NASA - National Aeronautics and Space Administration

Drought is an extreme weather and climate event that has been shown to cause the worsening of ozone (O3) air pollution. Using 15-year (2005–2019) surface O3 observations and weekly US Drought Monitor (USDM) indices, this study estimated that summertime US-mean surface O3 increased by 1.47 ppb per USDM level. It is revealed that O3 responses to drought display a spatial east-west variation: higher O3 enhancement in the Southeast (2.24 ppb/USDM), and no significant change or even a decrease in the west (e.g., −0.06 ppb/ USDM in California). The diurnal changes of O3 with drought also show an opposite pattern between the Southeast and California. Formaldehyde (HCHO) and nitrogen dioxide (NO2) column, two satellite-based O3 precursors proxies, show an increasing rate of 0.41 × 1015 molec/cm2/USDM and 0.03 × 1015 molec/ cm2/USDM in the Southeast, respectively, while these rates are not statistically significant in California. We at the sites with long-term observations in California and Georgia. Isoprene concentrations decreased by ∼37% explained this spatial discrepancy from the perspective of O3 chemistry by applying a zero-dimensional model under exceptional drought in California causing a reduction of O3 production (PO3) by ∼23.7% during daytime. On the contrary, isoprene increased by ∼41% in Georgia inducing a consequent increase of PO3 by ∼33.4% which accounts for more than half of the O3 enhancement. This study reveals the key role of biogenic isoprene on ozone chemistry under drought conditions.