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Ambient methane levels in Delhi
Chemosphere ± Global Change Science 2 (2000) 185±190
Ambient methane levels in Delhi Pratap Kumar Padhy, C.K. Varshney * School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India Received 13 May 1998; accepted 29 July 1999
Importance of this paper: Methane in the atmosphere is increasing by an average annual rate of 0.5±1% over its preindustrial level. Ambient methane levels vary widely locally and regionally in relation to the intensity of anthropogenic activity. Methane levels in the urban environment of Delhi were found to be well above the global average value. High ambient levels of methane in the urban environment, apart from its implications for global warming, also aect the atmospheric chemistry and air quality signi®cantly. The results of this study have highlighted the need for monitoring of ambient methane levels in the urban environment as well as preparation of its source inventory. Abstract The ambient levels of methane in the urban environment of Delhi were measured during November 1994±June 1995, at 13 sites varying in anthropogenic activities and trac density. The methane levels in the ambient urban environment of Delhi varied from 1703 to 9492 ppbv, with an average concentration of 4121 354 ppbv, exhibiting diurnal and seasonal variation. The likely reasons for the elevated methane level (above the global average of 1737 ppbv) in the urban environment of Delhi and its implications for air qualities have been discussed. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Air quality; Ambient urban environment; Anthropogenic sources; Greenhouse gas; Methane
1. Introduction Methane (CH4), a greenhouse gas, having a global concentration of 1737 ppbv (Khalil and Rasmussen, 1994), has increased about 115% over its pre-industrial level, with an annual growth rate of 0.5±1% (Engelman, 1994). The latest estimates of natural and anthropogenic global emission of CH4 are about 400±610 Tg/y (Cicerone and Oremland, 1988) and 505 Tg/y (Crutzen, 1991). In India, the emission from anthropogenic sources is estimated to be 13 Tg/y (Varshney and Padhy, 1999). In spite of the growing interest on CH4 as a greenhouse gas, information on its ambient levels in the urban environment is lacking. This paper deals with CH4 in the urban environment of Delhi, while information on *
Corresponding author. Fax: +91-11-6165886. E-mail address: [email protected] (C.K. Varshney).
TVOC has been reported elsewhere (Varshney and Padhy, 1998). 2. Materials and methods 2.1. The study area Delhi, the capital city, spread over an area of 1483 km2 , has grown at a phenomenal rate, having a population of over 10 million. From 1971 to 1993, the number of industries and automobiles have grown from 0.026 to 0.093 million and 0.2 to over 2.2 million, respectively (DSHB, 1995). 2.2. Methane monitoring sites The following 13 sites were selected for methane monitoring in Delhi. The trac density, average vehicle
1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 0 0 ) 0 0 0 0 5 - 2
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P.K. Padhy, C.K. Varshney / Chemosphere ± Global Change Science 2 (2000) 185±190
speed, and other anthropogenic activities including natural CH4 sources at the sampling sites vary from one another. JNU (S1 ): A university complex and a least impacted site situated within a vast tract of natural vegetation. DLTA (S2 ): An open area, having a complex of lawn tennis courts, sandwiched between an urban forest and a fast moving trac corridor. Chanakya Puri (S3 ): Lies on a wide two-lane fast trac road having sprawling lawns on either side of the road. WWF-India (S4 ): Situated on a two-way Lodi Road, in the vicinity of a large Lodi Garden, having low density fast moving trac. Vijay Chowk (S5 ): A large square, close to the Central Government Secretariat having moderate density of fast moving trac. Kamal Ataturk Marg (S6 ): Adjacent to a wide