Influence of typical weather conditions on the airborne particulate matters in urban forests in northern China

It is important to understand the effects of weather conditions on the flux and transportation of airborne particulate matters for assessing the role of urban forests may play in controlling those pollutants in the air.The daily concentration variation of airborne particulate matters with four diameters were measured under typical seasonal weather conditions throughout the year in urban forests with different structures located in the West Mountain of Beijing,China.Our objectives were to explore the influence of typical weather conditions on the concentration changes of airborne particulate matters in urban forests with different structures in four different seasons of the year.The results showed that: 1) the rainfall reduced the concentrations of coarser particulate matters as the total suspended particulate(TSP) concentration was only 0.58~0.68 of that measured on the days with clear sky conditions.The ratios of finer particulate matters in TSP increased in the clear daytime after rains.On the contrary,the concentrations of airborne particulate matters increased in the nighttime after rain due to the increased air humidity;2) snowfall decreased the concentrations of airborne particulate matters.After snow the concentrations of airborne particulate matters of four diameters were only 0.2 of that measured over the days under clear sky conditions;3) cloud and fog aggravated the pollution of airborne particulate matter,especially at night.Moreover,increased amplitudes of finer particulate matters were larger in foggy and cloudy days;4) the concentrations of finer airborne particulate matters and their ratios in TSP were increased under high humidity,stable,and muggy weather conditions in summer;5) wind reduced the concentrations of airborne particulate matters after rain to some degree.In contrast,the concentrations of airborne particulate matters was increased by wind under dry air conditions,specifically in cloudy days after wind;6) the concentrations of airborne particulate matters were higher in exposed deciduous forests in windy spring and winter,while they were higher in forest with higher canopy density in foggy and sauna days in summer and autumn.