Spatial Variation of Surface O<sub>3</sub> Responses to Drought Over the Contiguous United States During Summertime: Role of Precursor Emissions and Ozone Chemistry
Wei Li, Yuxuan Wang, James Flynn, Robert J. Griffin, Fangzhou Guo, Jordan Schnell
Abstract
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 (NO 2 ) column, two satellite‐based O 3 precursors proxies, show an increasing rate of 0.41 × 10 15 molec/cm 2 /USDM and 0.03 × 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 ∼37% under exceptional drought in California causing a reduction of O 3 production (PO 3 ) by ∼23.7% during daytime. On the contrary, isoprene increased by ∼41% in Georgia inducing a consequent increase of PO 3 by ∼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.