Aerosol Sulfate and Trace Elements in Urban Fog - Energy & Fuels

Mar 26, 2002 - Application of the SO42−/Se tracer technique to study SO2 oxidation in cloud and fog on a time scale of minutes. Liaquat Husain , Bad...
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Energy & Fuels 2002, 16, 640-646

Aerosol Sulfate and Trace Elements in Urban Fog O. V. Rattigan,† M. Ishaq Mirza,‡ B. M. Ghauri,‡ A. R. Khan,† Kamal Swami,† Karl Yang,† and Liaquat Husain*,†,§ Wadsworth Center, New York State Department of Health, Albany, New York 12201-0509, SUPARCO, Division of Space and Environment, P.O. Box 8402, University Road, Karachi 75270, Pakistan, and Department of Environmental Health and Toxicology, SUNY, Albany, New York 12201-0509 Received August 2, 2001

Lahore has been enveloped by intense fog for a period of 1 to 2 months during the previous three winters. The extensive nature of the fog has caused severe disruption to transport and the economy in Northeastern Pakistan. In this paper we report aerosol data from a field study carried out from 25 December 1999 to 8 January 2000 during the fog period. High pollutant concentrations were observed throughout the field study with SO2 from 0.3 to 24.7 ppb, SO42- from 4 to 141 µg/m3, and NO3- from 3 to 74.5 µg/m3. Exceptionally high trace element aerosol concentrations were also observed, for example, Se concentrations up to 258 ng/m3, As of 26 ng/m3, and Sb of 84.8 ng/m3. Pb concentrations up to several µg/m3 were observed well above the WHO guideline of 1 µg/m3. Maximum aerosol concentrations were observed during the daytime compared to nighttime which is consistent with increased urban and photochemical activity throughout the day. Source apportionment revealed four major groupings. Group one is comprised of the crustal elements Al, Mg, Fe, Co, Cr, Mn, Ca, and Ni and is associated with airborne soil material. The second grouping contained Pb, Sb, As, Zn, and Cu due to vehicular and various industrial processes. Approximately 70% of the Pb was ascribed to leaded gasoline. Group three contained SO42-, NO3-, and Ni which were ascribed to coal and oil combustion sources. The final group contained Se, As, and Sb with Se the highest loading suggesting a local source, probably industrial. The very high aerosol concentrations observed in Lahore pose a serious health risk in a highly populated urban center (population of approximately 7 million). An in-depth study of the fog chemistry and aerosol size measurements are required in order to develop appropriate control strategies and to assess possible impacts on climate and human health.

1. Introduction Due to their relatively small size, aerosol particles may be airborne for several days and transported over large distances and are therefore ubiquitous in the atmosphere. Their chemical composition includes a range of species, such as SO42-, NO3-, and NH4+, organic and elemental carbon, crustal and various trace elements.1 During the past few decades considerable effort has been directed toward atmospheric aerosol particulate measurements in various urban centers across Europe and the United States due in part to the detrimental effects of particulate on human health.2,3 In urban areas aerosol sources are often dominated by combustion and various industrial activities followed by gas-to-particle conversion processes. SO42- often accounts for 40-50% of the total aerosol composition, particularly in urban regions. The production of sulfate * Corresponding author. E-mail: [email protected]. † Wadsworth Center, New York State Department of Health. ‡ SUPARCO, Division of Space and Environment. § Department of Environmental Health and Toxicology, SUNY. (1) Pandis, S. N.; Wexler, A. S.; Seinfeld, J. H. Dynamics of Tropospheric Aerosols. J. Phys. Chem. 1995, 99, 9646-9659. (2) Schwartz, J. Air pollution and daily mortality: A review and meta analysis. Environ. Res. 1994, 64, 36-52. (3) Pope, C. A.; Thun, M. J.; Namboodiri, M. M.; Dockery, D. W.; Evans, J. S.; Speizer, F. E.; Heath, C. W., Jr. Particulate air pollution as a predictor of mortality in a prospective study of US adults. Am. J. Respir. Crit. Care 1995, 151, 669-674.

