Article pubs.acs.org/est
Selected Organochlorine Pesticides and Polychlorinated Biphenyls in Urban Atmosphere of Pakistan: Concentration, Spatial Variation and Sources Jawad Nasir,†,‡,§ Xiaoping Wang,*,† Baiqing Xu,† Chuanfei Wang,† Daniel R. Joswiak,† Said Rehman,§ Arifa Lodhi,§ Shoaib Shafiq,§ and Rehmatullah Jilani§ †
Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Earth Sciences Directorate, Pakistan Space and Upper Atmosphere Research Commission (SUPARCO), P.O. Box 8402, Karachi 75270, Pakistan S Supporting Information *
ABSTRACT: Robust knowledge on the occurrence and distribution of persistent organic pollutants (POPs) in the atmosphere of low-latitude regions is inevitable to forecast their transportation to pristine ecosystem and assess toxicological impacts upon local biota. Despite the earlier revelation of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in soils/sediments and water bodies in Pakistan, knowledge about atmospheric levels and sources of these POPs remains limited. For the first time, a network of XAD resin-based passive air samplers (PAS) was established across megacities of Pakistan, i.e., Karachi, the coastal city, and Lahore, lying in an agricultural region. Typical geographical locations of the two cities allowed assessing the influence of source regions on the occurrence and distribution patterns of selected POPs. Average concentrations (ng/PAS) in both cities ranged as endosulfan 39−101, DDTs 63−92, HCHs 33−65, heptachlor 10−26, and PCBs 48−61. High concentrations of endosulfan and lindane as observed throughout Lahore were certainly due to their ongoing applications in surrounding agricultural fields. Lower proportions of parental DDTs as compared to their metabolites were observed in both cities, suggesting inputs of DDTs from older or secondary sources. Owing to ultimate discharge of country’s agricultural/industrial waste through river streams in to Arabian Sea, the coastal region of Karachi was found potential source of weathered POPs that could be dissipated at regional/global scales by maritime advections. The study contributes to the pool of information on fate and geographical distribution of POPs in subtropical developing countries.
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INTRODUCTION Persistent organic pollutants (POPs) are semivolatile organic compounds that are originated locally but dispersed globally due to their moderate vapor pressure.1 Many of these compounds such as organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) have been found to be of toxic nature1 and are considered as hazardous environmental hormones,2 posing high risks to the ecosystem and human health.3 Their occurrence in environmental compartments largely depends on their regional application history and patterns as well as their long-rang atmospheric transport (LRAT) features.4 For instance, POPs discharged in tropical/ subtropical environments tend to be dissipated to polar and environmentally pristine regions by LRAT,5 so-called “global fractionation and condensation”.6 In this context, lower latitude countries have attracted attention as the potential source regions of global POPs and a number of studies7−10 have been carried out in tropical/subtropical regions (e.g., Australia, Botswana, China, Hong Kong, India, Japan, South Korea, etc.) to assess distribution and fate of atmospheric POPs. In © 2014 American Chemical Society
comparison with developed nations, high concentrations of POPs generally observed in developing countries were suggestive of extensive past or ongoing usage of pesticides for agricultural and vector-borne diseases as well as an inability to enforce regulation to restrict indiscriminant use and disposal of industrial/agro-chemicals.8,9,11 Pakistan (24−37°N/61−77°E) is a subtropical south Asian country. A major part of the country, i.e., up to 30°N comprises of an arid region, and the rest is semiarid and humid climatic zones (Figure 1). Agriculture is a vital sector of Pakistan’s economy, accounting for 21% of the country’s GDP.12 Crop protection measures are based on intensive use of pesticides with 83% of these are applied to cotton, rice, and sugar cane.13 In Pakistan, extensive use of agrochemicals has been practiced since 1954 with 254 metric tons that reached to 78 132 tons per Received: Revised: Accepted: Published: 2610
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Figure 1. Geographical locations of cities in focus and spatial distribution of sampling network.
