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Aerosol and Air Quality Research, 12: 359370, 2012Copyright Taiwan Association for Aerosol ResearchISSN: 1680-8584 print / 2071-1409 onlinedoi: 10.4209/aaqr.2011.07.0105

Carbonaceous and Secondary Inorganic Aerosols during Wintertime Fog andHaze over Urban Sites in the Indo-Gangetic Plain

Kirpa Ram 1,2* , M.M. Sarin 1, A.K. Sudheer 1, R. Rengarajan 1

1 Physical Research Laboratory, Ahmedabad-380 009, India2 Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan

ABSTRACT

The chemical composition of total suspended particulate (TSP) matter and secondary aerosol formation have been studiedduring wintertime fog and haze events from urban sites (Allahabad and Hisar) in the Indo-Gangetic Plain. The atmosphericabundances of elemental carbon (EC), organic carbon (OC), water-soluble OC (WSOC) suggest that organic matter is amajor component of TSP, followed by concentrations of sulphate and nitrate under varying meteorological conditions. Theconcentrations of EC, OC, and WSOC show a nearly 30% increase during fog and haze events at Allahabad and a marginalincrease at Hisar; whereas inorganic constituents (NH4+, NO3 and SO42) are 23 times higher than those during clear days at both the locations. The sulphur and nitrogen oxidation ratios (SOR and NOR) also exhibit significant increases suggesting possible enhancement of secondary formation of SO42 and NO3 during fog and haze events. The significant correlation between NH4+ SO42 (R 2 = 0.66, n = 61) and an NH4+/SO42 equivalent ratio 1 during fog-haze conditions suggest near-complete neutralization of sulphuric acid by ammonia. In contrast, NH4+/SO42 equivalent ratios are less than 1 during normaldays suggesting an NH3-deficient environment and the possible association of SO42 with mineral dust for neutralization.Secondary inorganic aerosol formation and their hygroscopic growth can have significant impact on atmospheric chemistry,air-quality and visibility impairment during fog-haze events over northern India.

Keywords: Elemental carbon; Organic carbon; Secondary inorganic aerosols; Sulphur and nitrogen oxidation ratios; Fog andhaze; Indo-Gangetic Plain.

INTRODUCTION

The Indo-Gangetic Plain (IGP) in northern India, stretchingfrom north-west to north-east, is a densely populated regionwith characteristic emissions from anthropogenic sources.Among the major sources of aerosol, emissions fromagricultural-waste and biomass burning dominate during thewintertime (DecFeb) (Venkataramanet al ., 2005; Rengarajan

et al ., 2007; Gustafssonet al ., 2009; Ram et al ., 2010a;Shakya et al ., 2010; Rajputet al ., 2011; Ram and Sarin,2011). The transport of mineral aerosol from arid and semi-arid regions of western India, during the summer (April early June), imparts strong temporal variability in the chemicalcomposition and optical properties of aerosol (Jethvaet al .,2005; Chinnamet al ., 2006). In addition, lack of year-round precipitation further leads to the high aerosol loading overthe entire Gangetic Plain. Measurements of aerosol opticaldepth (AOD) and absorption aerosol optical depth (AAOD)

*

Corresponding author. E-mail address: [email protected],[email protected]

have characterized IGP as a hotspot for anthropogenic aerosolin south-Asia (Ramanathanet al ., 2007).

The enhanced levels of pollutants lead to poor air-qualityand several environmental, health and socio-economicalissues in Asia (Hameedet al , 2002; Lee and Sequeira, 2002;Husainet al ., 2007; Bariet al ., 2010; Wanet al ., 2010; Ziaoet al ., 2010). Among these, fog-haze formation and reductionin visibility is of major concern (Watson, 2002; Zhanget al .,

2010). The formation of fog and haze is associated withhigh aerosol loading from anthropogenic sources as well asformation of secondary aerosols via gas-to-particle conversionunder favorable meteorological conditions (Reillyet al .,2001; Husainet al ., 2004; Sunet al ., 2006; Zhanget al .,2010; Fang et al ., 2011;). Recent studies suggest that asignificant fraction of atmospheric particulate matter (PM)in the Indo-Gangetic Plain are comprised of carbonaceousaerosol (~3035% of the PM) and water-soluble inorganicspecies (~1020% of the PM) over IGP and other locationsin India (Tareet al ., 2006; Rengarajanet al ., 2007; Rametal ., 2010a; Deshmukhet al ., 2011; Kothai et al ., 2011).During the wintertime, low ambient temperatures (range:

~1020C, at times dipping as low as 5C) and weakwinds (< 2 m/s) and boundary layer height (~500800 m)favour stable atmosphere, leading to poor convective


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mixing and accumulation of pollutants in the loweratmosphere (Nairet al ., 2007; Ramet al ., 2010a).

A large variability in aerosol optical properties andradiative forcing during wintertime haze and fog has beenreported over northern India (Ramachandranet al ., 2006;Tripathi et al ., 2006). The aerosol chemical compositionchanges through the aqueous phase-reactions on aerosolsurfaces (Sunet al ., 2006; Biswaset al ., 2008; Jihuaet al .,2009; Tan et al ., 2009) and subsequently, it affects aerosoloptical properties during fog-haze conditions. In thismanuscript, we report on the changes in chemicalcomposition of atmospheric aerosol, secondary speciesformation and neutralization processes during fog and hazeevents through simultaneous measurements of carbonaceousspecies (EC, OC and WSOC) and water-soluble inorganicionic constituents (cations: Na+, K +, NH4+, Ca2+ and Mg2+ and anions: Cl, SO42, NO3 and HCO3) from two urban

sites (Allahabad and Hisar) in the IGP. In addition, theformation of secondary inorganic aerosols (NO3 and SO42)have been assessed under different meteorological conditions(clear days, haze and fog events) by sulphur and nitrogenoxidation ratios (SOR and NOR, respectively).

