Comparative Study On Temporal And Spatial Variation Of Air Particulate Matter Concentration In Urban Green And Gray Space Based On Site Scale

Air pollution is one of the serious problems facing the urban environment in China,and the air pollution control at site scale is under enormous pressure.Some studies suggest that urban green space plays an important role in improving air quality,but some studies have questioned that green space vegetation exacerbates air pollution.In view of the inconsistency of conclusions,this study carried out a comparative analysis of particle concentration in green space and gray space.In this study,the"Danfeng garden"and the adjacent"Convention and Exhibition Center"square in Xian,China,were selected as the research sample plots.Three monitoring sample belts perpendicular to the road are set in the green space and square respectively.Each sample belt is 70m long.Sampling points are set at 5m intervals,with a total of 90 sampling points.Field sampling was conducted in two periods of plant dormancy and vigorous growth state（March and June 2021）,The concentration of particulate matter and meteorological factors at different horizontal distances from the road were monitored continuously for 24 hours for a week.The diurnal variation and influencing factors of particulate matter concentration in green space and gray space were compared.The results show that:（1）the 24-hour variation of particle concentration in the research sample plots is as follows.In spring,the particle concentration increased from 5:00 to 7:00,decreased from7:00 to 10:00.From 10:00 to 16:00,the particle concentration is low,and in the green space it is higher than the square,but increased at 16:00 gradually,and decreased gradually from24:00 to 5:00 the next day.In summer,the particle concentration in green space is generally lower than that in square,the concentration of PM_2.5、PM₁decreased gradually from 5:00 to22:00,increased from 22:00 gradually;The concentration of TSP and PM₁₀decreased from7:00 to 17:00 continuously,and which increased at 17:00 gradually.（2）The diurnal variation of particle concentration is affected by the periodic variation of meteorological factors.The threshold of humidity affecting particle concentration is 75%.When humidity exceeds the threshold,the particle concentration decreases.The dominant meteorological factor affecting the concentration of particulate matter during the day is wind,and at night is humidity.（3）In different seasons,the concentration of particulate matter in typical plant communities showed differences:in spring,semi-open green space>square>closed green space.However,in summer,square>semi-open green>closed green.（4）The spatial distribution characteristics of particulate matter at different horizontal distances show that:the concentrations of TSP and PM₁₀in the closed green space with the increase of distance during the active traffic emission period,showing a significant downward trend,while PM_2.5and PM₁form a certain accumulation in the sample plot,but the concentration in the closed green space is the lowest,indicating that green vegetation can effectively block the diffusion of exogenous particulate matter into the green space.However,in the period of less traffic emissions,there was no significant difference in the concentration of particulate matter with distance in the sample plot.（5）Positive and negative effects of green space on particulate matter reduction:the reduction effect of closed green space in spring was positive,and the reduction effect of semi-open green space was negative.The ability of two typical communities to reduce particulate matter in summer was positive.According to the above results,in practice,we can actively explore how to increase the humidity in the green space and improve the air quality during the period of strong traffic emission by establishing intelligent humidification facilities.Dense forest protection with high canopy density shall be established on the side of the park green space close to the road to block the diffusion of particles into the green space.Establish a sparse forest space or open lawn inside the green space,to build a good ventilation environment,promote the diffusion of internal particles,and avoid the establishment of leisure trails and other activity places near the road.The research results can provide theoretical basis and reference for the planning and design of urban green space guided by air purification at the site scale in the future.