in the atmosphere has received considerable interest due to the detrimental effects of acid precipitation on ecosystems. Aerosols appear to play a significant role in the Earth’s climate by scattering incoming solar radiation.4-6 Aerosol particles may also influence cloud cover and cloud lifetimes. It is well-known that SO42aerosols form smaller but much larger numbers of cloud drops and can prevent rainout. Radke et al.7 observed that in ship tracks there is a significant enhancement in total cloud drop number concentrations predominantly for the smaller size range compared to nonpolluted clouds. The larger number of small drops can lead to a reduction in precipitation and a corresponding increase in cloud lifetime.8 Despite the increasing emissions and large population centers in the developing countries, however, measurements there have been relatively sparse. India is cur(4) Charlson, R. J.; Schwartz, S. E.; Hales, J. M.; Cess, R. D.; Coakley, J. A., Jr.; Hansen, J. E.; Hofmann, D. J. Climate Forcing by Anthropogenic Aerosols. Science 1992, 255, 423-430. (5) Lelieveld, J.; Heintzenberg, J. Sulfate cooling effect on climate through in-cloud oxidation of anthropogenic, SO2. Science 1992, 258, 117-120. (6) Kiehl, J. T.; Briegleb, B. P. The relative roles of sulfate aerosols and greenhouse gases in climate forcing. Science 1993, 260, 311-314. (7) Radke, L. F.; Coakley, J. A.; King, M. D. Direct and remote sensing observations of the effects of ships on clouds. Science 1989, 246, 1146-1148. (8) Albrecht, B. A. Aerosols, cloud microphysics and fractional cloudiness. Science 1989, 245, 1227-1230.

10.1021/ef010199g CCC: $22.00 © 2002 American Chemical Society Published on Web 03/26/2002

Aerosol Sulfate and Trace Elements in Urban Fog

rently the fourth largest in SO2 emissions,9,10 and there are many urban areas in India and neighboring Pakistan which experience poor air quality. Northeastern Pakistan and India are the most industrialized parts of the sub-continent.11 Some measurements have been reported from Karachi, Pakistan,12,13 Lahore, Pakistan,14,15 and at remote mountain sites in Nathiagali and Changlagali, Pakistan.16 Lahore, which has a population of approximately 7 million has recently been covered by fog for a period of 1 to 2 months during winter. The fog covered much of Northeastern Pakistan and part of Northeastern India and extended over 1500 km. We reported 12 h aerosol measurements collected in Lahore toward the end of the winter fog period from January 1 to 5, 1999.17 Our measurements showed very high concentrations of aerosol SO42- (up to 100 µg/m3) and several trace elements. The SO42-/Se ratios were suggestive of longrange transport from several hundred kilometers away in neighboring India. Such high concentrations pose a serious health risk and require a more detailed study. We suggest that the fog may be intensified by the high aerosol concentrations. The objectives of this study were to obtain a more extensive set of aerosol measurements at Lahore and to identify the various pollution sources. Sampling was carried out over a two-week period from December 25, 1999, to January 8, 2000, compared to a five-day period in January 1999. Six-hour sampling of aerosols and gasphase sulfur dioxide were carried out simultaneously. The study provided analytical data on 14 trace elements, SO42-, and NO3- for source identification. 2. Experimental Method 2.1. Sampling. This field study was confined to aerosol and gas-phase SO2 measurements. Samples were collected at Lahore (31.6°N, 74.3°E), which is the capital of Punjab and the second largest city in Pakistan with a population of approximately 7 million. Sampling was carried out from December 25, 1999, to January 8, 2000. Total suspended (9) Hameed, S.; Dignon, J. Global emissions of nitrogen and sulfur oxides in fossil fuel combustion, 1970-1986. J. Air Waste Manage. Assoc. 1991, 42, 159-163. (10) Streets, D. G.; Tsai, N. Y.; Akimoto, H.; Oka, K. Sulfur dioxide emissions in Asia in the period 1985-1997. Atmos. Environ. 2000, 34, 4413-4424. (11) Arndt, R. L.; Carmichael, G. R.; Streets, D. G.; Bhatti, N. Sulfur dioxide emissions and sectorial contributions to sulfur deposition in Asia. Atmos. Environ. 1997, 31, 1553-1572. (12) Parekh, P. P.; Ghauri, B.; Siddiqui, Z. R.; Husain, L. The use of chemical and statistical methods to identify sources of selected elements in ambient air aerosols in Karachi, Pakistan. Atmos. Environ. 1987, 21, 1267-1274. (13) Parekh, P. P.; Ghauri, B.; Husain, L. Identification of pollution sources of anomalously enriched elements. Atmos. Environ. 1989, 23, 1435-1442. (14) Smith, D. J. T.; Harrison, R. M.; Luhana, L.; Pio, C. A.; Castro, L. M.; Tariq, M. N.; Hayat, S.; Quraishi, T. Concentrations of Particulate Airborne Polycyclic Aromatic Hydrocarbons and Metals Collected in Lahore, Pakistan. Atmos. Environ. 1996, 30, 4031-4040. (15) Harrison, R. M.; Smith, D. J. T.; Pio, C. A.; Castro, L. M. Comparative Receptor Modeling Study of Airborne Particulate Pollutants in Birmingham (United Kingdom), Coimbra (Portugal), and Lahore (Pakistan). Atmos. Environ. 1997, 31, 3309-3321. (16) Ghauri, B. M.; Mirza, M. I.; Richter, R.; Dutkiewicz, V. A.; Rusheed, A.; Khan, A. R.; Husain, L. Composition of Aerosols and Cloudwater at a Remote Mountain Site (2.8 KM) in Pakistan. Chemosphere (Global Change Science) 2001, 3, 51-63. (17) Hameed, S.; Ishaq Mirza, M.; Ghauri, B. M.; Siddiqui, Z. R.; Javed, Rubina; Khan, A. R.; Rattigan, O. V.; Qureshi, Sumizah; Husain, Liaquat. On the Widespread Winter Fog in Northeastern Pakistan and India. Geophys. Res. Lett. 2000, 27, 1891-1894.

