Impacts of transportation emissions on horizontal and vertical distributions of air pollutants in Shanghai: Insights from emission reduction in COVID-19 lockdown

Transportation is a major sector of anthropogenic emissions in urban areas and deteriorates air quality. The surface and vertical observational data were combined with the model results to reveal its impact on the horizontal and vertical variations of pollutants during the COVID-19 lockdown period. The evident reductions in ambient PM_2.5 (∼30%) and NO₂ (∼50%) concentrations but a ∼25% increase in O₃ concentration were observed at the transportation sites. On the vertical scale, a uniform decrease of ∼28% in PM_2.5 concentrations was observed within 600 m. However, the vertical profiles of NO₂ and O₃ exhibited increasing vertical variation rates with concentrations varying significantly within 400 m. Meanwhile, O_x shared a similar pattern of vertical profile with O₃, with a uniform increase (∼5%) within 600 m in the urban area. The WRF-CMAQ model reproduced the variations, and revealed that the reduction of transportation emissions was the key factor contributing to the increase of urban O₃ and O_x due to the weakened NO titration effect. The simulated vertical profile of NO₂ was featured by a decreasing curve, while that of O₃ exhibited the opposite trend. We find that the transportation emissions impact vertical concentrations of NO₂ and O₃ within at most 400 m. The process analysis revealed that the vertical transport and horizontal transport from bay areas contributed to O₃ in the urban area, while chemical processes mainly consumed it. The reduction in transportation emissions weakened the consumption and resulted in O₃ accumulation during rush hours and at night. The variation of planetary boundary layer height also favored the rise of urban O₃ by promoting vertical transport at daytime and trapping it at night. The reduction in NO_x emissions from the transportation enhanced O₃ pollution, suggesting that collaborative reductions in VOCs from multiple sectors should be conducted. This study also indicated that regional collaborations in emission reductions were necessary for comprehensive air pollution prevention.