Aerosols over the Foothills of the Eastern Himalayan Region during Post-monsoon and Winter Seasons

Sishir Dahal,Dipesh Rupakheti,Binod Kumar Bhattarai,Ram Kumar Sharma,Bhupesh Adhikary

doi:10.4209/aaqr.210152

Abstract

In this study a Microtops II sunphotometer is used for the first time over Birtamode, a rapidly growing city located in the eastern Himalayan foothills of Nepal, to measure aerosol optical depth (AOD). The average AOD for the observation period (October 2018–February 2019) was 0.68 ± 0.39 with the post-monsoon season having a higher value (0.74 ± 0.43) compared to the winter season (0.60 ± 0.32). The Angstrom exponent (α) for post-monsoon and winter are found to be 1.08 ± 0.10 and 1.11 ± 0.16 respectively. During the monitoring period, the majority of AOD values (47%) are above 0.60, indicating moderately polluted conditions in the region. Anthropogenic, biomass burning, and mixed aerosols are identified as the prevalent aerosol types in the study region. The observed aerosol classification is also explained in terms of CAMS near-real-time model datasets. The AOD values retrieved by MODIS, VIIRS, Himawari-8, and CAMS show a good correlation with the observed Microtops AOD with R2 values ranging from 0.60 to 0.94. Moreover, different MODIS aerosol products (DB, DT, and combined DB-DT) are evaluated based on a comparison of Collection 6.1 AOD with the ground truth obtained from Microtops. The spatial distribution of AOD as observed by various satellites are compared and the vertical distribution of aerosol is also explained with the extinction coefficient provided by the CALIOP lidar onboard CALIPSO and aerosol types provided by CAMS.

Highlights

Atmospheric aerosols are the solid particles and liquid droplets suspended in the atmosphere varying between 0.001 μm to 100 μm in size
The seasonal-average value of β signifies higher aerosol loading over the eastern Himalayan foothills region during the post-monsoon season, contrary to the observations recorded in the central Himalaya foothills region
Aerosols are classified based on the correlation of aerosol optical depth (AOD) and α, which shows that the contribution of anthropogenic aerosol type is the highest among all others

Summary

Introduction

Atmospheric aerosols are the solid particles and liquid droplets suspended in the atmosphere varying between 0.001 μm to 100 μm in size. These originate from both natural and anthropogenic activities. They are known to have both warming and cooling effects on the atmosphere as they perturb incoming solar radiation and outgoing terrestrial long wave radiation by scattering or absorption. Aerosols perturb radiative forcing indirectly through cloud properties (Penner et al, 2003). Aerosols such as mineral dust and black carbon are identified to cause warming by absorbing solar radiation (Kaufman et al, 2002; Lohmann and Feichter, 2005). The microphysical and chemical properties of atmospheric aerosols strongly influence the direct and indirect effects

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Conclusion