin@prl.res.in (M.M. Sarin).

0304-4203/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.marchem.2010.04.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 October 2009Received in revised form 9 April 2010Accepted 9 April 2010Available online 24 April 2010

Keywords:Bay of BengalAerosolsAerosol iron solubilityMineral dust

The chemical composition (involving water-soluble inorganic constituents, crustal elements and carbona-ceous species) of size-segregated aerosols (PM10 and PM2.5), collected from the marine atmosphericboundary layer (MABL) of the Bay of Bengal (BoB) during 27th Dec' 08–28th Jan' 09, has been studied toascertain the factors controlling the spatio-temporal variability in the fractional solubility of aerosol iron.Based on the air-mass back-trajectory (AMBT) analyses and chemical proxies, continental outflow from thetwo major source regions has been identified, viz: (1) outflow from the Indo-Gangetic Plain (IGP) sampledover north-BoB (N-BoB); and (2) south-east Asian (SEA) outflow over south-BoB (S-BoB). A significant linearrelationship among fractional Fe solubility [WS-Fe (%)] and nss-SO4

2− over N-BoB (characterized by higherabundance of aerosol iron (FeA) and SO4

2−) provides evidence for the acid processing of mineral dust duringatmospheric transport from IGP. The enhancement in the solubility of aerosol constituents is also evidentfrom a linear increase in nss-Ca2+ with nss-SO4

2−. In contrast, a temporal shift in the winds, representing theoutflow from south-east Asia and aerosol composition over south-BoB, exhibit enhanced fractional solubilityof aerosol Fe (range: 11.4 to 49.7%) associated with the lower abundance of dust (b100 ng m−3 of FeA) andnss-SO4

2− (b15 µg m−3). These observations suggest the dominance of combustion sources (biomass burningand fossil-fuel) in dictating the aerosol iron solubility over south Bay of Bengal. The impact of the anthropogenicsources is also ascertained based on the covariance of WS-Fe with K+ and OC (organic carbon); as well asenrichment factor of heavy metals (Pb and Cd) associated with the outflow from south-east Asia.

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1. Introduction

Atmospheric transport is considered as one of the major pathwaysfor the input of a variety of chemical elements andnutrients to the oceansurface and in stimulating the phytoplanktongrowth. The enhancementin ocean productivity can modulate CO2 sequestration and thusinfluence the global carbon cycle and climate (Jickells et al, 2005;Mahowald et al, 2005, 2009; Solmon et al, 2009). On average, ironconstitutes 3.5% of themineral aerosols (Duce and Tindale, 1991) and isrecognized as a key nutrient in supporting the primary production(Boyd et al, 2007). Numerous studies carried out over the oceanicregions have demonstrated the significance of iron in regulating thebiogeochemistry of ocean surface (Martin and Fitzwater, 1988; Coaleet al, 2004; Boyd et al, 2000, 2004; De Baar et al., 2005). A finite fractionof the aerosol Fe (referred as ‘soluble iron’ or ‘bioavailable iron’) is ofprimary interest in open ocean regions for phytoplankton growth. In thepast decade, several studies have attempted to assess the aerosol ironsolubility using different analytical techniques, and have suggested a

large range in the fractional solubility (0.01–90%) (Mahowald et al,2005).

It has been well recognized that fractional solubility of iron fromatmospheric deposition of desert dust and soil to the ocean isrelatively poor than that associated with remote marine aerosolsderived from anthropogenic sources (Zhuang et al, 1990; Baker et al,2006a; Bonnet and Guieu, 2004; Buck et al, 2006, 2008a). Theenhancement in iron solubility has been linked to chemical processing(in the presence of acidic components, e.g. SO2 and its oxidationproducts) of mineral dust particles during long-range atmospherictransport. This has led several workers to examine the relationbetween fractional solubility of aerosol iron and concentration ofacidic species based on real-time samples collected in the field (Handet al, 2004; Baker et al., 2006a,b; Baker and Jickells, 2006; Kumar andSarin, 2010a). However, these studies do not provide a firmconclusion in favour of acid processing as a primary control forenhanced fractional solubility of aerosol iron. The recent studies(Sedwick et al, 2007; Baker and Croot, 2008; Sholkovitz et al, 2009)have suggested anthropogenic emissions (biomass burning and fossil-fuel combustion) as a significant source of soluble aerosol Fe, incontrast to its supply from mineral aerosols (or processed dust). Thisconcept has been further strengthened by modelling and field-basedobservations (Chuang et al, 2005; Luo et al, 2008; Sedwick et al, 2007;

168 A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

Kumar and Sarin, 2010a; Trapp et al, 2010). More recently, Schrothet al. (2009), have documented supply of highly soluble iron (77–81%)from oil fly ash, based on laboratory experiments, thus confirmingfield and modelling efforts that have suggested fossil-fuel combustionproducts as a major source of bioavailable iron to the ocean surface.

In the recent years, there has been substantial interest in theanthropogenic emissions and atmospheric transport from south andsouth-east Asian regions (Galloway, 1995; Streets et al, 2003; Arimotoet al, 2004). The impact of anthropogenic emissions has beenobserved in the remote marine regions of the Arabian Sea and Bayof Bengal (Lelieveld et al., 2001; Johansen and Hoffmann, 2004;Kumar et al, 2008a,b). However, the issues concerning the solubility ofaerosol iron have not been addressed to cover these oceanic regions.In this study, we have examined the spatio-temporal variability in thefractional solubility of aerosol iron over Bay of Bengal, which isinfluenced by the continental outflow during the late north-eastmonsoon (January–April).

