Atmospheric aerosols are one of the primary air pollutants within the troposphere and usually have an adverse influence on hydrological cycle, crops, plants, human health and visibility. They are capable of influencing weather and climate over a particular region though their role is highly uncertain and difficult to understand (Stocker et al., 2013). They can modulate the total radiation the earth is exposed to through several direct and indirect ways. Directly, they scatter and absorb solar radiation and cause a cooling effect in the atmosphere whereas indirectly, their effect involves modification of cloud microphysical properties including droplet size, lifetime and cloud height, to serve as cloud condensation nuclei for cloud formation (Tao et al., 2012). This process is depicted in Figure 1.
Since developing countries like India and China, are undergoing massive industrialisation and economic growth, anthropogenic aerosol content in the atmosphere is found to be increasing (Kaskaoutis et al., 2012; Du et al., 2015), and aerosols are expected to modulate weather and climate over India.
How are aerosols produced?
Atmospheric aerosols are produced through natural and anthropogenic processes. Anthropogenic activities include fossil fuel and biomass burning, mining activities, modification of the natural land-use and cover, and changes in industrial and agricultural practices. Natural phenomena such as dust and sandstorms transport mineral dust particles to distant places from their source and contribute to atmospheric aerosol content. However, the contribution of anthropogenic aerosols far exceeds that of natural aerosols. Thus, the accumulation of aerosols over the northern hemisphere is largely due to industrial growth and agricultural practices (Satheesh & Moorthy, 2005).
In India, several regions such as the Indo-Gangetic Plain (IGP), eastern coastal areas and the western coast show significant variability in aerosol characteristics due to the presence of different types of aerosols. The IGP is among the most densely populated areas and heavily aerosol-laden regions of the world. Several satellite observations and ground based measurements have revealed an overall increase in aerosol optical depth over India—particularly in the IGP region (Satheesh et al., 2002). This is because of the continuous increase in fossil-fuel and bio-mass combustion owing to the growing energy demands of a rapidly increasing population (Lawrence & Lelieveld, 2010). Convection induced winds, driving desert and sandy dust into the atmosphere during the pre-monsoon season also contribute to this problem (Gautam et al., 2011). The increased aerosol content in the atmosphere over IGP is assumed to cause considerable changes in monsoonal climate (Gautam et al., 2009; Ramanathan et al., 2001).
Effect of aerosols over the Indian region
During a normal monsoon season, aerosols serve as a positive driving factor and help in strengthening the lower tropospheric zonal wind, resulting in a southward shifting of the monsoonal circulation during summer (Das et al., 2015). The increased loading of absorbing aerosols such as black carbon or dusts over IGP region during the pre-monsoon season increases the upper tropospheric heating, while a warm-core upper level anticyclone is formed over the Tibetan Plateau in the months of April and May (Lau & Kim, 2006). The resulting feedback mechanism helps in increasing the upward motion over the core monsoon region, thereby encouraging cloud formation. The subsequent advance of the monsoon in the northern Indian region results in rains.
The carbonaceous aerosols over IGP, the adjacent Himalayas and the Tibetan Plateau can form brown clouds and haze in this part of the world and may also enhance lower atmospheric solar heating (Ramanathan et al., 2002, 2005, 2007). However, the resultant heating may be season dependent; that is the warming effect may be up to 0.5 Wm-2 during the pre-monsoon season and the cooling effect may be realised during winter up to -1 Wm-2 (Arola et al., 2015). On the other hand, the prevailing local meteorological conditions and regional atmospheric flow processes could facilitate penetration of these aerosols throughout the Himalayan region (Cong et al., 2015) and impact the hydrological cycle or regional water budget, agriculture and health (Ramanathan et al., 2002, 2005, 2007). Further, it may also play a role in enhancing the monsoonal rainfall over northern India (Lau et al., 2009). The loading of black carbon aerosols in May-June may serve as the desired elevated heat source in order to act like a heat pump. This helps strengthen the monsoonal circulation over the region by increasing the inflow of moisture to the atmosphere resulting in high cloud formation and enhanced precipitation.
A study by Panda et al., (2009) suggested that dust particles are transported (from Thar Desert or Arabian region) to north and central India during pre-monsoon seasons. The prevailing topography and land-surface features favour the formation of desired circulation pattern that is appropriate for the accumulation of atmospheric aerosols over the said region (Dey et al., 2004). Organic aerosols like black carbon produced by bio-fuel and burning of biomass dominate over the Bay of Bengal and eastern Indian coasts. Similarly, sulphate aerosols are transported to the south-western coasts of India (Verma et al., 2008).
Since local and regional meteorological conditions govern the accumulation, distribution and transportation of aerosols (Panda et al., 2009; Gautam et al., 2011), which in turn play a vital role in cloud formation, the amount of rainfall during an extreme weather event or monsoon would be significantly modified due to aerosols (Vinoj et al., 2014) as described earlier.
It may be noted that there is little research done as far as the role of aerosols during extreme weather events such as pre-monsoon thunderstorms is concerned. However, it may be alluded that aerosols can have some positive impact on cloud formation during thunderstorms and thereby initiate rainfall over a particular region. On the other hand, the role of aerosols may be minimised during extremely heavy rainfall events. It should be noted, though, that such studies are yet to be carried out over the Indian region
Aerosols affect the weather and climate in several ways by controlling solar radiation, and affecting cloud formation. However, as yet, there are hardly any studies about their role and effects over the Indian region. Given that the rapidly industrialising economies like India and China are contributing to a massive accumulation of aerosols, it is important that studies are undertaken to assess the contribution of these particles to climate over the region.
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