Atmos. Chem. Phys. Discuss., 11, 15697-15743, 2011
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Accumulation of aerosols over the Indo-Gangetic plains and southern slopes of the Himalayas: distribution, properties and radiative effects during the 2009 pre-monsoon Season
R. Gautam1,2, N. C. Hsu2, S. C. Tsay2, K. M. Lau2, B. Holben2, S. Bell2,3, A. Smirnov2,4, C. Li2,5, R. Hansell2,5, Q. Ji2,5, S. Payra6, D. Aryal7, R. Kayastha8, and K. M. Kim2,9
1Universities Space Research Association, Columbia, MD 21044, USA
2NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
3Science Systems and Applications, Inc., Lanham, MD 20706, USA
4Sigma Space Corporation, Lanham, MD 20706, USA
5Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA
6Birla Institute of Technology Mesra, Extension Centre – Jaipur, Jaipur, India
7Tribhuwan University, Kathmandu, Nepal
8Kathmandu University, Dhulikhel, Nepal
9Morgan State University, Baltimore, MD 21251, USA

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/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 (SSA550 nm ~0.89) than the near-desert region in NW India (SSA550 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.

Citation: Gautam, R., Hsu, N. C., Tsay, S. C., Lau, K. M., Holben, B., Bell, S., Smirnov, A., Li, C., Hansell, R., Ji, Q., Payra, S., Aryal, D., Kayastha, R., and Kim, K. M.: Accumulation of aerosols over the Indo-Gangetic plains and southern slopes of the Himalayas: distribution, properties and radiative effects during the 2009 pre-monsoon Season, Atmos. Chem. Phys. Discuss., 11, 15697-15743, doi:10.5194/acpd-11-15697-2011, 2011.
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