Ozone Budget in the Lower and Middle Troposphere over North China

Photochemical reaction, transportation and deposition are the main processes that effect ozone concentrations in troposphere.Quantitative estimation of troposphere ozone budget and the effect of intercontinental transport are very important in order to reveal ozone formation mechanism and the impacts of transport on troposphere ozone over North China. MOZART-2, a global chemical transport model (model of ozone and related tracers, Version 2) is used to assess physical and chemical processes that influence the budget of lower and middle troposphere ozone in North China.Ozone sonde data obtained by TAPTO (The Transport of Air Pollution and Troposphere Ozone over China) field campaign are compared with model results for reference. The comparison shows that MOZART-2 represents the vertical distributions of ozone in the lower and middle troposphere over North China very well, while for the upper troposphere, ozone concentrations are overestimated by the model. Budget analysis indicates that in the lower troposphere over North China, photochemical production (41.5 Tg) contributes about 58.3 % of the total ozone sources, and oxidation of NO by HO₂ is the largest contributor especially. The largest consumption process in lower troposphere is dry deposition, accounting for about 43.2 % of the total ozone sinks. Ozone chemical budget varies with seasons notably. It reaches its maximum in summer due to strong photochemical reactions, while the minimum chemical budget occurs in winter because of low temperature and weak reactions.The chemical production of ozone is more than the chemical loss in the lower troposphere in the whole year. But for middle troposphere, chemical loss exceeds chemical production all year except in summer. In summer, ozone precursors in the boundary layer can be transported to upper level of troposphere due to the active convection, and the photochemical reactions of more ozone precursors lead to more ozone production. Net horizontal transport plays an important role in ozone budget in the low and middle troposphere over North China. About 41.6 % ozone in lower troposphere comes from net horizontal transport. As the height increases, wind speed rises, and the influence of net horizontal transport enhances. Nearly 81.5 % ozone in the middle troposphere comes from net horizontal transport. Constrained by simulating ability for sub-grid processes, the model tends to overestimate ozone concentration in upper troposphere and near the tropopause over middle to high northern latitudes. Modeling for sub-grid processes such as stratosphere-troposphere exchange (STE) should be improved to comprehend mechanism of ozone in the whole troposphere in depth.