Observing Severe Drought Influences on Ozone Air Pollution in California.

Drought conditions affect ozone air quality, potentially altering multiple terms in the O 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-polluted locations of Fresno and Bakersfield near the Sierra Nevada foothills. We separately assess drought influences on O chemical production ( PO) from O concentration. We show that isoprene concentrations, which are a source of O-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 becoming more NO -suppressed, leading to a decrease in PO of ∼20%. We infer reductions in atmospheric O loss of ∼15% during the most severe drought period, consistent with past observations of decreases in O uptake by plants. We consider drought-related trends in O variability on synoptic time scales by analyzing statistics of multiday high-O events. We discuss implications for regulating O air pollution in California and other locations under more prevalent drought conditions.