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

Abstract

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 (AOD), 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 AOD at 500nm in ozone episodes (~0.78) displayed 2.4 times higher than that in clean days (~0.32). Large Ångström exponent (~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 (~0.28) in fine mode (radius <0.6μm) was observed at 3p.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 (~0.91 at 440nm) to afternoon (~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 ~1.96km) was discovered during ozone-polluted days than clean condition (~1.4km). In episodes, the maximum extinction ratio (~0.5km−1) was presented at a height of 1.2km 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.