9. A Study of Optical and Microphysical Properties of Atmospheric Brown Clouds Over the Indo-Gangetic Plains

Atmospheric brown clouds (ABCs), a dense and widespread layer of air, have significant implications to human health, air quality and regional climate. Being a habitat of more than 350 million human population, the Indo-Gangetic Plains (IGP) is also one of the ABCs hotspots of the world. Considering the significant socio-economic impact of ABCs on lives of IGP’s populations, understanding of spatio-temporal characteristics of ABCs is necessary over the region. This study focuses on the frequency of ABCs occurrences and associated optical and microphysical properties using data from seven ground-based remote sensors situated across the IGP. We have used total ~5253 days of Level 2 aerosol measurements from seven AERosol Robotic NETwork (AERONET) sites [Karachi, Lahore, Jaipur, New Delhi, Kanpur, Gandhi college and Dhaka University] for three seasons (Winter, Pre-monsoon and Post-monsoon). A well-defined algorithm based on extinction and absorbing properties of particles is used to characterize extreme pollution days (ABCs days) for each sites. Further, we have used spectral dependency of aerosol optical depth (AOD) and absorption aerosol optical depth (AAOD) to characterize aerosol types during ABCs days over IGP. The results indicate a unique feature of ABCs occurrences over the IGP. In general, the maximum frequency of ABCs days is found during pre-monsoon seasons over all the sites. However, other seasons have specific features over specific locations, for example, the maximum occurrence of ABCs days was found over Dhaka in post-monsoon (65%), followed by Delhi during pre-monsoon (61.29%), and Kanpur in pre-monsoon (44.29%). The results show that while extreme pollution days in pre-monsoon is dominated by dust polluted aerosols, biomass burning is the main source during winter and post-monsoon. This study provides a comprehensive climatology of spectral nature of AOD, AAOD, single scattering albedo (SSA), asymmetry parameter and size distribution of ABCs across the region.