Effects of relative humidity and PM2.5 chemical compositions on visibility impairment in Chengdu, China.

To better understand the potential causes of visibility impairment in autumn and winter in Chengdu, relative humidity (RH), visibility, the concentrations of PM2.5 and its chemical components were on-line measured continuously in Chengdu from Nov. 2016 to Jan. 2017. Six obvious haze episodes occurred in Chengdu, with the total time of haze episodes accounted for more than 90% of the total observation period, and higher NO2 concentrations and RH were related to the high particle concentrations in haze episodes. The visibility decreased in a non-linear tendency under different RH conditions with the increase of PM2.5 concentrations, which was more sensitive to RH under lower PM2.5 concentrations. The threshold concentration of PM2.5 got more smaller with the increase of RH. During the entire observation period, organic matter (OM) was the largest contributor (31.12% to extinction coefficient (bext)), followed by NH4NO3 and (NH4)2SO4 with 28.03% and 23.01%, respectively. However, with the visibility impairment from Type I (visibility > 10 km) to Type IV (visibility ≤2 km), the contribution of OM to bext decreased from 38.12% to 26.77%, while the contribution of NH4NO3 and (NH4)2SO4 to bext increased from 19.09% and 20.20% to 34.29% and 24.35%, respectively, and NH4NO3 became the largest contributor to bext at Type IV. The results showed that OM and NH4NO3 were the key components of PM2.5 for visibility impairment in Chengdu, indicating that the control of precursors emissions of carbonaceous species and NH4NO3 could effectively improve the visibility in Chengdu.