What drives seasonal variations of organic aerosol over the Indo Gangetic Plain?

The Indo Gangetic Plain (IGP), home to more than 400 million people, encompasses most of northern and eastern India, the most populated parts of Pakistan, and Bangladesh. Cities in the IGP are among the most polluted in the world, with levels of particulate matter with diametres smaller than 2.5 microns (PM_2.5), often far exceeding human health recommendations. Seasonal changes in the physical and chemical environment over the IGP are dominated by the large-scale South Asian monsoon system, but also by seasonal sources such as lifting of dust from the Thar desert and agricultural stubble burning at the end of the growing seasons. Organic aerosol (OA) represents a major contribution to PM_2.5. They exist in a complex mixture, comprising of thousands of individual organic compounds. OA is made up of primary OA (POA), emitted directly to the atmosphere, and by secondary OA (SOA) formed by the gas-phase oxidation of volatile organic compounds. We use the WRF-Chem regional atmospheric chemistry model to study seasonal changes in the chemical properties of fine particulate matter over the IGP. In particular, we use the Volatility Basis Set (VBS) model in WRF-Chem to study both POA and SOA seasonal variations, and to quantify the importance of seasonal sources of OA to PM_2.5 over the IGP. We evaluate the model using satellite observations of aerosol optical properties.