Photochemical grid model estimates of lateral boundary contributions to ozone and particulate matter across the continental United States

- Photochemical model source apportionment provides credible boundary contributions.
- Inert tracer method overestimates ozone lateral boundary inflow contribution.
- Spring intermountain west ozone contribution largely from west/north mid/upper-troposphere.

Multiple approaches to characterize lateral boundary contributions to photochemical model predicted ozone (O3) and particulate matter less than 2.5 microns in diameter (PM2.5) are available in the Comprehensive Air quality Model with extensions (CAMx). Here, three approaches are used for O3: (1) a comprehensive source apportionment scheme for chemical boundary conditions and emissions (OSAT), (2) chemically reactive tracers (RTRAC), and (3) chemically inert tracers. Two approaches are used for PM2.5: (1) particulate source apportionment (PSAT) and (2) chemically inert tracers. The inert tracer approach resulted in higher O3 lateral boundary contribution estimates because the method does not account for any O3 destruction reactions. OSAT and RTRAC estimate generally similar monthly average contributions during the warmer months although RTRAC estimates higher urban area contribution during the cold months because this RTRAC implementation did not treat O3 titration by NO. Accurate representation of lateral boundary O3 impacts must include appropriate accounting for O3 destruction reactions. OSAT and RTRAC were configured to estimate the contribution to modeled O3 from each of the four lateral faces of the model domain. RTRAC was configured to further stratify the western and northern boundaries by groups of vertical layers. The RTRAC approach showed that the largest O3 contributions to the continental U.S. are from the mid-troposphere, with less contribution from the upper troposphere/lower stratosphere. Inert tracers compared more closely to reactive tracers on average for PM2.5 compared to O3. This close agreement for PM2.5 indicates most of the lateral boundary contribution is from PM2.5 rather than precursor inflow. A strong relationship exists between model predicted PM2.5 boundary contribution and model overestimates of nitrate and organic carbon at IMPROVE monitor locations suggesting global model estimates of these species were overestimated at some places and times.