CONTEMPORARY RESEARCH IN INDIA (ISSN ): VOL. 6: ISSUE: 2

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1 VARIATION OF RATE OF CHANGE OF TROPOSPHERIC OZONE WITH RAINFALL DURING DIFFERENT SEASONS OVER GANGETIC WEST BENGAL, INDIA S Goswami, Department of Chemistry, Dinabandhu Mahavidyalaya, North 24 Parganas, Bongaon, India S K Midya, Department of Atmospheric Sciences, University of Calcutta, Kolkata, India Indian Centre for Space Physics, 43 Chalantika, Kolkata, India Abstract: A critical analysis is done on the variation of tropospheric ozone with rainfall during pre-monsoon, monsoon and post-monsoon seasons for the period over Gangetic West Bengal, India. A significant observation has been found that during these aforesaid three seasons, total rainfall attains its maximum value when ozone gradient becomes nearly equal to 3.5 except for negative ozone gradient against total rainfall plot in pre-monsoon season. A possible explanation, based on chemical kinetics of O 3, Cloud Condensation Nuclei (CCN) is presented. Keywords: Tropospheric ozone, Cloud Condensation Nuclei (CCN), Flare index, Ice nuclei. 1. Introduction The south west monsoon that extends from June to September every year in India is known as Indian summer monsoon rainfall and accounts for approximately 80% of India s monsoon rainfall. This rainfall constitutes one of the significant components of the south Asian climate and majorly affects the production and life in this region. Indian monsoon rainfall is impacted upon to a great extent by the African orography (Chakraborty et al., 2002). Recently, the study of monsoon rainfall variation has become important area of study (Zhang et al., 2000). It has been pointed out that the rainfall is affected by the deposition of snow on the Tibetan plateau (Yang and Jiang, 2001 ; Zhao and Moore, 2004) whereas possible links has been found between Jiangnan region of China and Indian summer monsoon rainfall (ISMR) changes (Xiao and Gong, 2000; Lihua et al., 2007). A number of studies demonstrate the relationship between ISMR changes and meteorological parameters and phenomena e.g. surface temperature (Chattopadhyay et al., 1995), relative humidity and sea level barometric pressure (Parthasarathy et al.,1992; Bansod and Singh,1995), El Nino / Southern oscillations events (Mooley et al., 1985; Nichols, 1995; Gadgil et al., 2004), Quasi Biennial Oscillation (QBO), cloud condensation nuclei (CCN), aerosol concentration, relative sunspot number (SSN) ( Lihua et al., 2007; Hiremath and Mandi, 2004) and flare index (Midya et al., 2011). Atmospheric ozone plays a vital part in maintaining ecosystems on the Earth s surface. With a strong absorption at 9.6 µm in the outgoing long wave from the Earth, tropospheric ozone acts as a powerful greenhouse gas while it acts as a major air pollutant at the surface level and one of the main oxidants (Debaje and Kakade, 2006; Naja and Lal, 2002). The production of tropospheric ozone in the atmosphere caused mainly by its precursors emitted by the increased human activities in Indian subcontinent (Naja et al., 2003; Nair et al., 2002). O 3 concentration is massively influenced by meteorological parameters like wind speed, wind direction, temperature and relative humidity (Lyons and Cole, 1976). A decrease in the wheat crop yield in China has been related to an increase of O 3 (above 60 ppbv) at the ground level (Chameides et al., 1999).The present work is an attempt to study the effect of tropospheric ozone on rainfall over Gangetic West Bengal(GWB), India, during the time period

