Assessment of Aerosol Characteristics and Radiative Forcing Over Northwest Himalayan Region

Abstract

The study examined spectral aerosol optical depth (AOD) carried out using ground based measurements (MicrotopsII) and satellite data (MODIS&MISR) over Dehradun, part of Northwest Himalayas during January–December 2015. During premonsoon the region experiences high mean AOD500 (0.34 ± 0.09 to 0.63 ± 0.12) along with low mean Ångström exponent (α) (0.46 ± 0.09 to 0.66 ± 0.10) attributing to the dominance of mineral dust aerosols while moderate mean AOD500 and α during monsoon and post monsoon season are (0.38 ± 0.08 to 0.54 ± 0.16) and (0.78 ± 0.1 to 0.91 ± 0.11), respectively, indicating mixed type aerosols. During winter season low mean AOD500 (0.29 ± 0.06 to 0.46 ± 0.12) and high mean α (1.12 ± 0.09 to 1.30 ±0.1) is observed due to domination of fine mode aerosols coming from anthropogenic activities and agricultural biomass burning. Aerosol transport pathways were analyzed and complimented with Hysplit air mass back trajectory. Comparison of satellite derived AOD agrees well with Microtops measurements ( ${\rm{R\,= \,0.80}}$ and 0.71 for MODIS and MISR, respectively). MODIS data were used to generate spatial and seasonal distribution of AOD over northwest Himalayans and validated over selected stations in the region. The results show that AOD values are in agreement within an error of 11%–14% over entire region. The monthly and seasonal radiative forcing (ARF) was estimated over selected stations in the region. The annual mean atmospheric ARF over Dehradun was estimated to +25.83 Wm−2, while + 30.83 Wm−2 over Patiala, and + 14.97 Wm−2 over Kullu. The ratio of Surface to TOA forcing is found to be ∼2.4 to 3.7 indicating absorbing nature of aerosols predominantly existing over Northwest Himalayas.