基于树干液流技术的北京市刺槐冠层吸收臭氧特征研究

Ozone uptake at the canopy level in Robinia pseudoacacia in Beijing based on sap flow measurements

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。

The accelerating global urbanization caused severe air pollutions. In Beijing, the air pollution has been exacerbated due to the wide-spread construction activities and increasing number of vehicles. The increasing concentration of ozone （ O3 ） in the troposphere has been recognized as a source of air pollution, due to its adverse effects on human health and plant and animal growth, and its contribution to global climate change. O3 is known to impact forest trees in many ways including morphological and histological injuries, decreasing photosynthesis, increasing respiration, and alteration of carbonallocation and water balance. Considerable researches have been conducted to investigate ozone uptake by urban forests at the regional scale. On the other hand, trees are able to improve urban air quality by taking up and removing gaseous pollutants, and the O3 uptake by urban forests at the regional scale has been extensively studied. However, research on O3 uptake by urban trees at the canopy scale is rare. The main objectives of this study are ： （ 1 ） to quantify the whole-tree O3 uptake by Robinia pseudoacacia, one of urban greening tree species in Beijing ; and （2） to examine how O3 flux in R. pseudoacacia trees is regulated by the stomata and environmental conditions. In this study, the whole-tree O3 uptake in R. pseudoacacia trees during spring and summer was estimated based on sap flow measurements and the data of micro-climate and ambient O3 concentration were also collected. The diurnal ozone uptake rate （Fo3） by R. pseudoacacia showed a single peak pattern with the maximum rate occurring at around 15：00 pm. The diurnal Fo, showed a narrow peak during summer and a wide peak during autumn. The most obvious increase in accumulated stomatal ozone flux（AFst） occurred around noon time. Fog showed a seasonal pattern with higher values found in summer than in autumn. The increase in AFst was most obvious in summer than in autumn. The diurnal and seasonal patterns of O3 uptake were related to the temporal variations of ambient air O3 concentrations and canopy conductance （Go3）. Ambient air O3 concentration showed a similar diurnal and seasonal pattern to Fo3 Under a given ambient air O3 concentrations, the whole-tree Fo3 was dependent on canopy conductance, and hereby was further influenced by the vapour pressure deficit （D） and total radiation （Rs）. Go3 decreased exponentially with increasing D. High D caused low Go3, and thus low Fo3 in spite of relatively high ambient air O2 concentrations. On the contrary, Go3 was high under low D conditions, and thus Fo3 was high in spite of relatively low ambient air O3 concentrations. However, Fo3was relatively low under very low D conditions, such as in early mornings, which may be attributed to the weak photosynthesis and small stomatal apertures in the early morning at this time. Moreover, Go3 decreased rapidly with increasing Rs when R was higher than 600 W/m2. Similarly, ambient air O3 concentration decreased with increasing R when Rs was higher than 800 W/m2. Therefore, Fo3 exhibited an asymmetric single-peak pattern： Fo3 slightly increased with increasing Rs when R was below 800 W/m2, however, it decreased rapidly with increasing Rs when Rs was higher than 800 W/m2. The annual O3 uptake by R. pseudoacacia trees estimated in our study was 0.16 g/m2, which was much lower than the values estimated from the Urban Forest Effects Model. This difference suggests the necessity to consider the O3 uptake flux on canopy level when evaluate the O3 risks on urban trees.