A synchronous observation of enhanced aerosol and NO2 over Beijing, China, in winter 2015.

Despite extensive efforts into the characterization of air pollution during the past decade, concurrent real-time characterization of aerosol and NO2 as well as satellite observation above the urban canopy in the megacity of Beijing has sparsely been performed to date. We conducted a simultaneous real-time measurement of aerosol and NO2 in urban Beijing in a pollution episode from November 25, 2015 to December 2, 2015. The aerosol extinction coefficient was measured by a 532 Mie lidar and the NO2 concentration by a Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). The highest aerosol extinction coefficient was 1.55 (±0.16) km-1 at 532m at a height of 0.27km. The high average aerosol extinction was observed throughout the pollution period, probably due to the favorable static weather conditions under high surface high-pressure system and transportation sources. NO2 showed similar temporal variations between ground level and 0.3km, whereas a much weaker diurnal evolution was found for NO2. The boundary layer structures were observed, and the relatively homogeneous mixture maintained the pollution within the layer under 0.5km. Ozone Monitoring Instrument (OMI) NO2 products based on well-known DOAS trace gas fitting algorithm and WRF-Chem model further illustrated the simultaneous enhancement of NO2 during the observation period. Backward trajectories suggest that long-range pollution transportation from northwest China contributes more to this pollution event.