annum in 2003,14 depicting an alarming increase in pesticide consumption along with the number of sprays per crop.15 Pakistan ratified the Stockholm convention in 2008;16 however, so far there is no specific regularity mechanism to control and monitor OCP and PCB levels in various environmental matrices.17 Limited studies, so far conducted in Pakistan, reported significant amount of residual PCBs, DDTs, and HCH in urban soils, coastal sediments, river streams, and lakes.14,18−21 Explicitly, the high temperatures in tropical/ subtropical regions have been found to facilitate the rapid removal of POPs from soils, sediments, and water through volatilization followed by their globally atmospheric dispersion.7,22,23 Also, the understanding of the spatial distribution of air toxics is prerequisite for conducting accurate assessment of “hot spots” to better estimate personal exposure to air pollution and potential health risks.24 However, besides the absence of local monitoring campaigns for POPs, Pakistan has never been the part of any regional or global monitoring network for atmospheric POPs. Consequently, there is a dearth of knowledge about atmospheric concentrations, spatial variations, and potential sources of POPs in the country, signifying an important region at the crossroad of the Indian subcontinent, Central Asia, and the Middle East. XAD-based passive air samplers (PAS) have been used by several studies for spatial monitoring atmospheric of POPs at local,7 regional10 and continental25 scales. The prominent features of XAD-PAS besides simplicity, low cost, and electricity-free operation are their sampling rates, which are independent of wind speed; and PCBs and OCPs do not attain equilibrium during a year-round exposure.26,27 In this study, a network of XAD-resin based passive air samplers (PAS) was established simultaneously across two megacities of Pakistan
(Karachi and Lahore) over the period of 10 months in 2011. The objectives of the study were to (i) get the levels of target POPs (i.e., DDTs, endosulfan, HCHs, heptachlor and PCB congeners) across urban centers; (ii) compare atmospheric levels of the POPs with other regional/global cities and characterize their possible sources; (iii) elucidate inter- and intracity spatial distribution of target POPs. To our knowledge, this study provides the first and detailed atmospheric levels and spatial patterns of OCPs and PCBs in megacities of Pakistan.
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MATERIALS AND METHODS Study Sites and Sampling Programs. Karachi is the largest and most populous metropolitan city of Pakistan, covering an area of 3530 km2 with a population of about 18 million.28 It is located alongside the Arabian Sea coast in the extreme south of the country (Figure 1). The climate of Karachi is arid with mild winters and warm summers. Lahore is the second largest city of Pakistan with area of 1772 km2 and population of about 10 million.29 The city is located in approximately the central-east part of the country, near the Ravi River (tributary of Indus River) and is surrounded by agricultural plains. It has a hot semiarid climate with monsoon rains, hot summers, and dry and warm winters. Duplicate XADPAS were deployed at six sites in Karachi and four sites in Lahore (Figure 1). The total sampling period contains 10 months from January 15 to October 11, 2011, including the period with intensive pesticidal sprays (from July to September). Effort was made to encompass boundaries (north, south, east, west) and central zones of the cities and to cover all sorts of land-uses, i.e., residential, commercial, industrial, coastal/agricultural, etc. Field blanks were collected at six sampling sites, three in each city. Location and 2611
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Figure 2. Average wind fields over Karachi and Lahore at 850 mbar (Jan−Oct, 2011).
