Impact of urban heat island effect on ozone pollution in different Chinese regions

With urbanization, urban heat island effect (UHI) and ozone (O3) pollution have become hot issues in urban research. UHI affects the transformation of O3, but its mechanism of action is not clear. In this paper, we investigate the effects of UHI on O3 concentration in major cities across China using O3 data from March 2020 to February 2021 and ERA5-Land temperature data and single-level wind field data in the same period. The results show that (1) the daily variation pattern of O3 concentration is the same in all regions, with a "single peak and single valley" pattern, reaching a valley around 8:00 a.m. and a peak around 15:00 p.m. The seasonal distribution of O3 concentration varies among regions, with the highest O3 concentration in summer (149.0 mu g/m3, 183.4 mu g/m3, 146.5 mu g/m3, 158.0 mu g/m3, respectively) and the lowest in winter in Northeast, North, Northwest and East China; the highest O3 concentration in spring (144.6 mu g/m3, 151.0 mu g/m3, respectively) and the lowest in winter in Southwest and Central China; and the highest O3 concentration in winter (144.6 mu g/m3, 151.0 mu g/m3, respectively). In spring and the lowest in winter; the highest O3 concentration in autumn (142.0 mu g/m3) and the lowest in summer in South China. Nationwide, the areas with high O3 concentrations are concentrated in North, Central and East China. (2) The inter-day fluctuation of urban heat island intensity (UHII) was the smallest in East China (0.69 degrees C) and the largest in Southwest China (1.90 degrees C). There are significant differences in the daily variation of UHII among regions, including "single peak, single valley", "single peak", "single peak, double valley" and other types. In general, the UHII in northern China is larger than that in southern China, and the seasonal variation of UHII in eastern cities is small due to the regulating effect of ocean and sea breeze. (3) The correlation between O3 concentration and UHII in different cities in China has obvious differences. The O3 concentration and UHII in cities closer to the coastline or at higher elevation show a negative correlation, while the O3 concentration and UHII in cities far from the coastline and at lower elevation show a positive correlation. At the same time, there is a general lag between O3 concentration and UHII in major cities across the country, but the lag time still varies greatly. (4) This study shows that UHI affects O3 concentration in two ways: firstly, it changes the local heat balance and affects O3 production; secondly, it affects the urban wind field, which plays the role of diffusion, dilution and transmission for O3, forming regions with different O3 concentration values.