Energy & Fuels, Vol. 16, No. 3, 2002 641 particulate was collected on Whatman 41 filter papers using Andersen high-volume samplers employing procedures used at Whiteface Mountain, NY, over two decades.18 Due to the rather large orifice of high-volume collectors, some fog droplets must have also been entrained by the samplers. Inspection of the filters, however, showed no obvious signs of wrinkling indicating that the amount of water absorption must have been small. Nevertheless, the aerosol chemical composition reported invariably has some contribution from aqueous fog drops. Since there was no sampling of sized resolved aerosols during this field study it is not possible to quantify for effects caused by fog drop collection. From 2000 hours on December 25 to 0800 hours on January 4, there were two sampling time intervals. The intervals consisted of two 6 h periods from 0800 to 1400 hours and from 1400 to 2000 hours followed by a 12 h overnight period from 2000 to 0800 hours. From 2030 hours on January 5, sampling was conducted on 12 h intervals. The flow rate was controlled with a Sierra mass flow controller at a rate of ∼0.7 m3/min and was checked at the start and end of each sampling period. Sampled air volumes varied from 255 to 510 m3. The filters were transported to the laboratory inside sealed envelopes and analyzed within one to two weeks after collection. A portion of each filter (∼10 cm2) was extracted in double distilled deionized water and analyzed for anions SO42- and NO3- by ion-chromatography using a Dionex Model 500 equipped with Peaknet software.19 Another portion of each filter, ∼10 cm2, was analyzed for trace metals by Inductively Coupled Plasma Mass Spectrometry.20 Selected samples were also analyzed for trace metals using instrumental neutron activation analysis using the procedure described earlier.21 2.2. Aerosol Sampling Artifacts. Aerosol sampling techniques employing filters is susceptible to positive and negative artifacts. The most well-known of these is that concerning particulate NO3- sampling. NH4NO3 aerosol can decompose into NH3 and HNO3 under ambient temperatures,22-25 leading to losses of aerosol NO3- from the filter. Kim et al.25observed average aerosol NO3- losses from 10 to 20% (1.3 to 2.3 µg/m3) with maximum losses ranging from 6.4 to 22.5 µg/m3 during aerosol sampling in the south coast air basin of California. Their results indicated that nitrate losses may also occur by chemical reaction of NH4NO3 aerosol with acidic gases. Positive artifacts can occur when HNO3 is adsorbed onto the filter or reacts with previously adsorbed basic soil material on the filter.25,26 In addition, during periods of fog or high relative humidity, soluble gases such as HNO3 (and NH3) will dissolve in aqueous droplets. Unless some size selection is employed (e.g., cyclone to exclude micron-sized drops) these droplets can (18) Husain, L.; Dutkiewicz, V. A.; Das, M. Evidence for decrease in atmospheric sulfur burden in the Eastern United States caused by reduction in SO2 emissions. Geophys. Res. Lett. 1998, 25, 967-970. (19) Khwaja, H. A.; Khan, A. R.; Qureshi, S. Ion chromatographic determination of anions in environmental samples. Int. J. Environ. Anal. Chem. 1999, 75, 285-297. (20) Richter, R. C.; Swami, K.; Chace, S.; Husain, L. Determination of arsenic, selenium and antimony in cloudwater by inductively coupled plasma mass spectrometry. Fresenius J. Anal. Chem. 1998, 361, 168173. (21) Dutkiewicz, V. A.; Parekh, P. P.; Husain, L. An evaluation of regional elemental signatures relevant to the northeastern United States. Atmos. Environ. 1987, 21, 1033-1044. (22) Appel, B. R.; Tokiwa, Y.; Haik, M. Sampling of nitrates in ambient air. Atmos. Environ. 1981, 15, 283-289. (23) Appel, B. R.; Tokiwa, Y.; Kothny, E. L.; Wu, R.; Povard, V. Evaluation of procedures for measuring atmospheric nitric acid and ammonia. Atmos. Environ. 1988, 22, 1565-1573. (24) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to climate Change; Wiley: New York, 1998. (25) Kim, B. M.; Lester, J.; Tisopulos, L.; Zeldin, M. D. Nitrate Artifacts during PM2.5 Sampling in the South Coast Air Basin of California. J. Air Waste Manage. Assoc. 1999, 49, 142-153. (26) Forrest, J.; Spandau, D. J.; Tanner, R. L.; Newman, L. Determination of atmospheric nitrate and nitric acid employing a diffusion denuder with a filter pack. Atmos. Environ. 1982, 16, 1473-1485.