SAMPLING LOCATIONS, AEROSOL COLLECTIONAND METEOROLOGICAL DETAILS

Ambient aerosol samples were collected at Allahabad andHisar during Dec 2004 using a high-volume sampler (modelAPM 450, Environtech, India). The sampler does not havesize-selective inlet and thus, all samples correspond to total

suspended particulate (TSP) samples. The sampler wascalibrated before the field campaign and the flow-rate wasfound to vary between 0.9 to 1.2 m3/min (average flow rate:1.0 0.1 m3/min). A total of 42 and 19 samples werecollected from Hisar and Allahabad, respectively. Hisaraerosol samples include a 31 daytime samples (sampling hrs:8:00 AM to 7:00 PM, IST, Indian standard time) and 11nighttime samples (sampling time: 7:30 PM to 7:00 AM,IST) whereas all the samples (n = 19) were collected on 24hr basis at Allahabad.

The two sampling sites, Allahabad (25.4N, 81.9E) andHisar (29.2N, 79.5E), represent an urban pollutedenvironment in northern India and their locations are shown

in Fig. 1. As per Census India report in 2011, the populationsof Hisar and Allahabad are 0.3 and 1.2 million, respectively.Both sites are located in a highly agriculture productiveregion of India and high level of pollution is observed fromthe burning of cow-dung, agricultural waste and wood-fuelused for residential heating purposes (Badarinathet al ., 2009).

The sampling period (Dec 2004) represents wintertimeweather conditions over northern India and characterized by calm winds (wind speed < 2 m/s) and low ambienttemperature (range: 525C). In addition, lower boundarylayer height (~500800 m) (Nairet al ., 2007) and prevailingmeteorological conditions (lower temperature and calmwinds, and high relative humidity) are conducive to the

accumulation of aerosol and pollutants in the loweratmosphere over the IGP. The sampling period was alsocharacterized by a number of fog and haze weather conditions

Fig. 1. Sampling locations of Allahabad and Hisar in theIndo-Gangetic Plain of northern India during the Land-Campaign-II (Dec 2004). The thick haze advecting eastwardand then spreading out over the Bay of Bengal in the IndianOcean is seen in the picture. The Moderate ResolutionImaging Spectroradiometer (MODIS) on NASAs Terrasatellite captured this image of the region on December 17,2004.

in the Gangetic Plain (Ramachandranet al ., 2006; Lal etal ., 2008). Thus, these particulate samples provide an idealopportunity to understand the role of primary (derivedfrom the anthropogenic emissions within the IGP) andsecondary aerosol, and local meteorological conditions inthe modulation of fog-haze formation. The classification offog, haze and clear days is based on the visibility, relativehumidity (RH) and the notes in the log book during thesampling period (Ramachandranet al ., 2006; Lal et al .,2008). When the visibility was in the range of 0.20.5 km,12 km and ~10 km; the events were defined as fog, hazeand clear days, respectively (Ramachandranet al ., 2006).The diurnal plot of three hourly averaged temperature andrelative humidity data during clear, haze and fog events atHisar is shown in Fig. 2. Generally, relative humidity was

less than 70 and 80%, respectively, during clear and hazedays. In contrast, the RH was higher than 90% (especially inmorning hours; see Fig. 2) during foggy days. In addition,some of the intense fog and haze events (e.g. Fig. 1) in theIGP region were also captured by the Moderate ResolutionImaging Spectroradiometer (MODIS) onboard NASAs Terrasatellite (Badarinathet al ., 2011). Thus, the classification offog, haze and clear days at Hisar and Allahabad (Table 1) isfurther substantiated by the earlier published work and thereal time images taken by the MODIS satellite over the IGP(Ramachandranet al ., 2006; Lalet al ., 2008; Badarinathetal ., 2011). Based on the meteorological data, field andsatellite observations, 6, 8 and 15 Dec are defined as clear

days, 11, 17, 18 and 27 Dec are hazy and 12, 13, 14 and 22Dec 2004 as foggy days at Hisar whereas (9, 25 and 27Dec), (10, 11, 19 and 23 Dec) and (13, 17, 21 and 28 Dec)


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T e m p e r a t u r e

( C )

5

10

15

20

25

30Clear HazeFog

Local Time (hrs)

R e l a

t i v e

H u m

i d i t y

( % )

0

20

40

60

80

100

Clear HazeFog

2.5 5.5 8.5 11.5 14.5 17.5 20.5 23.5

(a)

(b)

sunrise

sunset

Fig. 2. Three hourly average diurnal plot of (a) temperatureand (b) relative humidity data during clear, haze and fogevents at Hisar.

are classified as clear, hazy and foggy days at Allahabad,respectively (Table 1).

