Surface aerosol optical properties during high and low pollution periods at an urban site in central China

Abstract

The aerosol scattering (σ sp ) and absorption (σ ap ) 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 σ sp (532 nm), σ ap (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 σ sp and σ ap 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. PM 2.5 was a major contributor to the large optical parameters. During pollution periods, the mean σ sp (624 Mm –1 ) was roughly 3 times that during clean periods (214 Mm –1 ), and the σ ap (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 σ sp and σ ap values for the pollution periods were accompanied by calm winds (0–2 m s –1 ), from 0° to 45°, whereas the low σ sp and σ ap values of the clean periods were observed with high wind speeds (above 4 m s –1 ), from 0° to 90°. Moreover, the σ ap showed a clear positive correlation (R 2 = 0.412) with the PM 2.5 concentrations for the clean periods, whereas the σ sp had a good positive correlation (R 2 = 0.406) with the PM 2.5 concentrations during the pollution periods.