• rain chemistry;
  • pH;
  • trace metals;
  • MSA;
  • major ions


  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[1] Rainwater samples for chemical analysis were collected over the Indian Ocean during the Indian Ocean Experiment (INDOEX) campaign January–March 1999 on board the research vessels Ronald H. Brown and Sagar Kanya. Samples were analyzed for major ions and some trace metals. The rainwater data are interpreted in terms of transport from potential source regions in Asia using air mass trajectories covering 10 days. A comparison is also made between the rainwater data and the concentration of aerosol components measured simultaneously on the ships. The concentrations of nonsea-salt (nss)-SO42−, NO3, NH4+, nss-K+, and nss-Ca2+ in rainwater over the Indian Ocean, while a factor of 2 to 3 lower than over the Indian continent, were still clearly influenced by pollution and soil sources in Asia. The concentration of nss-Ca2+ decreased more rapidly as the air moved southward from the continent out over the ocean, whereas the concentration of nss-SO42− became relatively more abundant. This was consistent with the observed higher acidity of the rainwater over the ocean (pH in the range 4.8 to 5.4) than over the Indian subcontinent, with NH4+ as the main cation (rather than Ca2+, as over land). Variations in the concentration of Al and Fe correlated well with those of nss-Ca2+, indicating a crustal source for these elements. The relation between Na+, Cl, and Br in the rainwater was close to that of seawater, implying no excess or deficit of the two halogen ions. The ratio between the concentration in rainwater and the concentration in surface air was systematically larger for aerosol components that exist in the coarse mode of sea-salt origin (Na+, Mg2+, and Cl) than those in the fine mode (NH4+, nss-K+, and nss-SO42−), indicating that fine-mode particles are scavenged mainly by in-cloud processes whereas coarse-mode sea-salt particles are scavenged also by falling raindrops under the clouds. Nss-Ca2+ and NO3 fall in a category in between, indicating that these compounds are not as effectively removed by below-cloud scavenging as sea-salt aerosols.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[2] Rain chemistry measurements serve the dual purpose of estimating the deposition of potentially harmful or beneficial components to ecosystems and of quantifying the wet scavenging of atmospheric constituents. Together with measured atmospheric concentrations of related constituents they can also be used to test regional and global chemical tracer transport models.

[3] The Indian Ocean Experiment (INDOEX) [Lelieveld et al., 2001; Ramanathan et al., 2001] was aimed at obtaining a comprehensive picture of the transport, composition, and radiation properties of atmospheric aerosols and related gaseous components over the Indian Ocean during the winter monsoon season. The research vessels Ronald H. Brown and Sagar Kanya provided a possibility to obtain precipitation samples with a minimum of contamination from the ship or from nearby terrestrial sources. The study area was Indian Ocean and Arabian Sea from 20°S to 20°N. Comprehensive rain chemistry measurements from this region have not previously been reported. Kulshrestha et al. [1999] reported data on pH (only) from rain samples collected without local contamination from the research vessel Sagar Kanya in 1998. Rain chemistry data have also been obtained regularly from the Minicoy Island north of the Maldives [Mukhopadhyay et al., 1992], but little was known in this case about the contribution from local sources, particularly from soil dust. Regular rain chemistry measurements are being carried out at the remote Amsterdam Island (40°S) in the southern Indian Ocean [Moody et al., 1991].

[4] The aim of this study has been to contribute to the understanding of scavenging by precipitation of pollutants (aerosols and soluble trace gases) as well as of naturally emitted components in airflowing out from the Asian continent over the Indian Ocean during the winter monsoon season. This is done by measuring the chemical composition of precipitation and relating it to observed concentrations of gaseous and particulate compounds in the same air masses and to measurements at upstream continental locations. Specific questions we wished to answer were the following: (1) How is the pH of rainwater modified as air is transported out from the Asian continent and which ionic constituents contribute to this modification? (2) What is the relative contribution of different man-made and natural sources to the chemical constituents found in rainwater over the ocean? (3) What are the scavenging ratios for the different ionic constituents in coarse and fine particle modes?

2. Route of the Expedition and Measurement Systems

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

2.1. Route, Observing Periods, Platform, and Sample Collectors

2.1.1. Observing Periods

[5] The measurements were made from Ronald H. Brown between 22 February (Day of year, DOY 53) and 30 March (DOY 89) and from Sagar Kanya between 21 January (DOY 21) to 10 March (DOY 69) 1999. The Ronald H. Brown cruise was divided into three legs. Leg 1 started from Port Louis, Mauritius (20.2°S, 57.5°E) on 22 February (DOY 53) and ended at Male in the Maldives (4.1°N, 73.3°E) on 28 February (DOY 59). Leg 2 started from Male on 4 March (DOY 63) and followed the Indian west coast northward to latitude 19 °N. Thereafter the ship headed southwards into the Indian Ocean to latitude 12°S and then returned to Male on 23 March (DOY 82). Leg 3 started from Male on 26 March (DOY 85) and went into the Bay of Bengal and returned to Male on 30 March (DOY 89). The Sagar Kanya cruise started from Panjim (15.7°N, 73.9°E) on 21 January (DOY 21) and followed longitude 77°E down to latitude 20°S and then headed west to reach Mauritius on 10 February (DOY 41). The second part started from Mauritius on 18 February (DOY 49) and headed north to latitude 15°N and then east toward India and to a final arrival in Panjim on 10 March (DOY 69). Figure 1 shows these cruise tracks and the location of the rain events.