2 2. Data and Methodology Four seasons over Gangetic West Bengal (GWB) designated by Indian Meteorological Department, are as follows: Winter, occurring in January and February. Pre-monsoon or summer seasons, lasting from 1 st March to 7 th June. Monsoon or rainy season lasting from 8 th June to 10 th October. Humid southwest summer monsoon which begins in the late May or early June sweeps slowly across the country and dominates monsoon or rainy season. Post-monsoon season lasting from 11 th October to December. The Indian summer monsoon rainfall (ISMR) sub-divisional rainfall data set used for this paper is taken from the Indian Institute of Tropical Meteorology, Pune (IITM). The time of the monsoon period (June to September) is covered by the data set. The ozone concentration over Dum Dum, Gangetic West Bengal, is taken from the Earth Probe Total Ozone Mapping Spectrometer (TOMS) website (TOMS V8 Tropospheric Column Ozone (DU) pac average 120W-120E). For pre-monsoon season the tropospheric ozone data is taken for March, April and May (MAM). For monsoon season tropospheric ozone data is taken for June to September (JJAS). For postmonsoon season tropospheric ozone data is taken from October to December (OND). Tropospheric ozone data is then plotted against months for premonsoon seasons i.e. March, April and May (MAM) which is shown in Fig.1. From Fig.1 ozone gradient is then calculated and ozone gradient is then plotted against total rainfall in pre-monsoon season which is shown in Fig. 2. Similarly Fig. 3 shows variation of tropospheric ozone against months in monsoon season i.e. June, July, August and September (JJAS). Fig. 4 shows variation of ozone gradient, calculated from Fig.3, against total rainfall in monsoon season. Fig. 5 shows variation of tropospheric ozone against months in post-monsoon season i.e. October, November and December (OND). Fig. 6 shows variation of ozone gradient, calculated from Fig. 5, against total rainfall in post-monsoon season. Ozone gradient, as calculated in the aforesaid method, for different seasons from years and corresponding total rainfall is tabulated in Table1. The tropospheric ozone gradient for each season is divided into two sub-categories (i) Positive values of ozone gradient and (ii) negative values of ozone gradient. Total rainfall for each season is then plotted against both positive and negative values of ozone gradient for that season in a particular year during the time period from It has been found that total rainfall attains its maximum value when ozone gradient becomes nearly equal to 3.5 except for negative ozone gradient against total rainfall (Fig. 2b) in pre-monsoon season. It has been reported that a direct relationship exists between total monsoon rainfall with Total Column Ozone (TCO) gradient (Midya and Saha, 2011). Although there are other parameters like temperature, relative humidity, El Nino, flare Index they have less pronounced effect on total monsoon rainfall (Midya et al., 2012). 3. Results and discussion There are two sources of tropospheric ozone (i) transport from the overlying stratosphere (Midya and Jana, 2002) and (ii) chemical production in troposphere. A fraction of stratospheric ozone passes to troposphere, transported by folding events. During photolysis of O 3 excited oxygen atoms O ( 1 D) are produced. O 3 + hν O( 1 D)+O 2 (1) 2

3 Ground state oxygen atoms O( 3 P) are formed from major portion of O( 1 D) by quenching. O( 1 D)+N 2 O( 3 P)+N 2 (2) O( 1 D)+O 2 O( 3 P)+O 2 (3) The O( 3 P) react with oxygen molecules to form O 3. O( 3 P)+O 2 +M O 3 +M (4) In the equation M represents a species which transfers excess energy from the transition state to stabilize potential O 3 molecule. It (M) may be molecular nitrogen, molecular oxygen or other gaseous species. Since troposphere contains high oxygen concentration and high pressure, reaction (4) occurs very fast and almost all of the O( 3 P) react with oxygen molecules. Although most of the O( 1 D) are quenched to reform O( 3 P) some portion of O( 1 D) react with water vapour to form OH radical which is the most important oxidant found in the troposphere. O( 1 D) +H 2 O 2OH (5) O( 3 P) is also produced by photolysis of NO 2. They react with oxygen molecules to produce O 3 through reaction (4). NO 2 +hν (λ<410 nm) NO+O( 3 P) (6) Reaction (7) acts as a sink reaction for O 3 and NO O 3 +NO NO 2 +O 2 (7) Tropospheric O 3 concentration is mainly controlled by reaction (6) and (7) and no net change in ozone concentration occurs as seen from these two reactions. Each of these reactions occurs rapidly on a time scale of 200 s or less (Sillman, 1999). When reactions (6) and (7) are in equilibrium, then the O 3 concentration is given by the following equation, on the basis of O 3 photo stationary state approximation. J NO2 [NO 2 ] [O 3 ] = K[NO] (8) Brackets indicates chemical concentration for the reactive species and J NO2 is the solar radiation dependent photolysis frequency of reaction (6) and K is the rate constant of reaction (7). The magnitude of J NO2 depends on the intensity of solar radiation and for this reason J NO2 follows the solar diurnal cycle. There are other processes involving volatile organic compounds (VOC), carbon monoxide and NOx (NO+NO 2 ) through which ozone formation and conversion of NO to NO 2 occurs other than reaction (7) (Sharkey et al., 2008). RH+OH+O 2 RO 2 +H 2 O (9a) RO 2 +NO RO+NO 2 (9b) RO+O 2 R'CHO+HO 2 (9c) HO 2 +NO OH+NO 2 (9d) 2(NO 2 +O 2 NO+O 3 ) (9e) Net reaction is: RH+4O 2 R'CHO+2O 3 +H 2 O (10) R'CHO represents intermediate organic species, aldehydes and ketones. The directly emitted 3 hydrocarbon and intermediate organic species are collectively known as volatile organic compounds

