Abstract Description: The Northern Wasatch Front (NWF) in Utah, which covers Salt Lake, Davis, Weber, and Tooele counties, is currently designated as a moderate ozone nonattainment for the 2015 ozone standard but will be bumped up to serious nonattainment in 2025. In its State Implementation Plan (SIP) for the NWF, the Utah Division of Air Quality (UDAQ) estimates minor ozone contributions derived from local controllable emissions within the NWF but more substantial contributions from regional emissions among neighbor states and foreign sources that are consistently present during exceedance episodes. Section 179B of the Clean Air Act provides some regulatory flexibility for nonattainment areas impacted by international contributions if they fail to attain despite reasonable efforts to control local emissions. Among many peer-reviewed articles that consistently characterize the amount of total global background contributions across the western US, only a few specifically assess and confirm the importance of deep vertical transport of global ozone from the middle and upper troposphere. However, these few studies modeled these processes at relatively coarse resolution both spatially and temporally. Furthermore, the importance of vertical transport has not been evaluated or quantified specifically for international anthropogenic (IA) ozone arriving in the NWF. Source apportionment modeling reported by UDAQ estimates how much IA ozone arrives in NWF but not it’s transport path. A mechanistic explanation is needed to understand why western US background ozone is consistently highest over Utah and to describe the specific processes that bring IA ozone into the NWF. Such information will bolster and add weight of evidence elements for any future 179B demonstrations for the area.
Ramboll directly assessed, via high-resolution photochemical modeling using the Comprehensive Air-quality Model with Extensions (CAMx), the amount of mid/upper-tropospheric IA ozone that is transported into the NWF. The approach utilized UDAQ’s SIP modeling platform with source apportionment, which resolves the atmosphere and local topography from 1 to 4 km scale over the NWF and Utah, respectively. We characterized daily variations in IA contributions from several different altitude ranges between the surface and the stratosphere. We discuss several topics, including the relative importance among transport pathways; the extent to which high pressure centered over Utah blocks or enhances background ozone entering the area; how the relative strengths of these transporting motions vary in time and modulate inter-daily variations of background ozone; and the extent to which transported peroxyacetyl nitrate (PAN) contributes to ozone in the NWF.
