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 (PO3) 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 PO3 becoming more NO-suppressed, leading to a decrease in PO3 of similar to 20%. We infer reductions in atmospheric O-3 loss of similar to 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.