Seasonal quantification of the inter-city transport of PM2.5 in the Yangtze River Delta region of China based on a source-oriented chemical transport model and the Michaelis-Menten equation

Regional transport plays a crucial role in the pollution of fine particulate matter (PM_2.5) over the Yangtze River Delta region (YRD). A practical joint regional emission control strategy requires quantitative assessment of the contribution of regional transport. In this study, the contribution of inter-city transport to PM_2.5 among the 41 cities in the YRD region were quantitatively estimated using a source-oriented chemical transport model, and then the relationship between the cumulative contribution of regional transport and the distance was examined using the Michaelis-Menten equation. The results show that the Michaelis-Menten equation is suitable to represent the relationship between the cumulative contribution and transport distance. The coefficient of determination (r²) of the fittings is greater than 0.9 in 71% of the cases in the six subregions and four seasons in YRD. Two key parameters in the Michaelis-Menten equation K₁, indicating the maximum contribution of regional transport, and K₂, indicating the distance to which the regional transport contribution reach half the maximum contribution, show substantial regional and seasonal variations. The average K₁ is 73.6%, with lower values observed in the northern part of the YRD and higher values in central Jiangsu. K₂ is larger in northern Jiangsu, as well as central and southern Zhejiang. The local contribution in autumn and winter is lower than that in spring and summer in the northern part of the YRD. Particularly in northern Jiangsu, the local contribution reaches 90.4% in summer but drops to 53.0% in autumn and winter, illustrating significant impacts of regional transport to PM_2.5 in autumn and winter in this area. K₂ is larger on polluted days, compared to clean days, indicating greater contributions from regional transport to PM_2.5 in YRD. The results can serve as a scientific foundation for implementing regional joint prevention and control measures in the YRD region.