Atmospheric Deposition NO2 Dry Deposition Fluxes in 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都会区凤凰城为研究对象，以1998年6月9日至13日为模拟时段，借助多尺度公共空气质量模型（Community Multiscale Air Quality Model, CMAQ），探究了城市植被对氮氧化物（NOₓ）源性干沉降通量的调控机制。研究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，以表征城市土地覆盖的空间分布范围与异质性特征；同时对沉降速度计算模块进行了调整，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，城区内25%的NOₓ源性干沉降通量沉降于植被表面。当模拟中移除城市植被要素后，由于不透水面的沉降速度显著低于植被覆盖表面，二氧化氮（NO₂）的沉降量降低了57%；而硝酸沉降量则相对保持稳定。本研究还借助基于城市空气质量监测站点输入数据构建的诊断模型，对1998年全年的每小时一氧化氮（NO）与二氧化氮（NO₂）干态氮沉降通量进行了模拟，结果显示其通量水平约为6 kg·ha⁻¹·yr⁻¹。夏季时段干沉降通量呈下降趋势，这主要源于污染物浓度降低，以及午后时段温度升高诱导植物气孔关闭，进而导致沉降效率降低。