road, passing through a low-density residential area, with a highly regulated fast trac. Safdarjang Fly Over (S7 ): A ¯yover, having fast moving heavy trac, across a railway line, in the vicinity of a vast stretch of open grounds of a sparingly used aerodrome. Pahar Ganj (S8 ): Located in a densely populated commercial and residential area, having narrow and congested road with mixed, slow moving heavy trac. Minto Bridge (S9 ): Located in a busy oce and commercial complex having heavy, slow and mixed trac. AIIMS (S10 ): An extremely busy trac intersection, having very heavy trac density. South Extension (S11 ): Lies on either side of a twolane trac corridor, having heavy and congested trac during most part of the day. Ashram (S12 ): An extremely busy trac intersection, having a slow moving dense trac. Darya Ganj (S13 ): A busy road intersection in a commercial-cum-residential locality having heavy, mixed and slow trac. 2.3. Air sampling procedure Grab samples were collected between 15:30 to 18:00 h at an interval of 15 days from November 1994 to June 1995, using disposable syringes. After repeated purging, sample containers were ®lled with ambient air and sealed with rubber stopper and aluminum jacket. To determine diel variation, 2 sites (S1 and S10 ) were selected and sampling was done at 3-h interval for 24 h every month. 2.4. Methane estimation The air samples were analysed using GC-FID equipped with Porapak-Q column. The GC (accuracy 99%) was calibrated each time prior to air sample
analysis using standard calibration gas mixture of CH4 (108 vpm) in air obtained from EDT Research, London. For each sample, a new syringe was used and the syringe was purged with the sample air prior to the drawing of aliquots for analysis. This procedure was followed to avoid contamination and syringe carry over. For each sample three aliquots were drawn for methane measurements. 3. Observations and discussions 3.1. Ambient methane levels in Delhi The average ambient CH4 levels at Delhi was 4121 354 ppbv, varying between 1703 and 9492 ppbv (November 1994±June 1995). The CH4 levels at all sites, except S1 , were well above its global average value (1737 ppbv, 1994). The reason for such high CH4 levels in the urban environment of Delhi is because of many anthropogenic sources, a quantitative estimation of which have been worked out on the basis of published emission factors and are described below. Transportation: The number of registered motor vehicles in Delhi is over 2.3 million (DSHB, 1995). CH4 is a product of gasoline and diesel fuel combustion in the automobile exhausts (Lawson et al., 1990). Annual CH4 emission from automobiles in Delhi is estimated to be about 4025 t (using emission factor: JEA/EPA, 1990). Solid waste disposal sites: Delhi produces huge quantity of solid waste, about 6000 t per day (SE, 1995) and about 0.88 million tonnes (mt) of biodegradable waste per annum. It is reported that one tonne of waste in a land®ll emits approximately 25±30 m3 of CH4 (Vanni and Esposito, 1982). Delhi, having six land®ll sites, are estimated to emit about 8760 t CH4 annually (emission factor used: USEPA, 1980; calculated after Orthofer, 1991). Waste dumpsites as well as primary waste collection sites also emit CH4 (Bingemer and Crutzen, 1987). Sewers: Often large sections of drains remain chocked, creating ideal anaerobic environment for CH4 formation. In April 1995, in the Connaught Place, excessive pressure developed in an underground drain leading to multiple explosions causing damage. It is calculated that about 2279 t of CH4 is emitted from drains whose estimated area is about 2:37 106 m2 (emission factor used: IPCC, 1995). Sewage treatment: Delhi generates about 1480 MLD of sewage water and only 745 million litres receive treatment (Trivedi et al., 1995). During sewage treatment, varying amounts of CH4 is released (USEPA, 1980; Orthofer, 1991). Based on IPCC (1995), it is estimated that from this source about 2970 t of CH4 is emitted annually.