2. Methodology

2.1. Sampling location and meteorological conditions

Aerosol samples were collected from the MABL of the Bay ofBengal (BoB) onboard ORV Sagar Kanya cruise (SK-254), during theperiod 27th December 2008–28th January 2009, in several transectsbetween 4 to 22 °N and 76 to 98 °E (Fig. 1a). The dots along thecruise tracks represent the ship's position at 05:30 UTC on each day.The three different wind patterns recorded during the cruiseduration are illustrated in Fig. 1b, c, and d. The relevantmeteorological parameters (wind speed, relative humidity, and airtemperature) were measured every hour at a height of 15 m fromthe sea level. The relative humidity varied from 49.1 to 79.8%, withmean of about 64.2% during the sampling period. The prevailingwinds, ranging from 1.2 to 6.3 m s−1, were corrected for the ship'sspeed; whereas surface-level pressure varied over a narrow range(1007 to 1017 mb).

2.2. Aerosol collection and analysis

The size-segregated aerosol samples (PM10 and PM2.5) werecollected simultaneously, on pre-combusted and pre-weighed PAL-LFLEX™ tissuquartz filters (20×25 cm2), by operating two high volumesamplers (Thermo-Andersen) onboard ORV Sagar Kanya. Each samplewas collected for a time period ranging from 20 to 22 hwhile cruising ata speed of ∼10 knots (or more), thus, conforming to the protocol thatthe relative wind direction is from the bow. Samplers were calibrated,once every week, to check on variations in the flow rate (if any).Typically, the flow rate varied from 1.08 to 1.18 m3 min−1, with anuncertaintyof±5%. Soonafter the sample collection,filterswerepackedin zip-lock bags, stored in the deep-freezer (at ca.−19 °C) until the timeof their chemical analyses.

In the laboratory, mass concentrations of PM10 and PM2.5 wereascertained gravimetrically by weighing the full filters (with aprecision of 0.1 mg). Prior to their weighing, all filters wereequilibrated at a relative humidity of 40±5% and temperature of 23±1 °C for 5–6 h. Subsequently, filters were cut into quarters under a cleanflow bench (Class 100) and were soaked in 50 mL of Milli-Q water(resistivity=18.2 MΩ cm) for 5–6 h following an initial ultrasonictreatment for 15 min. The water extract was centrifuged and superna-tant fraction was transferred to polypropylene bottles (precleaned inMilli-Q water for ∼72 h) and analyzed for water-soluble ions (Na+,NH4

+, K+, Mg2+, Ca2+, Cl−, NO3−, SO4

2−, and HCO3−). The anions (Cl−,

NO3−and SO4

2−) and cations (Na+, NH4+, K+, Mg2+, and Ca2+) were

analyzed on DIONEX® Ion-Chromatograph equipped with sup-pressed conductivity detector (ED-50) (Kumar et al., 2008a,b;Kumar and Sarin, 2010b). Using Na+ as a reference for sea-salts

correction, non-sea-salt (nss) component of individual constituentwas calculated as follows:

nss‐X = Xtotal½ �–f Naþh i

4 X=Nað Þseawatergwhere, X may be K+, Mg2+, Ca2+ and SO4

2−; [Xtotal] and [Na+] are themeasured concentrations of X and Na+ in water-extracts of aerosols;(X/Na)seawater is X to Na+ ratio in seawater (Keene et al, 1986).

The crustal constituents (Al, Fe, Ca and Mg) and trace elements (Pband Cd) were analyzed using ICP-AES (JobinYovan, ULTIMA-2) andGraphite Furnace-Atomic Absorption Spectrometer (GF-AAS; P-EModelAAnalyst 100 coupled to a HGA 800) respectively (Kumar and Sarin,2010a). In order to accomplish complete dissolution of aerosol samples,filter punches (∼1 cm diameter) were digested in Teflon vials using HFandHNO3under highpressure (∼100 bars) and temperature (∼250 °C).The acid extract wasmade to a final volume of 25 mLwithMilli-Qwaterand analyzed with respect to a commercial standard (Merck®, 23elements), which was used for making linear calibration plots.Reproducibility in the analytical data for crustal elements was within5% based on the repeat analysis of a number of samples and standards.The concentrations of crustal and trace elements were corrected forprocedural blanks (comprising of blank filters and analytical reagents).Based on blank concentrations and average volume of air filtered(∼1400 m3), the detection limits for the measured elements wereascertained (80, 11, 0.035 and 0.130 ng m−3 for Al, Fe, Cd and Pbrespectively). The concentrations of carbonaceous species, elementalcarbon (EC) and organic carbon (OC), were determined on EC–OCanalyzer using thermo-optical transmittance (TOT) protocol standard-ized in our laboratory (Rengarajan et al, 2007; Ram et al, 2008).

Thewater-soluble iron (WS-Fe)wasmeasured onGraphite Furnace-Atomic Absorption Spectrometer (P-EModel AAnalyst 100 coupled to aHGA 800) as described by Kumar and Sarin (2010a). In the adoptedanalytical protocol, about 15 cm2 of the sample filter (equivalent to 5-circular punches area=3.14 cm2) were treated with 10 mL Milli-Qwater in a 50 mL Savilex Teflon vial followed by an ultrasonic treatmentfor 15 min. Subsequently, water extract was filtered through 0.4 µmPTFE filter into a polyethylene vial and immediately acidified to pHb2using double distilled HNO3. The selection of Milli-Q water for WS-Fe,insteadof seawater, is basedon the rationale thatMilli-Qwater providesa consistent and reproducible leaching solution (Buck et al, 2006;Sedwick et al, 2007). The ultrasonic treatment during the water-extraction helps in disintegrating the aerosol particles from the filtersubstrate, thus achieving uniformity in the extraction procedure. TheWS-Fe data reported in this manuscript represent relative amount ofsoluble Fe (also referred as “operational solubility”) as compared to‘absolute’ or ‘true’ solubility. It is rather difficult to reproduce the in-situconditions that aerosol particles undergo subsequent to their depositionto the ocean surface. Several filter blanks were analyzed along with thesamples and blank levels were less than 5% of the minimumconcentration measured for WS-Fe.

