Modeling of the impacts of China's anthropogenic pollutants on the surface ozone summer maximum on the northern Tibetan Plateau

The current debate about the role of anthropogenic pollutants in a unique broad summer maximum pattern at a remote site (Waliguan, 3810 asl) on the northern edge of the Tibetan Plateau is continuing. This study quantifies the impacts of anthropogenic emissions and stratosphere-totroposphere exchange (STE) on surface ozone on the Tibetan Plateau using a regional chemical transport model with an on-line tracer tagging module. The model is shown to be well suited for simulating the unique seasonal variation at Waliguan. Model results clearly show that the regional transport of photochemically produced ozone in China is responsible for die summer maximum on the northern Tibetan Plateau with a contribution of ∼ 10-25 parts per billion by volume (ppbv) in summer, which is more than ∼5 ppbv of the contribution to STE. The weak local ozone photochemical production (-0.5-0.1 ppbv/day) suggests that this transport to the Tibetan Plateau mostly concentrates on photochemical products. The unique strong near-surface convergence and upper layer divergence driven by elevated surface heating and low air density over the Tibetan Plateau in summer is its dynamic cause. These transported anthropogenic pollutants are likely to rise to the upper troposphere and circulate globally. This study also demonstrates that the effect of convection on surface ozone over the northern Tibetan Plateau is compensated by the advective updrafts in summer.