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Table 1. Average Diurnal Aerosol Concentrations at Lahore, Pakistan, from December 25, 1999 to January 3, 2000 species

0800-1400

1400-2000

2000-0800

T (°C) RH (%) SO42- a NO3- a SO2 a Se As Sb Cu Zn Pb Al Fe Ca Mg Cr Mn Co Ni

13.9 69.4 37.9 28.5 22.2 10.4 11.2 18.5 799 2279 1035 16547 9376 9688 1321 24.5 211 3.73 16.6

16.6 56.6 38.3 37.3 34.6 38.1 14.1 12.1 820 2621 676 24216 14569 14816 1888 36.6 288 5.87 22.2

10.0 89.8 26.4 21.6 23.9 19.2 8.33 13.5 1125 2060 905 6189 4007 4485 573 11.5 92.9 1.58 8.45

a Concentrations of SO 2-, NO -, and SO are in µg/m3, others 4 3 2 in ng/m3.

be sampled in the aerosol stream and add to the aerosol concentration. With this in mind the reported aerosol NO3concentrations should be viewed with caution. Particulate SO42- measurements are not susceptible to the above sampling artifacts because of the high stability of the (NH4)2SO4 aerosol. Note that the aerosol concentrations reported here tended to be higher during periods of low relative humidity indicating that SO42- contributions from aqueous SO2 oxidation followed by fog drop entrainment must not have been a major contribution to the measured aerosol SO42- concentrations. 2.3. SO2. A filter pack system using Millipore Swinex holder trains run concurrently with the hi-vol sampler was used to sample SO2. The first stage of the filter train was Zefluor (2 µm supported PTFE) to collect aerosols, followed by two sets of Whatman 41 filters impregnated with K2CO3/glycerol to collect SO2. The flow rate was about 16 L/min. The flow rate (approximately 16 L/min) was established using a precalibrated rotometer and was checked at the start and end of each sampling period. The K2CO3 filters were extracted in double distilled deionized water. H2O2 was added to completely oxidize the adsorbed SO2 to SO42- which was subsequently analyzed by ion chromatography.

3. Results Sampling commenced at 2000 hours on December 25, 1999, and, except for a short period between 0800 on January 4 until 1430 on January 5, continued until 0830 on January 8, 2000. A total of 35 aerosol samples were collected during the field study. Except for a brief period from 0800 to 2000 hours on December 30, light to dense fog persisted throughout the campaign. The temperature varied from 9.5 to 17.4 °C and the relative humidity from 48.2 to 93.4%. The lowest temperatures occurred overnight from 2000 to 0800 hours; mean of 10 °C and the relative humidity was 89.9%. Table 1 shows the diurnal variations along with aerosol concentrations. Highest temperatures occurred from 1400 to 2000 hours with a mean 16.6 °C; the corresponding mean relative humidity was 56.5%. Wind speeds were generally very low with average daily values usually below 1 m/s except on December 30 when the average wind speed was 1.7 m/s with a maximum 6 h value of 3.6 m/s. There were 6 days when no measurable wind speed was reported.

Figure 1. Concentrations in µg/m3 of SO42-, NO3-, Se, and As, and SO42-/Se ratios in aerosol samples at Lahore, Pakistan.