20 °C min−1 to 140 °C, at 4 °C min−1 to 200 °C (10 min hold time), then at 4 °C min−1 to 300 °C and held for 2 min. All samples were quantified for the following target compounds: PCB 28, 52, 101, 138, 153, and 180; heptachlor (HEPT) and heptachlor epoxide (HEPX); o,p′-DDT, p,p′-DDT, o,p′-DDE, p,p′-DDE, o,p′-DDD and p,p′-DDD. Quantification of HCHs (α-HCH, β-HCH, γ-HCH), and α-, β-endosulfan were performed on the same instrument operated in negative chemical ionization mode with ammonia as the reagent gas. Quality Assurance/Quality Control (QA/QC). All analytical procedures were carried by using strict quality assurance and control measures. Laboratory blanks (pre-extracted XAD resin filled cellulose thimbles) and field blanks were extracted and analyzed in the same way as the samples. No targeted compound or its precursor was detected in laboratory blanks whereas o,p′-DDT, p,p′-DDT, o,p′-DDE, p,p′-DDE, α-HCH, γHCH, α-endosulfan, PCB 28 and PCB 52 were detected in some of the field blanks in both cities. Method detection limits (MDLs) were derived as 3 times the standard deviation of the mean field blank concentrations. In case any targeted compound or its precursor was not detected in field blanks, the concentration of the lowest calibration standard was substituted for the MDL. Concentrations of field blanks and MDLs are provided in Table SI-2 (Supporting Information). Recoveries were between 69% and 108% for TCmX and 74− 118% for PCB 209. The full data set for the POPs concentrations (including the values of duplicates) is available as Table SI-3 (Supporting Information). All reported values are field-blank corrected but not corrected for the recovery rates. Wind Direction/Back Trajectories. Figure 2 shows the average wind direction and wind speed over Pakistan including Karachi and Lahore during the sampling period, as determined by analyzing meteorological data from the National Center for
characteristics of sampling sites are illustrated in the Supporting Information (Table SI-1). Sample Preparation, Extraction, and Analysis. Prior to deployment, XAD resin was Soxhlet extracted subsequently by methanol, acetonitrile, and dichloromethane (DCM). The XAD resin (60 mL of wet XAD in methanol) was transferred to a precleaned stainless steel mesh cylinder and dried in a clean desiccator. Dry cylinders were kept in airtight stainless steel tubes with Teflon lids for transportation to sampling sites. Upon completion of sampling, harvested XAD cylinders were resealed in solvent rinsed stainless steel tubes and sent by courier to Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Beijing, China, and stored at −20 °C until extraction. Samples were extracted in a dedicated clean laboratory, which had filtered, charcoal stripped air and positive pressure conditions. Each sample was transferred into the glass Soxhlet thimble and spiked with recovery standards of 2,4,5,6tetrachloro-m-xylene (TCmX) and decachlorobiphenyl (PCB 209). Samples were Soxhlet extracted using DCM for 24 h. The extracts were first solvent-exchanged by hexane and loaded on the top of a column (2 g of 3% deactivated alumina/3 g of 6% deactivated silica). The column was eluted with a 30 mL mixture of DCM and hexane (1:1). The eluates were further cleaned by gel permeation chromatography (GPC, containing 6 g of Biobeads SX 3). Samples were finally solvent exchanged to 25 μL of dodecane containing known quantity of pentachloronitrobenzene (PCNB) as internal standard (20 pg/μL). All samples and blanks were analyzed on a gas chromatograph (GC) with an ion-trap mass spectrometer (Finnigan Trace GC/Polaris Q), using a CP-Sil 8CB capillary column (50 m) and operating under the MS-MS mode. Helium was used as the carrier gas at 1 mL min−1 under constant-flow mode. The oven temperature began at 100 °C for 2 min, ramped up at a rate of 2612
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Jan−Oct, 2011 Jan−Oct, 2011 Nov−Dec, 2008 July−Sep, 2006 July−Sep, 2006 July−Sep, 2006 2006−07 2006−07 2006−07 Sep−Nov, 2004 Sep−Nov, 2004 Sep−Nov, 2004 4−58 51−112 24−147 188−228
Polyurethane foam based passive air samplers. a
Karachi Lahore Azerbaijan32 Mumbai, India30 Kolkota, India30 Chennai, India30 Guangzhou, China33 Zhaoqing, China 33 Hongkong 33 Japan9 Singapore9 South Korea9
