An Exceptional Event Screening Tool to Estimate Natural Impacts on Ozone Exceedances

摘要

The EPA has put forth an Exceptional Events Rule that provides a mechanism by which air quality data can be excluded from regulatory decisions if an exceptional event has impacted the data such that it causes an exceedance of the NAAQS ozone standard. These exceptional events can be caused by natural phenomena such as wildfires and stratospheric intrusions. The current US EPA guidance for an exceptional event demonstration requires detailed modeling and analysis work to be done for each event. Thus, states such as Texas place an increasingly high value on identifying days when natural sources are impacting ozone exceedances, but they have limited resources to perform extensive investigations of every ozone exceedance. We have developed an exceptional events screening tool that quickly determines which ozone exceedance days were significantly impacted by fire emissions or stratospheric ozone intrusion upwind of the reporting monitor, wnich allows states to identify which events should be further investigated with a full photochemical model study. A back-trajectory component based on the Lagrangian particle dispersion model STILT (Stochastic Time Inverted Lagrangian Transport model) determines the multiple paths that different air masses took to get to the reporting monitor. the second component uses satellite observations to determine the impact or wildfires or stratospheric intrusions on the air mass. The tool provides three defined levels of likelihood that an exceptional event has occurred, allowing air quality managers to allocate modeling resources more efficiently and ultimately help in determining the contribution of natural sources to local ozone pollution. In this presentation we will demonstrate the capabilities of the tool and present results for case studies from Texas.