Seasonal trends in the wintertime photochemical regime of the Uinta Basin, Utah, USA

Abstract. Several lines of evidence indicate that the photochemical regime, i.e., the degree to which ozone production is either VOC- or NO_x-limited, varies with season in the Northern Hemisphere. VOC-sensitivity seems to be more likely in winter and NO_x-sensitivity in summer. For most regions, the question is patently academic, since excessive ozone occurs only in summer. However, the Uinta Basin in Utah, USA exhibits ozone in excess of regulatory standards in both winter and summer. We have performed extensive F0AM box modelling to better understand these trends. The models indicate that in late December the Basin’s ozone system is VOC-sensitive, and either NO_x-insensitive or NO_x-saturated. Sensitivity to NO_x grows throughout the winter, and in early March, the system is about equally sensitive to VOC and NO_x. The main driver for this trend is the increase in available solar energy as indicated by the noontime solar zenith angle. A secondary driver is a decrease in precursor concentrations throughout the winter, which decrease because of, first, a dilution effect as thermal inversions weaken, and second, an emission effect because certain emission sources are stronger at colder temperatures. On the other hand, temperature and absolute humidity are not important direct drivers of the trend.