Natural air can produce negative air ions in large amounts through radioactive material ionization, gases or cosmic rays (Tammet et al., 2006), the Lenard effect of water (Wu et al., 2001), natural lightning, frictional effects of wind and ground, and tip discharges of plants (Meng and Zhang, 2004; Wang et al., 2004). Negative air ions are beneficial for ster

ilization, reducing dust, purifying air, and enhancing immunity. Therefore, these ions are also referred toas “air vitamins and growth hormones”, adsorbing onto suspended matter and bacteria in the air (Alonso et al., 2009), purifying the air, and providing comfort for people (Bowers et al., 2018). In air quality assessment, negative air ions have been recognized as an important parameter for measuring air quality (Shao et al., 2005; Iwama, 2010; Wang et al., 2013).

Studies conducted worldwide on negative air ions have focused on the variation patterns of these ion concentrations, their influencing factors, and air quality evaluation (Liang et al., 2014; Cao et al., 2017). The evaluation methods adopted mainly include the unipolar coefficient evaluation method and amperometric air ion evaluation index method (Zhong et al., 1998), negative air ion coefficient (Shi et al., 2004), and relative density of air ions (Wu et al., 1998). Previous studies have also focused on the medical and health benefits of negative air ion concentrations (Goel and Etwaroo, 2006; Flory et al., 2010; Bowers et al., 2018). Recent studies conducted in domestic scholars have gradually moved focus to urban forests or green areas with considerably higher negative air ion concentrations than in nonforested land (Wu and Wang, 2007). Forest parks are naturally large oxygen bars, which have good health and body care benefits (Zhu et al., 2021). Si et al. (2014), Li et al. (2021), and Yu et al. (2021) have analyzed the changes in negative air ion concentration and its influencing factors and identified the spatial and temporal distribution characteristics of these ions as well as the influence of plant photosynthetic characteristics, relative humidity, temperature, and weather conditions. Feng et al. (2015) showed that the negative air ion concentration in broadleaved forests is higher than that in forests of both coniferous and broad leaves.

Currently, the common tree species considered for these studies conducted around northern China are Platycladus orientalis, Pinus tabuliformis, and Populus davidiana. In this study, we examined Juglans mandshurica, Betula dahurica, and Tilia tuan in rocky mountain areas in northern China because of the lack of adequate research conducted in these forest stands. We investigated the changes in negative air ion concentration over a period of time for three typical forest stands at the Songshan National Nature Reserve, Beijing. The air quality was measured using the amperometric air ion evaluation index method，which provides important significance for better guiding the adjustment of stand structures, rational allocation of stand resources, and further exerting its due ecological benefits.

The Songshan National Nature Reserve is at the northwest of the central city of Yanqing district, Beijing (115°43′44″- 115°50′22″E, 40°29′9″-40°33′35″N). The area has a complex topography and varying altitude, and it is a warm temperate continental monsoon climate zone. The local climate is spring from May to June, summer from July to August, and autumn from September to October. The area has low precipitation and high winds in spring; it is warm and rainy in summer and has low temperature in autumn but is cold and dry in winter. The annual average temperature is 8.86 ℃, and the main soil types are brown, cinnamon, and meadow soils. The area has a high forest coverage and wide variety of wildlife. Particularly, the study area is east of the Xidazhuangke Village, a forest vegetation. The low altitude and frequent human interference cause the vegetation composition in the area to be complex, having both planted and natural forests. The vegetation is patchily distributed, and some common trees found in the area are Juglans mandshurica, Tilia mongolica, and Populus davidiana; some shrubs include Syringa villosa, Spiraea salicifolia, and Lespedeza bicolor; and herbs include Elymus dahuricus and Saussurea japonica.

