Scattering and absorption properties of near - surface aerosol over Gangetic - 1 Himalayan region : the role of boundary layer dynamics and long - range transport

10 Light scattering and absorption properties of atmospheric aerosols are of vital importance in 11 evaluating their types, sources and radiative forcing. This is of particular interest over the 12 Gangetic-Himalayan (GH) region due to uplift of aerosol from the plains to the Himalayan range 13 causing serious effects on atmospheric heating, glaciology and monsoon circulation. In this 14 respect, Ganges Valley Aerosol Experiment (GVAX) was initiated at Nainital during June 2011 15 to March 2012 aiming to examine the aerosol properties, source regions, uplift mechanisms and 16 aerosol-radiation-cloud interactions. The present study examines the temporal (diurnal, monthly, 17 seasonal) evolution of scattering (sp) and absorption (ap) coefficients, their wavelength 18 dependence, and the role of the Indo-Gangetic plains (IGP), boundary-layer dynamics (BLD) and 19 long-range transport (LRT) in aerosol evolution via the Atmospheric Radiation Measurement 20 Mobile Facility. The analysis is separated for particles <10 μm and <1 μm in diameter in order to 21 examine the influence of the particle size on optical properties. The sp and ap exhibit a 22 pronounced seasonal variation with monsoon low and post-monsoon (November) high, while the 23 scattering wavelength exponent exhibits higher values during monsoon, in contrast to the 24 absorption Ångström exponent which maximizes in December-March. The elevated-background 25 measuring site provides the advantage of examining the LRT of natural and anthropogenic 26 aerosols from the IGP and southwest Asia and the role of BLD in the aerosol lifting processes. 27 The results reveal higher aerosol concentrations during noontime along with increase in mixing 28 height suggesting influence from IGP. The locally-emitted aerosols present higher wavelength 29 dependence of the absorption during October to March period compared to the rather well-mixed 30 and aged transported aerosols. Monsoon rainfall and seasonally-changing air masses contribute 31 to the alteration of the extensive and intensive aerosol properties. 32 33 34