Observing Severe Drought Influences on Ozone Air Pollution in California.

Drought conditions affect ozone air quality, potentially altering multiple terms in the O ₃ mass balance equation. Here, we present a multiyear observational analysis using data collected before, during, and after the record-breaking California drought (2011-2015) at the O ₃ -polluted locations of Fresno and Bakersfield near the Sierra Nevada foothills. We separately assess drought influences on O ₃ chemical production ( PO ₃ ) from O ₃ concentration. We show that isoprene concentrations, which are a source of O ₃ -forming organic reactivity, were relatively insensitive to early drought conditions but decreased by more than 50% during the most severe drought years (2014-2015), with recovery a function of location. We find drought-isoprene effects are temperature-dependent, even after accounting for changes in leaf area, consistent with laboratory studies but not previously observed at landscape scales with atmospheric observations. Drought-driven decreases in organic reactivity are contemporaneous with a change in dominant oxidation mechanism, with PO ₃ becoming more NO _x -suppressed, leading to a decrease in PO ₃ of ∼20%. We infer reductions in atmospheric O ₃ loss of ∼15% during the most severe drought period, consistent with past observations of decreases in O ₃ uptake by plants. We consider drought-related trends in O ₃ variability on synoptic time scales by analyzing statistics of multiday high-O ₃ events. We discuss implications for regulating O ₃ air pollution in California and other locations under more prevalent drought conditions.