Inter-annual and seasonal variations in optical and physical characteristics of columnar aerosols over the Pokhara Valley in the Himalayan foothills

This study reports comprehensive analysis of seasonal and inter-annual variations of aerosol optical and physical properties over the Pokhara Valley in the foothills of central Himalayas in Nepal utilizing the high-quality multi-year columnar aerosol data observed recently from January 2010 to December 2017. The influence of forest fires and agro-residue fires on aerosol properties is also investigated. Seasonal mean aerosol optical depth (AOD) over Pokhara is ≥0.3 during the year, highest in pre-monsoon and lowest in monsoon season. AOD exhibits a consistent seasonal cycle every year with a peak in pre-monsoon. Fine mode fraction contributing to AOD and Ångström exponent (α) corroborate seasonal pattern of AODs. AODs show good correlation with fire counts in the Pokhara Valley and surrounding region. The aerosol volume size distributions are bimodal. The peaks in volume size distribution and volume concentrations obtained in 2016 pre-monsoon are the highest during the 8-year observation period which coincided with peaks in the occurrence of highest number of fires in the Pokhara Valley and surrounding region. Effective radius of coarse mode aerosols is an order of magnitude larger than fine mode aerosols. The analysis of optical and physical properties of aerosols over the Pokhara Valley suggest that the aerosols observed over the Himalayan foothills are of urban/industrial, biomass burning and dust origin with proportional contributions varying in different seasons. The study provides observational constraints on aerosol physical and optical properties that can serve as inputs for model simulation of aerosol physiochemical processes and quantification of impacts on air quality, climate and sensitive ecosystems in this data-sparse region.