Relationship between negative air ion and PM_2.5 in Quercus variabilis under natural conditions

In recent years, PM_2.5 pollution has become a most important source of air pollution. Prolonged exposure to high PM_2.5 concentrations can give rise to severe health issues. Negative air ion (NAI) is an important indicator for measuring air quality, which is collectively known as the 'air vitamin'. However, the intricate and fluctuating meteorological conditions and vegetation types result in numerous uncertainties in the correlation between PM_2.5 and NAI. In this study, we collected data on NAI, PM_2.5, and meteorological elements through positioning observation during the period of June to September in 2019 and 2020 under the condition of relatively constant leaf area in Quercus variabilis forest, a typical forest in warm temperate zones. We investigated the spatiotemporal variation of PM_2.5 and NAI under consistent meteorological conditions, established the correlation between PM_2.5 and NAI, and explicated the impact mechanism of PM_2.5 on NAI in natural conditions. The results showed that NAI decreased exponentially with the increases in natural PM_2.5, with a significant negative correlation (y=1148.79x^-0.123). The decrease rates of NAI in PM_2.5 concentrations of 0-20, 20-40, 40-80, 80-100 and 100-120 μg·m^-3 were 40.1%, 36.2%, 9.4%, 2.4%, 5.1% and 6.8%, respectively. Results of the sensitivity analysis showed that the PM_2.5 concentration range of 0-40 μg·m^-3 was the sensitive range that affected NAI. Our findings could provide a scientific basis for better understanding the response mechanisms of NAI to environmental factors.