Characteristics of the imaginary part and single-scattering albedo of urban aerosols in northern China

An improved broadband diffuse solar radiation method for retrieval of the aerosol imaginary part (AIP) and its single-scattering albedo (SSA) is proposed. The first improvement is to determine diffuse radiation from combined pyrheliometer and pyranometer data. Secondly, available approaches to input parameters demanded for AIP retrieval are presented, including an approach to determine the aerosol extinction coefficient profile from visibility and aerosol optical depthdata. This method is used to retrieve AIP and SSA from routine pyrheliometer and pyranometer observation data measured in 1993–2001 at six meteorological observatories in northern China, all located around big or medium-sized cities. As shown in the retrieval results, 9-yr mean yearly-mean AIP during 1993–2001 changed from 0.0207 to 0.0301 forthe six cities, and the corresponding SSA changed from 0.851 to 0.803, with a mean value of 0.832. There are different variation trends in yearly-mean AIP and SSA during 1993–2001. At Harbin and Beijing observatories the AIP shows an increasing trend. At Shenyang and Zhengzhou observatories the AIP had an increasing trend during 1993–1995, buta decreasing trend after 1995.

Errors in the retrieved yearly-mean AIP and SSA were estimated. The main sources of AIP errors are systematic errors in the radiation measurement as well as uncertainty about the aerosol size distribution. The main error in SSA retrieval is the systematic error in the radiation data. Uncertainty about aerosol size distribution has a weak impact on SSA retrieval. It is estimated, according to sensitivity tests, that errors of yearly-mean AIP and SSA are usually within 0.0054 and 0.041 respectively.