Ozone dry deposition and chemical removal in the urban environment of Beijing: I. Chemical and meteorological controlling parameters.

Tropospheric ozone (O) is a major air pollutant that negatively affects human health and vegetation, and plays a central role in climate change and atmospheric chemistry. Current simulations of tropospheric O concentrations in climate and air-quality models are significantly limited by the inaccurate representation of O dry deposition rate-particularly in urban areas, where field measurements remain scarce. We hypothesize that O dry deposition in the urban environment is controlled by factors similar to those over vegetation, albeit via potentially different mechanisms. Accordingly, we performed O and reactive nitrogen oxide (NO = [NO] + [NO]) flux measurements using eddy covariance (EC) technique from a meteorological tower in an urban area of Beijing (IAP) during the spring and summer, complemented by EC flux measurements of volatile organic compounds (VOCs) using Vocus-PTR-TOF-MS. Our analyses indicated that the downward ozone velocity (Vdw,O) at the IAP measurement site is controlled by both gas-phase chemical reactions and surface uptake, with comparable contributions. The reaction of O with NO emissions dominated the chemical contribution to Vdw,O, with a significant contribution of NO near the ground, particularly in the morning. The effect of relative humidity (RH) on Vdw,O, likely via surface wetness accumulation similar to its effect on non-stomatal O deposition over vegetation, showed a logarithmic and hyperbolic dependency of Vdw,O on RH, for 5 % < RH < 30 % and RH > 70 %, respectively. Both NO emissions from elevated sources and RH < 30 %-conditions associated with a higher frequency of positive O flux events-dramatically limited Vdw,O.