Aerosol optical properties from the Atmospheric Radiation Measurement Mobile Facility at Shouxian, China

The Atmospheric Radiation Measurement program's Mobile Facility was deployed at Shouxian, China (32.56 degrees N, 116.78 degrees E, elevation 22.7 m), from May to December 2008. Aerosol optical properties derived from the Aerosol Observation System (AOS) measurements are analyzed. The median value of the light scattering coefficient (sigma(sp)) at 550 nm is 302.8 Mm(-1). The lowest value of sigma(sp) appeared in the summer and was mainly due to strong wet deposition and upward transport. Averaged over the measurement period from May to July (arithmetic mean +/- standard deviation), the aerosol light absorption coefficient, sigma(ap), is 29.4 +/- 31.1 Mm(-1) (550 nm) which is lower than most measurements made in urban areas in China but higher than those observed in background sites such as Shangdianzi, a suburb of Beijing. The mean single scattering albedo (omega(o)) is 0.92, which is greater than values found in the Pearl River Delta, the Yangtze Delta, and northern China, indicating less absorbing aerosols at the Shouxian site. The mean values and standard deviations of the Angstrom exponent (alpha, 700-550 nm), the submicron scattering ratio (R-sp), and the hemispheric backscattering ratio (b) at 550 nm are 1.19 +/- 0.23, 0.47 +/- 0.15, and 0.101 +/- 0.017, respectively. These values are lower than those observed at continental sites in the United States, implying the relatively large contributions of coarse mode particles at Shouxian. A distinct diurnal variation of aerosol optical properties is observed. Absorption coefficients are highest in the early morning and minimum values appear at noon. The omega(o) shows nearly the same cycle as the absorption coefficient but with the maximum and minimum reversed. Impacts of local agricultural residue burning and long-range transport of dust on aerosol loading and properties are presented using Moderate Resolution Imaging Spectrometer (MODIS) fire spot data, MODIS level-2 aerosol optical depth data and micropulse lidar data together with AOS data.