Despite air quality has been improved significantly after the implementations of clear air actions during the past decade, air pollution is still heavy in the Lanzhou urban basin in winter. To gain insight into the formation mechanism and evolution process of the winter haze pollution, the first synchronous real-time measurement focusing on the vertical difference in aerosol chemistry was conducted at two different heights (~640 m with a horizontal distance of ~5 km) in the basin during January 2021. Significant vertical differences in the temporal and diurnal variations of submicron aerosol (PM₁) chemical compositions and pollution characteristics were observed during heavy polluted periods. At the ground site, on average, primary emissions from residential cooking, traffic, and heating activities were prominent sources of PM₁ (42%), of which contribution decreased by ~18% at the mountaintop site. In contrast, secondary aerosols dominated (76%) the PM₁ at the mountain site. The vertical differences could increase up to ~40% during heavy polluted periods, suggesting the inhabiting of planetary boundary layer (PBL) on air pollution. Two heavy haze pollution events were elucidated to clarify the role of low PBL in accumulation of air pollutants in this mountain-valley city. Our findings for the first time highlighted quantitively the impact of PBL variation and vertical mixing process on air quality in the city, which was also useful for the validation and evaluation of results from model simulations.

气溶胶,边界层,垂直观测