CHEMICAL ANALYSES OF AEROSOL SAMPLESAND QUALITY CONTROL

All the filters were wrapped in the Aluminum foil aftersample collection and were packed in zip-lock bags. Sampleswere brought to the laboratory and stored in a refrigeratorat ~4C until the analysis. Prior to assessment of total aerosolmass and chemical analysis, samples were equilibrated in alaminar flow bench for ~1520 h under constant relative

humidity (35 5%) and temperature (22 1C) conditions.The particulate matter (PM) mass (g/m3) was ascertainedgravimetrically by weighing full-filter before and after theaerosol sampling, and dividing the aerosol mass (ing) bythe total volume of air sampled (in units of m3). The massconcentrations of elemental and organic carbon (EC and OC)in ambient aerosol were assessed on the EC-OC analyzer(Sunset Laboratory, USA) using thermo-optical transmittance(TOT) protocol (Rengarajanet al ., 2007; Ramet al ., 2008).The transmittance signal measured using a laser source (at678 nm) was used to define the split-point between EC-OCand to correct for the pyrolyzed carbon. Also, OCconcentrations were corrected for the carbonate carbon byacidifying the aerosol sample with 6M HCl in a desiccator(for ~6 hrs) and re-analyzing the acidified samples on theEC-OC analyzer. A number of replicate analyses of aerosolsamples (n = 6) provide good reproducibility (reported as

relative percentage deviation; r.p.d.) for OC (~6%), EC (~8%)and TC (~5%) measurements. In addition, the transmittancesignal at 678 nm of the laser source has been used for thedetermination of absorption coefficient (babs) and massabsorption efficiency (abs) of EC (Ram and Sarin, 2009).

The inorganic constituents, cations (Na+, K +, NH4+, Ca2+ and Mg2+) and anions (Cl, SO42 and NO3) in the water-extract of aerosol samples, were measured using an ionchromatograph (Dionex-500) whereas water-soluble organiccarbon (WSOC) was analyzed using a total organic carbon(TOC) analyzer (Shimadzu, TOC-5000A) (Ram and Sarin,2010). The mass concentration of bicarbonate ion (HCO3)was measured using a 0.005 M HCl and fixed endpoint

titration method in an auto-titration system. The experimentaldetails for the measurement of inorganic constituents andWSOC are described in our recent publication (Ram andSarin, 2010). The reproducibility (as r.p.d.) of replicateanalyses of water-extracts for K +, Ca2+, Mg2+, Cl, SO42 and NO3 were better than 3% while it was ~6% for NH4+ analysis (n = 4). All the reported data of carbonaceous species(EC, OC and WSOC) and inorganic ionic constituents(cations and anions) are corrected for their respective procedural blanks (Ram and Sarin, 2010; Ramet al ., 2010a).

Table 1. The 24-hr average meteorological data, NO2 and SO2 concentrations during the clear days, haze and fog events attwo sampling sites (Hisar and Allahabad) in northern India.

Species

Hisar # AllahabadClear days Haze events Fog events Clear days Haze events Fog events

(6, 8, 15 Dec) (11, 17, 18, 27Dec)(12, 13, 14, 22

Dec) (9, 25, 27 Dec)(10, 11, 19, 23

Dec)(13, 17, 21, 28

Dec)RH (%)$ 46.7 9.9 62.3 8.5 61.1 5.7 45.3 1.5 70.3 13.3 56.5 3.4

Temperature (C)$ 17.5 0.4 16.7 1.1 17.3 2.1 18.9 2.2 18.9 2.8 19.8 4.0Wind speed (m/s)$ 0.6 0.1 0.5 0.3 0.5 0.2

Visibility range (km) ~10.0 1.02.0 0.20.5 NO2 (g/m3)## 23.3 7.1 31.5 7.6 24.1 4.1 11.7 2.9 26.8 9.7 17.4 6.2SO2 (g/m3)## 11.0 1.7 15.6 4.6 17.2 10.9

#Classification of fog, haze events and clear days as per Ramachandranet al . (2006); Lalet al . (2008).$Daily-average data for relative humidity (RH), temperature and wind speed. The fog and haze events persist only for few

hrs (when RH > 80%; see Fig. 1), the 24-hr averaging resulted in lower RH during fog and haze events.##Daily-average NO2 and SO2 concentrations at Allahabad is adopted from Kulshreshthaet al . (2009) and those at Hisarfrom Lalet al . (2008).


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RESULTS AND DISCUSSION

Total Suspended Particulate MatterThe average mass concentrations of total suspended

particulate (TSP), carbonaceous species (OC, EC and WSOC)and inorganic species during clear days, haze and fog eventsare presented in Table 2 and their fractional contribution isshown in Fig. 3. Among the measured chemical species,organic matter (OM = 1.6 OC) contributes ~2530% ofthe TSP mass whereas the absorbing EC mass is only 2% atAllahabad and Hisar (Fig. 3). Organic matter often containsfunctional groups which comprise of elements such asoxygen, nitrogen and sulphur and an appropriate conversionfactor is needed to account for these elements. The conversionfactor (i.e. OM/OC ratio) is highly variable (range: 1.42.1)(Turpin and Lim, 2001) and depends on the sources oforganic aerosol, location and environment (urban, rural or

high-altitude). In this study, we have used a value of 1.6 toestimate the OM concentration at the urban sites (Rengarajanet al ., 2007; Ram et al ., 2010; Turpin and Lim, 2001).SO42 contributes ~7% to the TSP mass at both sites, followed by NO3 (8% at Hisar and 4% at Allahabad) and NH4+ (4%at Hisar and 2.5% at Allahabad). In addition, carbonaceousand inorganic species together contribute ~60% of theaerosol mass at Hisar whereas their contribution is ~50% atAllahabad (Fig. 3). The contribution of unidentified matter(UM), estimated by taking difference of OM, EC andinorganic constituents from the TSP, is of the order of 40%at Hisar and ~50% at Allahabad. The unidentified mattermainly consists of mineral dust with dominant contribution

from carbonate-rich minerals (such as CaCO3, MgCO3,dolomite [CaMg(CO3)2]), calcium sulphate (or Gypsum),and alumino-slilicates (Kruegeret al , 2004; Rengarajanetal , 2007; Zhang et al ., 2010). The high-level of Ca2+ concentration in our samples (Table 2) and neutralizationof acidic species by mineral aerosol (Table 3 and section

4.5) further attests our inference.

