Investigation of Ozone Formation Chemistry during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA)

Salt Lake City (SLC), UT, is an urban area where ozone (O₃) concentrations frequently exceed health standards. This study uses an observationally constrained photochemical box model to investigate the drivers of O₃ production during the Salt Lake Regional Smoke, Ozone, and Aerosol Study (SAMOZA), which took place from August to September 2022 in SLC. During SAMOZA, a suite of volatile organic compounds (VOCs), oxides of nitrogen (NO_x), and other parameters were measured at the Utah Technical Center, a high-NO_x site in the urban core. We examined four high-O₃ cases: 4 August and 3, 11, and 12 September, which were classified as a nonsmoky weekday, a weekend day with minimal smoke influence, a smoky weekend day, and a smoky weekday, respectively. The modeled O₃ production on 4 August and 3 September was highly sensitive to VOCs and insensitive to NO_x reductions of ≤50%. Box model results suggest that the directly emitted formaldehyde contributed to the rapid increase in morning O₃ concentrations on 3 September. Model sensitivity tests for September 11-12 indicated that smoke-emitted VOCs, especially aldehydes, had a much larger impact on O₃ production than NO_x and/or anthropogenic VOCs. On 11 and 12 September, smoke-emitted VOCs enhanced model-predicted maximum daily 8 h average O₃ concentrations by 21 and 13 parts per billion (ppb), respectively. Overall, our results suggest that regionwide VOC reductions of at least 30-50% or NO_x reductions of at least 60% are needed to bring SLC into compliance with the national O₃ standard of 70 ppb.