3. Results and discussion

3.1. Air-mass back-trajectory analyses

The airmass back-trajectories (AMBTs) for seven days are computedusing NOAA Air Resource Laboratory HYSPLIT-Model (GADS data set) atarrival heights of 100, 500 and 1000 m. The origin of each AMBT is set asthe mid point of the sampling interval and the corresponding ship'sposition. Thewind-trajectory analyses are useful to infer the origin of airmasses at various sampling locations. Fig. 1(b to d) presents AMBTs fordifferent dates representing the wind regimes during the campaignperiod. During the first half of the cruise track in the North Bay of Bengal(27th Dec '08 to 10th Jan '09; Fig. 1b), the air masses mainly originatedfrom the Indo-Gangetic Plain (north and north-eastern India) and from

Fig. 1. (a) The cruise track followed during 27 Dec' 08–30 Jan' 09 in the Bay of Bengal [under National Campaign for Aerosol, Trace Gases and Radiation Budget (ICARB-Winter)] campaign. Based on air-mass back-trajectories for seven days atthree (100, 500 and 1000 m) arrival heights, major-wind regimes are identified as (b) outflow from the Indo-Gangetic Plain and north/north-east of India (IGP); (c) outflow from south-east Asia (SEA); and (d) marine-air parcel originatingwithin the MABL (MAP).

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170 A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

Bangladesh. A conspicuous temporal shift in the wind regime wasrecordedduring11th Jan '09 to19th Jan '09,when airmasses originatingfrom south-east Asia were sampled over the southern Bay of Bengal(Fig. 1c). Towards the end of the cruise (19th Jan onwards), although airmasses originated from south-east Asia, themajor path traversed (morethan 5 days) was within the marine atmosphere (Fig. 1d). Based on theprevailing wind regimes, the three important air masses have beenidentified and represented as: i) outflow from the Indo-Gangetic Plainand north/north-east India (hereafter referred as IGP, Fig.1b); ii) windsoriginating from south-east Asia (Burma, Thailand and south-west ofChina), referred toasSEAoutflow, Fig. 1c); and iii) airmassesoriginatingwithin the MABL, referred as marine-air parcel (MAP, Fig. 1d).

3.2. Spatio-temporal variations and chemical characteristics of PM10 andPM2.5

The spatio-temporal variability is significantly pronounced in themass concentrations of PM10 and PM2.5 (range: 6.5 to 108 µg m−3 and

Fig. 2. Temporal and spatial variability in PM10 and PM2.5 for north and south Bay of Bengal ((WS-Fe), (e) nss-SO4

2−, (f) NO3−/nss-SO4

2−.

2.0 to 76.7 µg m−3; Fig. 2a). The higher mass concentrations ofaerosols over north Bay of Bengal (27th Dec to 10th Jan) areassociated with the outflow from IGP. A significant co-variationbetween PM10 and PM2.5 mass at open ocean locations in south-BoB,and PM2.5 mass approaching close to that of PM10, suggest significantdifferences in the aerosol size associated with the outflow from IGPand SEA. These changes in the aerosol characteristics, among the twocontinental outflows, could arise due to differences in the sourcestrength of the natural and anthropogenic emissions (mineral dust,combustion products of biofuels, agricultural waste and fossil-fuels).It is also noteworthy that air-mass back-trajectories computed for thesouth-east Asian outflow (Fig. 1c) represent farther distance from thesource regions; thus, relatively enriched in fine mode particles(PM2.5). The mass concentration of dust (assessed based on Alabundance in the aerosols; Kumar and Sarin, 2009, 2010a) is relativelysignificant in PM10 compared to PM2.5 (Fig. 2b). Also, we have used asuite of crustal elements (e.g. Al, Fe, Ca andMg) and their ratios for thecharacterization of mineral dust and its temporal variability over the

a) mass concentration, (b) dust component, (c) Aerosol iron (FeA) (d) water-soluble Fe

171A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

Bay of Bengal. Table 1 summarizes the elemental ratios (Fe/Al, Ca/Aland Mg/Al) in the dust component associated with different windregimes during the cruise duration. The Fe/Al ratios in both PM10 andPM2.5 exhibit large spatio-temporal variability (Table 1). The majorsource regions of mineral dust to the Bay of Bengal are Chinese loessand alluvial dust from the Indo-Gangetic Plain. On average, massfraction of dust in IGP outflow over north-BoB is 36% in the PM10 size;whereas it is 24% over south-BoB from SEA outflow and marine-airparcel (MAP).

Aerosol iron (FeA) also exhibits pronounced spatial and temporalvariability, quite similar to the abundance of mineral dust in PM10

(Fig. 2b, c), with relatively high concentration occurring in thenorthern Bay (FeA range: 48.6–1651 ng m−3) compared to thesouthern Bay of Bengal (FeA range: 30.6–382 ng m−3). In PM2.5,however, the co-variation of FeA with mineral dust is significant in theoutflow from IGP. In contrast, their abundances in samples fromsouthern transect, influenced by SEA and MAP winds, do not exhibitany meaningful relationship. It is, thus, conceivable that dust is one ofthe major sources of FeA in PM2.5 associated with IGP outflow (Fig 2band c). A comparison of the Fe/Al ratios associated with IGP and SEAoutflow, suggests that significant contribution of aerosol Fe fromfossil-fuel combustion and biomass burning is a plausible mechanismfor the relatively enriched Fe/Al ratio in PM2.5 (Table 1). The water-soluble iron (WS-Fe) concentration in PM10 is higher than that inPM2.5 for the samples associated with IGP outflow (Fig. 2d). Incontrast, WS-Fe abundance is nearly identical in both the sizefractions over south-BoB dominated by outflow from south-eastAsia (Fig. 2d). The high abundance of nss-SO4