P.K. Padhy, C.K. Varshney / Chemosphere ± Global Change Science 2 (2000) 185±190
Fossil fuels and biomass: About 0.3 million kilolitre of kerosene, 0.259 mt of LPG, and 0.04 mt of soft coke were consumed in Delhi (DSHB, 1995). CH4 is emitted during the combustion of fossil fuels in cook stoves using charcoal, kerosene and LPG (Smith et al., 1993). It is estimated that about 8000 t of garbage is burnt everyday by the street sweepers. CH4 is a major component of combustion gases emitted during biomass burning (Crutzen and Andreae, 1990). The annual emission from these sources has been estimated to be 9048 and 8160 t, respectively (calculated using the emission factors: JEA/EPA, 1990; Hegg et al., 1990; CONCAWE, 1986). Industry: Combustion of coal, oil and gas in thermal power plants and industry also emit CH4 . Delhi has three coal ®red thermal power plants with a total installed capacity of 950 MW. The annual CH4 emission from these sources is of the order of 70.7 t (using emission factor: JEA/EPA, 1990). Wetlands: There are many swampy habitats, shallow ditches, ponds and puddles including low lying areas along the bank of River Yamuna and in the vicinity of unauthorised residential colonies. The combined annual estimated emission of CH4 from various wetlands in Delhi is about 2939 t (using emission factor: Bartlett et al., 1990). Livestock and agriculture: Livestock form a signi®cant source of CH4 emission. The total livestock population in Delhi is over 0.33 million (DSHB, 1995). The estimated emission from livestock is about 21854 t (using emission factor: Crutzen et al., 1986). Paddy ®elds are important source of CH4 emission. In Delhi, paddy is cultivated over 2.4 ´ 104 ha, from which about 2328 t of CH4 is emitted (using emission factor: Bartlett et al., 1990). Forests, pasture, grazing, cultivable, and wastelands are estimated to emit 327 t of CH4 annually (using emission factor: ECE, 1990). 3.2. Inter-site variation in methane levels Based on the methane measurements in the ambient environment, the 13 sites could be grouped into three categories, namely, sites with high, moderate, and low CH4 levels. At sites S8 ±S13 , high CH4 levels (3299±9492 ppbv) were recorded. The maximum methane level (9492 ppbv) was observed at Darya Ganj (S13 ). High automobile exhaust due to idling and slow vehicular movement occurs in these localities. In addition, CH4 sources in the vicinity include poorly maintained solid waste dumps and collection sites, chocking of sewer, proximity to the river and riverine wetlands and thermal power plants. At sites S2 ±S7 , CH4 levels were moderate (1792±4901 ppbv). These sites have very few emission sources and they happen to be situated in relatively open localities.
187
Methane levels at site S1 were low (1703±2003 ppbv). The lowest concentration of 1703 ppbv was recorded from this site. Close to the site, there are no signi®cant sources of methane except for the diusion of CH4 from the city centre. 3.3. Diel variation Ambient CH4 levels exhibited variation on a 24-h cycle. At site S10 , CH4 build-up started early in the morning attaining the maxima at 0900 h (Fig. 1). Methane undergoes photochemical reactions in the atmosphere. At site S1 , CH4 level rises in the morning, attained maximum value during noon and thereafter, declines through the night (Fig. 2). The pattern of diel variation was similar in dierent months except in January, when the CH4 maxima was attained at 0900 h and also at 1500 h. Further probing is required to explain this. Methane maxima at S1 site was mostly attained at 1200 h but declined subsequently. In some isolated instances CH4 values receded marginally below the mixed air value (1750 ppbv), reported for 28°N latitude (Dlugokenchy et al., 1994). But diurnal ¯uctuations in CH4 level mostly swung to the higher side of the mixed air value. 3.4. Seasonal variation Methane levels ¯uctuated seasonally too (Fig. 3). The average methane level during winter (November±February) varied between 3997 1331 ppbv and 5253 2155 ppbv, attaining the peak value in November. During summer (March±June), the CH4 values were low, between 3172 1278 ppbv and 3508 1484 ppbv. The reasons of large standard deviation is that the average CH4 values have been derived by pooling the CH4 values of all the 13 sites. Relatively high CH4 values during the winter could be on account of: (1) low dispersion in the winter season due to stable atmosphere; and (2) short day lengths, low solar intensity and ambient temperature in the winter months reduces photochemical scavenging of CH4 . The range of variation in CH4 values during winter months (1743±793 ppbv, except for one occasion when a relatively higher value 9492 ppbv was observed) was more as compared to the range of variation observed during the summer months (1703±5876 ppbv). Such variations in the methane levels are in agreement with the seasonal cycle of CH4 and OH in the Northern Hemisphere (Khalil and Rasmussen, 1983). Photochemical loss rate of CH4 due to its reaction with OH is about 1:4 1011 molecules cmÿ2 sÿ1 or 5:8 108 t yrÿ1 (Logan et al., 1981).
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Fig. 1. Plot of average ambient air data showing the diurnal variation of methane at site S10 .
Fig. 2. Plot of average ambient air data showing the diurnal variation of CH4 at site S1 .
P.K. Padhy, C.K. Varshney / Chemosphere ± Global Change Science 2 (2000) 185±190
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Fig. 3. Seasonal variation of CH4 at Delhi (all sites combined).
