The correlation between negative air ions (NAI) and nitrogen oxides (NOx) exhibits significant seasonal characteristics and is non-static. Previous studies have shown that NAI concentration is highly sensitive to meteorological factors, while NOx concentration is also affected by meteorological factors, resulting in potential differences in their correlation under different meteorological conditions. To deepen the understanding of this relationship, this study explored the impact of different meteorological factors on the correlation between NAI and NOx. The main conclusions are as follows: (1) The interaction between NAI and NOx in urban forests is regulated by meteorological factors; the higher the temperature, humidity, and solar radiation, the larger the correlation coefficient, and the stronger the negative correlation between the two; (2) Under synergistic meteorological conditions, NAI concentration is high and NOx concentration is moderate, which is suitable for outdoor activities: Condition 1 is temperature > 20 °C, humidity 30–60%, air pressure > 940 kPa, solar radiation 30–60 W·m⁻², wind speed < 1 m·s⁻¹; Condition 2 is temperature > 20 °C, humidity > 60%, air pressure > 940 kPa, solar radiation > 60 W·m⁻², wind speed < 1 m·s⁻¹ (based on NAI and NOx concentration data and health standards: NAI ≥ 1000 cm⁻³ is beneficial to health, and NOx ≤ 80 μg/m³ meets WHO limits); (3) Temperature, humidity, and air pressure have regulatory effects on the relationship between NAI and NOx, among which air pressure exerts positive regulation, while temperature and humidity exert negative regulation.

Negative air ions (NAIs) have the effect of improving environmental quality and human health. This study for the first time constructed an evaluation system for forest release of NAIs employing five capacity indicators: release contribution rate (L), release coefficient (n), release rate (s), instantaneous standing stock (v), and total release amount (Z). These were applied to evaluate the forest’s ability to release NAIs in the suburban urban green space of Beijing—Xishan National Forest Park. The results showed that: (1) during the growing season of the forest, the value ranges of these indicators were as follows: L: 6.04~9.71%, n: 6.63~11.05%, s: 4.53 × 10³~7.49 × 10³/cm²/min, v: 4.48 × 10⁴~7.34 × 10⁴/cm², Z: 2.70 × 10⁵~4.40 × 10⁵/cm², with the spring and autumn “noon and evening” and summer “morning and evening” forests having the strongest effect and the highest release capacity of NAIs; (2) the daily changes of L, n, s, v, and Z are generally in a “bimodal” pattern, and the overall trend of “rise and fall, rise and fall” among various indicators is consistent, showing a “linkage”; (3) weather characteristics affect release capacity in the order of sunny > rainy > cloudy, with the strongest NAI release ability from forests at 6:00 on cloudy days (0.53%, 1.7%, 877.19/cm²/min, 3.56 × 10⁴/cm², 9.67 × 10⁴/cm²) and at 18:00 on rainy days (4.58%, 4.83%, 3.16 × 10³/cm²/min, 3.16 × 10⁴/cm², 1.90 × 10⁵/cm²), with poorer NAI release ability in the afternoon on cloudy and rainy days; (4) forests can produce over 100 million levels of NAIs throughout the year, with an average daily production of over one million levels of NAIs. From 2019 to 2021, NAI production showed an increasing trend year by year, and the increase rate increased year by year to 19.6% and 56.5%.; and (5) the five indicators are significantly positively correlated with solar radiation and temperature in the range of 0–200 w/m² and 5–20 °C, respectively. This study provides a new method to reveal the ability of forests to release NAIs, providing strong evidence for creating a livable ecological environment.

Negative air ions (NAIs) can purify the atmosphere and maintain human health. In this study, we selected six tree species, Pinus tabuliformis, Pinus bungeana, Acer truncatum, Sophora japonica, Koelreuteria paniculata, Quercus variabilis, Robinia pseudoacacia, and Populus tomentosa, and we established for the first time five “capacity indicators” to characterize and judge the capacity of plants to release negative ions: they comprised the release contribution rate L, release coefficient n, release rate s, instantaneous current number v, and total level of release Z. These indicators were used to assess the ability of the plants to release NAIs by themselves. The results showed the following. (1) The daily variations in L and n show “W” and “concave” shapes, respectively, and the contribution capacity at night is significantly higher than that during the day. The diurnal variations in s, v, and Z all showed a “bimodal” pattern. The NAI release rate and release level of each tree species during the day were significantly higher than those at night. (2) The trees released the most NAIs during the day at approximately 10:00, while Robinia pseudoacacia and Populus tomentosa peaked with a 2 h lag (12:00). The NAI release capacity of each tree species was the worst at 13:00. (3) During the growing season, the self-contribution effects L and n of the plants were the strongest in May. The release rates and release levels s, v, and Z were the lowest in August. The coniferous plants released NAIs at the fastest rate in September and broad-leaved plants in July, with the highest release levels. In this study, the plants released the most NAIs from 10:00 a.m. to 11:00 a.m., which is the best time to travel. Quercus variabilis was preferentially recommended in the pairing of species of tree with the quickest NAI release and the highest total number released, followed by Koelreuteria paniculata and Sophora japonica.