04 Mar 2022
04 Mar 2022
Status: this preprint is currently under review for the journal ACP.

The Impacts of Wildfires on Ozone Production and Boundary Layer Dynamics in California’s Central Valley

Keming Pan and Ian Faloona Keming Pan and Ian Faloona
  • Department of Land, Air, & Water Resources and the Air Quality Research Center, University of California, Davis

Abstract. We investigate the role of wildfire smoke on ozone photochemical production (P(O3)) and atmospheric boundary layer (ABL) dynamics in California’s Central Valley during June–September, 2016–2020. Wildfire events are identified by the Hazard Mapping System (HMS) and Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT). Air quality and meteorological data are analyzed from 10 monitoring sites operated by the California Air Resources Board (CARB) across the Central Valley. On average, wildfires were found to influence air quality in the Central Valley on about 20 % of the total summer days of the study. During wildfire influenced periods, maximum daily 8 h averaged (MDA8) O3 was enhanced by about 5.5 ppb or 10 % of the median MDA8 (once corrected for the slightly warmer temperatures) over the entire valley. Overall, nearly half of the total exceedances of the National Ambient Air Quality Standards (NAAQS) where MDA8 O3 > 70 ppb, occur under the influence of wildfires, and approximately 10 % of those were in exceedance by 5 ppb or less indicating circumstances that would have been in compliance with the NAAQS were it not for wildfire emissions. The photochemical ozone production rate calculated from the modified Leighton relationship was also found to be higher by 26 % on average compared to non-fire periods despite the average diminution of j(NO2) by ~7 % due to the shading effect of the wildfire smoke plumes. Furthermore, the in-situ ozone production rates are found to be elevated due to enhancement of both peroxy radicals and NO in near equal measure. Surface heat flux measurements from two AmeriFlux sites in the Northern San Joaquin Valley show midday surface buoyancy fluxes decrease by 30 % on average when influenced by wildfire smoke. Similarly, afternoon peak ABL heights measured from a radio acoustic sounding system (RASS) located in Visalia in the Southern San Joaquin Valley were found to decrease on average by 80 m (~15 %) with a concomitant reduction of downwelling shortwave radiation of 54 Wm-2, consistent with past observations of the dependence of boundary layer heights on insolation.

Keming Pan and Ian Faloona

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-144', Anonymous Referee #1, 22 Mar 2022
  • RC2: 'Comment on acp-2022-144', Anonymous Referee #2, 25 Mar 2022

Keming Pan and Ian Faloona

Keming Pan and Ian Faloona


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Short summary
This work represents a unique analysis of 10 existing air quality network sites, meteorological sites, two Ameriflux sites, and a radio acoustic sounding system in the Central Valley of California during 5 consecutive fire seasons, June through September, from 2016 to 2020. We find that the ozone production rate is found to increase by 26 % during wildfire influenced periods. Wildfire smoke also decrease the heat flux by 30 % and result in 12 % lower mixed-layer height.