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

摘要

Light scattering and absorption properties of atmospheric aerosols are ofvital importance for evaluating their types, sources and radiative forcing.This is of particular interest over the Gangetic–Himalayan (GH) region dueto uplift of aerosol from the plains to the Himalayan range, causing seriouseffects on atmospheric heating, glaciology and monsoon circulation. In thisrespect, the Ganges Valley Aerosol Experiment (GVAX) was initiated in Nainitalfrom 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 examines the temporal (diurnal, monthly, seasonal)evolution of scattering (σ) and absorption (σ_ap)coefficients, their wavelength dependence, and the role of the Indo-Gangeticplains (IGP), boundary-layer dynamics (BLD) and long-range transport (LRT)in aerosol evolution via the Atmospheric Radiation Measurement MobileFacility. The analysis is separated for particles <10 μm and<1 μm in diameter in order to examine the influence ofparticle size on optical properties. The σ_sp and σ_ap exhibit a pronounced seasonal variation between the monsoon low andpost-monsoon (November) high, while the scattering wavelength exponentexhibits higher values during the monsoon, in contrast to the absorption?ngstr?m exponent which maximizes in December–March. Theelevated-background measuring site provides the advantage of examining theLRT of natural and anthropogenic aerosols from the IGP and southwest Asiaand the role of BLD in the aerosol lifting processes. The results revealhigher aerosol concentrations at noontime along with an increase in mixingheight, suggesting influence from IGP. The locally emitted aerosols presenthigher wavelength dependence of the absorption in October–Marchcompared to the rather well-mixed and aged transported aerosols.Monsoon rainfall and seasonally changing air masses contribute to thealteration of the extensive and intensive aerosol properties.