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 (SSA550 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 characteristic 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.
Highlights
Aerosols over South Asia have received growing attention in recent years due to their potential effects on the seasonal evolution and long-term variability of the summer monsoon, as demonstrated by climate modeling studies (Menon et al, 2002; Ramanathan et al, 2005; Lau et al, 2006; Meehl et al, 2008; Randles and Ramaswamy, 2008; Collier and Zhang, 2009; Sud et al, 2009; Wang et al, 2009)
With the characterization of aerosols near the Thar Desert and the dust-dominated column aerosol loading over Northwestern India, we further investigate aerosol loading over the IndoGangetic Plains (IGP) and foothill/slope region, in terms of the size distribution, in order to determine the extent of the dust transport
Recent climate model-based studies have underscored the role of absorbing aerosols over South Asia in impacting the evolution and long-term variability of the summer monsoon rainfall (Ramanathan et al, 2005; Lau et al, 2006; Meehl et al, 2008) as well as cryospheric reservoirs of the HimalayasTibetan Plateau (Lau et al, 2010; Qian et al, 2011)
Summary
Aerosols over South Asia have received growing attention in recent years due to their potential effects on the seasonal evolution and long-term variability of the summer monsoon, as demonstrated by climate modeling studies (Menon et al, 2002; Ramanathan et al, 2005; Lau et al, 2006; Meehl et al, 2008; Randles and Ramaswamy, 2008; Collier and Zhang, 2009; Sud et al, 2009; Wang et al, 2009). We report simultaneous aerosol and broadband surface flux measurements over the IndoGangetic Plains (IGP) and southern slopes of the Himalayas during the pre-monsoon season of 2009, as part of the Radiation Aerosol Joint Observations – Monsoon Experiment in the Gangetic Himalayan Area (RAJO-MEGHA). This paper presents ground-based radiometric assessment of aerosol optical/radiative properties and regional distribution with an emphasis on the spatial variations from the flat plains to the elevated slopes of the Gangetic-Himalayan region. Simultaneous measurements in northern India and Nepal were carried out to characterize the aerosol loading and understand the variability of optical properties including aerosol size distribution, single scattering albedo (SSA), along with water vapor dynamics, during the course of the pre-monsoon season.
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