Quantifying negative air ion release from urban tree seedlings in Beijing using a stripping curve approach

Plants release negative air ions (NAI) through tip discharge and photosynthesis, thereby purifying the air. Previous studies have predominantly occurred in natural environments where isolating the plants’ specific contributions proves challenging. This study simulated a natural environment using an open-top chamber (OTC) to continuously monitor the negative air ion concentration (NAIC) released by eight common urban tree species. An experimental group with plants (EXP) and a control group without plants (CTRL) were established, leading to the first construction of the stripping curve to analyze the role of self-releasing NAI (SR-NAI) plants. The results showed that: (1) NAIC released by coniferous species was higher than that of broadleaf species, with the following order: Pinus tabuliformis (535.71 ± 90.26 ion·cm -3 ) > Pinus bungeana > Acer truncatum > Sophora japonica > Quercus variabilis > Koelreuteria paniculata > Robinia pseudoacacia > Populus tomentosa (261.92 ± 48.77 ion·cm −3 ). The self-release of NAI (SR-NAI) was 1.83–3.12 times that produced by atmospheric ionization (CTRL), reaching 74.88–90.24% of the experimental group with plants (EXP). (2) The daily change of NAIC followed the order EXP > SR-NAI > CTRL. The NAIC curve of the CTRL group exhibited a “single-peak” pattern, with the strongest atmospheric ionization occurring at 12:00 h (317.02 ± 34.81 ion·cm −3 ). At the same time, the EXP displayed a “double-peak” pattern, with the strongest release of NAIC by plants occurring at 10:00 h and 14:00 h (626.50 ~ 1892.33 ion·cm −3 ). The NAIC curve of the SR-NAI group exhibited a “double-peak” pattern, with the strongest atmospheric ionization occurring at 12:00 h (317.02 ± 34.81 ion·cm −3 ). Notably, Robinia pseudoacacia and Populus tomentosa peaks lagged by 2 h, with the highest release at 12:00 h, while the lowest NAIC release occurred at 13:00 h (376.00–493.67 ion·cm −3 ). (3) The self-release of NAIC by each seedling species varies, with the highest release in July (334.77–625.19 ion·cm -3 ), followed by August, June, September, May, October (228.32–484.80 ion·cm -3 ). The self-release of NAIC during the growing seasons was about 1.95–2.78 times that of the CTRL group. (4) The NAIC released by each seedling species was highest in summer (309.69–610.68 ion·cm −3 ) and lowest in spring (228.77–478.10 ion·cm −3 ). Most seedling species exhibited their strongest NAIC release at 10:00 h, making this the most suitable time for travel. Acer truncatum is recommended first and Sophora japonica second in urban landscape planning, recommended for urban landscape planning, with mature-tree validation required species selection and configuration, in order to improve the concentration of NAIC in the air, give full play to its role and improve the quality of life of residents. Coniferous species, particularly Pinus tabuliformis , are most effective for enhancing NAIC and should be prioritized in urban greening. Among broadleaf species, Acer truncatum is recommended first and Sophora japonica second, with the caveat that these findings based on seedlings require validation in mature trees before direct application.