Photolysis of surface O₃ and production potential of OH radicals in the atmosphere over the Tibetan Plateau

Abstract

With an area of 2.5 million km 2 and an average altitude of about 4000 m above sea level, the Tibetan Plateau is the largest and highest plateau in the mid‐latitudes of the Northern Hemisphere. The high altitude, low latitude, and large snow coverage create strong surface solar radiation on the plateau. It is therefore likely that the atmosphere over the plateau is very photochemically active, with a high production potential for OH radicals by ozone photolysis. We used the Tropospheric Ultraviolet‐Visible Radiation (TUV) model to calculate the photolysis rate of ozone in the Mt. Qomolangma (Everest) region under clear sky conditions. Field measurement data were used to calculate the production rate and concentration of OH radicals in the atmosphere. In the summer noon, over non‐snow‐covered area, surface ozone photolysis rates J (O 1 D) were 4–5 × 10 −5 s −1 , the OH radical production rate range was 1–4 × 10 6 s −1 , and the OH concentrations range was 10 6 to 10 7 cm −3 . Over areas covered with ice and snow, the J (O 1 D) and OH production rates could be as high as 1.3 × 10 −4 s −1 and 5.5 × 10 6 cm −3 s −1 , respectively, because of strong snow albedo. We also calculated the spatial distributions of J (O 1 D) and OH production over the whole Tibetan Plateau. Because of the large area covered with snow, the O 3 photolysis rates in winter could be equal to or even higher than the rates in summer and might create as photochemically active an atmosphere as in summer.