What makes the less urbanized city a deeper ozone trap: implications from a case study in the Sichuan Basin, southwest China

Abstract. The urban-rural gradient of surface ozone concentration is widely reported in global megacities. As yet, quantitative analyses for this gradient pattern have been lacking. Using near surface atmospheric pollutant reanalysis and remote sensing measurements, we demonstrate a dipole-like urban surface ozone trap pattern in two megacities (Chengdu and Chongqing) in the Sichuan Basin. During the study period of 2013–2019, the urban-rural gradients of surface ozone level in Chongqing were higher than in Chengdu despite Chongqing’s lower urbanization level. In winter, the ozone level in the core area of Chongqing/Chengdu is 16.4/22.1 μg m^-3, with an increasing rate of 8.98 %/5.19 % per 10 km towards the surrounding suburban area. However, the nitrogen dioxide level in Chengdu is higher than in Chongqing. Besides, the concentration levels of formaldehyde and ultraviolet-absorbing aerosol did not show comparable differences between these two cities. Regarding the meteorological conditions, atmospheric visibility, sunshine duration, and nighttime wind speed in Chongqing were all lower compared to Chengdu, the ozone trap pattern aligns more with meteorological condition rather than chemical condition. Our study characterized the ozone trap pattern for two megacities with different urbanization levels, providing a novel perspective on urban atmospheric environment assessment.