Abstract
Drought is an extreme weather and climate event that has been shown to cause the worsening of ozone (O-3) air pollution. Using 15-year (2005-2019) surface O-3 observations and weekly US Drought Monitor (USDM) indices, this study estimated that summertime US-mean surface O-3 increased by 1.47 ppb per USDM level. It is revealed that O-3 responses to drought display a spatial east-west variation: higher O-3 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 O-3 with drought also show an opposite pattern between the Southeast and California. Formaldehyde (HCHO) and nitrogen dioxide (NO2) column, two satellite-based O-3 precursors proxies, show an increasing rate of 0.41 x 10(15) molec/cm(2)/USDM and 0.03 x 10(15) molec/cm(2)/USDM in the Southeast, respectively, while these rates are not statistically significant in California. We explained this spatial discrepancy from the perspective of O-3 chemistry by applying a zero-dimensional model at the sites with long-term observations in California and Georgia. Isoprene concentrations decreased by similar to 37% under exceptional drought in California causing a reduction of O-3 production (PO3) by similar to 23.7% during daytime. On the contrary, isoprene increased by similar to 41% in Georgia inducing a consequent increase of PO3 by similar to 33.4% which accounts for more than half of the O-3 enhancement. This study reveals the key role of biogenic isoprene on ozone chemistry under drought conditions.