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

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.