Column-integrated aerosol optical properties during summer and autumn of 2012 in xi'an, China

Column-integrated aerosol optical properties were derived systematically from measurements made in Xi'an, which is located in Guanzhong Plain of central China with a ground-based CIMEL sun photometer from May to November 2012. Aerosol optical depths (AODs), Ångström exponents, water vapor contents, and aerosol optical and micro-physical properties, including aerosol volume size distribution, complex refractive indices and single scattering albedo (SSA), were determined. Daily variations in AODs at 440 nm (τ₄₄₀) generally followed those of the 24-hr PM_2.5 mass concentrations, but there were differences in the relationships in summer and autumn. August showed the highest monthly τ₄₄₀ (1.13) while the largest monthly Ångström exponent (α_440-870 = 1.30) and water vapor content (C_w = 4.28) both occurred in July. Monthly averages of the aerosol size distributions showed the dominance of coarse mode aerosols, except in July and August, when the contribution of the accumulation and coarse modes were fairly comparable. Monthly changes in the complex refractive index (including both real and imaginary parts) and SSA were also studied, including their wavelength dependences; these analyses implied changes in the abundances of the aerosol types. Finally, an episode involving urban and dust aerosols was analyzed using sun photometer aerosol retrievals; MODIS images captured by Aqua satellite and average wind vectors from the NCEP operational global analyses were also considered in the case study.