Measurement report: Investigation of regional pollutant transport to Beijing, China based on a unique 528 m platform

Abstract. Observations at elevated altitudes can capture the chemical characteristics of regional aerosols more effectively than ground-level measurements, but in situ measurements of aerosols over megacities remain scarce. In this work, aerosol composition and gaseous pollutants measured from 2020 to 2024 at a 528 m landmark tower in downtown Beijing, together with ground-level observations, are analyzed to understand regional pollutant transport. The results reveal that both aerosol mass concentration and composition differ significantly among air masses originating from different directions. Further analysis of sulfur and nitrogen oxidation ratios showed that both were significantly higher in air masses from the south and northeast compared to those from the northwest. This difference is likely attributable to higher relative humidity (RH) in the former, which promotes heterogeneous oxidation of SO2 and NO2 during transport. Regional aerosols were downwards transported to ground efficiently through planetary boundary layer (PBL) process during daytime, thereby exacerbating air pollution in Beijing. These findings underscore the critical role of regional transport in shaping Beijing's aerosol burden and highlight how the chemical signatures of transported aerosols reflect their diverse source regions and formation mechanisms.