3.1. SO42-, NO3-, Se, As, and Sb. Figure 1 shows aerosol concentrations of SO42-, NO3-, Se, As, and SO42-/Se ratios at Lahore. From December 25, 1999, to January 3, 2000, SO42- varied from 4 to 61 µg/m3 with an average of 34 µg/m3. Daytime SO42- concentrations were on average higher than nighttime values; 38.1 compared to 26.4 µg/m3 as observed in Table 1. From 1430 to 2030 hours on January 5, concentrations rose rapidly to 81 µg/m3 and later on to a maximum of 141.3 µg/m3 from 2030 to 0830 hours on January 6. Such a rapid increase in SO42- is due to enhanced SO2 oxidation either via aqueous pathways within the fog droplets and/or due to transport of a different air mass where significant gas-phase oxidation has occurred. During this period SO2 concentrations were very lowsbetween 2 and 7 µg/m3. SO42- concentrations decreased again and averaged 45 to 60 µg/m3 between January 6 to 8 indicating possible mixing and dilution with other air masses. Note that as for the earlier period, daily SO42concentrations from January 5 exceed night values; 81.6 compared to 68.4 µg/m3. NO3- showed a trend similar to that for SO42- with concentrations varying between 3 and 58 µg/m3 from December 5 to January 3. As observed previously for SO42-, daytime NO3- concentrations exceeded nighttime values with highest concentrations occurring from 1400 to 2000 hours, Table 1. A peak in NO3- concentration around January 5 to 6 was observed but it was not as pronounced as SO42-, reaching 74.5 µg/m3. This is not surprising as the formation mechanisms for NO3- and SO42- are quite different. Trace element Se shows some similarities with SO42and NO3-. In general, concentrations ranged from 1 to 10 ng/m3 from December 25 to January 4. However, two periods were observed when Se concentrations were extremely high: the first between 1400 and 2000 hours

Aerosol Sulfate and Trace Elements in Urban Fog

on December 29 when concentration reached 257.6 ng/ m3, and the second from 2000 on January 2 to 0800 h on January 3 when the concentration reached 121.8 ng/ m3. Peak concentrations may well have been significantly higher. Concentrations were elevated for one or two sampling periods afterward, before decreasing to around 5-10 ng/m3. Such dramatic changes in concentration were not observed for SO42- or NO3-. In both instances the SO42-/Se ratios dropped to very low values, ∼200, before recovering to around 4000-5000, Figure 1, indicating a local sporadic Se source for these short periods. Average daily surface wind speeds were generally very low except for these two periods. The highest wind speed occurred on December 29-30 with a mean of 1.7 m/s and between January 2 and 3 the mean was 0.6 m/s which are coincident with the very high Se concentrations. Maximum reported wind speeds were 3.6 m/s on December 29-30 and 3.1 m/s on January 2-3. Sb and to a lesser degree As showed significantly enhanced concentrations although not to the same extent as Se. Parekh et al.12,13 observed average Se concentrations from 1.0 to 2.6 ng/m3 at Karachi, Pakistan, during July 1985, March 1986, and February-March 1987. We have never observed such high aerosol Se concentrations during our 20 years of sampling at Whiteface Mountain in the Northeastern United States. Typically, concentrations of 0.5 to 4 ng/ m3 are observed at Whiteface Mountain during summer months. Such high concentrations at Lahore must be due to a relatively nearby source. Se concentrations showed a smaller peak of 21 ng/m3 on January 6 around the same time that SO42- and NO3- were a maximum. Similarly to Se, As and Sb (not shown) showed enhanced concentrations on December 29 but the second peak was observed in this case on January 1. Peak concentrations, however, were not as pronounced as for Se. The SO42-/Se ratio ranged from 4000 to 5000 from December 25 to January 4. From January 5 to 8 the ratio increased to 10000-14000 and decreased to around 5000 from January 8. Our measurements at Karachi12 and Nathiagali, Pakistan16 show that the SO42-/Se ratio in fresh aerosols is typically ∼1000-1500. This ratio will increase with distance from the source as SO2 is oxidized to SO42- via gas- and aqueous-phase mechanisms. However, since the extent of oxidation in the aqueous fog drops was not determined the origin of the source is difficult to quantify. 3.2. Diurnal Variation. Only aerosol data covering the period from December 25 to January 4 was used to check for diurnal trends because there were three measurement periods: 0800 to 1400 hours, 1400 to 2000 hours, and 2000 to 0800 hours. In addition, meteorological data are available only for this period. Following January 4, aerosol sampling was carried out over two 12 h periods and therefore these data provide much less information on diurnal changes. The aerosol data from December 25 to January 4 are shown in Table 1 along with temperature and relative humidity measurements. On average, SO42- concentrations were higher during daytime compared to nighttime values. Concentrations during the day periods 0800 to 1400 hours and 1400 to 2000 hours were about equal: 37.9 µg/m3 and 38.3 µg/ m3, respectively. Nighttime SO42- was approximately 70% of the daytime value, averaging 26.4 µg/m3. Even