Figure 1. Cruise track of Ronald H. Brown (R) and Sagar Kanya (S) during the INDOEX cruises 1999. Rain events are denoted with the day of the year (DOY).

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2.1.2. Rainwater Sampling and Platforms

[6] The rain was mainly collected in wet-only samplers. The wet-only collectors were of MISU (Department of Meteorology, Stockholm University, Sweden) design, consisting of a cylindrical part with a polypropylene funnel and a bottle inside with a lid in polypropylene making an almost tight seal against the collector.

[7] Onboard Ronald H. Brown rainwater was collected on an event basis with two wet-only collectors and occasionally with a manually operated bulk collector especially designed for trace metal analysis. The first wet-only collector-a was located just above the bridge in the forward port side corner about one meter outside the railing. The second wet-only collector-b was located in a bow tower 16 m above deck on the port side one meter outside the tower. The diameter of the collecting surface was 200 mm. During Leg 3 the tower collector-b was replaced with a larger unit with a 400 mm diameter opening to enable larger sample volumes. Both wet-only collectors could be rotated to face the wind and tilted to bring the opening perpendicular to the path of the raindrops. During ship operation, the opening of the lid was controlled by an optical rain gauge to make it open during rain events. In order to limit the sampling to clean air, unperturbed by possible contaminated episodes, the opening of the lid was also controlled by the ship's pollution sensor [Quinn et al., 2002]. The bulk collector was located opposite collector-a just above the bridge and mounted on a plastic arm on the starboard side about 2 m outside the railing. The bulk collector could also be positioned with the collecting surface perpendicular to the rain. For each sampling event a new funnel (50 mm diameter) and bottle was used.

[8] Onboard Sagar Kanya two wet-only collectors were located at the railing in the forward part of the deck below the bridge, one on each side. Lid opening and closing was initiated manually by a remote control, operated by a man on watch.

[9] Ten rain events were sampled from Ronald H. Brown of which eight were retained after scrutinization. From Sagar Kanya five samples were retained out of six possible. The retained samples were analyzed for 13 components. Additionally four samples were obtained from Ronald H. Brown for trace metal analysis.

2.1.3. Aerosol Mass Sampling and Platform

[10] In this study only aerosol data collected in connection with rain events on board Ronald H. Brown will be used. The air intake for collection of particulate mass was located at the top of a 6 m mast in front of the bridge. The height of the collection point was 18 m above sea level. The lower 1.5 m of the mast was heated to dry the aerosol to 55 ± 5 % RH. For details of the sampling manifold, see Quinn et al. [2002]. The same pollution sensor as used for the rain collectors excluded direct contamination from the ship.

[11] Ambient submicron aerosol (D50% = 0.9 μm equivalent aerodynamic diameter (EAD), here referred to as fine particle mode, fpm) mass concentration was determined using a filter pack set (flow rate 50 dm3 min−1) mounted on a sliding tray next to the entrance of the mast. The set held three units: two of them collected samples and one served as a sampling and an analytical blank. Each of the units consisted of one 47 mm Millipore Teflon aerosol particle filter with 1.0 μm pore size held in a polyacetal (Deldrin®) filter holder. More details are given by Leck and Persson [1996]. Supermicron aerosol mass (1.1 μm < D50 (55% RH) < 10μm EAD, here referred to as coarse particle mode, cpm) was collected with an impactor located at the end of the heated compartment of the mast. The supermicron impactor stage used a 90-mm Tedlar film substrate. Each sample prolonged for about 6 hours.

2.2. Rainwater Handling, Analysis, and Quality Control

[12] All wet-only collectors were cleaned every morning and late evening with a plastic brush and de-ionized water (18 MΩ cm) from a spray bottle. At times the wash water was collected for analysis to serve as a check for possible contamination (see below). The water amount of each sample was determined by weighing on an electronic balance. Sample volumes corresponded to precipitation amount ranging from 0.4 to 41 mm. Conductivity and pH of the rainwater was measured on board both ships. The samples were transferred to 50 cm3 bottles. To prevent biological degradation of the organic acids a preservative was added (300 mg thymol per dm−3 [Gillet and Ayers, 1991]). All samples were cold stored until analysis in the MISU laboratory after the cruise. Plastic gloves were used during all handling of collectors and sample bottles. The brush was kept in a plastic bag between each cleaning.

[13] The bulk funnel and bottle collectors onboard Ronald H. Brown were thoroughly acid washed (in advance) after which each unit was kept ready to use in a plastic bag. After collection the funnel was replaced with a lid, the bottle put in a plastic bag and stored in the cold with no further treatment until analysis.

2.2.1. Analyses of Major Cations, Anions, and Week Acids

[14] The rainwater samples were analyzed for major cations (ammonium: NH4+, sodium: Na+, potassium: K+, magnesium: Mg2+, and calcium: Ca2+) and anions (fluoride: F, chloride: Cl, bromide: Br, formate: HCHO, acetate: CH3CHO, methane sulfonate: CH3SOO (will be referred to as MSA), nitrate: NO3 and sulfate: SO42−) by ion chromatography (IC, Dionex DX100 (cations)/DX500 (anions)). The analysis of cations was made with Dionex CG12A/CS12A columns and a CRSR-I auto suppressor with external water mode and 20 mM MSA eluent (isocratic) with a flow of 0.25 cm3 min−1. The anions were analyzed with Dionex AG11/AS11 columns and an ASRS-1 autosuppressor mode, eluent NaOH 2.5 mM at 0.5 cm3 min−1 for weak anions, gradually increasing to 44 mM to elute strongly retained anions.