2− (range: 2.2 to35.0 µg m−3 in PM10) over north-BoB and its temporal variability inPM10 and PM2.5 (Fig. 2e) suggest that nss-SO4

2− is associated with finemode (PM2.5) aerosols, thus, establishing its anthropogenic source.The temporal variability of NO3

−/nss-SO42− is summarized in Fig. 2f;

suggesting that, on an average, the concentration of NO3− is less than

10% in PM2.5 and PM10.The fractional solubility of aerosol Fe (defined as [WS-Fe/FeA]

100), in general, shows an increasing trend with decrease in FeA, forboth PM10 and PM2.5, with relatively high soluble-Fe fraction in theoutflow from SEA (Fig 3a). The enhanced fractional solubility of Fe[WS-Fe (%); Fig. 3a] is associated with relatively low abundance ofdust and FeA (less than 100 ng m−3); whereas nss-SO4

2− accounts foras much as 67% of the water-soluble inorganic constituents in SEAoutflow. In contrast, higher abundance of dust associated withadvective transport from IGP exhibits lower fractional solubility ofaerosol iron. The fractional solubility of Fe for different wind regimesis summarized in Table 1. A wide range of data for water-soluble-Fefraction have been reported in case of field-basedmeasurements fromdifferent oceanic regions (Siefert et al., 1999; Johansen and Hoffmann,2003; Chen and Siefert, 2004; Baker et al, 2006a, b; Sedwick et al,2007; Buck et al, 2006, 2008a,b; Trapp et al, 2010) using differentmeasurement techniques. These studies suggest that enhancedfractional solubility of Fe is related to chemical alteration of the

Table 1Major element composition and fractional solubility of aerosol Fe over Bay of Bengal.

Sampling date Air massa WS-Fe(%)

Fe/Al Ca/Al Mg/Al

27 Dec' 08–10 Jan' 09 IGP PM10 1.4–15.3 0.31–0.53 0.14–0.33 0.07–0.17PM2.5 3.5–21.2 0.26–1.47 0.09–0.63 0.06–0.57

11 Jan' 09–22 Jan' 09 SEA PM10 5.6–23.9 0.26–0.54 0.17–0.51 0.07–0.21PM2.5 29.7–49.7 0.14–1.15 0.10–0.64 0.05–0.37

23 Jan' 09–28 Jan' 09 MAP PM10 2.3–13.6 0.15–0.39 0.04–0.28 0.03–0.17PM2.5 11.4–26.8 0.22–0.87 0.17–0.37 0.04–0.16

a Different air masses identified are: IGP outflow (winds originating from Indo-Gangetic Plain); SEA (outflow from south-east Asia); and MAP (winds originatingwithin themarine atmospheric boundary layer) during the cruise in Bay of Bengal (27thDec, 2008–28th Jan, 2009).

Fe-bearing particles derived from combustion sources, rather thanthe chemical processing of dust. More recently, an inverserelationship between WS-Fe (%) and FeA has been reported from ahigh-altitude site in semi-arid western India (Kumar and Sarin,2010a), a feature similar to that reported in the literature. In thisstudy, we have examined the contribution of water-soluble Fe frommineral dust (via chemical processing) vis-à-vis anthropogenicsources (fossil-fuel combustion/biomass burning).

In order to assess the contribution from anthropogenic sources, wehave examined the Cd/Al, Pb/Al ratios and elemental carbon (EC)/aerosol mass ratios. The Cd/Al and Pb/Al show strong spatial andtemporal variability (Fig. 3b and c) in PM2.5 when compared to PM10;thus, asserting the impact of anthropogenic sources over Bay ofBengal. These results also establish the dominance of anthropogeniccomponents in PM2.5; with relatively high concentrations over S-BoBinfluenced by the outflow from south-east Asia. This is consistent withthe enhanced fractional solubility of aerosol Fe over S-BoB due tocontribution of soluble Fe from combustion sources. A similarenrichment of EC in PM2.5 is evident over S-BoB (Fig. 3d), furthercorroborating the influence of biomass and fossil-fuel combustionsources. However, nss-SO4

2−/PM2.5 ratio does not exhibit strongspatial or temporal variability; with ratio averaging around 0.31±0.08 in PM2.5 (Fig. 3e).

3.3. Controls on aerosol Fe solubility over Bay of Bengal

In this section, we have considered the PM2.5 samples collectedduring the cruise campaign in 2009 (Fig. 1). The concentrations ofWS-Fe, heavy metals (Cd and Pb), water-soluble inorganic constitu-ents and carbonaceous species have been used to examine the variousfactors affecting the Fe solubility (Table 2). The chemical compositionof PM2.5 suggests higher contribution of dust in the samples overnorth-BoB dominated by the outflow from IGP (Table 2). Asignificantly large difference is also observed with respect to thefractional solubility of Fe (Table 2). The WS-Fe (%) is nearly threetimes higher in the outflow from SEA compared to that from IGP.These differences emphasize the role of anthropogenic sources as wellas various physico-chemical transformation processes (acid pro-cessing or physical processing). The enrichment factors for Cd and Pbshow exceptionally high values over the entire Bay of Bengalcompared to the other oceanic regions (Table 2). However, southernsector of BoB (dominated by outflow from SEA) shows relatively highenrichments for Cd and Pb. The OC/EC ratios ranged from 1.9 to 4.9over the northern Bay and relatively lower values (0.2–3.9) in thesouthern region. A similar decreasing trend (from north to south) inOC/EC ratio has been reported over BoB in an earlier study by Sudheerand Sarin (2008).