Energy & Fuels, Vol. 16, No. 3, 2002 643

though relative humidities were highest at night, ∼90%, SO42- concentrations were a minimum indicating that aqueous SO42- from droplet entrainment may not be a major contribution, although this cannot be verified. NO3- concentrations were highest in the afternoon/ evening period, averaging 37.3 µg/m3, and, as with SO42-, lowest at night: 21.6 µg/m3. The high NO3during periods of high temperature and low relative humidity is somewhat unexpected as NH4NO3 is susceptible to decomposition under these conditions. As shown in the lower part of Table 1, basic soil components comprised a considerable amount of the measured TSP (equivalent to aerosol NO3- concentration) and it is possible that a significant fraction of the NO3- was associated with these soil components. These aerosols are considerably more stable than NH4NO3 under daytime conditions observed in Lahore. SO2 concentrations were highest in the afternoon period (34.6 µg/m3), probably reflecting increased activities from vehicular traffic and other fossil fuel combustion processes. The high aerosol SO42- and NO3- concentrations during the day likely reflect increased photochemical and anthropogenic activities resulting in oxidation of the source species SO2 and NOx. Table 1 shows that the other aerosol species also have highest concentrations during the day, Cu being the exception. This may be more to do with a difficulty in the measurement of Cu rather than to the diurnal variation. Most aerosol species showed highest concentrations in the afternoon/evening period, Sb and Pb, however, peaked during the earlier part of the day. The maximum in the afternoon for the crustal components Al, Fe, Ca, etc., is as expected since this corresponds to a period of relatively high air turbulence caused by rising temperatures and urban activity. The interpretation of this data set should be viewed with some caution, however, because of the limited number of measurements and the large day-to-day variability in aerosol concentrations as observed in Figure 1. 3.3. Partitioning of S. Since SO2 is oxidized to SO42-, the partitioning of sulfur between gaseous SO2 and particulate SO42- can be used to indicate the extent of oxidation. The partitioning of S at Lahore is shown in Figure 2. For ease of comparison, concentrations are shown in ppb. From December 25 to January 4, SO42varied from under 1 to 14.8 ppb with a mean of 8.2 ppb and SO2 varied from 0.3 to 24.7 with a mean of 9.6 ppb. Total S during this period was 18.2 ppb of which approximately 45% was aerosol SO42-. During the period January 5 to 8 the partitioning was significantly different. SO42- concentrations increased to 33.9 ppb between January 5-6 and decreased to 10.8 ppb between January 6-7 and averaged 11.9 ppb for the rest of the event. SO2 concentrations were low during this time, averaging 1.6 ppb. Approximately 90% of total S was aerosol SO42-. These changes are possibly due to increased aqueous oxidation within the fog drops and/ or to a change in air mass where considerable SO2 oxidation has occurred. The percent SO42- tended to increase as the relative humidity increased, providing some evidence for oxidation. Total S increased to 34 ppb from January 5 to 6, which is almost a factor of 2 higher than in the previous period and averaged 13.4 ppb for

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Rattigan et al. Table 3. Crustal Enrichment Factors of Trace Elements at Lahore, Pakistan

element

crustal concentrationa (ppm)

aerosol concentration (ng/m3)

EFb

R2

Al Fe Ca Mn Cr Ni Co Zn Cu Pb Se As Sb

81300 50000 36300 950 100 75 25 70 55 13 0.05 1.8 0.2

13392 8176 8696 177.1 22.1 16.5 3.33 2333 843.9 846 20.6 10.8 13.3

1 0.99 1.45 1.13 1.34 1.33 0.81 202.3 93.2 395 2498 36.5 405

1 0.940 0.819 0.895 0.90 0.91 0.927 0.096 0.253 0.020 0.011 0.388 0.015

a Crustal element concentration from ref 27. b Enrichment factor ) (Xair/Alair)/(Xcrust/Alcrust).

Figure 2. Partitioning of sulfur between gaseous and particulate phases at Lahore, Pakistan. Table 2. Mean Concentration and Range of Trace Species during Fog from December 1999 to January 2000 at Lahore, Pakistan species 2- a

SO4 NO3- a SO2 a Se As Sb Cu Zn Pb Al Fe Ca Mg Cr Mn Co Ni

mean

max

min

43.2 31.7 21.2 26.1 10.6 13.1 843.9 2200 846 13391 8176 8696 1078 22.1 177.1 3.33 16.5

141.3 74.5 68.5 257.6 25.97 84.8 5305 7796 4786 54351 24197 30811 4409 67.6 503.9 9.53 55.6