[15] An analytical quality check of the analyses was done with adding both internal and external reference samples (NIS). An additional reference was obtained from seawater collected during the expedition, which was diluted 1:1000 and 1:5000 to equal 470 and 94 μmol−1 Na+ respectively. This to match general levels in the precipitation samples. The calibration was adjusted so that the composition obtained for the seawater samples agreed with standard seawater composition.

[16] The precision in the analyses of major anions, cations, and MSA was better than ±3%, ±2% and ±3% (1 sigma), respectively. The overall analytical accuracy was better than 5%, 7% and 15% for the major anions, cations, and MSA, respectively. The analytical detection limits for the various ions, defined as twice the level of peak-to-peak instrument noise, were 0.20, 0.05, 0.10, and 0.25, 0.02, 0.01, and 0.0005 μmol dm−3, for Na+, NH4+, K+, and Cl, NO3, SO42−, and MSA, respectively.

[17] The nonsea-salt (nss) fractions of the ionic components were calculated from Na+ measured in the sample and bulk seawater composition (with the following molar ratios: K+/Na+: 0.0212, Mg2+/Na+: 0.113, Ca2+/Na+: 0.022, F/Na+: 0.000145, Cl/Na+: 1.165, Br/Na+: 0.00177 and SO42−/Na+: 0.060). Although it is difficult to estimate the uncertainties of the calculation of nss fractions, due to possible fractionation during sea-salt production and small errors in calibration, we believe that these uncertainties do not compromise the conclusions.

2.2.2. Analyses of Trace Metals

[18] For trace metal analysis, the samples were acidified at arrival to the laboratory with 2 cm3 14 M nitric acid (HNO3: distilled by sub-boiling in a quartz apparatus) per liter of sample to dissolve any precipitated components. The trace metals (vanadium, V; manganese, Mn; thallium, Tl; aluminum, Al; iron, Fe; cadmium, Cd; lead, Pb; and zinc, Zn) were analyzed with inductively coupled plasma mass spectrometry (Perkin-Elmer Sciex Elan 5000). The analyses of iron were made by graphite furnace atomic absorption spectrometry (Perkin-Elmer Z3030) with duplicate injections to improve sensitivity. The method thus measures soluble compounds after extraction in strong acid. In addition, it can not be ruled out that some particulate material is also volatilized and determined.

2.2.3. Quality Control

[19] To show possible contamination, wash water was collected and analyzed both from Ronald H. Brown and Sagar Kanya. The funnel was washed with a spray bottle after 12 h with the lid closed representing contribution from any dry deposition and from handling and washing, referred to as “w1”. Washing was repeated after the funnel had been cleaned, representing contamination during handling and washing, referred to as “w2”. The wash water from the spray bottle was also collected for analysis. It had concentrations less or much less than 0.1 μmol dm−3. On Ronald H. Brown the concentrations in w2 were usually slightly lower than in w1 in absolute numbers, w2/w1 ratios ranging from 0.1 to 0.5. Compared to rain samples with Southern Hemisphere origin (SHo-see further on classification under Trajectories below), which usually had the lowest concentrations, the w1/SHo ratios were 0.04 or less for Na+, NH4+, Mg2+, acetate, Cl, NO3, and nss-SO42− and 0.1 for MSA. For nss-K+ no ratio could be given since SHo value was close to zero. Blank concentrations of these compounds were thus without any practical influence. For nss-Ca2+ the ratio was higher, 0.7. Formate concentrations in w1 were variable with a median w1/SHo ratio of 0.8. On Sagar Kanya the concentrations in w2 were similar to those on Ronald H. Brown but the w1 concentrations were generally higher and more variable with median w1/SHo ratios less than 0.2 for Na+, NH4+, Mg2+, Cl, NO3, and nss-SO42− and 0.4 for formate. The ratio was around 2 for nss-K (relative to uncertainty level), nss-Ca, acetate, and MSA. The overall conclusion is that the samples could be collected without handling errors (w2) and that the lid, especially on Ronald H. Brown with automatic operation, provided a good seal against contamination during dry periods. For samples from Sagar Kanya with low concentrations, the data for nss-K, MSA, acetate, formate, and nss-Ca2+, may be associated with some positive bias. However, this bias is small enough not to affect the conclusions drawn below, for instance about origins of compounds and removal during transport.

[20] The trace metal concentrations in wash water (w1) were below those in SHo samples in case of Al (ratio of 0.5), about equal for Mn, Fe, and Pb and below detection for the remaining V, Cd, and Tl. Compared to the men values from samples of Northern Hemisphere origin, the wash water concentration ranged from a ratio of 0.3 (V and Mn) to 0.05 (Tl). There was thus probably a positive bias due to artifacts during sampling which made the results for the lowest values uncertain. In the case of Zn the blank values were too high and all data were rejected.

[21] As a further indication on the possible risk of contamination due to sampling and handling a comparison of the simultaneous sampling at two locations (collector-a and -b) was made. On Ronald H. Brown the deviation was usually less than 10% but larger differences were also found in the following cases. On one occasion the difference was a factor 10 for Na+ and 5 for nss-SO42−. As no other explanation than contamination in the sampling and/or handling was offered these samples were removed. On one other occasion the concentration ratio (collector-a/b) was 1.7 for the sea-salt elements and 0.7 for NH4+, NO3, and nss-SO42−. However, this difference could be explained by the fact that collector-b started half an hour later due to technical problems. In this case only data from collector-a was used.

