A study of aerosol optical properties during ozone pollution episodes in 2013 over Shanghai, China

Aerosol optical property is essential to the tropospheric ozone formation mechanism while it was rarely measured in ozone-rich environment for a specific study. With the retrieved products of the sun-photometer, a comparative investigation was conducted on aerosol optical depth (ADD), single scattering albedo (SSA) and size distribution during ozone-polluted episodes and clean background. Contrary to expectations, aerosol loading was found to be positively-correlated with ozone concentration: daily averaged ADD at 500 nm in ozone episodes (similar to 0.78) displayed 2.4 times higher than that in clean days (similar to 032). Large Angstrom exponent (similar to 1.51) along with heavy aerosol loading indicated a considerable impact of fine particles on optical extinction. The dynamic diurnal fluctuation of these parameters also implied a complex interaction between aerosols and photo-chemical reactions. The bimodal lognormal distribution pattern for aerosol size spectra exhibited in both ozone-polluted and clean days. The occurrence of maximum volume concentration (similar to 0.28) in fine mode (radius <0.6 mu m) was observed at 3 p.m. (local time), when ozone was substantially generated. Pronounced scattering feature of aerosol was reproduced in high-concentration ozone environment. SSA tended to increase continuously from morning (similar to 0.91 at 440 nm) to afternoon (similar to 0.99), which may be associated with secondary aerosol formation. The scattering aerosol (with moderately high aerosol loading) may favor the ozone formation through increasing solar flux in boundary layer. Utilizing the micro-pulse lidar (MPL), a more developed planet boundary layer (PBL, top height similar to 1.96 km) was discovered during ozone-polluted days than clean condition (similar to 1.4 km). In episodes, the maximum extinction ratio (similar to 0.5 km(-1)) was presented at a height of 12 km in the late afternoon. The humidity profile by sounding also showed the extreme value at this altitude. It suggested that optical extinction was mainly attributed to the aerosol in middle PBL, where the intense photochemical reactions and hydroscopic growth may occur. (C) 2014 Elsevier B.V. All rights reserved.