Spatial And Temporal Variation Characteristics And Influencing Factors Of Air Negative Ion Concentration In Urban Forest Of Beijing

Air negative ion(NAI)has the functions of purifying air,disinfecting and sterilizing,and is an important indicator to measure the quality of air.Forests have the ecological effect of releasing NAI and are the main continuous natural source of NAI.The urban forest is closer to the city and is an important vegetation component of the urban ecosystem.It can directly provide greater ecological value to the city and plays a key role in improving the comfort of the urban environment.As the capital of China,Beijing is seriously polluted,has great environmental pressures,and lacks recreational space for residents.It needs a higher quality urban green space ecosystem to serve the population.Therefore,there is an increasing demand for increasing the air negative ion concentration(NAIC)of urban forests.Studying the changes of NAI and the influencing factors in urban forests are of great significance for ecological assessment and ecological environment optimization.In this paper,the NAI automatic monitoring instrument is used to monitor the urban forest NAIC in different regions of Beijing at different times to obtain the NAIC variation characteristics in urban forests.Combined with the data of meteorological stations and air quality monitoring stations,the air pollutants,meteorological factors and urban forest release are explored.The relationship between NAI reveals the influencing factors of NAI release from urban forests,and verifies the purification function of NAI in urban forests.The hand-held NAI measuring instrument is used to measure the NAIC of common greening tree stands in Beijing,combined with the characteristics of different tree species.Analyze the difference of NAI release from different greening tree stands,and explore the NAI potential of urban forest ecosystems to provide a theoretical basis for tree species selection and allocation.The release of NAI from urban forests can provide a reference for improving urban environmental comfort and effectively promoting the rational use of NAI resources.The results show that(1)the annual average NAIC difference of urban forests in different polluted areas in Beijing is not significant,and the concentration value is 2120-2248 ions/cm3,indicating that urban forests have certain ecological carrying capacity.The seasonal variation of NAIC in urban forests is the highest in summer,with the highest reaching 2831 ions/cm3,the lowest in winter,and the lowest at 1994 ions/cm3.The NAIC in spring and autumn is between winter and summer,and the difference is not much.The average annual NAIC value in the forest margin is 7.20% higher than that in the forest.The daily variation of NAIC in summer forest edge is higher than that in forest area.The NAIC in the winter forest is slightly higher than that in the forest edge area,and the difference in concentration inside and outside the forest mainly exists at night.(2)The correlation between NAIC and meteorological factors in sunny days is different under different seasonal conditions.In winter,NAIC is positively correlated with temperature and opposite in summer.NAIC is inversely proportional to humidity in autumn and winter,and winter humidity has the most obvious negative effect on NAIC(r=-0.503).The amount of solar radiation is positively correlated with NAIC,and the amount of solar radiation in the summer is negatively correlated with NAIC.Air pressure is positively correlated with NAIC.The urban forest NAIC shows a certain difference with the weather,and its correlation with meteorological factors is different from that of sunny days.The average daily NAIC value of rainy days is 2134/cm3,which is 4.56% higher than the sunny NAIC,which is mainly affected by temperature and rainfall.Compared with the sunny day in winter,the foggy day is about 2075 ions/cm3 in the daytime and the NAIC is only 1948 ions/cm3 in the haze,which is mainly affected by the amount of solar radiation.The peak of NAIC in windy weather is 5.37% higher than that of breeze,and the daily average is greater than the breeze weather,mainly affected by air pressure.(3)The urban forest NAIC has certain correlation with the concentrations of atmospheric pollutants NO2,O3 and PM2.5,and the impact mechanism is different according to the difference of pollutant concentration levels.NAIC was significantly negatively correlated with NO2,PM2.5 and O3,and the correlation coefficients were-0.412,-0.366,0.207,respectively,and NO2 was the main influencing factor of NAIC change.NAIC is highly sensitive to PM2.5 and is sensitive to feedback on changes in PM2.5 concentration.When the concentration of NO2 is lower than 40?g/m3,NAI has a certain bearing capacity for NO2,and the NAIC range is about 2000-2800 ions/cm3.When O3 concentration is lower than 50 ?g/m3,NAIC is positively correlated with O3,r is 0.315.When O3 concentration is higher than 50 ?g/m3,NAIC is negatively correlated with O3,r=-0.304.(4)The average daily NAIC of different tree species was significantly higher than that of the open-ground control group,and the NAI effect of the turf was also significantly lower.The NAIC difference of different tree species was more obvious: the largest of the spruce,860 ions/cm3;the peony was the smallest,only 480 ions/cm3;sea otter is the highest NAIC among broadleaf species.Different forest species characteristics of different tree species will lead to different NAI effects.The distance between the measuring point and the plant canopy,the height of the canopy canopy,and the difference between the needle and broadleaf of the leaf will have an effect on the NAI effect.