[22] On one occasion (DOY 54) rainwater was fortuitously collected both on Ronald H. Brown (54.9R) and Sagar Kanya (54.3S) when they were in the same area, in clean SHo air. All nss components agreed satisfactorily except for one unexplained high NO3 value on board Sagar Kanya (54.3S) that probably was due to pollution. The value is marked “cv” in Table 1 and was omitted in any calculation of mean values.

Table 1. Concentrations in Rainwater Samples During INDOEX 1999a
DOYTrajectory ClassPrecipitate Amount, mmpHNa+NH4+nss-K+Mg2+nss-Ca2+nss-FAcetateFormateMSAClBrNO3nss-SO42−
  • a

    Concentrations are given in micromoles per cubic decimeter and taken as mean values for the two wet-only collectors at each ship. NHo and SHo are referred to as Northern and Southern Hemisphere origin respectively. References from (1) Norman et al. [2001] and (2) Granat et al. [2001]. Here, nd = not detected; cv = contaminated value; #d = number of days since last contact with land.

78.9RA, 7d (NHo)0.44.931223.
79.3RA, 7d (NHo)1.25.19941.
87.4RA, >10d (NHo)41.05.08374.
78.3Rc Ind, >10d (NHo)1.55.141266.
79.9Rc Ind, 6d (NHo)0.45.181308.20.614.
81.5Rc Ind, 3d (NHo)1.04.9411613.40.813.
28.7Sse Ind, 2d (NHo)3.35.37824.
29.2Sse Ind, 3d (NHo)4.25.26731.
29.3Sse Ind, 3d (NHo)3.05.021751.8−
58.9Rs Ind, 4d (NHo)8.64.791811.
87.5R87.4R, late9.05.17121.
87.5R/87.4Rratio0.22 0.330.350.000.320.500.500.670.502.800.350.000.250.69
medianA, 7d0.85.11082.
medianA, >10d41.05.1374.
medianc Ind1.05.11258.20.813.
medianse Ind3.85.1773.
ref. (1)Pune (west) 5.45329.03.11.312.0    6 18.026.8
ref. (2)Bhubaneswar (east, 1yr) 5.601217.   13 10.68.7
mean ref. (1) and ref. (2)Pune (west) & Bhub. 5.52823.    10 14.317.8

[23] As a final quality control the ion balance (Σc − Σa)/(Σc + Σa), where c and a represent cations and anions, was calculated for each sample. The agreement was better than 1% for all samples except for three samples which showed better than 3% and one with 5% deviation (less anions).

2.3. Aerosol Mass Procedures and Analysis

[24] All filter substrate, samples, and blanks used were carefully handled in a glove box (free from particles and gases such as ammonia (NH3) and sulfur dioxide (SO2)) both prior to and after collection. After exposure all filters and substrates were extracted (inside the glove box) by wetting with 0.5 cm3 methanol and 4.5 cm3 de-ionized water. The extracts were then analyzed onboard for major cations and anions and MSA using chemically suppressed IC [Quinn et al., 1998]. The analysis of cations used Dionex CG12A/CS12A columns and a CRSR-I auto suppressor. Strong anions were analyzed with Dionex AG4A/AS4A columns and a CSRS-1 membrane suppressor. MSA was analyzed with a Dionex AG4/AS4 column using a stepwise elution. A Dionex ATC-1 column was used prior to the injection valve in order to retain ionic contaminants in the eluent during the run.

[25] The sea salt contribution was on average less than 4% of the total submicrometer SO42− concentrations. The average particulate MSA, nss-SO42− and NH4+, and blank concentrations were <3%, 5%, 15% of the sample, respectively. Duplicate samples for the submicron aerosol agreed on average, within 15%.

2.4. Supporting Data

2.4.1. Trajectories

[26] In order to interpret the chemistry data in terms of source origin and transport we used calculated air trajectories arriving at the location of the ship (at 950 hPa) at each rain event. The calculations were made with the McGrath [1989] trajectory model, using 3-dimensional analyzed wind and mass fields from the ECMWF (European Center for Medium Range Weather Forecasts).

[27] The following five patterns of trajectories were selected (see Table 1) in an attempt to identify source regions and duration of transport over sea:

  1. Arabian Peninsula and surrounding areas followed by about seven days over the Arabian Sea (A, 7d).
  2. All 10 days over the sea, east and south of India. (A, >10d). The samples in these two first categories were collected in the SH but north of the ITCZ and in air masses with a NH origin (NHo).
  3. Central India followed by 3.5 days over sea, preceded by transport over Africa, Middle East, and northern India within the free troposphere. To this group was also allocated two events (78.3R and 79.9R) where the trajectories oscillated between central India and Arabian Peninsula (c Ind, #d).
  4. Traversing southern tip of India and then two to three days over the sea preceded by transport over Africa, Middle East, northern India, and Bay of Bengal. Before reaching northern India the transport was well within the free troposphere, (se Ind, #d).
  5. Thailand, traversing Bay of Bengal, passing the southern tip of India and then 4 days over the sea
  6. Southern Hemisphere origin south of the intertropical convergence zone, ITCZ (SHo).

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[28] The rainwater concentration of major components is given in Table 1 and of trace metals in Table 2. Ship and decimal day number (DOY) identify the rain events. Samples represent the whole duration of each rain event as encountered on the ship except for one case (87.5R) when two fractions were collected. For comparison, we have selected some results from rain chemistry measurements in India which we consider to be representative for rural conditions. These are median concentrations from station Pune (in the western part of India) during days with trajectories indicating transport from the Bay of Bengal across India to Pune [Norman et al., 2001] and a yearly weighted mean from station Bhubaneswar in eastern India [Granat et al., 2001].

