Scattering and absorption properties of near-surface aerosol over Gangetic–Himalayan region: the role of boundary-layer dynamics and long-range transport

Abstract

Light scattering and absorption properties of at- mospheric aerosols are of vital importance for evaluating their types, sources and radiative forcing. This is of partic- ular interest over the Gangetic–Himalayan (GH) region due to uplift of aerosol from the plains to the Himalayan range, causing serious effects on atmospheric heating, glaciology and monsoon circulation. In this respect, the Ganges Val- ley Aerosol Experiment (GVAX) was initiated in Nainital from June 2011 to March 2012 with the aim of examining the aerosol properties, source regions, uplift mechanisms and aerosol–radiation–cloud interactions. The present study ex- amines the temporal (diurnal, monthly, seasonal) evolution of scattering (σsp ) and absorption (σap ) coefficients, their wavelength dependence, and the role of the Indo-Gangetic plains (IGP), boundary-layer dynamics (BLD) and long- range transport (LRT) in aerosol evolution via the Atmo- spheric Radiation Measurement Mobile Facility. The anal- ysis is separated for particles < 10 μm and < 1 μm in diame- ter in order to examine the influence of particle size on op- tical properties. The σsp and σap exhibit a pronounced sea- sonal variation between the monsoon low and post-monsoon (November) high, while the scattering wavelength expo- nent exhibits higher values during the monsoon, in contrast to the absorption Ångström exponent which maximizes in December–March. The elevated-background measuring site provides the advantage of examining the LRT of natural and anthropogenic aerosols from the IGP and southwest Asia and the role of BLD in the aerosol lifting processes. The results reveal higher aerosol concentrations at noontime along with an increase in mixing height, suggesting influence from IGP. The locally emitted aerosols present higher wavelength de- pendence of the absorption in October–March compared to the rather well-mixed and aged transported aerosols. Mon- soon rainfall and seasonally changing air masses contribute to the alteration of the extensive and intensive aerosol prop- erties.