4 (VOC). So it is clear from reaction (10) that with increase in ozone concentration the concentration of anthropogenic (and biogenic also) hydrocarbon increases, which may act as Cloud Condensation Nuclei (CCN) or control the size of other CCN in proper dimension so that rainfall increases. It agrees fairly well with the previous results (Midya et al., 2012). The main ozone precursors (CO, hydrocarbons and NO X ) are emitted as byproducts of human activities. These anthropogenic emissions also influence HOx, affecting ozone destruction. Ozone destruction occurs mainly via reactions with water vapour and peroxy and hydroxyl radicals (HOx i.e. HO 2 and OH). High humidity leads to a high rate of OH production by reaction of O( 1 D), produced from O 3 photolysis, with water (Seinfeld and Pandis, 2006). It is known that clouds form in regions of the atmosphere where water vapour is supersaturated. Super saturation of water vapour is produced by cooling occurs through expansion in updraft regions and radiative cooling. Cloud droplets formed from pre-existing particles in the atmosphere, known as aerosols. Aerosols that can become droplets are called Cloud Condensation Nuclei (CCN). At a given mass of soluble material in the particle there is a critical value of the ambient water vapour super saturation below which the particle exists in a stable state and above which it spontaneously grows to become a cloud droplet having diameter of 10µm or more. The role of ice nuclei, a much smaller sub-set of atmospheric aerosols than CCN, in precipitation formation in certain clouds is critical. During monsoon and post-monsoon season the concentration of H 2 O molecules increases in the atmosphere with the rise of surface temperature. Since ozone formation is an endothermic process, it helps to increase ozone concentration near the surface. So there exists a direct relationship between rainfall and ozone concentration. When ozone gradient becomes nearly equal to 3.5, probably the CCN molecules reached their critical size and enormous rainfall occurs. Pre-monsoon period is different from other seasons because different mesoscale phenomena (thunderstorm, squall) take place which have significant role in rainfall pattern. Hence in premonsoon season no such relation between ozone gradient and total rainfall exists. 4. Conclusion From the analysis the following conclusion can be made that there exists a relationship between tropospheric ozone gradient and total rainfall during different seasons over Gangetic West Bengal except pre-monsoon season. Tropospheric ozone gradient varies with total rainfall during pre-monsoon, monsoon and post-monsoon. During monsoon and post-monsoon season maximum rainfall occurs when ozone gradient attains a value of around 3.5.There are many factors which control rainfall. Premonsoon rainfall mainly takes place due to thunderstorms which is a mesoscale phenomenon. So it is quite expected that the correlation coefficient will be poor in pre-monsoon season. Acknowledgements The authors acknowledge with thanks for tropospheric ozone data to NASA and Indian Institute of tropical meteorology (IITM), Pune for making sub-divisional rainfall data available. References Bansod, S. D. and Singh, S. V., 1995, Pre-Monsoon Surface Pressure and Summer Monsoon Rainfall Over India, Theor. Appl. Climatol., 51, Chakraborty, A., Nanjundiah, R. S. and Srinivasan, J., 2002, Role of Asian and African orography in Indian summer monsoon, Geophys. Res. Lett., 29, 20, pp doi: /2002 GL

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