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

Abstract

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.

Synergistic Approach for the Aerosol Monitoring and Identification of Types over Indo-Gangetic Basin in Pre-Monsoon Season

“…during March for all altitude levels in almost every year. Vertically extended (up to 4 km altitude) aerosol profiles have been found during each month of PrM season, which are generally associated with the increased thermal/mechanical convective activities and long-range transportation of dust particles during this time period (Gautam et al, 2011;Mishra and Shibata, 2012a). Similar role of prevailing meteorology on aerosol vertical distribution has been found during the monsoon season over Delhi using CALIOP observations by Srivastava et al (2012a).…”

“…The lower values of SSA could be a result of mixing of dust with pollution over Delhi, which is also reported by Gautam et al (2011) for north-west IGB (SSA 550 < 0.9). In order to add to robustness of our discussion section, the aerosol classification results of this study have been compared with .…”

“…Recently, Gautam et al (2011) reported the distribution of aerosol properties and their radiative effects using several ground-based radiometric measurements in the IGB region as well as over the southern slopes of the Himalayas during the PrM season. A positive gradient in aerosol optical depth (AOD) and Ångström Exponent (AE) from central IGB to eastern IGB with the advancement of the PrM season has been reported by Srivastava et al (2011).…”

“…Previous studies on the IGB region have focused on both natural and anthropogenic aerosols, whose dominance is seasonally dependent Jethva et al, 2005;Mishra and Shibata 2012a;. The significant aerosol types during the pre-monsoon (PrM) season are urban-industrial (U-I) pollution (locally generated and regionally transported secondary organic carbon and black carbon) and dust particles from nearby arid agricultural land and the Thar Desert (western India), along with some longrange transported mineral dust from the deserts of Sahara, the Arabian peninsula and Iran (Middleton, 1986;Chu et al, 2003;Bond et al, 2004;Singh et al, 2005;Gautam et al, , 2011Arola et al, 2011). Chinnam et al (2006) and Prasad and Singh (2007) have also reported that the dust aerosols over the IGB originate from three major sources (Oman, the southwest Asian basins and the Thar Desert) during the PrM season.…”

Global and regional evaluation of over-land spectral aerosol optical depth retrievals from SeaWiFS

“…The aerosol loading is linked with large-scale meteorology and seasonality differs between the Indian subcontinent and the eastern end of the region. In the Indian pre-monsoon (April-June), westerly winds blow dust across the Indo-Gangetic plain, where it is trapped by the Himalayas; the winter monsoon is associated with fog and thick haze from urban pollution and biomass burning aerosols (Prospero et al, 2002;Gautam et al, 2007Gautam et al, , 2011. Over the Indo-China peninsula, intense biomass burning during the pre-monsoon (February-April), in combination with urban emissions, leads to an optically-thick absorbing haze layer (Carmichael et al, 2003;See et al, 2006); monsoon rains during the summer washout much of the aerosol and cloud cover limits SeaWiFS's coverage.…”