A New System for Evaluating the Ability to Release Negative Ions—Taking Urban Forests as an Example

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.