Table 2. Trace Metal Concentration Measured on Board Ronald H. Browna
SampleTrajectory classAlVMnFeCdTlPb
  • a

    Trace metal concentrations are in nanomoles per cubic decimeter.

87.4RA, >10d990.84.6930.080.0101.6
79.3RA, 7d2540.44.71810.04<0.0051.2
58.9Rse Ind, 4d3172.814.11610.330.0188.4
54.9RSHo68< 0.41.6180.12< 0.0050.5
mean NHo(A, >10d; A, 7d; se Ind, 4d)2231.37.81450.150.0103.7

3.1. Spatial Distribution of pH, nss-SO42−, NH4+, and nss-Ca2+

[29] As shown in Figure 2a the variation in pH was modest, ranging from 4.8 to 5.4. This narrow range of pH was in sharp contrast to most measurements over the Indian subcontinent, which in general showed much higher pH values [Rao, 1997]. Many of the measurements summarized by Rao were made using bulk collectors - often in urban environments - and are likely to have been affected by local sources of alkaline dust. pH values in better agreement with the ones observed during this study have been reported by two recent studies, one outside Bhubaneswar in eastern India [Granat et al., 2001] and one from Pune in western India during easterly winds [Norman et al., 2001]. In these two studies great care was taken to obtain values representative for larger regions.


Figure 2a. Measured pH values and associated 10 days backwards trajectories for each rain sample. Data on pH from a compilation of measurements in India and nearby island were taken from (1)Norman et al. [2001], (2)Granat et al. [2001], and (3)Rao [1997].

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[30] The acidity of precipitation is dependent on a balance between the uptake of acid and alkaline compounds. The concentration of H+ in the raindrop equals the difference between the sum of the negative ions and all other positive ions. In India, as in many other continental regions, the dominating negative ions are nss-SO42−, NO3, and HCO3 and the dominating positive ions, in addition to H+, are nss-Ca2+ and NH4+. For the range of pH encountered over the ocean the contribution of HCO3 was neglected. NO3 did contribute to the balance over the ocean, but its importance was less than that of SO42−. As a consequence, we have focused our discussion mainly on the remaining three ions, i.e., nss-Ca2+, NH4+, and nss-SO42−, assuming that the sea salt components do not contribute to the balance. The contribution from acetate and formate was usually relatively small, with exception of the “A” cases (air from Arabian Peninsula) and case 58.9R, see Table 1.

[31] The association between the trajectories and the concentrations of nss-Ca2+, NH4+, and nss-SO42− is shown in Figure 2b. No obvious spatial pattern was evident in the concentrations of these ions, but they vary systematically with trajectory type, c.f. Table 1. For example, the average concentration of NH4+ during days with trajectories originating from NH and SH was 3.3 and 0.4 μmol dm−3, respectively. The corresponding difference for nss-Ca2+ and nss-SO42− was much less. The influence of trajectory types will be discussed in more detail below.


Figure 2b. Same as in (a) but for concentrations in μmol dm−3 of nss-Ca2+, NH4+, and nss-SO42, respectively.

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3.2. Sources and Transport

[32] The sources of the ionic components and metals observed in the rainwater samples can be either terrestrial or marine. Terrestrial sources include fossil fuel combustion and other industrial activities, biomass burning and other agricultural activities, and wind-blown soil-dust. The Indian subcontinent can be suspected to be the most important origin in these cases, but more distant sources can not be excluded. Of the analyzed components the most important marine sources are primary sea salt particles from sea spray and secondary aerosols produced from gaseous precursors like dimethyl sulfide (DMS). Great care was taken to avoid pollution from the ships themselves, cf. section 2.1.

[33] In order to interpret the variability of the observed concentrations of the various constituents, as exemplified in Figures 2a and 2b, the statistical correlation between their concentrations and their relation to trajectories were investigated. A separate discussion of the sea salt source is also included in this section.

3.2.1. Correlation Between Components

[34] A factor analysis (principal component analysis with Varimax rotation) including all analyzed rain components gave three factors with the following components (see Table 3 for values of multiple correlation coefficients, as r): (1) NH4+, nss-K+, nss-Ca2+, NO3, and nss-SO42−, (2) acetate and formate, and (3) MSA.

Table 3. Correlation Coefficients for Ionic Components in Rain

[35] This statistical analysis alone could not distinguish between soil derived components, such as nss-Ca2+ and those derived from gaseous precursors, such as nss-SO42− and NO3, implying that their source regions are similar.

[36] MSA did not correlate with any component including nonsea-salt fractions, with the exception of Na+ where a modest correlation was found (0.72). The correlation with Na+ may be due to a larger aerosol surface area enhancing the transfer from vapor phase as previously suggested [e.g., Quinn et al., 1993; Huebert et al., 1993].

[37] Acetate and formate were well correlated with each other (0.92). Molar concentration of formate was about 1.4 times that of acetate. Both components seem to have derived from the same source located in the Northern Hemisphere (NH) but not related to sources of the other compounds. This could be explained by a gradual production from long-lived gaseous precursors, which can be expected to be more abundant in NH than in SH.

[38] Nss-F was lower in SHo samples but did not correlate well with any other compound. Its presumed origin in combustion and process industry did not show up as a correlation with nss-SO42−.

[39] Among the trace metals two groups could be distinguished (not shown here). One consisting of Fe and Al, well correlated with each other and with nss-Ca2+, probably representing crustal material. The other consisted of V, Cd, Tl, and Pb that were well correlated with each other (r > 0.9 uncertain due to few samples) and with nss-SO42−, thus indicating anthropogenic sources.