3.5. Implications for air quality
Acknowledgements
The ground level O3 build-up is largely on account of increasing emissions of ozone forming precursors more particularly, VOCs, which have been shown to have high levels in the ambient environment in Delhi (Varshney and Padhy, 1998). In Delhi, the ground level O3 concentration has been also shown to be high (20±273 lg mÿ3 ) and 1 h WHO standard has been violated on many occasions (Varshney and Aggarwal, 1992). Methane levels in the ambient urban environment were mostly above its average global concentration of 1737 ppbv. High levels of methane in the urban environment ranging from 1200±15000 ppbv (SCDPH, 1967), 2000±20000 ppbv (Altshuller, 1968), 2000±6000 ppbv (Graedel et al., 1986), and 3300±3500 ppbv (Rowland et al., 1990) also have been reported. High ambient CH4 levels in Delhi indicate that CH4 emission from various sources in the city exert considerable local eect on CH4 levels. Reliable quantitative data on methane emission from dierent anthropogenic sources are required for air quality modelling studies as well as for predicting formation of tropospheric ozone, which is attracting increasing scienti®c attention.
The fellowship provided by the University Grant Commission, New Delhi, to one of the authors (Pratap K. Padhy) during the period of research work is gratefully acknowledged.
References Altshuller, A.P., 1968. Atmospheric analysis by gas chromatography. Adv. Chromatogr. 5, 229±262. Bartlett, K.B., Crill, P.M., Bonassi, J.A., Richey, J.E., Harriss, R.C., 1990. Methane ¯ux from the Amazon River ¯oodplain: emissions during rising water. J. Geophys. Res. 95, 16773±16788. Bingemer, H.G., Crutzen, P.J., 1987. The production of methane from solid waste. J. Geophys. Res. D 92, 2181±2187. CONCAWE, 1986. Volatile organic compound emissions: an inventory for Western Europe. Report No 2/86, Den Haag. Cicerone, R.J., Oremland, R.S., 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochem. Cycles 2, 299±327. Crutzen, P.J., 1991. Methane's sinks and sources. Nature 350, 381±382.
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Crutzen, P.J., Aselmann, I., Seiler, W., 1986. Methane production by domestic animals, wild ruminants, other herbivorous fauna, and humans. Tellus 38B, 271±284. Crutzen, P.J., Andreae, M.O., 1990. Biomass burning in the tropics: Impact on atmospheric chemistry and biogeochemical cycles. Science 250, 1669±1678. Dlugokencky, E.J., Steele, L.P., Lang, P.M., Masarie, K.A., 1994. The growth rate and distribution of atmospheric methane. J. Geophys. Res. 99 D 8, 17021±17043. DSHB, Delhi Statistical Hand Book, 1995. Bureau of Economics and Statistics. Government of National Territory of Delhi. ECE, Economic Commission for Europe, 1990. VOC task force, emissions of volatile organic compounds from stationary sources and possibilities of their control. Final Report, Karlsruhe. Engelman, R., 1994. Stabilizing the atmosphere, population, consumption and greenhouse gases. Population and Environmental Programme, Population Action International. Graedel, T.E., Hawkins, D.T., Claxton, L.D. (Ed.), 1986. Atmospheric Chemical Compound ± Sources, Occurrences and Bioassay. Academic Press, Orlando, FL, p. 732. Hegg, D.A., Radke, L.F., Hobbs, P.V., Rasmussen, R.A., Riggon, R.J., 1990. Emissions of some trace gases from biomass ®res. J. Geophys. Res. 95, 5669±5675. IPCC, Inter-governmental Panel on Climate Change, 1995. IPCC Guidelines for National Greenhouse Gas Inventories. JEA-EPA, Methane emissions and opportunities for control, 1990. In: Workshop Results of IPCC, Response Strategies Working Group. Japan Environment Agency-Environmental Protection Agency, Washington DC, p. 129. Khalil, M.A.K., Rasmussen, R.A., 1983. Sources, sinks, and seasonal cycles of atmospheric methane. J. Geophys. Res. 88 C 9, 5131±5144. Khalil, M.A.K., Rasmussen, R.A., 1994. Global methane record derived from six globally distributed locations. Trend '93 a compendium of data on global change. CIAC-65, WDC, pp. 268±272. Lawson, D.R., Groblicki, P.J., Stedman, D.H., Bishop, G.A., Guenther, P.L., 1990. Emissions from in-use motor vehicles in Los Angeles: A pilot study of remote sensing and the inspection and maintenance program. J. Air Waste Management Assoc. 40, 1096±1103. Logan, J.A., Prather, M.J., Wofsy, S.C., McElroy, M.B., 1981. Tropospheric chemistry: a global perspective. J. Geophys. Res. 86 C 8, 7210±7254.
