Executive Summary : | Air pollution is a primary concern worldwide, and most Indian cities exceed air quality standards. A holistic scientific understanding of atmospheric phenomena associated with air quality is currently lacking. Therefore, there is an evident need to meet the complex air pollution challenges in India owing to the rapid urbanisation and industrialisation, which have significantly changed air pollution levels. Further, stagnant atmospheric conditions could also lead to severe air pollution episodes on a local or regional scale [Kanawade et al., 2020]. Highly non-linear processes, such as atmospheric chemistry and aerosol dynamics, transform the urban pollution cocktail and generate secondary pollutants (e.g., ozone) and secondary aerosols from new particle formation (NPF) [Guo et al., 2014] which rapidly increases ultrafine particles (UFPs, particles with diameter less than 100 nm) in the atmosphere. The major sources of UFPs include direct emission from anthropogenic practices such as fossil-fuel and biomass combustion [Paasonen et al., 2016] and gas-to-particle conversion via NPF [Kulmala et al., 2013]. NPF frequently occurs in the polluted boundary layer in India [Kanawade et al., 2022], indicative of the rapid physio-chemical transformation that can lead to haze formation under conducive atmospheric conditions. Very little is known about the processes leading to NPF and airborne production of aerosol mass (haze) in India. Together with emission reductions, the way forward is to make long-term and comprehensive observations on aerosols, trace gases, atmospheric oxidants, and meteorological conditions. In India, the observation networks are very sparse and limited to a few locations. Therefore, the surface measurements alone are inadequate for air quality management at a regional scale. Several studies demonstrated the utility of satellite-derived aerosol products to estimate surface PM2.5 and complement the surface measurements [e.g. Mhawish et al., 2020 and references therein]. However, satellite AOD-based estimates of PM less than 1.0 µm (PM1.0) are currently unavailable globally. Further, the spatio-temporal network of PM1.0 or particle number size distribution measurements is extremely sparse in India. Previous studies evidently demonstrated the applicability of satellite data to study local-to-regional-to-global scale air quality studies [Dey et al., 2020; Lee et al., 2016]. NPF and a variety of anthropogenic activities are significant sources of aerosol particles of a diameter smaller than 0.1 µm [Kerminen et al., 2018]. Researchers have used a variety of satellite observations to indirectly obtain estimates of UFP number concentrations; however, these works have been limited to observations of USA, Europe, and Southern Africa [e.g. Crippa et al., 2017; Kulmala et al., 2011; Sundström et al., 2015]. Therefore, this project proposes to develop and evaluate satellite-based estimates for UFP concentrations, first-of-its-kind, over India. |