Surface Aerosol Optical Properties during High and Low Pollution Periods at an Urban Site in Central China

The aerosol scattering (σsp) and absorption (σap) coefficients and single scattering albedo (SSA) were continuously measured and analyzed at an urban site in Wuhan of central China from November 2014 to July 2017. The average σsp (532 nm), σap (532 nm), and SSA in Wuhan during the study period were 244 ± 212 Mm–1, 27 ± 17 Mm–1, and 0.86 ± 0.09, respectively. The aerosol optical properties had pronounced and distinctive diurnal cycles in the Wuhan area. The σsp and σap exhibited the highest values between 06:00 and 08:00 local time (LT) and the lowest values between 14:00 and 16:00 LT, which was mainly due to being coupled with increasing traffic emissions. PM2.5 was a major contributor to the large optical parameters. During pollution periods, the mean σsp (624 Mm–1) was roughly 3 times that during clean periods (214 Mm–1), and the σap (58 Mm–1) was about 2.5 times that during clean periods (24 Mm–1). The wind speed and direction also strongly affected the aerosol optical properties during the different periods. The high σsp and σap values for the pollution periods were accompanied by calm winds (0–2 m s–1), from 0° to 45°, whereas the low σsp and σap values of the clean periods were observed with high wind speeds (above 4 m s–1), from 0° to 90°. Moreover, the σap showed a clear positive correlation (R2 = 0.412) with the PM2.5 concentrations for the clean periods, whereas the σsp had a good positive correlation (R2 = 0.406) with the PM2.5 concentrations during the pollution periods.