Orthofer, R., 1991. Abschatzung der Methan-Emissionen in Osterreich (Estimation of methane emissions in Austria). Report No. OEFZS-A-1965, Austrian Research Centre Seibersdorf, Seibersdorf. Rowland, F.S., Harries, N.R.P., Blake, D.R., 1990. Methane in cities. Nature 347, 432±433. SCDPH (State of California Department of Public Health, Air and Industrial Hygiene lab), 1967. Ranges of hydrocarbon values expected in urban air masses. No. 9A, Berkeley Recommended Method (tentative). SE, 1995. Survey of Environment. The Hindu, Chennai. Smith, K.R., Khalil, M.A.K., Rasmussen, R.A., Thorneloe, S.A., Manegdeg, F., Apte, M., 1993. Greenhouse gases from biomass and fossil fuel stoves in developing countries: a Manila pilot study. Chemosphere 26, 479±505. Trivedi, R.C., Pandey, M., Bhardwaj, R.M., 1995. Waste water generation and treatment status, economic value and cost of waste water treatment plant in class I and II urban units (a study conducted by CPCB, Delhi). A Paper Presented at First Expert Group Meeting on Capacity Building for Urban Development held at Chandigarh, 13±15 January 1995, India. USEPA, 1980. Volatile Organic Compound (VOC) Species Data Manual, second ed. Report No, EPA-450/4-80-015, US Environmental Protection Agency. Vanni, A., Esposito, A., 1982. Land®ll gas recovery: state of the art in Italy; a feasibility study for one of the biggest Italian land®lls. In: Proceedings of the Conference on Resour. Recov. Solid Wastes, 10±12 May, Miami Beach, FL, New York, pp. 417±424. Varshney, C.K., Aggarwal, M., 1992. Ozone levels in the urban atmosphere of Delhi. Atmos. Environment 26 B 3, 291±294. Varshney, C.K., Padhy, P., K, 1998. Total volatile organic compounds (TVOC) in the urban environment of Delhi. J. Air Waste Management Assoc. 48, 174±179. Varshney, C.K., Padhy, P.K., 1999. Emission of total volatile organic compounds from anthropogenic sources in India. J. Industrial Ecology 2 (4), 93±105. C.K. Varshney is a senior professor of ecology at the School of Environmental Sciences (SES), Jawaharlal Nehru University, New Delhi, and actively involved in teaching and research. P.K. Padhy is a Senior Research Fellow at the School of Environmental Sciences (SES), working in the area of air pollution and plants.
Ambient methane levels in Delhi Pratap Kumar Padhy, C.K. Varshney * School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India Received 13 May 1998; accepted 29 July 1999
Importance of this paper: Methane in the atmosphere is increasing by an average annual rate of 0.5±1% over its preindustrial level. Ambient methane levels vary widely locally and regionally in relation to the intensity of anthropogenic activity. Methane levels in the urban environment of Delhi were found to be well above the global average value. High ambient levels of methane in the urban environment, apart from its implications for global warming, also aect the atmospheric chemistry and air quality signi®cantly. The results of this study have highlighted the need for monitoring of ambient methane levels in the urban environment as well as preparation of its source inventory. Abstract The ambient levels of methane in the urban environment of Delhi were measured during November 1994±June 1995, at 13 sites varying in anthropogenic activities and trac density. The methane levels in the ambient urban environment of Delhi varied from 1703 to 9492 ppbv, with an average concentration of 4121 354 ppbv, exhibiting diurnal and seasonal variation. The likely reasons for the elevated methane level (above the global average of 1737 ppbv) in the urban environment of Delhi and its implications for air qualities have been discussed. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Air quality; Ambient urban environment; Anthropogenic sources; Greenhouse gas; Methane
1. Introduction Methane (CH4), a greenhouse gas, having a global concentration of 1737 ppbv (Khalil and Rasmussen, 1994), has increased about 115% over its pre-industrial level, with an annual growth rate of 0.5±1% (Engelman, 1994). The latest estimates of natural and anthropogenic global emission of CH4 are about 400±610 Tg/y (Cicerone and Oremland, 1988) and 505 Tg/y (Crutzen, 1991). In India, the emission from anthropogenic sources is estimated to be 13 Tg/y (Varshney and Padhy, 1999). In spite of the growing interest on CH4 as a greenhouse gas, information on its ambient levels in the urban environment is lacking. This paper deals with CH4 in the urban environment of Delhi, while information on *
Corresponding author. Fax: +91-11-6165886. E-mail address: [email protected] (C.K. Varshney).
