Column-Integrated Aerosol Optical Properties during Summer and Autumn of 2012 in Xi’an, China

Xiaoli Su,Zhengqiang Li,Meijing Lin,Gehui Wang,Junji Cao Cao

doi:10.4209/aaqr.2013.03.0093

Abstract

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 PM2.5 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 (α440–870 = 1.30) and water vapor content (Cw = 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.

Highlights

Atmospheric aerosols are important in the global climate system due to the key roles they play in the atmospheric radiation budget, cloud formation and hydrological cycle
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 considered in the case study
The daily Aerosol optical depths (AODs) co-varied with the 24-hr average PM2.5, but the correlation was stronger during the summer months

Summary

Introduction

Atmospheric aerosols are important in the global climate system due to the key roles they play in the atmospheric radiation budget, cloud formation and hydrological cycle. The effects of aerosols on climate are normally quantified in terms of aerosol radiative forcing, which depends strongly on the particles optical properties (OPs). The high spatial and temporal variability of the number, size and composition of the aerosols makes their effects one of the greatest sources of uncertainty in climate modeling and prediction (Formenti et al, 2011; Mahowald et al, 2011). This is despite the fact that much attention has been paid to these issues, and. Numerous studies have been conducted in recent years to investigate the aerosol OPs over China,

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