Mass Concentrations of Carbonaceous and Inorganic Species

The average mass concentrations of OC and EC are 49.0 14.1 and 6.2 2.0gC/m3, respectively at Allahabad andthose at Hisar are 30.9 17.9 and 3.8 1.4gC/m3,respectively. Although, mass concentrations of OC and ECare relatively higher at Allahabad; their fractional contributionis similar at both the urban sites (Fig. 3). In addition, massconcentrations of OC and EC at Hisar and Allahabad arealso similar with a recent data acquired during the wintertimefrom Kanpur, an urban site located at the center of the IGP(Ram et al ., 2010a). However, mass concentrations of OCand EC at urban sites in the IGP are an order of magnitudehigher compared to those at high-altitude site (Manora Peak)in the central Himalaya (Ramet al ., 2010b). The average

mass concentration of WSOC is 17.7 5.9 and 15.3 7.6gC/m3 at Allahabad and Hisar, and average WSOCcontributes ~32 and 36% of total OC, respectively.

The mass concentrations of cations follow the trend: NH4+ > Ca2+ > K + > Na+ > Mg2+ at Allahabad and Hisar.Trend for anions is: SO42 >NO3 > HCO3 > Cl for thesamples collected at Allahabad and NO3 > SO42 > HCO3 > Cl at Hisar. The average water-soluble inorganic species(WSIS: sum of the cations and anions) mass is 61.0 and 45.6g/m3 at Allahabad and Hisar, and contribute ~20 and 25%of the TSP mass, respectively. The fractional contribution ofindividual cation and anion (to WSIS mass) at both samplingsites is presented in Fig. 4. Among anions; SO42, NO3

and HCO3

are the major contributor to the WSIS masswhereas NH4+, Ca2+ and K + cations contribute significantlytoward the WSIS mass (Fig. 4). The mass concentrationsof NH4+, SO42 and NO3 at Allahabad contribute ~12, 33and 20% of the WSIS mass, respectively (Fig. 4). Thecontribution of SO42 and NO3 is similar (~28 and 29% of

Table 2. Mass concentrations ( 1) of carbonaceous (OC, EC and WSOC) and selected inorganic species (in the units ofg/m3) during the clear days, haze and fog events at two sampling sites in northern India. Aerosol absorption coefficient (babs;unit: M/m) and mass absorption efficiency (abs; unit: m2/g) of EC, at 678 nm, is also given. The scattering coefficient (bscat; at678 nm) is estimated using an average mass scattering efficiency of 3.0 m2/g (Ramet al ., 2012).

Species Hisar Allahabad

Clear Haze Fog Clear Haze FogTSP 147 21 199 19 162 29 212 45 247 42 302 95OC 31.5 9.7 34.9 12.8 31.6 8.8 33.8 9.5 45.1 8.5 60.1 12.7EC 3.5 1.0 4.0 1.4 4.1 1.3 5.0 2.1 6.4 2.5 6.9 2.6

WSOC 9.4 3.9 11.3 3.7 11.8 2.1 12.5 4.8 19.0 5.4 21.3 1.7 Na+ 0.7 0.1 0.8 0.2 0.7 0.1 0.4 0.1 0.8 0.2 0.8 0.1K + 2.5 0.7 2.6 0.6 2.7 0.7 1.8 0.4 2.5 0.4 2.9 0.5

Ca+2 3.4 0.4 4.4 0.8 2.7 1.0 5.8 1.2 5.2 2.2 6.2 2.4Mg+2 0.3 0.1 0.4 0.1 0.3 0.1 0.5 0.1 0.6 0.1 0.6 0.1

NH4+ 3.2 2.6 6.8 1.9 9.7 3.8 2.9 1.1 7.3 2.3 8.5 2.8Cl 0.4 0.4 0.4 0.1 0.4 0.1 0.1 0.1 0.1 0.1 0.3 0.2

NO3 9.9 5.3 16.7 4.2 17.1 5.6 5.5 1.9 9.9 3.7 12.4 6.0SO42 7.0 2.9 12.7 1.8 17.3 6.1 10.8 2.8 18.6 2.6 23.6 8.6

ba s 39.4 6.7 40.0 15.7 49.3 6.2 56.4 14.2 72.9 25.5 71.9 10.6abs 12.2 1.0 11.2 3.1 12.0 2.8 11.9 3.0 12.0 3.0 11.4 4.1 bscat 441 63 597 57 486 87 636 135 741 126 906 285


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Fig. 3. TSP mass closure at urban Hisar and Allahabad forthe samples collected during Dec 2004. OM refers to theorganic matter (= 1.6 OC), WSIS# (sum of inorganicspecies excluding SO42, NO3 and NH4+) and unidentifiedmatter (UM) is defined as TSP OM EC WSIS#.

the WSIS mass) and ~14% from NH4+ at Hisar (Fig. 4).Thus, secondary inorganic aerosol (SIA: NH4+, SO42 and NO3) contribute ~65 and 70% of the WSIS mass atAllahabad and Hisar, respectively.