TVOC has been reported elsewhere (Varshney and Padhy, 1998). 2. Materials and methods 2.1. The study area Delhi, the capital city, spread over an area of 1483 km2 , has grown at a phenomenal rate, having a population of over 10 million. From 1971 to 1993, the number of industries and automobiles have grown from 0.026 to 0.093 million and 0.2 to over 2.2 million, respectively (DSHB, 1995). 2.2. Methane monitoring sites The following 13 sites were selected for methane monitoring in Delhi. The trac density, average vehicle
1465-9972/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 5 - 9 9 7 2 ( 0 0 ) 0 0 0 0 5 - 2
186
P.K. Padhy, C.K. Varshney / Chemosphere ± Global Change Science 2 (2000) 185±190
speed, and other anthropogenic activities including natural CH4 sources at the sampling sites vary from one another. JNU (S1 ): A university complex and a least impacted site situated within a vast tract of natural vegetation. DLTA (S2 ): An open area, having a complex of lawn tennis courts, sandwiched between an urban forest and a fast moving trac corridor. Chanakya Puri (S3 ): Lies on a wide two-lane fast trac road having sprawling lawns on either side of the road. WWF-India (S4 ): Situated on a two-way Lodi Road, in the vicinity of a large Lodi Garden, having low density fast moving trac. Vijay Chowk (S5 ): A large square, close to the Central Government Secretariat having moderate density of fast moving trac. Kamal Ataturk Marg (S6 ): Adjacent to a wide road, passing through a low-density residential area, with a highly regulated fast trac. Safdarjang Fly Over (S7 ): A ¯yover, having fast moving heavy trac, across a railway line, in the vicinity of a vast stretch of open grounds of a sparingly used aerodrome. Pahar Ganj (S8 ): Located in a densely populated commercial and residential area, having narrow and congested road with mixed, slow moving heavy trac. Minto Bridge (S9 ): Located in a busy oce and commercial complex having heavy, slow and mixed trac. AIIMS (S10 ): An extremely busy trac intersection, having very heavy trac density. South Extension (S11 ): Lies on either side of a twolane trac corridor, having heavy and congested trac during most part of the day. Ashram (S12 ): An extremely busy trac intersection, having a slow moving dense trac. Darya Ganj (S13 ): A busy road intersection in a commercial-cum-residential locality having heavy, mixed and slow trac. 2.3. Air sampling procedure Grab samples were collected between 15:30 to 18:00 h at an interval of 15 days from November 1994 to June 1995, using disposable syringes. After repeated purging, sample containers were ®lled with ambient air and sealed with rubber stopper and aluminum jacket. To determine diel variation, 2 sites (S1 and S10 ) were selected and sampling was done at 3-h interval for 24 h every month. 2.4. Methane estimation The air samples were analysed using GC-FID equipped with Porapak-Q column. The GC (accuracy 99%) was calibrated each time prior to air sample
analysis using standard calibration gas mixture of CH4 (108 vpm) in air obtained from EDT Research, London. For each sample, a new syringe was used and the syringe was purged with the sample air prior to the drawing of aliquots for analysis. This procedure was followed to avoid contamination and syringe carry over. For each sample three aliquots were drawn for methane measurements. 3. Observations and discussions 3.1. Ambient methane levels in Delhi The average ambient CH4 levels at Delhi was 4121 354 ppbv, varying between 1703 and 9492 ppbv (November 1994±June 1995). The CH4 levels at all sites, except S1 , were well above its global average value (1737 ppbv, 1994). The reason for such high CH4 levels in the urban environment of Delhi is because of many anthropogenic sources, a quantitative estimation of which have been worked out on the basis of published emission factors and are described below. Transportation: The number of registered motor vehicles in Delhi is over 2.3 million (DSHB, 1995). CH4 is a product of gasoline and diesel fuel combustion in the automobile exhausts (Lawson et al., 1990). Annual