北方地区典型天气对城市森林内大气颗粒物的影响

以北京西山几种游憩型城市森林为例,在一年四季选择典型天气条件,对4种粒径的大气颗粒物浓度进行全天24h监测,研究北方地区不同季节典型天气因素白昼不同时段对不同结构的城市森林内不同粗细粒径的大气颗粒物浓度变化的影响.结果发现:降雨使大气颗粒物浓度减少,尤其粗颗粒物(TSP只为连续晴天的0.58~0.68),所以雨后晴天粒径较小的颗粒物所占比例会增加.不过,有时在雨后的夜间由于空气湿度大大气颗粒物浓度也会增加.雪能够降低大气颗粒物浓度,“雪后晴天”4种粒径颗粒物浓度均只有连续晴天的0.2倍.多云和雾霾天气使大气颗粒物污染加重,尤其在夜间;雾霾和多云对小粒径颗粒物浓度的增加效果明显.夏季高温高湿静风、闷热的“桑拿天”能使郁闭度较大的林地内大气颗粒物特别是细颗粒物浓度及其所占的比例显著增加,PM2.5的浓度是“连续晴天”的2.53倍.风在雨后能使大气颗粒物在一定程度上扩散减少,而在天气干燥时刮风会增加城市森林内大气颗粒物的浓度,且多云会加重干燥天气刮风后大气颗粒物的污染程度.春、冬季林地裸露的落叶阔叶树在刮风时大气颗粒物浓度较四季常绿、地表覆盖物多的针叶林高,夏、秋季桑拿天和雾霾天郁闭度大的侧柏林大气颗粒物浓度较林地结构开阔的黄栌林高.

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 “sauna” 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.