Regional transport of air pollutants from stereoscopic remote sensing observation

  In addition to local emissions and atmospheric chemical reactions, regional transport is also an important source of atmospheric pollutants. Satellite-based remote sensing can obtain the spatial distribution of air pollutants in a large range. However, due to the influence of cloud coverage, there is the problem of data missing, which make it difficult to directly use satellite data for quantitative assessment of regional transport. In this study, we used satellite remote sensing from TROPOMI to update the emissions of WRF-Chem modeling. Then, the spatial distribution of regional transport fluxes was obtained through the updated modeling, and the output areas of air pollutants were identified in a large range of regional pollution events. We find that with the overall improvement of air quality in the Beijing-Tianjin-Hebei region, the contribution of external input to air pollution in Beijing from surrounding cities shows a downward trend, while the impact of local emissions become more prominent. Besides, the transports through elevated altitude were investigated through ground-based remote sensing. We found that the transport heights and source regions for different pollutants are quite different. Aerosols and sulfur dioxide (SO₂) are significantly affected by the long-distance transport across the upper boundary layer; nitrogen dioxide (NO₂) is mainly from local emissions and transport from surrounding area across the lower boundary layer; while the regional transport of formaldehyde (HCHO) is not obvious. Moreover, regional transport through elevated altitudes not only directly brings air pollutants, but also can cause the inversion of the vertical structure of aerosols. The inverse structure of aerosols can further induce adverse meteorological conditions through the interaction between pollution and meteorology, and then aggravate air pollution.