Latitudinal variation of aerosol properties from Indo-Gangetic Plain to central Himalayan foothills during TIGERZ campaign

As part of TIGERZ campaign, latitudinal variation of aerosol optical properties was analyzed over Indo-Gangetic Plains (IGP) to central Himalayas during premonsoon of 2008 and 2009. Measurements of aerosol optical depth (AOD) were performed using Aerosol Robotic Network Sun photometer at four sites with different aerosol environments. The AOD increases from Nainital located in central Himalayas to Kanpur located in IGP region. Further, aerosol size varies spatially with dominance of coarse-mode aerosols at Kanpur compared to fine-mode aerosols dominated at Nainital. Spectral variation of single-scattering albedo suggests that during premonsoon, dust is the dominant species in the IGP with exception of Pantnagar, where absorbing aerosols are dominant. The optical properties of aerosols are calculated, and shortwave clear-sky aerosol radiative forcing (ARF) is estimated. An insignificant difference is found in columnar ARF and columnar heating rate (HR) when vertical profiles of aerosols are included in radiative transfer models. Over Nainital, average ARF is estimated to be −7.61, −45.75, and 38.14 W m−2 at top of atmosphere (TOA), surface (SUR), and in the atmosphere (ATM), respectively. Average ARF is less negative at Kanpur compared to Pantnagar and Bareilly with values −17.63, −73.06, and 55.43 W m−2 at TOA, SUR, and ATM, respectively. ARF shows positive gradient from the highlands to the IGP sites; larger TOA and SUR cooling were observed at the three sites compared to the highland site. This translates into large columnar HR with estimated average values as 1.07, 1.41, 1.58, and 1.56 K d−1 for Nainital, Pantnagar, Bareilly, and Kanpur, respectively