Chemical and Absorption Properties of Aerosol duringClear Days, Haze and Fog Events

In general, total suspended particulate (TSP) andconcentrations of selected chemical species (EC, OC, WSOC, NH4+, SO42 and NO3) increases during haze and fogevents. Although, fog and haze events persist only for few

hrs (when RH > 80%; see Fig. 1), the increase in daily mass

concentrations of chemical species were significant at bothsites. The average ratios of mass concentrations of selectedchemical constituents during fog and haze events, withrespect to those during clear days, at Allahabad and Hisarare shown in Fig. 5. The average TSP mass increases by~2550% during haze-fog events at Hisar and Allahabad.The mass concentrations of OC, EC and WSOC also exhibitan increase of ~30% (Fig. 5). However, their fractionalcontribution (to TSP) increases only 5% during foggy days.The OC/EC and WSOC/OC ratios are also similar duringclear days, haze and fog events at both the sampling sites(Table 3).

Secondary organic carbon (SOC), estimated using theEC tracer method, contributes ~30% of total OC at urbansites in the Gangetic Plain during wintertime (Ram andSarin, 2010). The EC tracer approach provides an easierand convenient method to estimate SOC concentration.

However, the estimated SOC concentration can be associatedwith some degree of uncertainty due to varying nature andsource strength of primary emissions (biomass vs. fossilfuel emissions) and changes in the primary OC/EC ratio.Biomass burning emissions dominate in the IGP duringwinter months (Rengarajanet al ., 2007; Gustafssonet al .,2010; Ram and Sarin 2010; Ramet al ., 2012). Therefore, primary OC/EC ratio from biomass burning emission sourcesis reasonably constant so that the estimation of SOC is quitereliable in the present study. In addition, mass concentrationsof K +, a tracer of biomass burning emissions, shows anincrease (~3560%; Table 2) at Allahabad suggesting thatstrength of biomass burning emissions was higher during

haze and fog events. In addition, absorption coefficient(babs) also exhibits an increase by ~2530% during hazeand fog events (Table 2). Since, mass absorption efficiency(abs) remains similar during haze and fog events; theincrease in absorption coefficient is attributed to theincrease in EC mass concentration.

The mass concentrations of secondary inorganic aerosol(SIA: NH4+, SO42 and NO3) show a factor of 23 increaseduring haze and fog events (Table 2, Fig. 5). Amongsecondary inorganic aerosol; NH4+ shows the maximumincrease followed by SO42 and NO3 at both sites (Fig. 5).The fractional contribution of EC, OM and secondaryinorganic aerosol during clear days, haze and fog events

are presented in Fig. 6. It is observed that the contribution Table 3. Ratios of selected chemical species during clear days, haze and fog events at Hisar and Allahabad.

Species Hisar AllahabadClear Haze Fog Clear Haze FogOC/EC 8.9 0.5 8.6 0.4 7.7 0.9 7.2 2.2 7.7 2.5 9.2 1.9

WSOC/OC 0.29 0.05 0.33 0.03 0.38 0.05 0.36 0.10 0.42 0.07 0.37 0.08WIOC/EC 6.3 0.6 5.8 0.5 4.8 0.7 4.4 0.6 4.5 1.4 5.8 1.6

NH4+/SO42 (equivalent) 0.9 0.6 1.4 0.2 1.5 0.2 0.7 0.1 1.05 0.2 1.0 0.1 NO3/Ca2+ (molar) 1.5 0.7 2.2 0.7 3.6 1.4 0.6 0.1 1.3 0.2 1.3 0.5SO42/Ca2+ (molar) 0.8 0.3 1.1 0.3 2.1 0.8 0.8 0.4 1.8 1.0 1.9 1.1

NF (NH4+)# 0.48 0.32 0.70 0.08 0.82 0.12 0.50 0.14 0.74 0.21 0.70 0.18

NF (Ca2+

)#

0.72 0.49 0.41 0.06 0.23 0.14 0.94 0.24 0.47 0.20 0.49 0.25 NF (Mg2+)# 0.10 0.07 0.07 0.01 0.04 0.02 0.14 0.03 0.09 0.02 0.08 0.03 # NF stands for neutralization factor.


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Fig. 4. Mass closure of water-soluble inorganic species(WSIS) at Hisar and Allahabad.

of SIA increases from 12.5% (during clear days) to 23%(during fog events) at Hisar and 9.4 to 16% at Allahabad.

The increase in mass concentrations of NH4+

, SO42

and NO3 can not be attributed to the simultaneous increase inaerosol loading as their fractional contribution is also higherduring haze and fog events (Fig. 6). In addition, fractional

contribution of UM during fog events decreases to 30% atHisar and 40% at Allahabad. Furthermore, SO42/Ca2+ and NO3/Ca2+ molar ratios are also higher during haze and fogevents (Table 3). The mass concentration of Ca2+ is similarduring clear days, haze and fog events at both sites (Table2). Thus, the increase in mass concentrations of SO42 and NO3 can be attributed to secondary formation from theirrespective precursor gases.