It has been argued that biomass burning and fossil-fuel combustionare the potential sources of aerosol Fe (Chuang et al, 2005; Guieu et al,2005; Luo et al, 2008; Sholkovitz et al., 2009) in enhancing the fractionalsolubility of Fe as compared to themineral dust. In Fig. 3, we documentthat the abundances of Pb, Cd (normalized toAl) andEC/Mass (proxy forcombustion sources) show an increasing trend from north to south Bayof Bengal, similar to the increase in fractional solubility of aerosol Fe. It isimportant to reiterate that, on average, themass fraction ofmineral dustin PM2.5 is 12.5% (range: 4.6 to 39.2%) in the IGP outflow and 7.6%(range: 2.2 to 20.4%) in the outflow from south-east Asia (SEA). It istherefore, relevant to consider the chemical processing of dust particles,during long-range transport, mediated by the presence of acidiccomponents (e.g. SO2 and its oxidation products) and sunlight (Zhuanget al., 1992; Johansen et al, 2000; Meskhidze et al, 2003). The potentialdominance of acid processing can be ascertained from the inter-relationship between fractional solubility of Fe and the abundance ofacidic species. In Fig. 4a, nss-SO4

2− plotted against WS-Fe (%) exhibitslinear relationship (r2=0.56) for the samples collected during theperiod of 27th Dec–10th Jan from the northern Bay (influenced by the

Fig. 3. Temporal and spatial variability of (a) fractional solubility of aerosol Fe [WS-Fe (%)]; (b) Cd/Al; (c) Pb/Al; and (d) EC/Mass and (e) nss-SO42−/Mass in PM10 and PM2.5.

172 A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

outflow from IGP). This supports the hypothesis of acid processing ofmineral dust and its significant contribution to fractional solubility of Fe.The concept of acid-uptake by mineral dust is also borne out from theenhanced solubility of Ca2+ and linear relation between nss-SO4

2− and

Table 2Chemical characteristics of PM2.5 over Bay of Bengal representing different continentaloutflows.

IGP SEA MAP

Range AMa Range AM Range AM

WSICb/PM2.5

Mass (%)19.7–58.6 43.0 41.4–59.0 47.7 38.6–99.5 52.4

Dust (%) 4.6–39.2 12.5 2.2–20.4 7.6 3.1–29.9 14.7Fe/Al 0.26–1.47 0.52 0.14–1.15 0.56 0.22–0.87 0.4WS-Fe/Fe (%) 3.5–21.2 13.1 27.4–49.7 38.6 11.4–26.8 21.8OC/EC 1.9–4.9 2.9 0.9–3.9 2.5 0.2–1.0 0.6Cd E.F.c 400–5800 2900 710–15,000 5900 732–4300 2740Pb E.F.c 60–1700 500 270–2900 1160 335–1070 630

a Arithmetic mean.b Water-soluble inorganic constituents.c Enrichment factor.

Ca2+ (r2=0.65; Fig. 4b) in PM2.5. It is important that, on average, NO3−

abundance in IGP outflow is about 10 times lower than total acidiccomponent (NO3

−+nss-SO42−). Therefore, much of the acid processing

need to be invoked through reaction with sulphuric acid. However, anintercept of about 5% on y-axis (Fig. 4a) is significant and accounts fornearly 1/3 of the average fractional Fe solubility (Table 2); thussuggesting that soluble-Fe derived from combustion sources also haveimpact on the aerosol Fe solubility. Unlike the plot shown in Fig. 4a, alarge scatter is observed in the data from southern Bay (influenced bythe SEA outflow and winds within the MABL) and show enhancedfractional solubility of Fe at relatively low acid concentration (at nss-SO4

2−≤10 µg m−3; Fig. 4c). Likewise, three of the data points collectedduring 4–6 Jan, 2009 (Fig. 1a); representing the IGP outflow, showrelatively high fractional solubility at lower nss-SO4

2− concentration(Fig. 4a).

Looking from a different perspective, the fractional solubility ofaerosol Fe increases at lower FeA concentration (Fig. 5), suggesting aninverse relationship with the atmospheric mineral dust (a dominantsource of aerosol Fe). Similar inverse relationship has been reported byBaker and Jickells (2006), who have argued in favour of physicalremoval (as against chemical processing) of mineral dust in enhancing

Fig. 4. (a) A linear relationship among WS-Fe (%) and nss-SO42− for IGP outflow,

suggesting the role of chemical processing in the enhancement of fractional solubility ofFe over north Bay of Bengal, (b) acid uptake by mineral dust is also evident fromincrease in the solubility of Ca2+ as a function of nss-SO4

2− in PM10 (c) the enhancedfractional solubility of aerosol Fe, associated with outflow from SEA, varies independentof nss-SO4

2−; thus suggesting the dominance of combustion sources.

173A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

the fractional Fe solubility; and Sedwick et al. (2007) and Kumar andSarin (2010a), supporting the contribution of highly soluble aerosol Federived from combustion sources. In order to get further insight into thefractional solubility of aerosol Fe, we have examined the chemicalparameters (Table 2) associated with distinctly different wind regimes

Fig. 5. (a) A general inverse relationship betweenWS-Fe (%) and FeA for the IGP outflowis explained as a two end-member mixing of soluble Fe from mineral dust [higher FeAand lower WS-Fe (%)] and combustion sources [lower FeA and higher WS-Fe (%)].Enhanced fractional solubility at lower FeA concentration during SEA and MAP windregimes is mainly dictated by soluble Fe from combustion sources.

