Rapid photochemical production of ozone at high concentrations in a rural site during winter

Ozone is an air pollutant that can cause severe respiratory health effects. Photochemical ozone production near the Earth's surface is considered a summertime, urban phenomenon(1-3), where hourly average ozone concentrations can exceed 150 p.p.b., compared with background values of about 50 p.p.b., and wintertime ozone concentrations in the US are usually in the range of 35-50 p.p.b. (refs 1-3). Here we report rapid, diurnal photochemical production of ozone during air temperatures as low as 17 degrees C, in the rural Upper Green River Basin, Wyoming, in the vicinity of the Jonah-Pinedale Anticline natural gas field. We find that hourly average ozone concentrations rise from 10-30 p.p.b. at night to more than 140 p.p.b. shortly after solar noon, under the influence of a stagnant, high-pressure system that promotes cold temperatures, low wind speeds and limited cloudiness. Under these conditions, an intense, shallow temperature inversion develops in the lowest 100 m of the atmosphere, which traps high concentrations of ozone precursors at night. During daytime, photolytic ozone production then leads to the observed high concentrations. We suggest that similar ozone production during wintertime is probably occurring around the world under comparable industrial and meteorological conditions.