Enhancements of Urban Ozone during Wildfire Events in the Pacific Northwest

The Pacific Northwest region experiences elevated ozone periods during summers, sometimes due to the emissions of VOCs and NOx (ozone precursors) from seasonal wildfires. AIRPACT5 is an air-quality forecasting system using the CMAQ photochemical grid model to forecast ozone, PM2.5 and related species in the Pacific Northwest with explicit treatment of wildfire emissions. This modeling system tends to significantly overestimate surface ozone during wildfire periods, compared to non-wildfire periods. AIRPACT5 ozone output was compared to measurements made during an intensive measurement campaign at a site near Boise Id which was strongly affected by wildfire smoke during August and September 2017. The main goals of this study are to explore the issues with simulating ozone during wildfires and to find ways to improve ozone forecasting during such events.

Two likely complications for urban area ozone simulations during wildfire periods are changes in the VOC-to-NOx abundance that impacts ozone photochemistryand aerosol effects on photolysis rates at high PM levels. A significant portion of NOx in a wildfire plume can be converted to peroxyacetyl nitrate (PAN); in turn, PAN can be transported downwind and NOx can be reconstituted, contributing to ozone production. So, correct treatment of NOx-PAN dynamics from emission, through plume evolution and transport and during ozone production downwind appears essential to reducing errors in ozone prediction during such wildfire events.

Reduction in downward shortwave (including UV) radiation reaching the surface due to smoke leads to reduction of both photolysis and surface temperature, and as a result reduction in ozone production. Therefore, using corrected photolysis rates and temperature profiles may reduce ozone overestimation. This can be investigated using an offline, decoupled approach as in AIRPACT5, where WRF simulations are used to drive the AQ model, by use of meteorological forecasts that utilize aerosol data to attenuate radiation. Alternatively, this can be done using fully coupled met-AQ modeling, as in WRF-Chem or coupled WRF-CMAQ.