6−66 43−82 19−42 524 262 620 211−671 198−272 120−169 1.4−70 1.3−53 ΣDDTs > ΣHCHs > ΣPCBs > Σheptachlor. As the biggest sea port and industrial city of Pakistan, PCBs were dominant over OCPs except aged DDTs in Karachi; whereas, being surrounded by agricultural fields, an abundance of OCPs in Lahore is more evident. The fresh use of OCPs in Karachi appeared to be limited, which was supported by the overall lower OCPs concentration (∼half of that observed for Lahore). In recent past, the oceanic waters and marine sediments are found to become secondary sources of POPs due to their volatilisation back to the atmosphere.43,54 A study21 conducted on coastal sediments in Karachi reported higher residual contamination of pesticides including DDT, HCH, and heptachlor. Karachi receives untreated sewage of millions of residents, industrial effluent, and runoff from agriculture lands through the Lyari and Malir river streams, which finally drains into the Arabian Sea (Figure 4).55,56 High temperatures in Karachi throughout the sampling period, ranging between 18 and 32 °C (Table SI-4, Supporting Information) was enough to aid revolatilization of residual POPs. This could be the reason of elevated levels of weathered OCPs in Karachi. Air circulation patterns over Karachi and Lahore are different, which might influence the dispersal of atmospheric POPs. Back trajectory analysis revealed that during the sampling period, 87% of the air masses that arrived in Karachi originated from west and southwest directions over the Arabian Sea (Figure SI1a, Supporting Information). The average wind speed for Karachi can reach up to 6−7 m/s from May to September (Table SI-4, Supporting Information). In this respect, Karachi may be presumed as gateway of maritime winds that are certainly capable of flushing out the atmospheric POPs accumulated in city’s atmosphere. This might be the reason that overall lower concentrations of POPs were found in Karachi despite its multiple pollution sources. On the basis of its extensive industrial emissions, we expected high levels of PCB in Karachi; however, similar levels of PCB were found between Karachi and Lahore (Figure SI-2, Supporting Information). This can also be explained by the strong atmospheric convection in Karachi. On the contrary, 43% of the air masses that arrived in Lahore were associated with westerlies; 36% traveled across Indian region at eastern side; and the remaining 21% were dominated by monsoon winds reached at Lahore from its south passing across agricultural regions of Pakistan and western India (Figure SI-1b, Supporting Information). During the sampling period, the average wind speed across Lahore and its surrounding region was noted between 1 and 3 m/s, resulting in more stable conditions than in Karachi (Figure 2 and Table SI-4 (Supporting Information)). Thus, the distinct agricultural sources coupled with the stable climate lead to the high levels of OCPs in the atmosphere of Lahore. Intracity Variations. Spatially distributed PAS were deployed to identify source regions of POPs on the basis of concentration gradients across the study area.57 Site specific levels of OCPs and PCBs across Karachi and Lahore are shown in Figures 4 and SI-3 (Supporting Information), respectively. One-way analysis of variance (ANOVA) was performed to determine the statistical differences in the mean values of individual chemicals among different sampling sites (Table SI-8, Supporting Information). For chemical compounds with significant difference, Tukey’s post hoc test (Table SI-9,
Supporting Information) was further used to determine which sites differ significantly from others.58 Karachi. ANOVA showed significant variation (p < 0.05) in sample means for ΣDDTs, ΣHCHs, Σendo, and Σhept (Table SI-8, Supporting Information). Conversely, the ANOVA test for ΣPCBs showed no significant differences (p > 0.05, Table SI-8, Supporting Information) among the sampling sites (Figure 4), which could be attributed to their local emissions from industrial zones existing throughout the city. In addition, random sites for open burning of municipal wastes may also be the source of PCBs in the ambient air of Karachi.52 Tukey posttesting indicated significant differences between the south (K1, K2, K4) and north (K3 and K5) sites (Table SI-9, Supporting Information) of Karachi. In addition, Figure 4 represents the spatial distribution map of POPs in Karachi, with higher levels at the southern region (K1, K2, K4). The southern zone marks the Arabian coastline, sea ports, industries, and a small proportion of agricultural land. In earlier studies,59,60 a higher concentrations of residual DDTs, especially p,p′-DDE, HCHisomers, and other pesticides, have been recorded in samples from Arabian seawater and sediments. Pandit et al.54 