3.78 3.10 0.83 0.73 1.94 0.69 29.4 111.5 31.1 1004 741.8 1312 101 1.44 16.0 0.28 1.53

refs 14,15 18.4 12.8 29.4 420 27700 3920 37400 9930 4060 490 113 350 79.7

a Concentrations of SO 2-, NO -, and SO are in µg/m3, others 4 3 2 in ng/m3.

the remainder of the event, indicating a change in air mass has occurred. 3.4. Comparison with Previous Measurements. In addition to Se, As, and Sb, concentrations of 11 other trace element were determined. Table 2 shows the mean concentrations and ranges of all species measured. The results are compared with data from Smith et al.14 and Harrison et al.15 Concentrations of most species are in general similar to the annual means reported by Smith et al.14 and Harrison et al.,15 especially considering in their case samples were collected every 6th day and the sampling period was 24 h. Concentrations of SO42- and NO3- appear to have increased by approximately a factor of 2 compared to Smith et al.14 in 1996, even after considering the large range in concentration. This may be the result of increasing use of fossil fuel combustion.

However, care must be used in comparing two-week sampling with annual means since species concentration can vary significantly with season. Note that concentrations of trace elements Zn, Pb, and Ni are a factor of 5 to 10 lower than previously observed by Smith et al.14 From the measured aerosol concentration the SO42fraction accounted for 36%, NO3- and the crustal elements both accounted for 26%, Cl- was 9%, and the remaining 3% was non crustal elements. Aerosol samples were not analyzed for carbon. 3.5. Enrichment Factors. Table 3 shows concentrations of the elements compared to crustal abundances from Mason.27 The enrichment factor (EF) is the enrichment of the element in airborne particles compared to that in Earth’s crust, using Al as a reference element. An enrichment of unity indicates a soil-derived element, whereas elements derived from high temperature combustion processes will be more volatile and have enrichment factors significantly above one. As can be seen in Table 3, six elementssFe, Ca, Mn, Cr, Ni and Coshave enrichment factors very near one, (ranged from 0.8 to 1.5) and are therefore considered soil derived. Elements Zn, Cu, Pb, Se, As, and Sb are all significantly enriched (enrichment factors from 37 to 2500) and therefore their sources must be mainly anthropogenic. Note the very high enrichment factor for Se of 2498. The fifth column in Table 3 shows the correlation between Al and the other elements. The soilderived elements are well correlated with Al (correlation coefficient above 0.8) whereas the non crustal elements listed above, particularly Zn, Pb, Se, and Sb, are poorly correlated with Al. 3.6. Source Apportionment. 3.6.1. Crustal Elements. Note the high concentrations of the elements Al, Fe, Ca, and Mg averaging 13.4, 8.18, 8.70, and 1.08 µg/m3, respectively. This is not surprising since these elements are abundant in soil and considering the very arid climate in this region sampling of wind-blown dust is most likely the source. The concentrations of crustal elements exhibit a distinct diurnal pattern with highest concentrations during the day and lowest at night, as shown for Al, Mg, and Ca in Figure 3. The variations ranged from a factor of 2 to 10-fold with a mean of 5.7. (27) Mason, B. Principles of Geochemistry, 3rd ed.; Wiley: New York, 1996.

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Energy & Fuels, Vol. 16, No. 3, 2002 645

Figure 3. Temporal variations of Al, Mg, and Ca in aerosol samples at Lahore, Pakistan. Figure 5. Temporal variations of Pb and Sb in aerosol samples at Lahore, Pakistan. The asterisk denotes two periods of high Sb which are not correlated with Pb. Table 4. Rotated Factor Analysis of Aerosol Data from Lahore, December 1999-January 2000a species

1

Al Mg Fe Co Cr Mn Ca Ni As Pb Sb Zn SO42NO3Se Cu

0.980 0.977 0.967 0.963 0.952 0.935 0.923 0.695 0.522

a

Figure 4. Temporal variations of Zn, Cu, and Pb in aerosol samples at Lahore, Pakistan.

Concentrations of Co, Cr, Mn, and Ni, although significantly lower, showed a similar diurnal pattern. High concentrations of Cu, Zn, and Pb were observed reaching µg/m3 concentrations. Their temporal variations were significantly different from the crustal elements, Figure 4. Fluctuations in any one element was often not correlated with changes in the other two. This indicates these elements have distinct sources or a combination of sources such as industrial and vehicular traffic. 3.6.2. Pb and Sb. Sources of Pb in urban environments include vehicular exhaust and battery manufacture. Pb plates used in battery manufacture usually contain about 4-5% Sb.13 Pb and Sb generally showed similar temporal behavior with some noticeable differences around December 29-30 and again on January 2-3, as shown by the asterisk in Figure 5. If we assume that the major source of aerosol Sb is from lead battery

