Temporal dynamics and relationship between negative air ions and environmental factors in subtropical forests, China

Negative air ions (NAIs) in forest air contribute to air purification, improve respiratory system function, enhance immunity, reduce fatigue, and alleviate stress and anxiety. However, NAIs are easily influenced by meteorological factors, air quality factors, and radiation factors, and the mechanisms of these influences remain unclear. To investigate the dynamic changes of NAIs and their driving factors, this study focuses on the long-term monitoring of NAIs in the Yunzhong Mountain forest of the South Asian subtropical region in China. Using linear mixed models, structural equation models, and other ecological niche models, the study explores the characteristics of the dynamic changes in NAIs and their responses to meteorological factors, air quality factors, and radiation factors in the Yunzhong Mountain forest. The results showed that: (1) The seasonal variation of NAIs in the Yunzhong Mountain forest followed the pattern of summer > autumn > winter > spring. In terms of monthly dynamics, the highest concentrations of NAIs occurred in August. Regarding daily dynamics, the concentration of NAIs gradually increased from early morning, reaching its peak at 8:00 AM, after which it declined, reaching the lowest concentrations between 11:00 AM and 12:00 PM. (2) Correlation analysis showed that the concentrations of NAIs were significantly negatively correlated with NO₂, air temperature (AT), PM₁, PM_2.5, and PM₁₀, and significantly positively correlated with air humidity (AH). However, generalized linear mixed analysis indicated that PM_2.5, O₃, NO₂, and air pressure (AP) had significant negative effects on the seasonal, monthly, and daily concentrations of NAIs, while air humidity and temperature exhibited significant positive effects. Moreover, air quality factors had a greater impact than meteorological factors on seasonal and monthly concentrations, whereas meteorological factors had a more pronounced influence on daily negative air ion concentrations. (3) The latent variable of air quality factors had a direct negative effect on the concentration of NAIs, with O₃, PM₁, and PM_2.5 exerting significant indirect negative effects through the air quality factors. The latent variable of meteorological factors had a significant positive effect on the concentration of NAIs. Additionally, radiation factors did not have a direct significant effect on negative air ions, but radiation factors (ultraviolet radiation and net radiation) produced significant indirect positive effects on NAIs through meteorological factors and air quality. The concentration of NAIs in the Yunzhong Mountain forest is highest between 7:00 and 8:00 AM during July and August in the summer, making it the most suitable time for outdoor activities. Meteorological factors, air quality, and radiation factors all influence the NAIs in Yunzhong Mountain. Even in the absence of direct effects, these factors can have indirect impacts through other variables.