reported the volatilization of OCPs from the Arabian Sea water to the atmosphere, which may lead to the higher levels of OCPs near south coastal zone of Karachi. In addition, the DDT-containing antifouling paints have been reported to be used on fishing ships for preventing the adhesion of halobios (e.g., barnacles, mollusks, and algae),61 which could be a potential source of DDTs. All these factors could be possible reasons of elevated levels of residual OCPs, as observed in the southern region of Karachi. Higher levels of endosulfan and HCHs, specifically noted at PAS sites K2 and K4 (Figure 4), were most probably due to the ongoing usage of these OCPs at agricultural fields in the vicinity. Because the suburban site K4 is located downwind of coastal (K1) and agricultural (K2) sites, comparatively higher levels of POPs, as noted therein, were not surprising. On the basis of the prevailing wind patterns (Figure 2) and elevated levels of POPs, as observed near the coastal region, it may be inferred that transportation of POPs in the country generally follows a cyclic pathway as: (1) first by the discharge of agricultural/industrial POPs into rivers, (2) then the ultimate discharge into Arabian Sea through river streams, (3) thereafter the revolatilization of aged signatures from coastal ecosystem, and then moving toward neighbor regions (inlands of Pakistan) through westerlies and/or monsoon winds. Thus, the coastal belt of Pakistan can be ascribed as a potential source region of POPs, which constitutes an alluring area for further studies, for instance, continuous monitoring of concentrations and compositions of POPs in air, seawater, and sediments. Lahore. In Lahore, comparatively small interquartile ranges of box-and-whisker plots (Figure 3) and the ANOVA test (p > 0.05, Table SI-8 (Supporting Information)) indicate the uniform levels of OCPs and PCBs throughout the city. The city is located in the agricultural region, hence it might have been influenced with OCPs by either their fresh applications or weathered residuals revolatized from surrounding agricultural soils. For instance, consistently higher concentrations of endosulfan and lindane may be associated with their ongoing applications in agricultural fields of Chenab and Ravi river basins lying at northwest and southwest of Lahore, well-known for cotton and rice production.19 A slight gradient of increasing DDT and HCH levels at the north urban-suburban transect (sites L1 and L2, Figure SI-3 (Supporting Information)) was consistent with an earlier study,14 which characterized the same 2616
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(4) Wang, J.; Guo, L.; Li, J.; Zhang, G.; Lee, C. S. L.; Li, X.; Jones, K. C.; Xiang, Y.; Zhong, L. Passive air sampling of DDT, chlordane and HCB in the Pearl River Delta, South China: Implications to regional sources. J. Environ. Monit. 2007, 9 (6), 582−588. (5) Rajendran, R. B.; Imagawa, T.; Tao, H.; Ramesh, R. Distribution of PCBs, HCHs and DDTs, and their ecotoxicological implications in Bay of Bengal, India. Environ. Int. 2005, 31 (4), 503−512. (6) Wania, F.; Mackay, D. Peer reviewed: Tracking the distribution of persistent organic pollutants. Environ. Sci. Technol. 1996, 30 (9), 390A−396A. (7) Shunthirasingham, C.; Mmereki, B. T.; Masamba, W.; Oyiliagu, C. E.; Lei, Y. D.; Wania, F. Fate of pesticides in the arid subtropics, Botswana, Southern Africa. Environ. Sci. Technol. 2010, 44 (21), 8082− 8088. (8) Iwata, H.; Tanabe, S.; Sakai, N.; Nishimura, A.; Tatsukawa, R. Geographical distribution of persistent organochlorines in air, water and sediments from Asia and Oceania, and their implications for global redistribution from lower latitudes. Environ. Pollut. 1994, 85 (1), 15− 33. (9) Jaward, F. M.; Zhang, G.; Nam, J. J.; Sweetman, A. J.; Obbard, J. P.; Kobara, Y.; Jones, K. C. Passive air sampling of polychlorinated biphenyls, organochlorine compounds, and polybrominated diphenyl ethers across Asia. Environ. Sci. Technol. 2005, 39 (22), 8638−8645. (10) Baek, S.-Y.; Jurng, J.; Chang, Y.-S. Spatial distribution of polychlorinated biphenyls, organochlorine pesticides, and dechlorane plus in Northeast Asia. Atmos. Environ. 