3.2.2. The Sea Salt Source

[40] The production of sea salt aerosols due to bursting of bubbles in seawater is known to be related with wind speed. Two different size ranges of particles have been identified, film drops and jet drops. Day and Lease [1968] found the film drops, released into the air when the bubble film of water burst, to be around 100 nm in diameter while Blanchard and Woodcock [1957] found jet drops that were detached from the up moving water replaced the bubble film around 1 μm in diameter tailing toward larger sizes with increasing wind speeds. Several studies have found the logarithm of the aerosol number or mass concentration to be linearly related to wind speed. We found a similar relation to hold also for the concentration of sea-salt in precipitation:

  • equation image

[41] The best correlation (r2 = 0.69) was found if the maximum 24 hours average wind speed during the last 3 days backwards along the trajectories was used instead of the local wind speed. This indicates that the Na+ in the rain samples was not only of local origin. This studies rainwater slope of 0.11 was in good agreement with those reported for both number and sodium mass over the central Arctic Ocean [Nilsson et al., 2001, [Leck et al., 2002] and applied to the jet drop mode sized particles.

[42] The relative concentration of the sea salt components Cl and Br in rain may be modified from that of seawater by atmospheric processes or by contributions from terrestrial sources. Chlorine can be lost from the aerosol as gaseous HCl when sulfate, produced by aqueous oxidation of SO2 or condensation of sulfuric and methane sulfonic acids, displaces chlorine [Keene et al., 1998].

[43] The molar ratio for a specific particle of pure seawater NaCl introduced into the atmosphere is 1.16. The ratio at a later time will depend on how long it has been in the atmosphere, its original mass, and the rate at which it accumulates acid. Table 4 shows the mean of Cl/Na+ molar ratios encountered as a function of aerosol size (fpm and cpm). The mean fpm ratios as very different from the cpm ratio suggesting that the submicron particles were in general more likely to have an acid component than the supermicron particles. This in agreement with reported results elsewhere (e.g., Leck et al., submitted manuscript, 2000). The mean cpm Cl/Na+ ratio was slightly lower than that of pure seawater, which suggests the importance of a local or near local NaCl aerosol source.

Table 4. Mean Aerosol Concentration for Fine Particle Mass (fpm) and Coarse Particle Mass (cpm) During the Rain Eventsa
Aerosol concentrations, nmol m−3Cl/Na+Na+NH4+K+nss-K+Mg2+Ca2+nss-Ca2+MSAClNO3SO42−nss-SO42−
  • a

    Mean aerosol concentrations are in nanomoles per cubic meter.

total (fpm + cpm)1.0390.
% fpm of total 210049952111467112899

[44] The nss-Cl concentrations in rainwater from Ronald H. Brown were not significantly different from zero except for a slight deficit (5% excess of cations) in one event (79.3R). A similar deficit was then also obtained for Mg2+ and Ca2+ indicating an “extra” contribution of Na+ rather than a loss of Cl. On Sagar Kanya an excess nss-Cl was found in event (54.3S) and a deficit in event SK1 (28.7S). No simple explanation could be given in these cases and it was concluded that there was no overall indication of any loss of Cl in precipitation compared to seawater composition. The result is further consistent with a wind speed generated jet drop source of rainwater sea-salt.

[45] Nss-Br concentration can be decreased in the aerosol as a result of bromine oxidation followed by escape of volatile bromine compounds. The rain chemistry data showed that any deficit or excess Br was less than 10%, which is the limit of analytical accuracy due to interference from varying concentrations of NO3.

3.2.3. Relation to Air Trajectories

[46] Trajectories where the air parcel had been exposed to Indian sources (c Ind) showed the highest concentration of NH4+, nss-K+, nss-Ca2+, NO3, and nss-SO42−, cf. Table 1. For nss-F, formate, acetate, and MSA other events had similar or higher concentrations. After a transport of three to four days over sea the concentrations of NH4+, NO3, and nss-SO42− were 0.4 to 0.5 of those over land if we choose the data from Pune (west) and Bhubaneswar (east) as reference. For nss-Ca2+ the ratio was somewhat lower, about 0.3 of this land-based reference. The faster depletion of nss-Ca2+ from land to sea was in agreement with its expected distribution on larger particles.

[47] Many rain chemistry measurements in India have given nss-Ca2+ concentrations up to several times higher than our reference. Compared to those the nss-Ca2+ concentrations over sea reported here were very low indeed. This emphasized the possibility of local influence of many continental measurements, especially of soil dust components.

[48] The molar ratios of NH4+ and NO3 to nss-SO42− were 1.3 and 1.0 respectively while we calculate the ratios of emitted precursors (NH3 and NOx to SO2) in India to be 5.7 and 1.1 respectively. The emission estimates were taken from Zhao and Wang [1994], van Aardenne et al. [1999], and Streets et al. [2000], respectively, and adjusted to 1997. The high emission of NH3 compared to SO2 and NOx is noteworthy. NH3 was obviously depleted more rapidly than the other two components. Data from rural India on formate and acetate concentrations in rain during winter do not seem to be available. Compared to measurements in a semiurban area (Dayalbagh, near Agra, [Kumar et al., 1996]) our data were a factor 10 lower.

