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 NOx-limited, varies with season in the Northern Hemisphere. 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 Framework for 0-D Atmospheric Modeling (F0AM) box modeling to better understand these trends. The models indicate that, in late December, the Basin's ozone system is VOC-sensitive and either NOx-insensitive or NOx-saturated. Sensitivity to NOx grows throughout the winter, and, in early March, the system is about equally sensitive to VOC and NOx. The main driver for this trend is the increase in available solar energy as indicated by the noontime solar zenith angle (SZA). A secondary driver is a decrease in precursor concentrations throughout the winter, which decrease because of, firstly, a dilution effect as thermal inversions weaken and, secondly, an emission effect because certain emission sources are stronger at colder temperatures. On the other hand, temperature and absolute humidity (AH) are not important direct drivers of the trend.