2

3

4

0.573 0.590 0.965 0.828 0.661

0.414 0.477 0.958 0.841 0.951

0.469

0.470

Factor loadings less than 0.4 have been omitted.

manufacture and the lead plates contain 5% Sb, then the vehicular Pb component can be estimated using the equation: Pb(veh) ) Pb(tot) - (20 × Sb). The mean aerosol Pb concentration at Lahore is 846 ng/m3 of which 70% is estimated to be vehicular exhaust derived and 30% from battery manufacture. For approximately 20% of aerosol samples the vehicular component is several µg/m3 which exceeds the WHO guideline of 1 µg/m3. Since sources of Sb in urban air other than lead battery manufacture exist, as evidenced from the Sb peaks around December 29-30 and January 3, the estimates of vehicular derived Pb are lower limits. The results are in agreement with the annual mean concentrations of 3.92 µg/m3 at Lahore reported by Smith et al.14. 3.6.3. Factor Analysis. To gain more information on source apportionment, factor analysis was performed using the statistical package SYSTAT version 6.0. Rotated factor analysis yielded 4 distinct factors as shown in Table 4. Group one is comprised of the crustal elements Al, Mg, Fe, Co, Cr, Mn, Ca, and Ni. Note that As and Cu were also associated with this grouping,

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although these elements showed a higher association with group 2 (0.590 and 0.470) indicating they have more in common with vehicular and industrial sources. The high enrichment factor of As (36.5) and Cu (93.2) in Table 3, also points to anthropogenic sources. This could indicate some contamination of airborne dust has occurred in the urban environment. Group 2 showed high loadings for Pb and Sb which are commonly associated with lead battery manufacturing plants. However, the widespread use of leaded gasoline also contributes to the high aerosol lead concentrations in Lahore14 as discussed above. Since As and Sb are commonly associated with coal and oil combustion the presence of As in this group is not surprising. The lower loading of As indicates Sb and Pb have a stronger association with vehicle exhaust. Zn and Cu were also associated with group 2. The very high concentrations of Zn and Cu up to 7.8 µg/m3 and 5.3 µg/m3 respectively, indicates a local industrial source, probably metallurgi- and Ni belong to group 3 which is cal. SO24 , NO3 associated with fossil fuel combustion and secondary aerosol formation. However, Ni showed a higher loading with the crustal elements in group 1, indicating its main source is probably soil material. Support for the crustal source is indicated by its enhancement factor of 1.33 in Table 3. The final group consisted of Se, Sb and As, with Se showing the highest loading (0.951). This possibly indicates a separate source for Se (most likely from smelting processes) which also includes Sb and As. However, Sb and As showed much higher loadings with group 2, vehicular and industrial sources. In summary, factor analysis revealed four major source categories, soil material, fossil fuel combustion, vehicular exhaust, and various industrial processes. Some elements however were associated with more than one group indicating that there was more than one source e.g. Ni which was linked with the crustal elements and with fossil fuel combustion sources.

Rattigan et al.

4. Conclusions Concentrations of aerosol SO24 , NO3 and 14 trace elements and gas-phase SO2 were determined from 25 December 1999 to 8 January 2000 at Lahore, Pakistan. High species concentrations were observed during the field study, for example, SO24 concentrations ranged from 4 to 141 µg/m3, NO3- from 3 to 74.5 µg/m3 and gaseous SO2 from 0.3 to 24.5 ppb. Several trace elements e.g. Se, As and Sb also showed elevated concentrations. Pb concentrations up to 4.8 µg/m3 were observed. Most aerosol species concentrations were highest during the daytime compared to night which is consistent with increased urban and photochemical activity during the day. Source apportionment led to four main groupings. Group one was comprised of the crustal elements Al, Mg, Fe, Co, Cr, Mn, Ca, and Ni, which are associated with the resuspension of soil dust in the arid climate. The second group contained Pb, Sb, As, Zn, and Cu, which are associated with vehicular and various industrial processes. Approximately 70% of aerosol Pb was ascribed to leaded gasoline. Group three contained SO42-, NO3-, and Ni which are associated with coal and oil combustion and secondary aerosol formation processes. The final group contained Se, As, and Sb with Se showing the highest loading (0.951). This indicates a local industrial Se source, possibly smelting. Appropriate control measures need to be implemented soon because of the potential health risk to the inhabitants of the region where millions of people live. However, a more detailed study of the fog chemistry and sizeresolved aerosol measurements are required in order to fully assess the impact on human health, economy of the region, and climate.

Acknowledgment. The work was partially funded by NSF through Grant ATM9712436. EF010199G