Modeled dry deposition flux of nitrogen (NO) in central Arizona, USA (1998)

The role of urban vegetation on NOx-derived dry deposition fluxes was investigated for the arid Phoenix (Arizona, USA) metropolitan area using the Community Multiscale Air Quality Model (CMAQ) (9-13 June 1998). A new land cover classification and updated land cover data were introduced in the model to account for spatial extent and heterogeneity of urban land cover. Adjustments were made in the deposition velocity calculations to consider the adaptation of local plants to the environmental conditions of Central Arizona. According to the simulations 25 % of the NOx derived dry deposition fluxes in the urban area were deposited on vegetation. When urban vegetation was excluded from the simulations NO2 deposition was reduced by 57 % because of the significantly lower deposition velocities of impervious compared to vegetated surfaces; nitric acid deposition was relatively unchanged. Using a diagnostic model with input data from urban air quality monitoring sites, hourly NO and NO2 dry N deposition fluxes were simulated for the entire year 1998 to ~6 kg ha-1 yr-1. Dry deposition declined during the summer months, due to lower pollutant concentrations and temperature-induced closure of the plant stomata during afternoon hours.

本研究以美国亚利桑那州干旱型凤凰城都会区为研究区域，采用社区多尺度空气质量模型（Community Multiscale Air Quality Model, CMAQ），于1998年6月9日至13日开展了城市植被对氮氧化物（NOₓ）衍生干沉降通量调控作用的探究。研究中针对城市下垫面的空间范围与异质性特征，在模型中引入了全新的土地覆盖分类方案与更新后的土地覆盖数据集。为适配亚利桑那州中部的环境条件，结合本地植物的环境适应特性，对沉降速度的计算方案进行了针对性调整。模拟结果显示，该都会区内25%的氮氧化物衍生干沉降通量沉降于植被表面。若在模拟场景中移除城市植被，则二氧化氮（NO₂）的沉降量将降低57%，这是由于不透水下垫面的沉降速度显著低于植被覆盖表面；而硝酸沉降量则相对保持不变。本研究还采用基于城市空气质量监测站点输入数据的诊断模型，对1998年全年的小时尺度一氧化氮（NO）与二氧化氮（NO₂）干态氮沉降通量进行了模拟，结果约为6 kg·ha⁻¹·yr⁻¹。夏季时段干沉降通量呈下降趋势，这归因于污染物浓度降低，以及午后时段温度诱导的植物气孔闭合效应。