Fractal analysis of impact of PM_2.5 on surface O₃ sensitivity regime based on field observations

Properties of PM_2.5 that can change aerosol chemistry and photolysis rates have great impacts on O₃ sensitivity regime, further affecting the production rate of surface O₃. However, responses of O₃ sensitivity regime to changes in PM_2.5 levels are difficult to be accurately determined, due to the complexity and nonlinearity of atmospheric chemistry. Here, based on long-term time series (2016-2020) of air quality variables in north and south Taiwan, fractal analysis along with Pearson correlation analysis are used to directly reveal the impacts of PM_2.5 on O₃ sensitivity regime in real atmosphere, by capturing the nonlinear dynamic relations among air pollutants. Great regional and seasonal difference in impacts of PM_2.5 on O₃ sensitivity regime may be ascribed to meteorological factors, PM_2.5 components and levels of SO₂, NO, NO₂, etc. For north Taiwan, increased PM_2.5 level can enhance the sensitivity of O₃ formation to VOC in spring and summer, whereas the opposite effect can be observed in winter. But for south Taiwan, the influence of PM_2.5 on O₃ sensitivity regime is not statistically significant, excluding spring. Furthermore, feasibility and availability of fractal analysis is tested by simulations with Empirical Kinetics Modeling Approach (EKMA). The results demonstrate the capability of fractal analysis to identify the impacts of PM_2.5 on O₃ sensitivity regime in real atmosphere, which can provide suggestions for PM_2.5-O₃ coordinated control strategies in regions suffering combined air pollution.