Aerosol absorption coefficient (babs), measured at 678nm, varies from 24.9 to 54.6 M/m (1 M/m = 106 1/m) atHisar with an average of 39.9 9.1 M/m during Dec 2004.Similarly, babs ranges from 36.8 to 109.4 M/m (av.: 66.1 17.2 M/m) at Allahabad. Furthermore, absorption coefficientis relatively higher during fog and haze events at both urbansites (Table 2). In contrast, average mass absorptionefficiency (abs) of EC at both urban sites (11.1 2.6 m2/gat Allahabad and 11.3 2.2 m2/g at Hisar) are similar

during clear, fog and haze events (Table 2). Thus,relatively higher babs at Allahabad are attributed to thehigher EC mass concentrations. Theabs values, obtainedin the present study, are relatively higher compared tothose reported over Chinese regions; for example 7.7 m2/g(in PM2.5 samples collected at urban Guangzhou) (Andreaeet al ., 2008), 9.3 1.4 m2/g (for PM1.0) at urban Xinken(Cheng et al , 2008) and 8.45 1.7 m2/g for aerosol overBejing during wintertime (Chenget al , 2011). There is alack of scattering coefficient (bscat) measurements overnorthern India. However, more recently, Ramet al . (2012)have reported an average mass scattering efficiency of 3.0 0.9 m2/g for PM10 samples collected during OctNov

2008 at Kanpur. Assuming this value applicable for TSPsamples also, scattering coefficient (at 678 nm wavelength)ranges from 450 to 1600 M/m (at Allahabad) and 201 to 885M/m (at Hisar) during the observation period. The average

Fig. 5. The mass ratios of selected aerosol constituents, with respect to clear days, during fog and haze events: (a) and (b)at Hisar; (c) and (d) at Allahabad.


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Fig. 6. Aerosol chemical composition during fog, hazeevents and clear days at two urban locations in the IGP.Abbreviations are same as referred in Fig. 3.

absorption and scattering coefficients are summarized inTable 2 and both optical parameters are relatively higherduring fog and haze events at Hisar and Allahabad.

Sulphur and Nitrogen Oxidation Ratios ( SOR and NOR ) The 24-hr average meteorological data including ambient

temperature, relative humidity (RH) and wind speed duringclear days, haze and fog events at both the sampling sites are presented in Table 1. Although, only NO2 data is availableat Hisar; concentration of SO2 and NO2 gases are availableat Allahabad. Kulshresthaet al . (2009) have reported thatdaily-average concentrations of SO2 and NO2 varied from

5 to 43 g/m3

and 2 to 45 g/m3

, respectively during thesampling period at Allahabad. The average concentration ofSO2 at Allahabad during clear days, haze and fog, eventsare: 11.0 1.7, 15.6 4.6 and 17.2 10.9g/m3 whereasthat of NO2 are: 11.7 2.9, 26.8 9.7, and 17.4 6.2g/m3.It is observed that concentration of precursor gases as wellas those of SO42 and NO3 increases significantly duringhaze and fog events. Relative humidity is relatively higherduring haze and fog events at Allahabad (Fig. 1). Thus, itis evident that higher RH and concentrations of precursorgases during haze and fog events led to enhanced formationof SO42 and NO3 aerosols.

The acidic species, SO42 and NO3, are generally produced

by the oxidation of their precursor gases (SO2 and NOx,respectively) in the atmosphere under the favorableatmospheric conditions via homogeneous and heterogeneousoxidation processes. Homogeneous oxidation processesinvolves the gas-phase reaction of SO2 with OH radicalsand is a temperature dependent process. On the other hand,heterogeneous oxidation reaction of SO2 can take place ondust aerosol surfaces (Baueret al ., 2004; Kruegeret al .,2004), aqueous transformation though catalytic oxidation(via H2O2/O3 oxidation in the presence of Mn2+ and Fe3+)(Martin and Good, 1991; Seinfeld and Pandis, 1998; Tursicet al ., 2004) and in-cloud scavenging processes (Warneck,1999). It has been found that gas phase oxidation is more

important prior to fog formation and heterogeneous oxidationtake over after fog formation (Rattiganet al ., 2001a, b).Thus, the conversion of SO2 to sulfate depends on the

concentrations of precursor gases and oxidants, cloud cover,availability of surface area and meteorological conditionssuch as temperature and relative humidity (Liang andJacobson, 1999; Rattiganet al ., 2001a, b). For example,Rattiganet al . (2001b) investigated SO

2 oxidation in clouds

as a function of droplet size, and found that SO2 oxidationwas dominated by hydrogen peroxide when the pH of thecloud water was lower (in the range of 2.8 to 4.7).

Model simulations suggest that aqueous phase reactionscontribute ~two-thirds of the total sulfate production in thetroposphere (Warneck, 1999; Ungeret al , 2006). Since,ambient temperature is similar during clear days, haze andfog events (Table 1); the higher sulfate formation duringhaze and fog events can not be fully explained only by thehomogeneous gas-phase oxidation of sulphur dioxide. Tursicet al . (2004) concluded that heterogeneous reactions underhaze conditions can contribute significantly to formation of

secondary sulfate aerosol. Recent studies have reportedthat PM1.0 and PM2.5 contribute ~6580% of the PM10 mass at Kanpur, an urban site in the Gangetic Plain (Tareet al ., 2006; Ram and Sarin, 2011). Thus, conversion ofSO2, through heterogeneous reactions (either on surface oraqueous phase) is expected to be higher under favorablemeteorological conditions (lower ambient temperature andwind speed, high RH and SO2) and in the presence ofhigher particle concentration. However, this propositionand heterogeneous reactions needs a detailed investigationin the IGP.