2013, 64 (0), 40−46. (11) Castillo, L. E.; de la Cruz, E.; Ruepert, C. Ecotoxicology and pesticides in tropical aquatic ecosystems of Central America. Environ. Toxicol. Chem. 1997, 16 (1), 41−51. (12) Ministry of Finanace. Pakistan Economy Survey 2011−12, Government of Pakistan. http://www.finance.gov.pk/survey_1112. html (accessed January 1, 2013). (13) Khan, M. A.; Iqbal, M.; Ahmad, I.; Soomro, M. H.; Chaudhary, M. A. Economic evaluation of pesticide use externalities in the cotton zones of Punjab, Pakistan. Pak. Develop. Rev. 2002, 683−698. (14) Syed, J.; Malik, R. Occurrence and source identification of organochlorine pesticides in the surrounding surface soils of the Ittehad Chemical Industries Kalashah Kaku, Pakistan. Environ. Earth Sci. 2011, 62 (6), 1311−1321. (15) Tariq, M.; Hussain, I.; Afzal, S. Policy measures for the management of water pollution in Pakistan. Pak. J. Environ. Sci. 2003, 3, 11−15. (16) Stockholm Convention. The 12 initial POPs under the Stockholm Convention. http://chm.pops.int/Convention/ThePOPs/ The12InitialPOPs/tabid/296/Default.aspx (accessed January 1, 2013). (17) Malik, R.; Rauf, S.; Mohammad, A.; Eqani, S.-A.-M.-A.; Ahad, K. Organochlorine residual concentrations in cattle egret from the Punjab Province, Pakistan. Environ. Monit. Assess. 2011, 173 (1), 325−341. (18) Ahad, K.; Mohammad, A.; Khan, H.; Ahmad, I.; Hayat, Y. Monitoring results for organochlorine pesticides in soil and water from selected obsolete pesticide stores in Pakistan. Environ. Monit. Assess. 2010, 166 (1), 191−199. (19) Eqani, S. A. M. A. S.; Malik, R. N.; Zhang, G.; Mohammad, A.; Chakraborty, P. Polychlorinated biphenyls (PCBs) in the sediments of the River Chenab, Pakistan. Chem. Ecol. 2012, 28 (4), 327−339. (20) Iram, S.; Ahmad, I.; Ahad, K.; Muhammad, A.; Anjum, S. Analysis of pesticides residues of Rawal and Simly lakes. Pak. J. Bot. 2009, 41 (4), 1981−1987. (21) Bano, A.; Siddique, S. Chlorinated hydrocarbons in the sediments from the coastal waters of Karachi (Pakistan). Pak. J. Sci. Ind. Res. 1991, 34, 70−74. (22) Larsson, P.; Berglund, O.; Backe, C.; Bremle, G.; Eklöv, A.; Järnmark, C.; Persson, A. DDTFate in tropical and temperate regions. Naturwissenschaften 1995, 82 (12), 559−561. (23) Odabasi, M.; Cetin, B.; Demircioglu, E.; Sofuoglu, A. Air−water exchange of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) at a coastal site in Izmir Bay, Turkey. Mar. Chem. 2008, 109 (1), 115−129.
zone as the hotspot of DDT and HCH, as thousands of kilogram of these chemicals were dumped off in the soils around a local pesticide manufacturing unit after their ban in the 1990s. Syed et al.37 attributed these soils as secondary sources of residual DDTs in the atmosphere. Regarding PCB levels, although Lahore is not as heavily industrialized as Karachi, overall comparable PCB levels to Karachi could be largely attributed to the waste/straw burning process compounded with the re-evaporation of lighter congeners from secondary sources. Because the agricultural regions around Lahore could be regarded as potential POPs sources and the usage of pesticides is related to seasonal crops, the atmospheric levels of OCPs are expected to vary seasonally. Therefore, the seasonal monitoring of legalized/nonlegalized OCPs is recommended in agricultural regions around Lahore. This preliminary study explicated the status of Stockholm Convention POPs in the atmospheres of the central and southern urban regions of Pakistan and provided the baseline of their atmospheric concentrations. The study outcomes can be integrated with regional/global atmospheric monitoring campaigns for evaluating the effectiveness of international efforts to reduce POPs globally. The agricultural regions and coastal zone of Pakistan are suspected as hotspot of fresh/ weathered OCPs that need further elucidation.
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ASSOCIATED CONTENT
S Supporting Information *
Description of sampling sites, meteorological conditions during sampling period, concentration data and spatial variation of POPs in Lahore. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*X. Wang. Phone: +86-10-84097120. E-mail: wangxp@itpcas. ac.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (41222010 and 41071321). We are especially thankful to Lahore University of Management Sciences (LUMS); University of Engineering and Technology, Lahore; Defence Housing Authorities, Karachi and those household residents who provided their premises for installing passive air samplers. We also would like to appreciate and pay thanks to the staff at SUPARCO offices in Karachi and Lahore for their devoted efforts in successfully deploying and retrieving the passive air samplers.
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REFERENCES
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dx.doi.org/10.1021/es404711n | Environ. Sci. Technol. 2014, 48, 2610−2618
