California underwent its worst drought on record between 2011 and 2015, with a parching combination of low precipitation and high temperatures. Conditions of drought can have complicated effects on ozone air quality, so to better comprehend the process; scientists have examined data from two ozone-polluted cities before, during and after the California drought. Their results have been reported in ACS’ journal Environmental Science & Technology.
Even though ozone in the stratosphere shields the earth from UV radiation, at ground level the molecule is a detrimental air pollutant to humans, plants, and animals. Ground-level ozone develops when nitrogen oxide compounds, mainly from motor vehicle emissions, react with volatile organic compounds (VOCs) from anthropomorphic and natural sources. Isoprene, a VOC released by plants, is a major contributor to ozone production during summer months in many regions worldwide. However, plants also lower air ozone levels by absorbing the gas up via pores in their leaves. Since drought conditions influence both of these plant-related processes, Angelique Demetillo, Sally Pusede, and colleagues were keen to study air concentrations of isoprene and ozone—as well as leaf area index, nitrogen dioxide and meteorology—before, during, and after the California drought.
For their research, the scientists examined freely available data amassed from the ground and satellites in Fresno, an ozone-polluted city near an oak savanna, and Bakersfield, California. They learned that isoprene concentrations did not alter considerably during the early drought, but they decreased by over 50% during the most extreme drought conditions. The effects of drought on isoprene were also reliant on atmospheric temperature. The scientists discovered that drought changed ozone production such that the process turned out to be chemically more sensitive to the reduction in isoprene and other drought-affected VOCs. These aspects led to an estimated overall reduction in ozone production of about 20% during the extreme drought. But, this reduction was offset by a comparable decrease in ozone uptake by plants, resulting in only a 6% reduction in ozone levels overall during the extreme drought period. These results indicate that drought impacts on ozone pollution are complex and are subject to drought severity and length, the scientists say.