[49] The nss-K+ to nss-SO42− molar ratio in rain in air of NH origin was 0.10. This can be compared with molar ratios of K+ to SO2: (1) from biomass burning ranging from 0.3 to 1.6 depending on type of fire, with the lowest ratio for biofuel [Andreae and Merlet, 2001], (2) with 0.05 from fossil fuel combustion, and (3) with 0.07 in rain in Northern Europe with almost no biomass burning. This suggests that there was only a small, less than 15%, contribution to nss-K+ and nss-SO42− from biomass burning. This is in line with the results from the more comprehensive aerosol measurements during INDOEX, from which Mayol-Bracero et al. [2002] estimated that fossil fuel contribution to nss-SO42− in aerosol over the Indian Ocean is 60–90%. Transport from Arabia and some surrounding areas followed by about seven days over the sea gave generally low concentrations of the nss components. Since the emissions of NOx, SO2, and probably also NH3 are much lower, and of Ca much higher, in this area compared to India [Benkovitz et al., 1996; Tegen and Fung, 1994], one would expect a corresponding influence on the rainwater concentrations. However, the concentration of nss-SO42− in trajectory group A, 7d was only a factor two to three lower than in air from India.

[50] Two of the samples from the SHo had the lowest concentrations of all events for nss components except MSA. Concentrations in the third SHo sample (37.9S) were higher, comparable to those from the NHo with regard to nss-SO42− and nss-Ca2+. The SHo samples were characterized by low values of the following molar ratios: NH4+/nss-SO42− (0.1), nss-K+/nss-SO42− (<0.05), and NO3/nss-SO42− (0.1). The nss-Ca2+/nss-SO42− ratio (0.14) was low but not much lower than in other trajectory groups.

[51] The contribution of NO3 to acidity (molar ratios) ranged from one third to about equal of nss-SO42− in NHo but was insignificant in SHo. NH4+ was a factor of two to three more important than nss-Ca2+ for neutralizing the acids. The sum of acetate and formate was similarly important as NO3 for the acidity in the SHo samples. The contribution from MSA itself to the ion balance was insignificant but the formation of SO42− from DMS may have been important in air with SHo.

3.3. Aerosol Scavenging Ratios

[52] The simultaneous observations of the chemical composition of rainwater and size resolved aerosols provide a unique opportunity to draw conclusions about the aerosol scavenging processes. This has been done by considering the aerosol scavenging ratio (SR) defined as [Galloway et al., 1993]:

  • equation image

where Crain equals the concentration of a component in rain (nmol kg−1), Caerosol its concentration in air (nmol m−3), and Da the density of air. SR is thus expressed in kg of air (kg of water)−1. Table 5 shows SR calculated for aerosol fpm and cpm separately. Fpm SRs were calculated for components dominating the submicron fraction (Table 4), that is NH4+, nss-K+, and nss-SO42−. Cpm SRs were first calculated for Na+, Mg2+, and Cl which had their mass almost entirely in the supermicron fraction. In the case of nss-Ca2+ and MSA, which were distributed over both size fractions, the rainwater concentrations were subtracted with the contribution from fpm based on the SR for nss-SO42− before calculation of SRcpm:

  • equation image
Table 5. Scavenging Ratios (SR) for Fine Particle Mass (fpm), Coarse Particle Mass (cpm), and Total Particle Mass (total) from Ronald H. Browna
 Precipitate amount, mmPrecipitate intensity, mm h−1fpmfpmfpmtotalcpmcpmcpmcpmcpmcpm
NH4+ SRnss-K+ SRnss-SO42− SRMSA SRNO3 SRNa+ SRMg2+ SRCl SRnss-Ca2+ SRMSA SR
  • a

    Scavenging ratios are in kilograms air per kilogram water.

54.9R15.420.0427  16810669465675816991
78.9R0.4 256278441424670776767835 510
79.3R1.2 25867110299397504417427200296
79.9R0.4 3703193802654758828869431330101
81.5R1.0 279170236273274105510631560332 

[53] If both the submicron and supermicron aerosol were assumed to be released from the atmospheric boundary layer and processed by the clouds with the same efficiency, the below-cloud scavenging of cpm could be taken as the difference between SRcpm and SRfpm. This assumption neglects possible enrichment of soil dust particles due to ice phase processes as reported by Puxbaum and Tscherwenka [1998] for colder regions.

[54] A wide variability in SRfpm was observed with a tendency for lower SRs with higher rainfall intensity, as shown in Table 5. The SRs decreased with rainfall intensity approximately according to:

  • equation image

with SR in kg kg−1 and i, the rainfall intensity, in mm h−1. The variation on event basis was similar for nss-K+ and nss-SO42− while that of NH4+ showed a more narrow range. The median SRfpm (Table 5) was very similar for the three components NH4+, nss-K+, and nss-SO42− (∼250 kg kg−1).

[55] The median SRcpm (∼750 kg kg−1) for Na+, Cl, and Mg2+ varied between events in a similar fashion as the submicron aerosol components, with a tendency for the SRs to decrease with rainfall intensity approximately as:

  • equation image

[56] The SRtotal for nss-Ca2+ (distributed mainly in cpm) gave a median of 330 kg kg−1, lower than that for the sea salt and with event-to-event variations much better correlated with the SRfpm for nss-SO42− than with the SRcpm for Na+.

[57] In the case of MSA the SRtotal was even closer to the nss-SO42− SRfpm median value. This is probably because MSA, although appearing in both fpm and cpm, was mainly distributed in the submicron size fraction.