CH4 emission from automobiles in Delhi is estimated to be about 4025 t (using emission factor: JEA/EPA, 1990). Solid waste disposal sites: Delhi produces huge quantity of solid waste, about 6000 t per day (SE, 1995) and about 0.88 million tonnes (mt) of biodegradable waste per annum. It is reported that one tonne of waste in a land®ll emits approximately 25±30 m3 of CH4 (Vanni and Esposito, 1982). Delhi, having six land®ll sites, are estimated to emit about 8760 t CH4 annually (emission factor used: USEPA, 1980; calculated after Orthofer, 1991). Waste dumpsites as well as primary waste collection sites also emit CH4 (Bingemer and Crutzen, 1987). Sewers: Often large sections of drains remain chocked, creating ideal anaerobic environment for CH4 formation. In April 1995, in the Connaught Place, excessive pressure developed in an underground drain leading to multiple explosions causing damage. It is calculated that about 2279 t of CH4 is emitted from drains whose estimated area is about 2:37 106 m2 (emission factor used: IPCC, 1995). Sewage treatment: Delhi generates about 1480 MLD of sewage water and only 745 million litres receive treatment (Trivedi et al., 1995). During sewage treatment, varying amounts of CH4 is released (USEPA, 1980; Orthofer, 1991). Based on IPCC (1995), it is estimated that from this source about 2970 t of CH4 is emitted annually.
P.K. Padhy, C.K. Varshney / Chemosphere ± Global Change Science 2 (2000) 185±190
Fossil fuels and biomass: About 0.3 million kilolitre of kerosene, 0.259 mt of LPG, and 0.04 mt of soft coke were consumed in Delhi (DSHB, 1995). CH4 is emitted during the combustion of fossil fuels in cook stoves using charcoal, kerosene and LPG (Smith et al., 1993). It is estimated that about 8000 t of garbage is burnt everyday by the street sweepers. CH4 is a major component of combustion gases emitted during biomass burning (Crutzen and Andreae, 1990). The annual emission from these sources has been estimated to be 9048 and 8160 t, respectively (calculated using the emission factors: JEA/EPA, 1990; Hegg et al., 1990; CONCAWE, 1986). Industry: Combustion of coal, oil and gas in thermal power plants and industry also emit CH4 . Delhi has three coal ®red thermal power plants with a total installed capacity of 950 MW. The annual CH4 emission from these sources is of the order of 70.7 t (using emission factor: JEA/EPA, 1990). Wetlands: There are many swampy habitats, shallow ditches, ponds and puddles including low lying areas along the bank of River Yamuna and in the vicinity of unauthorised residential colonies. The combined annual estimated emission of CH4 from various wetlands in Delhi is about 2939 t (using emission factor: Bartlett et al., 1990). Livestock and agriculture: Livestock form a signi®cant source of CH4 emission. The total livestock population in Delhi is over 0.33 million (DSHB, 1995). The estimated emission from livestock is about 21854 t (using emission factor: Crutzen et al., 1986). Paddy ®elds are important source of CH4 emission. In Delhi, paddy is cultivated over 2.4 ´ 104 ha, from which about 2328 t of CH4 is emitted (using emission factor: Bartlett et al., 1990). Forests, pasture, grazing, cultivable, and wastelands are estimated to emit 327 t of CH4 annually (using emission factor: ECE, 1990). 3.2. Inter-site variation in methane levels Based on the methane measurements in the ambient environment, the 13 sites could be grouped into three categories, namely, sites with high, moderate, and low CH4 levels. At sites S8 ±S13 , high CH4 levels (3299±9492 ppbv) were recorded. The maximum methane level (9492 ppbv) was observed at Darya Ganj (S13 ). High automobile exhaust due to idling and slow vehicular movement occurs in these localities. In addition, CH4 sources in the vicinity include poorly maintained solid waste dumps and collection sites, chocking of sewer, proximity to the river and riverine wetlands and thermal power plants. At sites S2 ±S7 , CH4 levels were moderate (1792±4901 ppbv). These sites have very few emission sources and they happen to be situated in relatively open localities.