The secondary NO3 formation depends on theconcentrations of NOx and NH3, ambient temperature, RH,

OH radical (in daytime) and the photochemical activity. Inaddition; nighttime secondary NO3 formation involves thehydrolysis of dinitrogen-pentaoxide (N2O5) through theheterogeneous reaction on aerosol surfaces and higherrelative humidity enhances the heterogeneous hydrolysisof N2O5 (Ravishankaraet al ., 1997; Jacob, 2000; Pathaketal ., 2009). Brown et al . (2004) have suggested that theconversion efficiency of NOx to HNO3 during nighttimecan be equivalent to the daytime photochemical conversion.Moreover, Vrekoussiset al . (2006) have found that ~55 65% of the total production of nighttime HNO3 and NO3 were through the heterogeneous reactions of NO3 radicalin an anthropogenically-influenced eastern Mediterranean

marine boundary layer.

The higher abundances of NOx andhigh RH may facilitate the secondary NO3 NO3 formationduring the wintertime in the polluted atmosphere of the IGP(Ram and Sarin, 2011). The presence of particulate nitrate isa temperature dependent equilibrium between ammoniaand HNO3 with lower ambient temperature favouring theexistence of ammonium nitrate (NH4 NO3) (Seinfeld andPandis, 1998). Furthermore, this equilibrium also depends onconcentration of sulfuric acid as NH4 NO3 formation followsammonium sulfate and ammonium bisulfate (see section 4.5).

In order to understand the oxidation of SO2 and NO2 precursor gases, we have investigated the efficiency ofoxidation of SO2 to SO42 and NO2 to NO3 during clear

days, haze and fog events at Allahabad. The sulphur andnitrogen oxidation ratios (SOR and NOR) were calculatedusing the following equations (Wanget al ., 2006; Tan et


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al ., 2009):

2 24 4 2[ ] /{[ [ ]}SOR SO SO SO

(1)

3 3 2[ ] /{[ [ ]} NOR NO NO NO (2)

where daily-average concentrations of all the chemicalspecies are reported in molar units. The sulphateconcentration represents non-sea salt (nss)-SO42 (wherenss-SO42 = [SO42] 0.2514 [Na+]) concentration and iscorrected for the contribution from sea-salt (Ramet al .,2010). However, the contribution of sea-salt SO42 is verylow (~0.10.2 g/m3) and contributes only 1% to total SO42 in our samples. The average SOR and NOR, at Allahabad,during clear days, haze, and fog events are presented inTable 4. The SOR and NOR show an increase during hazeand fog events (with the highest values during fog events)

and thus, suggesting higher oxidation of SO2 and NO2 during haze and fog events. The average SOR, at Allahabad,during clear, haze and fog events are 0.39 0.05, 0.45 0.12 and 0.52 0.15, respectively. Similarly, the average NOR, at Allahabad, during haze and fog events are 0.20 0.04 and 0.33 0.11 whereas those at Hisar are 0.30 0.04and 0.39 0.08, respectively. The NOR values are 0.25 0.08 and 0.23 0.15 during clear days at Allahabad andHisar, respectively. These values are compared with thoseover the Chinese region (Table 4). It is noteworthy that totalaerosol mass as well as concentrations of SO42 and NO3 is similar over urban sites in India and China butconcentrations of SO2 and NO2 are factor of 34 times

higher over China (Wanget al ., 2006; Rengarajanet al .,2007; Jihuaet al ., 2009; Tanet al ., 2009). However, NORand SOR over Chinese regions, during clear days, aremuch lower (except at Beijing) than those at Allahabadand Hisar. Thus, the oxidation efficiency of SO2 to SO42 and NO2 to NO3 aerosol are relatively higher over Indianregions. The diurnal variability of O3 concentration wererelatively higher (by ~11 ppbv) during the haze condition(compared to clear days) (Lalet al ., 2008). This observationsuggests an increase in photochemical activity withsimultaneous increase in OH production and thus, results inthe lower concentrations of non-methane hydrocarbons and NOx species (Lalet al ., 2008). These differences in oxidation

efficiency could arise due to the meteorology, photochemicalactivity, and the levels of OH radical which are expected to be higher in India than China due to the meridional variationsin solar flux (Volzet al. , 1981; Lalet al ., 2008).

Ion-charge Balance and Neutralization of Acidic SpeciesThe ratio of equivalent concentrations of cations (+: sum

of cations) and anions (: sum of anions) varies from 0.66to 1.05 at Allahabad. However, the average/+ ratio(0.90 0.13) and average HCO

3

concentration (10.3 3.4g/m3) suggest the alkaline nature due to the presence ofsignificant amount of mineral dust (also supported by highconcentrations of Ca2+ at Allahabad; Table 2). In addition,the long-range transport of aerosol from the Thar Desert (inwestern India) and African Deserts was observed during thesampling period at Allahabad (Ram and Sarin, 2010). Thisconclusion was also corroborated by high concentrations ofunidentified matter (UM), especially in Allahabad samplesand the contribution of mineral aerosol can be as high as50% of the TSP mass at Allahabad (Fig. 3).

The scatter plot between the equivalent concentrationsof NH4+ and SO42 at Allahabad and Hisar is shown in Fig.

