Relationship between negative air ion and PM in under natural conditions.

In recent years, PM pollution has become a most important source of air pollution. Prolonged exposure to high PM 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 and NAI. In this study, we collected data on NAI, PM, 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 forest, a typical forest in warm temperate zones. We investigated the spatiotemporal variation of PM and NAI under consistent meteorological conditions, established the correlation between PM and NAI, and explicated the impact mechanism of PM on NAI in natural conditions. The results showed that NAI decreased exponentially with the increases in natural PM, with a significant negative correlation (=1148.79). The decrease rates of NAI in PM concentrations of 0-20, 20-40, 40-80, 80-100 and 100-120 μg·m were 40.1%, 36.2%, 9.4%, 2.4%, 5.1% and 6.8%, respectively. Results of the sensitivity analysis showed that the PM concentration range of 0-40 μg·m 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.