Modeled dry deposition flux of nitrogen dioxide (NO2) 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地区凤凰城都会区为研究对象，采用多尺度公共空气质量模型（Community Multiscale Air Quality Model, CMAQ），于1998年6月9日至13日开展模拟实验，探究城市植被对氮氧化物（NOₓ）源性干沉降通量的调控作用。研究中引入全新的土地覆盖分类方案与更新后的土地覆盖数据，以表征城市下垫面的空间分布范围与异质性。研究人员对沉降速度的计算方法进行了调整，以适配本地植物对亚利桑那州中部环境条件的适应性。模拟结果显示，城市区域内25%的氮氧化物源性干沉降通量沉积于植被表面。当模拟实验中剔除城市植被要素后，由于不透水下垫面的沉降速度显著低于植被覆盖表面，二氧化氮（NO₂）的沉降量减少了57%；而硝酸沉降量则相对保持不变。本研究还借助基于城市空气质量监测站点输入数据的诊断模型，对1998全年的一氧化氮（NO）与二氧化氮（NO₂）逐小时干态氮沉降通量进行模拟，结果约为6 kg ha⁻¹ yr⁻¹。夏季时段干沉降通量有所下降，这主要归因于污染物浓度降低，以及午后时段植物气孔因温度升高而关闭。