(Fig. 1b, c and d). The plot betweenWS-Fe (%) and total aerosol Fe (FeA)suggests that a general inverse relationship in Fig. 5 can be explainedas a two end-member mixing curve — one dominated by mineral dust[high FeA, low WS-Fe (%)] and other end-member dominated bycombustion sources [low FeA, high WS-Fe (%)]. This is consistent withour earlier interpretation arguing in favour of acid processing of dustin case of north-BoB samples for which both FeA (mineral dust) andnss-SO4

2− are high; and, hence, an increase in WS-Fe (%) for the datapoints falling away from the dotted curve (Fig. 5). Chen and Siefert(2004) and Baker et al. (2006a), had reported similar inverserelationship between fractional Fe solubility and mineral dust fromAtlantic Ocean. However, no significant correlation among WS-Fe (%)and sulphate concentration was reported in their study. We, thus,infer that SO4

2− rich aerosols (which accounts for 65% of the water-soluble inorganic constituents) in the outflow from the Indo-GangeticPlain provide ideal conditions for the chemical processing of dust(Fig. 4a). Nevertheless, the role of combustion sources (widespreadburning of agricultural waste and biofuel in the Indo-Gangetic Plain)is well substantiated based on a general increase in concentration ofWS-Fe as a function of K+ and OC, the two tracers of biomass burning(Fig. 6a). Our argument in favour of two different processes (acidprocessing and combustion based emissions) in the IGP outflow is ofutmost relevance to explain the large spatio-temporal variability inthe fractional solubility of Fe (Table 2; Fig. 5); which is associatedwith the variability in the regional source strength of the emissions.

On the contrary, outflow from the south-east Asia, sampled overthe southern Bay, is depleted in the mass fraction of mineral dust(FeAb100 ng m−3) but enriched in fine mode aerosols with charac-teristic higher fractional solubility of Fe (Figs. 2 and 3). The physicalsorting of mineral dust during the long-range transport from south-east Asia is a plausible mechanism for the depletion of aerosol iron(FeA). Alternatively, the enhanced fractional solubility of Fe in PM2.5 isassociated with the chemical alteration of particles from combustionsources. The fractional solubility of Fe ranged from 11.4 to 49.7% in thefine mode aerosols over south-BoB (Figs. 3 and 5). The evidence forbiomass burning as a dominant source is borne out from thesignificant linear relationship between WS-Fe, OC and nss-K+

(Fig. 6b) in the outflow from south-east Asia. The data plotted inFig. 6b would also imply that the aerosol Fe derived from biomasscombustion sources has characteristic higher solubility. The fractionalsolubility, as high as 50% associated with SEA outflow over south-BoBis among the highest values reported from the oceanic regions. It is,thus, concluded that combustion sources dominate the fractionalsolubility of aerosol Fe over Bay of Bengal. Nevertheless, our data setprovides a first field evidence for the conceptual acid processing ofmineral dust, as documented for the advective outflow (rich insulphate aerosols) from the Indo-Gangetic Plain. Recently, it has beenshown by Schroth et al. (2009), that more than 75% of Fe is water-soluble in the combustion products. However, based on the chemicalcomponent data set, it is difficult to quantitatively estimate thecontribution of WS-Fe from processed dust vis-à-vis anthropogenic(combustion of fossil-fuel and biomass burning) sources.

4. Conclusions

The spatio-temporal variability in the chemical characteristics ofthe ambient aerosols (PM2.5 and PM10), collected from the MABL ofBay of Bengal, has been documented based on the major-windregimes representing the continental outflow. On average, nss-SO4

2−

constitutes ∼65% of the total water-soluble ionic species and isprimarily associated with the fine mode (PM2.5) aerosols. Theanalytical data for PM2.5, associated with the outflow from the Indo-Gangetic Plain (northern India), yields a general linear relationshipbetween nss-SO4

2− (range: 1.6 to 28.5 µg m−3) and fractionalsolubility of Fe (range: 3.5 to 21.2%) over north Bay of Bengal. Basedon these findings, we provide an unambiguous evidence for the

Fig. 6. Linear relationship amongWS-Fe, nss-K+ and OC for samples collected during (a) IGP outflow and (b) SEA+MAP outflow, suggesting contribution of soluble Fe from biomassburning. The dotted line in (b) represents the best-fit line.

174 A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

chemical processing of mineral dust by anthropogenic acidic species(nss-SO4

2−) in controlling the fractional solubility of Fe during long-range atmospheric transport. The aerosol characteristics oversouthern Bay region are dominated by the outflow from south-eastAsia (Burma, China, Thailand) and exhibit enhanced fractionalsolubility of Fe (range: 11.4 to 49.7% in PM2.5). The relatively lowabundance of SO4

2− (range: 1.3 to 12.1 µg m−3), mass fraction ofdust (range: 2.2 to 29.8%) and significant linear relation between OC,nss-K+ and WS-Fe suggest the dominance of combustion sources(biofuel and agricultural waste burning) in enhancing the fractionalsolubility of Fe. The continental outflow from the Indo-GangeticPlain and south-east Asia, emphasizing the role of biomasscombustion sources and chemical processing of mineral dust, hasimplications to the supply of soluble Fe to the surface Bay of Bengalin the present-day scenario of rapid industrialization, demand forenergy consumption and increase in fossil-fuel combustion.

Acknowledgements

This study was supported by the office of ISRO-GBP (Bangaluru,India) as a part of the national programme on Integrated Campaign onAerosols Trace Gases and Radiation Budget. We wish to thank chiefscientist (Dr. C.B.S. Dutt) and crew members of the ORV Sagar Kanya(cruise # SK-254). We are greatly benefited by the constructivecomments and suggestions provided by the two anonymousreviewers.

References

Arimoto, R., Zhang, X.Y., Huebert, B.J., Kang, C.H., Savoie, D.L., Prospero, J.M., Sage, S.K.,Schloesslin, C.A., Khaing, H.M., Oh, S.N., 2004. Chemical composition of atmosphericaerosols from Zhenbeiitai, China, and Gosan, South Korea, during ACE-Asia.J. Geophys. Res. 109, D19S04. doi:10.1029/2003JD004323.

