Two Decades of Changes in Summertime Ozone Production in California's South Coast Air Basin

Tropospheric ozone (O-3) continues to be a threat to human health and agricultural productivity. While O-3 control is challenging, tracking underlying formation mechanisms provides insights for regulatory directions. Here, we describe a comprehensive analysis of the effects of changing emissions on O-3 formation mechanisms with observational evidence. We present a new approach that provides a quantitative metric for the ozone production rate (OPR) and its sensitivity to precursor levels by interpreting two decades of in situ observations of the six criteria air pollutants(2001-2018). Applying to the South Coast Air Basin (SoCAB), California, we show that by 2016-2018, the basin was at the transition region between nitrogen oxide (NOx)-limited and volatile organic compound (VOC)-limited chemical regimes. Assuming future weather conditions are similar to 2016-2018, we predict that NOx-focused reduction is required to reduce the number of summer days the SoCAB is in violation of the National Ambient Air Quality Standard (70 ppbv) for O-3. Roughly, & SIM;40% (& SIM;60%) NOx reductions are required to reduce the OPR by & SIM;1.8 ppb/h (& SIM;3.3 ppb/h). This change would reduce the number of violation days from 28 to 20% (10%) in a year, mostly in summertime. Concurrent VOC reductions which reduce the production rate of HOx radicals would also be beneficial.

Automated summary

This article provides a comprehensive analysis of the effects of changing emissions on the formation mechanisms of tropospheric ozone (O-3) and presents a new approach to quantify the ozone production rate and its sensitivity to precursor levels. The study reveals that in the South Coast Air Basin, reducing nitrogen oxide (NOx) emissions is crucial to decrease the number of summer days with ozone violations, and reducing volatile organic compound (VOC) emissions would also be beneficial.