The application of remote sensing techniques for air pollution analysis and climate change on Indian subcontinent

India is home to an extraordinary variety of climatic regions, ranging from tropical in the south to temperate and alpine in the Himalayan north, where elevated regions receive sustained winter snowfall. The subcontinent is characterized by high levels of air pollution due to intensively developing industries and mass fuel consumption for domestic purposes. The main tropospheric pollutants (O3, NO2, CO, formaldehyde (HCHO) and SO2) and two major greenhouse gases (tropospheric O3 and methane (CH4)) and important parameters of aerosols, which play a key role in climate change and affecting on the overall well-being of subcontinent residents. In light of considering these facts this paper aims to investigate possible impact of air pollutants over the climate change on Indian subcontinent. Satellite derived column aerosol optical depth (AOD) is a cost effective way to monitor and study aerosols distribution and effects over a long time period. AOD is found to be increasing rapidly since 2000 in summer season that may cause adverse effect to the agricultural crops and also to the human health. Increased aerosol loading may likely affect the rainfall which is responsible for the observed drought conditions over the Indian subcontinent. Carbon monoxide is emitted into the atmosphere by biomass burning activities and India is the second largest contributor of CO emissions in Asia. The MOPITT CO retrievals at 850 hPa show large CO emission from the IG region. The development of convective activity associated with the ASM leads to large scale vertical transport of the boundary layer CO from the Indian region into the upper troposphere. TCO over the Indian subcontinent during 2007 has a systematic and gradual variation, spatial as well as temporal. Higher amount of TCO in the northern latitudes and simultaneous lower TCO at near equatorial latitudes indicates depletion of ozone near the equator and accumulation at higher latitudes within the subcontinent. In addition, changes in stratospheric ozone and atmospheric abundances of aerosols alter the energy balance of the climate system.


Introduction
Major sources of energy in India include coal, petroleum, biomass, nuclear power and hydropower. Combustion of these fuels is the main source of pollution in the natural environment. India has a large variation in climate from region to region, due to its Geographical nature. Extremes of climate and weather events are increasingly being recognized as key aspects of climate change and they adversely affect humans and ecosystems, with serious socioeconomic consequences [1].The complexity of the climate system is enormous. The thermal gradient caused by the lower tropospheric warming and corresponding cooling over the upper troposphere largely regulates the atmospheric general circulation patterns. In the changing climatic scenario, the recent sharp rising trend in the surface air temperature over the Indian landmass is attributed to the large-scale declining trend in Indian summer monsoon rainfall [2]. Not only the temperature gradient but also the pressure gradients across the Indian landmass plays a major role in controlling the climate system in and around the Indian subcontinent [3].Anthropogenic changes in the atmosphere can have a profound impact on the climate and the consequences are assumed to occur at present [4]. Climate change may result in an increase in the frequency of floods in many regions. However, natural changes in the Earth's climate might have been augmented by the enhanced greenhouse effect, caused by manmade changes in the Earth's environment. Greenhouse gases and aerosols are the two most important elements that affect the radiation balance of the earth's atmosphere. It is well established now that the anthropogenic emissions of greenhouse gases (most importantly CO 2 ) into the atmosphere cause global warming of the troposphere [1].
In this paper we highlighted and put together role of air pollutant on climate change over the Indian subcontinent. Aerosols play a crucial role in the climate of the Earth-atmosphere system by means of their direct and indirect impacts, and are considered to be one of the largest uncertain components of the global climate system [4]. The overall composition of aerosol density is likely to affect the largescale heating and pressure gradients in the atmosphere, which can amend the circulation pattern, cloud dynamics, precipitation and so on [5]. CO is another important atmospheric pollutant for a number of reasons. The major source of CO emission in India is traditional bio-fuel use, which is almost 50% of the total Indian CO emission [6]. Carbon monoxide can be measured from space and can be used to identify sources of air pollution. The industrialization of eastern Asia has influenced the chemical climate of India through transport processes and thus through the Indian monsoon [7]. The small belt of ozone around the globe filters the ultraviolet radiations reaching the ground and thereby protects the whole biosphere from harmful effect [8].

Aerosols mechanism and its influence on climate
In the monsoonal countries like India and China, aerosol problem is ever becoming acute due to increased loading of atmospheric pollutants from anthropogenic as well as natural sources [9].They interact with various monsoonal activities, for example, energy balance, cloud formation processes and so on [10] and perturb their normal occurrences. The magnitude and sign of aerosol radiative impact depend on physical, chemical and radiative properties of these aerosols. Aerosols are classified into natural and anthropogenic, according to their origin. Sea salt, dust, natural sulphates, etc., are naturally occurring aerosols, whereas soot, industrial sulphates, black carbon, etc., are of anthropogenic origin. Soot is an absorbing aerosol whereas dust and organic matter are partly absorbing. Interactions between dust aerosols and clouds in the upper troposphere and lower stratosphere can produce a significant impact on atmospheric radiation budget, and hence on global and regional climates.
Carbonaceous aerosols have received much attention recently because of their potential role in regional climate change [11].Black carbon is a light-absorbing aerosol, that is, the by-product of incomplete combustion of carbonaceous fuel. Because of its absorptive nature, Black Carbon accounts directly for the reduction in incoming short-wave solar radiation at the Earth's surface, leading to heating of the atmosphere and thus possibly changing the temperature structure in the troposphere, which in turn affects the cloud microphysical properties and thereby rainfall mechanisms [11].Global warming produced by greenhouse gases is partly suppressed by aerosols; they therefore have a substantial role in the radiation budget and climate. The radiative effects of aerosols on the Earth's atmospheric system are governed by the quantity of aerosols in the atmosphere, their vertical distribution, size distribution and single scattering albedo, and the reflectivity of the underlying surface. Climate change is one of the most burning issues globally; aerosols have great potential to bring out changes in climatic conditions at regional and global scales [12].