Baker, A.R., Croot, P.L., 2008. Atmospheric andmarine controls on aerosol iron solubilityin seawater. Mar. Chem. doi:10.1016/j.marchem.2008.09.003.

Baker, A.R., Jickells, T.D., 2006. Mineral particle size as a control on aerosol ironsolubility. Geophys. Res. Lett. 33, L17608. doi:10.1029/2006GL026557.

Baker, A.R., Jickells, T.D., Witt, M., Linge, K.L., 2006a. Trends in the solubility of iron,aluminium, manganese and phosphorus in aerosol collected over the AtlanticOcean. Mar. Chem. 98, 43–58.

Baker, A.R., French, M., Linge, K.L., 2006b. Trends in aerosol nutrient solubility along awest–east transect of the Saharan dust plume. Geophys. Res. Lett. 33, L07805.doi:10.1029/2005GL024764.

Bonnet, S., Guieu, C., 2004. Dissolution of atmospheric iron in seawater. Geophys. Res.Lett. L03303. doi:10.1029/2003GL018423.

Boyd, P.W., et al., 2000. A mesoscale phytoplankton bloom in the polar Southern Oceanstimulated by iron fertilization. Nature 407, 695–702.

Boyd, P.W., et al., 2004. The decline and fate of an ironinduced subarctic phytoplanktonbloom. Nature 428, 549–553.

Boyd, P.W., et al., 2007. Mesoscale iron enrichment experiments 1993–2005: synthesisand future directions. Science 315, 612–617.

Buck, C.S., Landing, W.M., Resing, J.A., Lebon, G.T., 2006. Aerosol iron and aluminumsolubility in the northwest Pacific Ocean: results from the 2002 IOC cruise.Geochem. Geophys. Geosyst. 7, Q04M07. doi:10.1029/ 2005GC000977.

Buck, C.S., Landing, W.M., Resing, J.A., Measures, C.I., 2008a. The solubility anddeposition of aerosol Fe and other trace elements in the North Atlantic Ocean:observations from the A16N CLIVAR/CO2 repeat hydrography section. Mar. Chem.doi:10.1016/j.marchem.2008.08.003

Buck, C.S., Landing,W.M., Resing, J.A., 2008b. Particle size and aerosol iron solubility: a highresolution analysis of Atlantic aerosols. Mar. Chem. doi:10.1016/j.marchem.2008.11.002

Chen, Y., Siefert, R.L., 2004. Seasonal and spatial distributions and dry deposition fluxesof atmospheric total and labile iron over the tropical and subtropical North AtlanticOcean. J. Geophys. Res. 109, D09305. doi:10.1029/2003JD003958.

Chuang, P.Y., Duvall, R.M., Shafer, M.M., Schauer, J.J., 2005. The origin of water solubleparticulate iron in the Asian atmospheric outflow. Geophys. Res. Lett. 32, L07813.doi:10.1029/2004GL021946.

Coale, K.H., et al., 2004. Southern Ocean iron enrichment experiment: carbon cycling inhigh- and low-Si waters. Science 304, 408–414.

De Baar, H.J., et al., 2005. Synthesis of iron fertilization experiments: from the iron agein the age of enlightenment. J. Geophys. Res. 110, C09S16. doi:10.1029/2004JC002601.

Duce, R.A., Tindale, N.W., 1991. Atmospheric transport of iron and its deposition in theocean. Limnol. Oceanogr. 36, 1715–1726.

Galloway, J.N., 1995. Acid deposition: perspectives in time and space. Water Air SoilPollut. 85, 15–24.

Guieu, C., Bonnet, S., Wagener, T., Loye-Pilot, M.D., 2005. Biomass burning as a source ofdissolved iron to the open ocean? Geophys. Res. Lett. 32, L19608. doi:10.1029/2005GL022962.

175A. Kumar et al. / Marine Chemistry 121 (2010) 167–175

Hand, J.L., Mahowald, N.M., Chen, Y., Siefert, R.L., Luo, C., Subramaniam, A., Fung, I., 2004.Estimates of atmospheric-processed soluble iron from observations and a globalmineral aerosol model: biogeochemical implications. J. Geophys. Res. 109, D17205.doi:10.1029/2004JD004574.

Jickells, T.D., An, Z.S., Andersen, K.K., Baker, A.R., Bergametti, G., et al., 2005. Global ironconnections between desert dust, ocean biogeochemistry and climate. Science 308,67–71.

Johansen, A.M., Hoffmann, M.R., 2003. Chemical characterization of ambient aerosolcollectedduring thenortheastmonsoon seasonover theArabian Sea: labile-Fe (II) andother trace metals. J. Geophys. Res. 108 (D14), 4408. doi:10.1029/2002JD003280.

Johansen, A.M., Hoffmann, R., 2004. Chemical characterization of ambient aerosolcollected during northeast monsoon season over the Arabian Sea: anions andcations. J. Geophys. Res. 109, D05305. doi:10.1029/2003JD004111.

Johansen, A.M., Siefert, R.L., Hoffmann, M.R., 2000. Chemical composition of aerosolscollected over the tropical North Atlantic Ocean. J. Geophys. Res. 105 (D12),15,277–15,312.

Keene, W.C., Pszenny, A.P., Gallloway, J.N., Hawley, M.E., 1986. Sea salt correction andinterpretation of constituent ratios in marine precipitation. J. Geophys. Res. 91,6647–6658.

Kumar, A., Sarin, M.M., 2009. Mineral aerosols from western India: temporal variabilityof coarse and fine atmospheric dust and elemental characteristics. Atmos. Env. 43,4005–4013. doi:10.1016/j.atmosenv.2009.05.014.

