Aerosol radiative effects over an urban location and a remote site in western India: Seasonal variability

Seasonal variability in radiative effects due to aerosols in the shortwave and longwave regions is studied utilizing measured aerosol optical properties and models over two distinct environments, namely, Ahmedabad (urban, industrialised location) and Gurushikhar (high altitude remote site) in western India. Top of the atmosphere (TOA) forcing in shortwave varies from positive to negative over Ahmedabad, while over Gurushikhar it is negative throughout the year because of lower surface reflectance and higher single scattering albedo (SSA). Aerosol radiative forcing at the surface (SFC) is >−30 Wm−2 over Ahmedabad. In Gurushikhar SFC forcing increases from winter (−7 Wm−2) to premonsoon (−14 Wm−2), which is attributed to the increase in the amount of dust aerosol loading. Aerosol radiative forcing in the atmosphere (ATM) peaks during postmonsoon with a value of 54 Wm−2 over Ahmedabad, while ATM warming over Gurushikhar peaks during premonsoon (9 Wm−2). Shortwave heating rate is >0.9 K d−1 over Ahmedabad while it is an order of magnitude lower over Gurushikhar.Aerosols produce a positive TOA and SFC forcing in the longwave and cool the atmosphere. Anthropogenic influence is evident from the aerosol radiative effects obtained over Ahmedabad and Gurushikhar. The abundance of fine (coarse) mode aerosols over Ahmedabad (Gurushikhar) results in higher (lower) aerosol optical depth, lower (higher) SSA, and higher (lower) aerosol radiative forcing. Owing to the dissimilar nature of shortwave and longwave radiative effects of aerosols, when coarse mode aerosols are abundant over a region, longwave radiative effects can balance shortwave atmospheric warming produced by aerosols.

► Top of the atmosphere forcing varies from positive to negative over Ahmedabad while it is negative over Gurushikhar.
► Abundance of fine mode absorbing aerosols give rise to higher atmospheric warming in Ahmedabad.
► Shortwave heating rate is an order of magnitude lower over Gurushikhar than Ahmedabad.
► Longwave cooling balances the shortwave atmospheric warming when coarse mode aerosols are abundant.