Taking Another Look at Low-Level Stratospheric Intrusions and Wildfire Development during CABOTS 2016

As the climate changes and more extreme drought and fire seasons are predicted to occur, predicting the arrival of dry stratospheric ozone-rich air into the lower troposphere gains importance. An improved diagnostic technique is presented that could be applied to future air quality forecasting models to help predict days with poor air quality and wildfire danger. A period from late July to the middle of August 2016 was analyzed for a period of moderate to unhealthy air quality in the Sacramento non-attainment zone. Using MERRA-2 reanalysis data, a vertical cross-section analysis of 0.5 to 5.0 potential vorticity unit (PVU) was performed to analyze the low-level intrusion of stratospheric air to the region of the Soberanes Fire of Monterey County and the Cold Fire of Yolo County. The cross-sections indicate the arrival of 1 PVU on July 22, 2016 as the Soberanes Fire ignited, and on August 2, 2016 as the Cold Fire ignited. During CABOTS 2016 near-daily ozonesonde launches were performed at the coastal site of Bodega Bay, CA collecting a valuable dataset of baseline ozone. Changes in the background ozone concentration due to upper-level stratospheric intrusions have been identified. A moderately strong correlation in afternoon ozone was discovered between elevated ozonesonde measurements between 0.5 - 1.0 km vertical column above Bodega Bay and the Placerville surface ozone observations located within the Sacramento non-attainment zone. The Cold Fire was located on the east side of the Coastal Ranges and on the west side of the Sacramento Valley, near center between the coastal Bodega Bay observations and the Placerville surface monitoring site in the lower Sierra Nevada foothills. The Soberanes Fire was located about the same distance due south of the Cold Fire. Dates which both Bodega Bay and Placerville exhibited similar daily afternoon ozone increases were further evaluated. Increases occurred with the arrival of low-level high PV values along the vertical cross-sections. Other instances occurred with the arrival of the simulated fire plume. It was concluded that these increases in ozone were an indication of either stratospheric ozone or wildfire influences, or both. Gaining a better understanding of the low-level dispersion of stratospheric air following stratospheric intrusion events at the west coast of North America is important as our climate evolves. An advancement in air quality forecasts which could predict stratospheric ozone transport to the surface would benefit the public, and allow forecasters and researchers to better quantify the influence of stratospheric ozone on low-level ozone pollution either directly or indirectly through wildfire influence.