Vertical Distributions of Primary and Secondary Aerosols in Urban Boundary Layer: Insights into Sources, Chemistry, and Interaction with Meteorology

Abstract

Vertical measurements are essential for the characterization of aerosol and boundary layer interactions; yet, our knowledge of vertical profiles of primary and secondary aerosol species in megacities is limited. Here, we conducted comprehensive vertical measurements of aerosol particle composition on a 325 m meteorological tower with two aerosol chemical speciation monitors in winter in urban Beijing. The simultaneous measurements at ground level, 140, and 240 m illustrated similar aerosol bulk composition at these three heights. However, the vertical ratios varied significantly among different aerosol species. Particularly, the vertical ratios of the aqueous phase and photochemical-related secondary organic aerosol (SOA) (aqOOA/OOA) decreased significantly, accompanied by the increases in ratios of secondary to primary OA, highlighting different chemical properties of OA between ground level and aloft, and the large impacts of vertical changes in meteorology and gaseous precursors on SOA formation. The vertical changes in NO 3 /SO 4 ratios, however, were mostly insignificant, likely due to the low relative humidity and aerosol water content that inhibited nocturnal heterogeneous reactions in the residual layer. Considerable increases in the ratios of 240 m to ground level in the early morning were also observed for most aerosol species, demonstrating impact of residual layer on the air pollution of 2nd day.

Vertically Resolved Aerosol Chemistry in the Low Boundary Layer of Beijing in Summer

“…This is further supported by the higher daytime vertical ratio of Chl, a species with similar volatile properties as nitrate, than that of nighttime. In addition, most primary species showed higher vertical ratios during daytime, owing to the stronger vertical convection that has been demonstrated by many previous studies. ,, …”

“…The vertical ratios of aerosol species between 240 m and the ground level (ratio 240m/Ground ) that are shown in Figure can show the vertical differences more clearly. In summer, the ratio 240m/Ground of nitrate was generally higher than that of other inorganic species, suggesting a greater potential for nitrate production in city aloft. , Comparatively, the ratio 240m/Ground of secondary inorganic species was ubiquitously above 1 in winter, likely due to the influences of regional transport from the heavily polluted region in the south and southwest and the penetration of the residual layer . The vertical differences in OA factors were more pronounced than those of inorganic species in both summer and winter.…”

“…Particularly, three haze episodes with the concentrations of organics and nitrate up to 25 μg m –3 were observed. Consistent with winter and previous studies, ,, the formation of these haze episodes was associated with southerly winds, high RH, and low WS. Comparatively, the clean periods were characterized by strong northerly winds and the dominance of organics in aerosol composition.…”

“…Two vertical campaigns were carried out on the 325 m meteorological tower at the Institute of Atmospheric Physics, Chinese Academy of Sciences. The winter measurements were performed from 30 December 2018 to 14 January 2019, which were detailed in our previous study . The summer measurements were conducted from 30 May to 20 June 2021.…”

“…For example, the differences in aerosol chemistry between 260 m and the ground level and the evolution of the winter pollution process were elucidated by simultaneous real-time measurements on the tower. − A few studies also used the tower container to investigate the vertical profiles of aerosol composition, gaseous species, and optical properties in the urban boundary layer. Wang et al first identified four typical vertical profiles during two severe pollution episodes in Beijing wintertime, and Lei et al then characterized the vertical profiles of primary and secondary aerosols in winter with the first deployment of an aerosol chemical speciation monitor in the container. Similarly, the vertical profiles of NH 3 , HONO, N 2 O 5 , O 3 , and NO 2 were also measured during specific periods to characterize aerosol formation, for example, nitrate and ammonium, and the impacts on chemical processes. − Despite these efforts, our understanding of the vertical changes in aerosol properties in summer is very limited due to the absence of vertical measurements.…”