Abstract
The transport of pollutants has a substantial impact on the atmospheric environment in megacity clusters. However, owing to the lack of knowledge of the vertical pollutant structure, our quantification of transport processes and understanding of their impacts on the environment remain inadequate. In this study, we retrieved the vertical profiles of aerosol, NO2, and HCHO using multi-axis differential optical absorption spectroscopy (MAX-DOAS) and analyzed three typical transport phenomena. We found as follows: (1) The main transport layer (MTL) of aerosol, NO2 and HCHO along the southwest–northeast transport pathway in the Jing-Jin-Ji region were approximately 400–800 m, 0–400 m and 400–1400 m, respectively. The maximum transport flux of HCHO appeared in Wangdu (WD), oppositely, the minimum transport fluxes of aerosol and NO2 also occurred in this station. (2) The North China Plain (NCP) was usually affected by severe dust transport. The transported dust suppressed dissipation and boosted pollutant accumulation, converting the vertical profiles into an exponential shape. Furthermore, dust can indirectly affect trace gas concentrations by weakening optical intensity. For stations with higher optical intensity, the reduced NO2 levels were closely associated with its heterogeneous reactions on dust and aerosol surfaces. Comparatively, for other stations with low solar radiation, the decreased optical intensity favored NO2 concentration increase by inhibiting NO2 photolysis. The reduced solar radiation favored local HCHO accumulation in Shijiazhuang (SJZ) due to the dominant contribution of primary HCHO. (3) A back-and-forth transboundary transport between the NCP and Yangtze River Delta (YRD) was found. The YRD-to-NCP and NCP-to-YRD transport processes mainly occurred in the 500–1500 m and 0–1000 m layers, respectively. This transport, accompanied by the dome effect of aerosol, produced a large-scale PM2.5 concentration increase, further validating the haze-amplifying mechanism by practical observations.
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