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 (O-3) concentrations frequently exceed health standards. This study uses an observationally constrained photochemical box model to investigate the drivers of O-3 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 (NOx), and other parameters were measured at the Utah Technical Center, a high-NOx site in the urban core. We examined four high-O-3 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 O-3 production on 4 August and 3 September was highly sensitive to VOCs and insensitive to NOx reductions of <= 50%. Box model results suggest that the directly emitted formaldehyde contributed to the rapid increase in morning O-3 concentrations on 3 September. Model sensitivity tests for September 11-12 indicated that smoke-emitted VOCs, especially aldehydes, had a much larger impact on O-3 production than NOx and/or anthropogenic VOCs. On 11 and 12 September, smoke-emitted VOCs enhanced model-predicted maximum daily 8 h average O-3 concentrations by 21 and 13 parts per billion (ppb), respectively. Overall, our results suggest that regionwide VOC reductions of at least 30-50% or NOx reductions of at least 60% are needed to bring SLC into compliance with the national O-3 standard of 70 ppb.