Kumar, A., Sarin, M.M., 2010a. Aerosol iron solubility in semi-arid region: temporaltrend and impact of anthropogenic sources. Tellus 62B, 125–132. doi:10.1111/j.1600-0889.2009.00448.x.

Kumar, A., Sarin, M.M., 2010b. Atmospheric water-soluble constituents in fine andcoarse mode aerosols from high-altitude site in western India: long-rangetransport and seasonal variability. Atmos. Environ. 44, 1245–1254. doi:10.1016/j.atmosenv.2009.12.035.

Kumar, A., Sarin, M.M., Sudheer, A.K., 2008a. Mineral and anthropogenic aerosols inArabian Sea-atmospheric boundary layer during inter-monsoon: sources and spatialvariability. Atmos. Environ. 42, 5169–5181. doi:10.1016/j.atmosenv.2008.03.004.

Kumar, A., Sudheer, A.K., Sarin, M.M., 2008b. Chemical characteristics of aerosols inMABL of Bay of Bengal and Arabian Sea during spring inter-monsoon: acomparative study. J. Earth Syst. Sci. 117 (S1), 325–332.

Lelieveld, J., et al., 2001. The Indian Ocean experiment: widespread air pollution fromSouth and Southeast Asia. Science 291, 1031–1036.

Luo, C., Mahowald, N., Bond, T., Chuang, P.Y., Artaxo, P., Siefert, R., Chen, Y., Schauer, J.,2008. Combustion iron distribution and deposition. Global Biogeochem. Cycles 22,GB1012. doi:10.1029/2007GB002964.

Mahowald, N.M., Baker, A.R., Bergametti, G., Brooks, N., Duce, R.A., Jickells, T.D.,Kubilay, N., Prospero, J.M., Tegan, I., 2005. Atmospheric global dust cycle and ironinputs to the ocean. Global Biogeochem. Cycles 19, GB4025. doi:10.1029/2004GB002402.

Mahowald, N.M., Engelstaedter, S., Luo, C., Sealy, A., Artaxo, P., et al., 2009. Atmosphericiron deposition: global distribution, variability, and human perturbations. Annu.Rev. Mar. Sci. 1, 245–278. doi:10.1146/annurev.marine.010908.163727.

Martin, J.H., Fitzwater, S.E., 1988. Iron deficiency limits phytoplankton growth in thenorth-east Pacific subarctic. Nature 331, 341–343.

Meskhidze, N., Chameides, W.L., Nenes, A., Chen, G., 2003. Iron mobilization in mineraldust: can anthropogenic SO2 emissions affect ocean productivity? Geophys. Res.Lett. 30 (21), 2085. doi:10.1029/2003GL018035.

Ram, K., Sarin, M.M., Hegde, P., 2008. Atmospheric abundances of primary andsecondary carbonaceous species at two high-altitude sites in India: sources andtemporal variability. Atmos. Environ. 42 (28), 6785–6796. doi:10.1016/j.atmosenv.2008.05.031.

Rengarajan, R., Sarin, M.M., Sudheer, A.K., 2007. Carbonaceous and inorganic species inatmospheric aerosols during wintertime over urban and high-altitude sites inNorth India. J. Geophys. Res. 112, D21307. doi:10.1029/2006JD008150.

Schroth, A.W., Crusius, J., Sholkovitz, E.R., Bostick, B.C., 2009. Iron solubility driven byspeciation in dust sources to the ocean. Nat. Geosci. doi:10.1038/NGE0501.

Sedwick, P.N., Sholkovitz, E.R., Church, T.M., 2007. Impact of anthropogenic combustionemissions on the fractional solubility of aerosol iron: evidence from the SargassoSea. Geochem. Geophys. Geosyst. 8, Q10Q06. doi:10.1029/2007GC001586.

Sholkovitz, E.R., Sedwick, P.N., Church, T.M., 2009. Influence of anthropogeniccombustion emissions on the deposition of soluble aerosol iron to the ocean:empirical estimates for island sites in the North Atlantic. Geochim. Cosmochim.Acta 73, 3981–4003. doi:10.1016/j.gca.2009.04.029.

Siefert, R.L., Johansen, A.M., Hoffmann, M.R., 1999. Chemical characterization ofambient aerosol collected during the southwest and intermonsoon seasons overthe Arabian Sea: labile-Fe(II) and other trace metals. J. Geophys. Res. 104,3511–3526.

Solmon, F., Chuang, P.Y., Meskhidze, N., Chen, Y., 2009. Acidic processing ofmineral dustiron by anthropogenic compounds over the north Pacific Ocean. J. Geophys. Res.114, D02305. doi:10.1029/2008JD010417.

Streets, D.G., et al., 2003. An inventory of gaseous and primary aerosol emissions in Asiain the year 2000. J. Geophys. Res. 108, 8809. doi:10.1029/2002JD003093.

Sudheer, A.K., Sarin, M.M., 2008. Carbonaceous aerosols in MABL of Bay of Bengal:influence of continental outflow. Atmos. Environ. 42, 4089–4100. doi:10.1016/j.atmosenv.2008.01.033.

Trapp, J.M., Millero, F.J., Prospero, J.M., 2010. Trends in the solubility of iron in dust-dominated aerosols in the equatorial Atlantic trade winds: importance of ironspeciation and sources. Geochem. Geophys. Geosyst. 11, Q03014. doi:10.1029/2009GC002651.

Zhuang, G., Duce, R.A., Kester, D.R., 1990. The dissolution of atmospheric iron in surfaceseawater of the open ocean. J. Geophys. Res. 95, 16,207–16,216.

Zhuang, G.S., Yi, Z., Duce, R.A., Brown, P.R., 1992. Link between iron and sulfur cyclessuggested by detection of Fe (II) in remote marine aerosols. Nature 355, 537–539.