7(a). Also, 0.5:1 and 1:1 equivalent lines for the existenceof ammonium sulfate [(NH4)2SO4] and ammonium bisulfate[NH4HSO4], respectively are shown in the figure. Asignificant correlation between NH4+ SO42 (R 2 = 0.66, n =61; Fig. 1) and NH4+/SO42 equivalent ratio 1 suggestnear-complete neutralization of sulphuric acid by ammonia.Typically, NH3 neutralizes particle-phase sulfuric acid toform ammonium sulfate and ammonium bisulfate. The NH4+/SO42 equivalent ratio varies from 0.2 to 3.6 (av.: 1.3 0.6) at Hisar whereas it varies from 0.6 to 1.2 (av.: 0.9 0.2) at Allahabad. The observed equivalent ratios suggesttheir probable existence as ammonium sulfate [(NH4)2SO4],[NH4HSO4] and NH4 NO3 salts in the urban environment of

the IGP. The formation of NH4 NO3 involves theheterogeneous reaction on existing primary and secondaryaerosol particles. A short-living complex, formed by protontransfer from HNO3 to NH3, produces an ion-pair of(NH4+) and (NO3) under high relative humidity conditions.The data points with NH4+/SO42 equivalent ratios greaterthan unity indicate the presence of excess-NH3 gas which can neutralize nitric acid. The major sources of gaseousammonia (NH3) are human and animal excretion duringthe metabolism processes; and those emitted from industrialactivities and fertilizers (urea, ammonium phosphate) usedfor agriculture purposes in the IGP (Ramet al ., 2010). Thegaseous ammonia is trapped in the lower atmosphere due to

the shallow boundary layer height and provides ammonia-rich environment during fog-haze conditions. In contrast,data points with NH4+/SO42 equivalent ratios less thanunity, especially Allahabad samples, suggest NH3-deficient

Table 4. Intercomparison of nitrogen and sulfur oxidation ratios during clear days, haze and fog events.

Site Time-period NOR ## SOR ## ReferenceClear Haze Fog Clear Haze Fog

Allahabad Dec-04 0.25 0.08 0.20 0.04 0.33 0.11 0.39 0.05 0.45 0.12 0.52 0.15 This studyHisar Dec-04 0.23 0.15 0.30 0.04 0.39 0.08 This study

Guangzhou winter-2002 0.06 0.15 0.08 0.15 Tanet al . (2009)summer-2002 0.04 0.22 0.1 0.16 Tanet al . (2009)

Beijing 2001-04 0.18 0.29 0.17 0.27 Wanget al . (2006)Guangzhou Dec 07Jan 08 0.09 0.24 0.22 0.29 Jihuaet al . (2009) ## Nitrogen and sulfur oxidation ratios.


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environment and the possible association of NO3 with Ca2+ and Mg2+ as calcium nitrate [Ca(NO3)2] and [Mg(NO3)2],respectively. This is further reflected by the scatter plot between NH4+ and [NO3 + SO42] at the two sampling sites(Fig. 7(b)). All the data points, except one, fall below the linewith slope of unity indicating that ammonia is not sufficient tocompletely neutralize the acidic gaseous species and invokethe possible role of mineral aerosol toward the neutralization process. The scatter plot between [Ca2+ + NH4+] and [NO3 + SO42] indicate nearly complete neutralization of acidicspecies (Fig. 7(c)). Furthermore, we have calculated theneutralization by the cations (NH4+, Ca2+ and Mg2+) duringclear days, haze and fog events, and neutralization factors(NF; defined as the equivalent ratio of cation to the acidicspecies) are presented in Table 3. Although, ammonia is themajor neutralizing agent of the acidic species at Allahabadand Hisar; Ca2+ is the second neutralizing agent. In addition,

the neutralization by Ca2+

becomes more prominent duringclear days at both the sampling sites (Table 3).

Fig. 7. Scatter plots between (a) NH4+ and SO42 (in eq/m3

units), and (b) NH4+

and [SO42

+ NO3

], and (c) [Ca2+

+ NH4+] and [SO42 + NO3] at Allahabad and Hisar duringthe wintertime.

CONCLUSIONS AND IMPLICATIONS

Mass concentrations of carbonaceous and inorganicspecies, together with the oxidation ratios of SO2 and NO2,studied during wintertime, have led us to infer that organicmatter is a major constituent of aerosol (~2530% of TSPmass) in the Indo-Gangetic Plain. Water-soluble inorganicspecies contribute ~20 and 25% of the TSP mass atAllahabad and Hisar, respectively. A factor of 23 increasein the mass concentration of secondary inorganic aerosols(NH4+, SO42 and NO3) is observed at the sampling sites inthe IGP during haze and fog events. The scatteringcoefficient is relatively higher at urban sites in the IGP andis closely related to meteorological conditions and aerosolchemical composition. Carbonaceous and inorganic aerosols,associated with high mass scattering efficiencies, contributesignificantly to total aerosol mass and scattering coefficient

(as evident from the present study). Therefore, the reductionof carbonaceous and inorganic aerosols could effectivelyimprove the visibility over the Gangetic plain duringwintertime.

The sulphur and nitrogen oxidation ratios (SOR and NOR)are significantly higher during haze and fog events indicatingenhanced oxidation efficiency of precursor gases of SO42 and NO3. Secondary inorganic aerosol contributes ~65 and70% of the WSIS mass at Allahabad and Hisar, respectively.The prevailing meteorological conditions (higher RH) andthe presence of high levels of pollutants and condensationnuclei may be a likely cause for the fog-haze formationover the IGP which is also favoured under secondary

aerosol formation and their hygroscopic growth during thewintertime. Subsequently, changes in morphological featuresand physico-optical properties of ambient aerosol in the presence of high concentrations of acidic species of theIGP requires detailed investigation in order to understandoptical properties and their impact on radiation budget andregional climate.

ACKNOWLEDGEMENT

Technical help provided by UC Kulshreshtha in collectionof aerosol sample at Allahabad is highly appreciated. Theauthors would like to acknowledge the financial support

provided from ISRO-GBP office (Bengaluru, India). We arethankful to two anonymous reviewers for their constructivecomments and suggestions to our manuscript.

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Received for review, July 22, 2011

Accepted, February 15, 2012

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