Objective To explore the variation characteristics of Negative air ion (NAI) of typical plantations and the relationship between NAI and environmental factors in growing period and non-growing period in rocky mountain area of north China, and reveal environmental factors affecting NAI in different seasons in this area.

Method The variation characteristics of NAI of typical plantations in Henan Xiaolangdi Earth Critical Zone National Research Station were measured from May to December in 2021 using the air negative ions, PM 2.5, PM 10 and meteorological data. Besides, the main environmental factors and variable importance measures affecting NAI of typical plantations in the area were analyzed by random forest algorithm.

ResultsThe diurnal variation of NAI in Quercus variabilis showed single peak curve in growing period but not obvious in non-growing period. The NAI of Platycladus orientalis showed single peak cure during the experiment period. The NAI concentration of Quercus variabilis plantations (740.32 ion·cm⁻³) was higher than that of Platycladus orientalis (703.74 ion·cm⁻³) during the observation period. The daily NAI of Quercus variabilis (858.94 ion·cm⁻³) was higher than that of Platycladus orientalis (724.33 ion·cm⁻³) during the growing period. The daily NAI of Quercus variabilis (621.70 ion·cm⁻³) was lower than that of Platycladus orientalis (683.16 ion·cm⁻³) during the non-growing period. The meteorological factors such as Air temperature (Ta), Relative humidity (RH), Vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) in the growing period were higher than those in the non-growing period, while the particulate matter (PM2.5 and PM10) in the non-growing period was higher than that in the growing period. The concentration of PM10 was higher than that of PM2.5 during the experiment period. There was no significant difference between Wind speed (WS) between the growing period and non-growing period. The random forest method revealed that the main environmental factors affecting the NAI concentrations of Quercus variabilis and Platycladus orientalis in the growing period were VPD, PAR and WS, and their variable importance measures were 20.22, 15.08 ,14.71, respectively, and 25.08, 16.76, 16.49, respectively. The main environmental factors affecting the NAI concentration of Quercus variabilis and Platycladus orientalis during the non-growth period were PM 2.5, WS and PM 10, and their variable importance measures were 33.36, 17.58, 14.28, respectively, and 15.89, 17.51, 14.62, respectively.

Conclusion The diurnal variation of NAI concentration of Quercus variabilis and Platycladus orientalis in growing period both showed a single peak curve; the diurnal variation of NAI concentration of Quercus variabilis plantations was not obvious, while the diurnal variation NAI concentration of Platycladus orientalis plantations showed a single peak curve in non-growing period. There were significant differences in NAI concentration between Quercus variabilis and Platycladus orientalis plantations, NAI concentration of Quercus variabilis was higher than that of Platycladus orientalis during the growing period, while NAI concentration of Quercus variabilis was lower than that of Platycladus orientalis during the non-growing period. The NAI concentration of Quercus variabilis was higher than that of Platycladus orientalis during the observation period. Differences of environmental factors affecting NAI of typical plantations in the area were obvious. VPD and PAR were the key factors during the growing period, while PM2.5, PM 10 and WS were the key factors during the non-growing period.