Observations of ozone vertical profiles and corresponding precursors in the low troposphere in Beijing, China

A comprehensive investigation of ozone, its precursors as well as aerosols in the low troposphere was conducted over the University of Chinese Academy of Sciences site (abbreviated as UCAS site, a northern suburban site) in Beijing during the period of October 26 to November 6, 2014, with a focus on the comparison between polluted and clean conditions. The vertical profiles of ozone and aerosol extinction coefficients in the low troposphere were measured by differential absorption lidar, and the tropospheric vertical column densities (VCDs) as well as vertical profiles of HCHO and NO2 were measured by ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). O3 in the boundary layer (0.3-1.0 km) had a positive gradient with height and presented a stratified structure at night on haze days, in contrast to a uniform distribution during clean episodes. The polluted conditions were characterized by higher levels of atmospheric pollutants (O3, aerosol, HCHO and NO2) and more dynamic variations with respect to clean periods. During lightly to moderately polluted periods, elevated concentrations of O3 within the boundary layer were observed with a maximum exceeding 60 ppbv. In haze days, the air masses from urban Beijing and southern Hebei industrial and urbanized areas could bring large amounts of precursors, enhancing O3 formation over the lidar site. The photochemical production of O3 within the boundary layer was controlled by a VOC-limited regime in haze days and turned into a mixed-VOC-NOx-limited regime on clean days. According to sensitivity analysis, when air masses came from southern polluted regions, the sensitivity of ozone production was dominated by VOC-limited regime. The reduction of VOC emissions from urban Beijing and Hebei industrial and urbanized areas would favor the mitigation of O3 pollution at UCAS site.