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.