Observing Severe Drought Influences on Ozone Air Pollution in California

Drought conditions affect ozone air quality, potentially altering multiple terms in the O3 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 O3-polluted locations of Fresno and Bakersfield near the Sierra Nevada foothills. We separately assess drought influences on O3 chemical production (PO3) from O3 concentration. We show that isoprene concentrations, which are a source of O3-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 NOx-suppressed, leading to a decrease in PO3 of ∼20%. We infer reductions in atmospheric O3 loss of ∼15% during the most severe drought period, consistent with past observations of decreases in O3 uptake by plants. We consider drought-related trends in O3 variability on synoptic time scales by analyzing statistics of multiday high-O3 events. We discuss implications for regulating O3 air pollution in California and other locations under more prevalent drought conditions