[58] The SRcpm for NO3 was substantially lower (median 335 kg kg−1) than for the cpm components of sea salt origin. In view of a possible contribution from gaseous HNO3 a higher value might have been expected. There was thus a clear indication that nitrates are removed primarily by in-cloud scavenging, as in the case of nss-SO42−. The reason for the inefficient removal below cloud could be that the nitrate was distributed toward the smaller end of the supermicron size fraction compared to the other sea salt components.

[59] Both the calculated SRcpm subcloud and in-cloud scavenging and the SRcpm for below-cloud scavenging alone (taken as the difference between SRcpm and SRfpm) were within the estimated range given by Seinfeld and Pandis [1997]. Nevertheless, the larger SRcpm calculated for the supermicron sea salt (Na+, Mg2+, and Cl) components relative to the major submicron components indicates that the sea salt aerosols were removed by both in-cloud and subcloud scavenging.

[60] On one occasion the rain was divided into sequential periods (87.4R in Table 1 represents the total rain and 87.5R the last 9 of the total 41 mm). Concentrations at the end were generally lower but the remaining fraction differed between the components, about 0.3 for sea salt, NH4+, and NO3 but larger, 0.7, for nss-SO42−. For nss-K+ and nss-Ca2+ the result was scattered (0.0 and 0.5) but the uncertainty in the calculation is large for these ions.

4. Summary and Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[61] The following conclusions are based on measured chemical composition in 13 rain events sampled on board Ronald H. Brown and Sagar Kanya during January–March 1999 as part of the INDOEX intensive field phase in the Indian Ocean between 5°N and 20°S.

  1. Rain in air masses of NH origin show, even as far south as 10°S, a clear influence from land based sources. Concentrations of NH4+, nss-K+, NO3, and nss-SO42− in rain, after transport over sea for some 2000 km (3.5 days) since last encounter with land (western India), were 0.4 to 0.5 of concentrations measured over land under similar flow conditions. The concentration of nss-Ca2+ had decreased to 0.3 of the same terrestrial reference, indicating a more rapid removal of nss-Ca2+. This depletion corresponds to an average residence time of 4 and 3 days, respectively. The molar ratio NO3/nss-SO42− was similar to the ratio of estimated emissions of the corresponding precursors in India. On the other hand, the NH4+/nss-SO42− molar ratio was substantially lower than expected from the emissions. The average concentration for all rain events in NHo air compared to the land based reference was 0.2 for nss-SO42− and 0.1 for the other three components. The longer the air was advected over ocean the more dominating appeared the nss-SO42− compared to the other ions. The main reason is likely to be the successive oxidation of SO2 of continental origin and not the contribution SO2 or sulfuric acid from DMS produced over the ocean. No important contribution of nss-Ca2+ was seen in flow that had passed over the Arabian Peninsula at high altitude (>5 km).
  2. An influence on rainwater composition from terrestrial sources could be inferred also in the SH (south of ITCZ) but was found much weaker than in the NHo. Concentrations of NH4+, nss-K+, NO3, and nss-SO42− were generally low and the relative importance of nss-SO42− was much higher than in the NHo.
  3. The rain was slightly acidic, with pH ranging from 4.8 to 5.4. This is much lower (less alkalinity) than is usually reported from rain over India, but more in line with recent rural measurements in areas in India protected from local dust emissions. Nss-SO42− was the main contributor to acidity (with occasional contributions from acetic and formic acid). The main neutralizer was NH4+ rather than nss-Ca2+, which has been reported to be the dominant cation in areas with dust emission.
  4. Several trace metals (Al, Fe V, Mn, Cd, Tl, and Pb) were found in significant concentrations. Al and Fe correlated with nss-Ca2+ indicating an origin from crustal material. The other metals correlated with ionic components of anthropogenic origin. The concentration of the important oceanic nutrient Fe was about 20 nmol dm−3 in SHo and 150 nmol dm−3 in NHo.
  5. MSA was found in concentrations up to 0.2 μmol dm−3 and was not correlated to other components except for a weak correlation with Na+.
  6. Acetate and formate were well correlated with each other (0.92). Both components seem to have derived from the same source located in the NH but not related to sources of the other compounds. This could be explained by a gradual production from long-lived gaseous precursors, which can be expected to be more abundant in NH than in SH.
  7. The concentration of sea salt (estimated from Na+) in rainwater was approximately an exponential function of the wind speed.
  8. The relation between Na+, Cl, and Br in the rainwater was close to that of seawater, implying no excess or deficit of the halogen ions.
  9. Scavenging ratios were similar (240 kg kg−1) for NH4+, nss-K+, and nss-SO42− which are found in the fine-mode particles, clearly higher (750 kg kg−1) for Na+, Mg2+, and Cl which are coarse-mode components of sea-salt origin and in between (330 kg kg−1) for nss-Ca2+ and NO3. The scavenging ratio for MSA, which was found mainly in the fine mode, was 270 kg kg−1.


  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References

[62] We thank Teresa Miller, Derek Coffman, Trish Quinn, and Timothy Bates (PMEL/NOOA) for kindly providing the analyses of the coarse particle mass and for fruitful collaboration on board Ronald H. Brown. The dedicated assistance from the crew on board the Ronald H. Brown is greatly appreciated. Financial support from the Swedish Natural Science Council (contract G5103-1238/1999) and from the Swedish International Development Co-operation Authority (Sida) is gratefully acknowledged. We are obliged to ECMWF for the use of their database and computing facilities. We thank Ray McGrath for allowing us to use his trajectory model.


  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Route of the Expedition and Measurement Systems
  5. 3. Results and Discussion
  6. 4. Summary and Conclusions
  7. Acknowledgments
  8. References
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