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全球范围内加速的城市化导致空气质量严重退化。随着北京市建设范围不断扩大和机动汽车数量迅猛增长,空气污染日益严重。浓度不断增加的近地层臭氧作为影响全球气候变化的重要因素和危害人类健康、动植物生长的二次污染物,受到广泛关注。城市树木能够有效地去除大气污染物,进而提高空气质量。目前已有很多研究关于区域尺度上城市树木吸收臭氧,然而,冠层尺度上城市树木吸收臭氧特征少有研究。因此,基于树干液流技术,结合天气变化和大气臭氧浓度分析,研究夏秋季节北京市典型绿化树种刺槐(Robinia pseudoacacia)整树冠层吸收臭氧特征及环境影响因素。结果表明,在日尺度上,刺槐吸收臭氧速率变化呈单峰曲线,于15:00左右达到峰值;夏季峰值范围较宽,秋季峰值范围较窄;中午前后累积吸收臭氧量增加最明显。在季节尺度上,夏季刺槐吸收臭氧速率高于秋季;夏季累积吸收臭氧量显著增加,秋季略有增加。刺槐吸收臭氧的时间变化规律取决于大气臭氧浓度和冠层对臭氧的导度。臭氧浓度日变化和季节变化明显,导致刺槐吸收臭氧速率时间变化格局与之接近。在一定的臭氧浓度下,刺槐吸收臭氧速率的变化主要由冠层对臭氧的导度调控,进而受水汽压亏缺和总辐射的影响。随着水汽压亏缺降低,刺槐冠层对臭氧的导度明显下降;总辐射大于600 W/m2,冠层对臭氧的导度迅速下降。研究树种刺槐单位冠层投影面积上年吸收臭氧量约为0.16 g/m2,明显低于基于模型得到的结果,表明评估森林受臭氧危害的风险应考虑树种冠层臭氧通量。
Accelerated urbanization on a global scale has led to serious deterioration of air quality. With the continuous expansion of construction in Beijing and the rapid growth of motor vehicles, air pollution is worsening. The increasing concentration of near-surface ozone has drawn extensive attention as an important factor affecting global climate change and as a secondary pollutant that endangers human health, animal and plant growth. Urban trees can effectively remove atmospheric pollutants, thereby improving air quality. Much research has been done on the absorption of ozone by urban trees on a regional scale, however, few studies have examined the ozone absorption characteristics of urban trees on the canopy scale. Therefore, based on the sap flow technology, combined with the changes of weather and atmospheric ozone concentrations, the canopy ozone absorption characteristics of the canopy of Robinia pseudoacacia, a typical afforestation species in Beijing during summer and autumn, and the environmental factors were studied. The results showed that on the daily scale, the rate of ozone uptake by Robinia pseudoacacia showed a single peak curve and peaked at about 15:00. The summer peak range was wide and the autumn peak range was narrow. The accumulated ozone uptake increased most obviously at noon. On the seasonal scale, the rate of ozone uptake by summer locusts is higher than that in autumn; the cumulative ozone uptake increased significantly in summer and slightly increased in autumn. Robinia pseudoacacia ozone absorption time variation depends on the atmospheric ozone concentration and canopy on the ozone conductivity. The diurnal variation of ozone concentration and the seasonal variation were obvious, which led to the similar temporal pattern of ozone uptake by R. pseudoacacia. Under a certain ozone concentration, the change of ozone absorption rate of Robinia pseudoacacia was mainly controlled by the canopy conductance of ozone, which was further affected by vapor pressure deficit and total radiation. With the reduction of vapor pressure deficit, the Robinia pseudoacacia canopy significantly reduced the ozone’s conductance; when the total radiation was greater than 600 W / m2, the canopy’s ozone conductivity decreased rapidly. The canopy area of canopy acacia trees in study tree was about 0.16 g / m2, which was significantly lower than the model-based results. It is suggested that the canopy ozone flux should be taken into account when assessing the forest risk of ozone damage.