False Alarms and Missed Events: A Root Cause Analysis of Ozone Forecast Challenges for the Bay Area Air Quality Management District

The Bay Area Air Quality Management District aims to create a healthy breathing environment for every Bay Area resident while protecting and improving public health, air quality, and the global climate. The Air District has a forecast area that covers the nine counties surrounding San Francisco Bay: Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, southwestern Solano, and southern Sonoma counties. During the summer months (May 1 through October 31), air quality meteorologists forecast ozone concentrations for the Bay Area (including densely populated areas from Santa Rosa south to San Jose, and east to Livermore). The U.S. National Ambient Air Quality Standards (NAAQS) are exceeded when the 8-hour average ozone concentration is greater than 70 ppb. A Spare the Air Alert is issued when an exceedance of the NAAQS is expected. During these Spare the Air days, the Air District encouraged residents to cut back on any activities that cause air pollution and limit their time outdoors.

Meteorologists in the Air District are skillful in forecasting Spare the Air days. However, there are cases of false alarms and missed events, especially when ozone concentration is around the 70-ppb exceedance threshold. A root cause analysis was performed to evaluate atmospheric conditions that are conducive for false alarms and missed events from 2005 through 2018. The findings from this study were then applied to data in 2019. This study will help the Air District minimize the number of false alarms and missed events for the ozone season starting in 2020.