Abstract.We examine the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/southern slopes of the Himalayas during the 2009 pre-monsoon season. Enhanced dust transport extending from the Southwest Asian arid regions into the IGP, results in seasonal mean (April-June) aerosol optical depths of over 0.6 -highest over Southern Asia. The influence of dust loading is greater over the Western IGP as suggested by pronounced coarse mode peak in aerosol size distribution and spectral single scattering albedo (SSA). Transported dust in the IGP, driven by prevailing westerly airmass, is found to be more absorbing (SSA 550 nm <0.9) than the near-desert region in Northwestern (NW) India suggesting mixing with carbonaceous aerosols in the IGP. On the contrary, significantly reduced dust transport is observed over eastern IGP and foothill/elevated Himalayan slopes in Nepal where strongly absorbing haze is prevalent, as indicated by lower SSA (0.85-0.9 at 440-1020 nm), suggesting presence of more absorbing aerosols compared to IGP. Additionally, our observations show a distinct diurnal pattern of aerosols with characCorrespondence to: R. Gautam (ritesh.gautam@nasa.gov) teristic large afternoon peak, from foothill to elevated mountain locations, associated with increased upslope transport of pollutants -that likely represent large-scale lifting of absorbing aerosols along the elevated slopes during pre-monsoon season. In terms of radiative impact of aerosols, over the source region of NW India, diurnal mean reduction in solar radiation fluxes was estimated to be 19-23 Wm −2 at surface (12-15 % of the surface solar insolation). Furthermore, based on limited observations of aerosol optical properties during the pre-monsoon period and comparison of our radiative forcing estimates with published literature, there exists a general spatial heterogeneity in the regional aerosol forcing, associated with the absorbing aerosol distribution over northern India, with both diurnal mean surface forcing and forcing efficiency over the IGP exceeding that over Northwestern India. Finally, the role of the seasonal progressive buildup of aerosol loading and water vapor is investigated in the observed net aerosol radiative effect over Northwestern India. The radiative impact of water vapor is found to amplify the net regional aerosol radiative forcing suggesting that the two exert forcing in tandem leading to enhanced surface cooling. It is suggested that water vapor contribution should be taken into account while assessing aerosol forcing impact for this region and other seasonally similar environments.

We examine the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/slopes of the Himalayas during the 2009 pre-monsoon season. Enhanced dust transport extending from the Southwest Asian arid regions into the IGP, results in seasonal mean (April–June) aerosol optical depths of over 0.6 – highest over southern Asia. The influence of dust loading is greater over the western IGP as suggested by pronounced coarse mode peak in aerosol size distribution and spectral single scattering albedo (SSA). The transported dust in the IGP, driven by prevailing westerly airmass, is found to be more absorbing (SSA_{550 nm} ~0.89) than the near-desert region in NW India (SSA_{550 nm} ~0.91) suggesting mixing with carbonaceous aerosols in the IGP. On the contrary, significantly reduced dust transport is observed over eastern IGP and foothill/elevated slopes in Nepal where strongly absorbing haze is prevalent, associated with upslope transport of pollution, as indicated by low values of SSA (0.85–0.9 for the wavelength range of 440–1020 nm), suggesting presence of more absorbing aerosols compared to IGP. Assessment of the radiative impact of aerosols over NW India suggests diurnal mean reduction in solar radiation fluxes of 19–23 Wm⁻² at surface (12–15 % of the surface solar insolation). Based on limited observations of aerosol optical properties during the pre-monsoon period and comparison of our radiative forcing estimates with published literature, there exists spatial heterogeneity in the regional aerosol forcing, associated with the absorbing aerosol distribution over northern India, with both diurnal mean surface forcing and forcing efficiency over the IGP exceeding that over NW India. Additionally, the role of the seasonal progressive buildup of aerosol loading and water vapor is investigated in the observed net aerosol forcing over NW India. The radiative impact of water vapor is found to amplify the net regional aerosol radiative forcing suggesting that the two exert forcing in tandem leading to enhanced surface cooling. It is suggested that water vapor contribution should be taken into account while assessing aerosol forcing impact for this region and other seasonally similar environments

The aerosol properties retrieved from the AErosol RObotic NETwork (AERONET) measurements during the period 2009 to 2012 over Jaipur (26.9°N, 75.8°E, 450m asl) in Northwestern India are used for the first time to identify the types of aerosols. In order to consider the appropriate threshold of aerosol optical thickness (τ) at 500 nm (τ 500 ) and Angstrom exponent (α) in the spectral band 440-870 nm, a novel approach has been conducted and applied for the identification process. Five prevailing aerosol classes are identified: desert dust, biomass, maritime, arid background and mixed aerosols. Arid background and desert dust type aerosols are the most common at Jaipur (34.7% and 13.6%, respectively), with a wide variability in both τ and α. In about 8.4% of the cases, aerosols can be classified as maritime, although mixing with other aerosols (33.6%) is substantial. The ground-based spectral optical thickness and the refractive index estimated at visible and near-infrared wavelengths are used to account for the type of atmospheric aerosols. They are compared with four more AERONET sites located in India based upon their geographical distribution and extensive data availability. Simultaneously, single scattering albedo of dust is also inferred for all the available AERONET sites for the same period over India. The comparison results suggest that Jaipur arid background is more scattering in nature than Northern and Western regions in India. Finally, the absorption is less in summer than in winter over the Jaipur site.