The negative air ions concentration is considerably higher in forested land than in the nonforested land because the rocks and soil in the forest produce vast amounts of radioactive materials. The tip discharge, transpiration, and dustfall of plants also increase negative air ion concentration (Jovanić and Jovanić, 2001; Shi et al., 2010a, 2010b). The fluctuation pattern of negative air ion concentration in each forest stand was as “double peaks”, and the first peak value was attained between 9:00 and 11:00. This phenomenon was probably influenced by plant photosynthesis, whereby the concentration increased as solar radiation increased in the morning, resulting in the generation of a large amount of negative air ion. The trough value was attained at about 13:00, resulting from the “siesta” phenomenon of plants at noon when solar radiation is strong. Additionally, the temperature in the forest stands was higher, and the relative humidity was lower, resulting in a decrease in negative air ion concentration. The second peak value was attained between 16:00 and 17:00 when solar radiation was weak in the afternoon. Hence, as plant photosynthesis gradually increased, the relative humidity in the stands and negative air ion concentration increased (Wang et al., 2010). These findings are consistent with those of Tammet et al. (2006), Huang et al. (2013), Zhao et al. (2018). In contrast, Liu et al. (2019) found that the negative air ion concentration in the Square Park area attained trough value between 9:00 and 10:00. This phenomenon may have occurred because of the high number of visitors at the park in the morning and solar radiation is still relatively weak at that time. However, compared with the rising trend of air negative ion concentration in the Songshan National Nature Reserve during this period, the influence of human activities on it is still obvious. The negative air ion concentrations of the Songshan National Nature Reserve forest stands in the different seasons ranged from high to low: summer > autumn > spring, which is consistent with the findings of Wu et al. (2006), Pawar et al. (2010) and Li et al. (2022). This is consistent with a high photoel, ectric effect occurring in the summer (Li et al., 2019; Shi et al., 2022) when environmental conditions are more suitable for plant growth than spring and autumn. However, other scholars have reported winter > spring > autumn > summer (Cong and Sun, 2010), which may related to the special weather conditions of the four seasons in Dalian. The air quality evaluation results obtained through the amperometric air ion evaluation index are generally consistent with the negative air ion concentration results: The forested land had better air quality, and the nonforested land had acceptable air quality range, signifying that forest stands play a crucial role in regulating atmospheric quality and exploiting the health benefits of forests. Likewise, the air quality of the three forest stands was in the order of Juglans mandshuric forest > Tilia mongolica forest > Betula dahurica forest, providing guidance for future forest resource management for the Songshan National Nature Reserve to improve ecological service value.

The selected three typical forest stands in the Songshan National Nature Reserve, Beijing, are all deciduous broad-leaved forests; Shao et al. (2005) suggested that negative air ion concentration in broadleaved forests is higher than that in coniferous forests in spring and summer, whereas the opposite is true in autumn and winter seasons. This may be because the leaf area index of broadleaved forest species is higher in spring and summer during the growing season. Similarly, several studies have been conducted to compare the negative air ion concentrations between coniferous and broadleaved forests, but the underlying negative air ion concentration differences between broadleaved forests is yet to be investigated. The unipolar coefficient and amperometric air ion evaluation index techniques are typically used to evaluate the air quality of negative air ion concentrations. However, Shi et al. (2004) and others have shown that these techniques have their limitations and are more suitable for measuring air ion concentrations in urban and residential areas than in forests. Therefore, the application of such indices for measuring forest air quality should be further verified.

The daily negative air ion concentration characteristics is “bimodal” in all forest stands at the Songshan National Nature Reserve, Beijing, except in the Betula dahurica forest in spring. Generally, the peak values for each stand primarily occur between 9:00 and 11:00 and between 16:00 and 17:00, with the troughs occurring at around 13:00. The months of July, August, and September have the highest levels of negative air ions both inside and outside the forest, with slight variations between the Juglans mandshuric forest and the Tilia mongolica and Betula dahurica forests. The magnitude of daily negative air ion concentration fluctuations varied among the seasons. Relatively sharp fluctuations were observed in summer and autumn while more stable fluctuations were observed in spring; Using the amperometric air ion evaluation index to evaluate air quality, we found both forested and nonforested land to be in the order of summer > autumn > spring, with clean air quality in summer and autumn and moderately clean air quality in spring. Similarly, we found that Juglans mandshuric forest > Tilia mongolica forest > Betula dahurica forest > nonforested land, with clean air quality in the Juglans mandshuric and Tilia mongolica forests, and moderately clean air quality in the Betula dahurica forest while the nonforested land air quality fell within an acceptable quality range.