AOD profile over Subcontinent
The dominance of the monsoon climate over the Indian subcontinent controls the seasonal highs and lows of aerosol concentration. Due to the climatic characteristics, during the winter and summer dry seasons (prior to the onset of the monsoon season), the concentration of aerosol particles remains high in the atmosphere. Hence the aerosol properties and their effects on weather and the climate of Indian Subcontinent are certainly different from rest of the world.AOD distribution during pre-monsoon affects cloud formation and hence rainfall distribution which is found to be prominent in last 4 years. Complex interaction between aerosols, clouds, climate and vegetation needs to be addressed in the light of harmful effects observed in several studies by different workers [14]. Organizations (WMO) recognize dust as a major component of atmospheric aerosol, which is an essential climatic variable to study. Mineral dust emitted from arid and semi-arid regions plays an important role in climate and also cause changes in cloud properties, such as the number, concentration, and size of cloud droplets, which can alter both cloud albedo and cloud lifetime. The phenomenon termed the 'indirect aerosol effect'. [10], using aerosol forcing derived from atmospheric brown clouds field experiments, suggested that aerosol induced cooling decreases surface evaporation and reduces the north south surface temperature gradient over the Indian Ocean, leading to a weakened monsoon circulation. [5] Found that an abundant amount of dust aerosols from the Thar Desert and the Middle East deserts are transported into northern India, during the summer season (April through early June).Forced by the prevailing wind against the steep topography of the Himalayas, the dust aerosols pile up against the foothills and spread over the Indo-Gangetic Plain (IGP). The airborne dust particles become even more absorbing when transported over megacities of the IGP and coated by fine black carbon aerosols from local emissions [15]. The Indo-Gangetic Plain is an aerosol "super hotspot", hosting the world's highest population density and concentration of coal-firing industrial plants. AOD is found to be very high (>.6) in Ganga basin with increasing aerosol concentration at an alarming rate in eastern part of basin shown in Figure1

Atmospheric ozone and climate
Atmospheric ozone is a strong oxidizing agent. Together with water vapor, as a precursor of the hydroxyl radical, ozone has a strong influence on the oxidizing power of the atmosphere and hence on the rate at which many natural and anthropogenic compounds are eliminated from the atmosphere [22]. Tropospheric ozone impacts air quality and human health, atmospheric radiative forcing and ecosystem productivity with resulting impacts on food and climate. It also modifies the "oxidizing capacity" of the troposphere, impacting the lifetime and radiative forcing of methane. The small belt of ozone around the globe filters the ultraviolet radiations reaching the ground and thereby protects the whole biosphere from harmful effect [8]. The ozone located in the vicinity of tropopause where temperature is considerably lower than near the surface is the largest contributor to the 'greenhouse' warming [23]. The presence of ozone in the Earth's atmosphere protects the human biological life system from harmful solar UV-B radiation and also plays a pivotal role in controlling the thermal structure of the stratosphere [24]. Changes in UV irradiance can influence the structure of the middle atmosphere through modification of photochemical dissociation rates; with associated effects on ozone [22].Solar UV radiation, apart from its dependence on stratospheric ozone, is closely associated with tropospheric ozone abundance [8]. The Indian region has indicated a possible rise in tropospheric ozone, which could be related to increasing industrialization, transportation growth, and urbanization. .Spatial gradients in TCO observed during this period, gradually increases from a low value near the equator to a maximum at high latitude. This observed latitudinal variation pattern of TCO over the Indian subcontinent may be due to the geographical and meteorological characteristics of the region which strongly influence the transportation as well as the production and loss of TCOshown in Figure 6 during monsoon period.

Conclusions
These study summaries the following aspects which influence the climate change over the subcontinent due to increased level of air pollution. AOD is found to be very high in the Indo-Gangetic region with increasing aerosol concentration in eastern part India. AOD is increasing from southern part of Indian subcontinent to northern part up to Himalaya. The increasing aerosol loading over the Indian subcontinent affects the rainfall distribution which is responsible for agricultural production and human health. MOPITT CO daytime retrievals provide sufficient information about the vertical and horizontal transport of CO and the deep convective activities during the monsoon period. Higher values of CO mixing ratios over the central part of India are observed low due to less industrial activities and low biomass burning. Spatial gradients in TCO during winter period, gradually increases from a low value near the equator to a maximum at high latitude. The variation pattern of TCO over the Indian subcontinent may be due to the geographical and meteorological characteristics of the region which strongly influence the transportation as well as the production and loss of TCO. The geographical diversity of the Indian Peninsula may be one of the major causes of spatial variability of total ozone concentration.

Alknowledgement
The authors are thankful to the Indian Institute of Tropical Meteorology for their valuable guidance and encouragement during the work.