187
Methane levels at site S1 were low (1703±2003 ppbv). The lowest concentration of 1703 ppbv was recorded from this site. Close to the site, there are no signi®cant sources of methane except for the diusion of CH4 from the city centre. 3.3. Diel variation Ambient CH4 levels exhibited variation on a 24-h cycle. At site S10 , CH4 build-up started early in the morning attaining the maxima at 0900 h (Fig. 1). Methane undergoes photochemical reactions in the atmosphere. At site S1 , CH4 level rises in the morning, attained maximum value during noon and thereafter, declines through the night (Fig. 2). The pattern of diel variation was similar in dierent months except in January, when the CH4 maxima was attained at 0900 h and also at 1500 h. Further probing is required to explain this. Methane maxima at S1 site was mostly attained at 1200 h but declined subsequently. In some isolated instances CH4 values receded marginally below the mixed air value (1750 ppbv), reported for 28°N latitude (Dlugokenchy et al., 1994). But diurnal ¯uctuations in CH4 level mostly swung to the higher side of the mixed air value. 3.4. Seasonal variation Methane levels ¯uctuated seasonally too (Fig. 3). The average methane level during winter (November±February) varied between 3997 1331 ppbv and 5253 2155 ppbv, attaining the peak value in November. During summer (March±June), the CH4 values were low, between 3172 1278 ppbv and 3508 1484 ppbv. The reasons of large standard deviation is that the average CH4 values have been derived by pooling the CH4 values of all the 13 sites. Relatively high CH4 values during the winter could be on account of: (1) low dispersion in the winter season due to stable atmosphere; and (2) short day lengths, low solar intensity and ambient temperature in the winter months reduces photochemical scavenging of CH4 . The range of variation in CH4 values during winter months (1743±793 ppbv, except for one occasion when a relatively higher value 9492 ppbv was observed) was more as compared to the range of variation observed during the summer months (1703±5876 ppbv). Such variations in the methane levels are in agreement with the seasonal cycle of CH4 and OH in the Northern Hemisphere (Khalil and Rasmussen, 1983). Photochemical loss rate of CH4 due to its reaction with OH is about 1:4 1011 molecules cmÿ2 sÿ1 or 5:8 108 t yrÿ1 (Logan et al., 1981).
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Fig. 1. Plot of average ambient air data showing the diurnal variation of methane at site S10 .
Fig. 2. Plot of average ambient air data showing the diurnal variation of CH4 at site S1 .
P.K. Padhy, C.K. Varshney / Chemosphere ± Global Change Science 2 (2000) 185±190
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Fig. 3. Seasonal variation of CH4 at Delhi (all sites combined).
3.5. Implications for air quality
Acknowledgements
The ground level O3 build-up is largely on account of increasing emissions of ozone forming precursors more particularly, VOCs, which have been shown to have high levels in the ambient environment in Delhi (Varshney and Padhy, 1998). In Delhi, the ground level O3 concentration has been also shown to be high (20±273 lg mÿ3 ) and 1 h WHO standard has been violated on many occasions (Varshney and Aggarwal, 1992). Methane levels in the ambient urban environment were mostly above its average global concentration of 1737 ppbv. High levels of methane in the urban environment ranging from 1200±15000 ppbv (SCDPH, 1967), 2000±20000 ppbv (Altshuller, 1968), 2000±6000 ppbv (Graedel et al., 1986), and 3300±3500 ppbv (Rowland et al., 1990) also have been reported. High ambient CH4 levels in Delhi indicate that CH4 emission from various sources in the city exert considerable local eect on CH4 levels. Reliable quantitative data on methane emission from dierent anthropogenic sources are required for air quality modelling studies as well as for predicting formation of tropospheric ozone, which is attracting increasing scienti®c attention.
The fellowship provided by the University Grant Commission, New Delhi, to one of the authors (Pratap K. Padhy) during the period of research work is gratefully acknowledged.
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