Key results from the salt lake regional smoke, ozone, and aerosol study (SAMOZA).

ABSTRACT

The Northern Wasatch Front area is one of ~ 50 metropolitan regions in the U.S. that do not meet the 2015 O3 standard. To better understand the causes of high O3 days in this region we conducted the Salt Lake regional Smoke, Ozone and Aerosol Study (SAMOZA) in the summer of 2022. The primary goals of SAMOZA were Measure a suite of VOCs, by Proton Transfer Reaction Mass Spectrometry (PTR-MS) and the 2,4-dinitrophenylhydrazine (DNPH) cartridge method.Evaluate whether the standard UV O3 measurements made in SLC show a positive bias during smoke events, as has been suggested in some recent studies.Use the observations to conduct photochemical modeling and statistical/machine learning analyses to understand photochemistry on both smoke-influenced and non-smoke days.Implications The Northern Wasatch Front area is one of ~50 metropolitan regions in the U.S. that do not meet the 2015 O3 standard. To better understand the causes of high O3 days in this region we conducted the Salt Lake regional Smoke, Ozone and Aerosol Study (SAMOZA) in the summer of 2022. A number of policy relevant findings are identified in the manuscript including role of smoke and NOx vs VOC sensitivity.

We found no significant difference in the O3 measurements using a "scrubber-less" UV instrument compared to the standard O3 measurements at PM2.5 concentrations up to 60 µg m−3.On days with smoke, we found that PM2.5, CO, O3 and nearly all VOCs were significantly enhanced. On average, NOx was also enhanced on days with smoke, but this was complicated by day of week effects.Photochemical modeling of O3 production rates at the Utah Tech Center demonstrates a strong sensitivity to VOC concentrations and less sensitivity to NOx. For non-smoke days, achieving the current O3 standard would require regional reductions in VOCs of ~40% or reductions in NOx ~ 60%.The photochemical modeling shows that formaldehyde and other OVOCs, along with alkenes, were the most important O3 precursors.Generalized Additive Modeling (GAM) gave similar MDA8 O3 enhancements on smoky days as the photochemical modeling. Analysis of the GAM results show that 23% of the smoke days have GAM residuals that exceed the U.S. EPA's criteria for inclusion as exceptional event documentation.