Distinct urban-rural gradients of air NO and SO concentrations in response to emission reductions during 2015-2022 in Beijing, China.

Nitrogen dioxide (NO) and sulfur dioxide (SO) are two major air pollutants in urban environment. Emission reduction policies have thus been implemented to improve urban air quality, especially in the metropolises. However, it remains unclear whether the air concentrations of NO and SO in and around large cities follow a same spatial pattern and how their characteristics change over time in response to the emission reductions. Using ground-based monitoring datasets of air NO and SO concentrations in Beijing, China, we tested the hypothesis of urban air pollutant islands and evaluated their seasonal and inter-annual variations during 2015-2022. The results showed that air NO concentrations increased significantly towards the urban core, being in line with the hypothesis of urban air pollutant island, while air SO concentrations showed no such spatial patterns. The urban air NO island varied seasonally, with larger radius and higher air NO concentrations in spring and winter. In response to the emission reduction, the annual mean radius of the urban air NO island showed a rapid decrease from 45.8 km to zero km during the study period. The annual mean air NO concentration at the urban core showed a linear decrease at a rate of 4.5 μg m yr. In contrast, air SO concentration decreased nonlinearly over time and showed a legacy in comparison to the emission reduction. Our findings suggest different urban-rural gradients of air NO and SO concentrations and highlight their distinct responses to the regional reductions of anthropogenic emissions.