Observed suppression of ozone formation at extremely high temperatures due to chemical and biophysical feedbacks.

Ground level ozone concentrations ([O(3)]) typically show a direct linear relationship with surface air temperature. Three decades of California measurements provide evidence of a statistically significant change in the ozone-temperature slope (Δm(O3-T)) under extremely high temperatures (> 312 K). This Δm(O3-T) leads to a plateau or decrease in [O(3)], reflecting the diminished role of nitrogen oxide sequestration by peroxyacetyl nitrates and reduced biogenic isoprene emissions at high temperatures. Despite inclusion of these processes in global and regional chemistry-climate models, a statistically significant change in Δm(O3-T) has not been noted in prior studies. Future climate projections suggest a more frequent and spatially widespread occurrence of this Δm(O3-T) response, confounding predictions of extreme ozone events based on the historically observed linear relationship.