Ozone Formation and Dry Deposition in Urban Environments: Insights from WRF-Chem Modeling and Eddy Covariance Flux Measurements in Beijing

Tropospheric ozone (O₃) is a major air pollutant that negatively affects human health and vegetation, while also playing a central role in atmospheric chemistry and climate change. Dry deposition, a process by which gases are deposited on a surface by air turbulence and gravity, accounts for about 20–25% of tropospheric O₃ removal. However, the mechanisms controlling the O₃ dry-deposition velocity (Vd,O₃) in urban areas are poorly understood, largely due to the scarcity of measurements in such environments.

We hypothesized that: (i) Combining direct O₃ flux measurements with source apportionment of factors controlling ozone levels (e.g., NO and volatile organic compounds [VOCs]) based on comprehensive field measurements in an urban environment is essential for disentangling the simultaneous effects of emission sources and meteorological conditions on Vd,O₃. (ii) Integrating atmospheric chemistry model simulations with flux measurements can elucidate how emissions and environmental conditions influence O₃ formation and removal, providing critical insights for air-quality assessment and urban planning.

Accordingly, we conducted direct eddy covariance measurements of O₃, VOCs (via Vocus PTR-TOF-MS), and NOx ([NO] + [NO₂]) fluxes at a height of 102 m on a meteorological tower in Beijing between April 28 and June 26, 2023. Vertical profiles of meteorological parameters were measured at 15 levels along the tower. Source apportionment analysis of VOCs and NOx was conducted using the positive matrix factorization (PMF) model. Additionally, the Weather Research and Forecasting model with Chemistry (WRF-Chem) was applied to evaluate the contributions of anthropogenic and biogenic emissions to O₃ formation. WRF-Chem model simulations were validated against data from nearby air-quality monitoring stations.

Our results show that Vd,O₃ in the urban environment was primarily controlled by chemical reactions, including O₃ titration by NO and contributions from anthropogenic VOCs. Surface wetness was identified as a key factor influencing Vd,O₃, consistent with our findings from measurements in vegetated environments [1,2]. Comparing urban and vegetated settings highlighted the influence of air turbulence, relative humidity, and chemical interactions on Vd,O₃. These findings provide valuable insights for improving urban O₃ simulations and for air-quality management and urban planning strategies.

 

1. Li, Qian, et al. "Measurement-based investigation of ozone deposition to vegetation under the effects of coastal and photochemical air pollution in the Eastern Mediterranean." Science of the Total Environment 645 (2018): 1579-1597.‏

2. Li, Qian, et al. "Investigation of ozone deposition to vegetation under warm and dry conditions near the Eastern Mediterranean coast." Science of the Total Environment 658 (2019): 1316-1333.‏