Summer ozone in the northern Front Range metropolitan area: weekend-weekday effects, temperature dependences, and the impact of drought

Contrary to most regions in the US, ozone in the northern Front Range metropolitan area (NFRMA) of Colorado was either stagnant or increasing between 2000 and 2015, despite substantial reductions in NOx emissions. We used available long-term ozone and NOx data in the NFRMA to investigate these trends. Ozone increased from weekdays to weekends for a number of sites in the NFRMA with weekend reductions in NO2 at two sites in downtown Denver, indicating that the region was in a NOx-saturated ozone production regime. The stagnation and increases in ozone in the NFRMA are likely due to a combination of decreasing NOx emissions in a NOx-saturated environment and increased anthropogenic volatile organic compound (VOC) emissions in the NFRMA. Further investigation of the weekend-weekday effect showed that the region outside of the most heavily trafficked Denver area was transitioning to peak ozone production towards NOx-limited chemistry. This transition implies that continued NOx decreases will result in ozone being less sensitive to changes in either anthropogenic or biogenic VOC reactivity in the NFRMA. In contrast to anthropogenic VOCs, biogenic VOCs are unlikely to have increased in the NFRMA between 2000 and 2015, but are temperature dependent and likely vary by drought year. Ozone in the NFRMA has a temperature dependence, albeit smaller than many other US locations, consistent with biogenic VOC contributions to ozone production in the region. We show that while ozone increased with temperature in the NFRMA, which is consistent with a NOx-saturated regime coupled to temperature-dependent VOCs, this relationship is suppressed in drought years. We attribute this drought year suppression to decreased biogenic isoprene emissions due to long-term drought stress. Thus, while anthropogenic NOx and VOCs likely dominate ozone production regimes in the NFRMA, biogenic VOCs may also impact regional ozone and its temperature dependence.