Temperature dependent emissions of agricultural and dairy VOCs in the San Joaquin Valley measured by Airborne Eddy Covariance

The San Joaquin Valley is a major agricultural production area in California, with various crops, large cattle and dairy farms, as well as oil and gas production facilities. It is among the most frequent nonattainment areas of US air quality standards for ozone and particulate matter (PM) in the United States. Emission inventories of agricultural precursors to ozone and PM such as volatile organic compounds (VOCs) are subject to large uncertainties, and deposition rates of many oxidation products are poorly constrained. We measured fluxes of a wide range of VOCs by Vocus PTR-ToF-MS from a Twin Otter aircraft over the San Joaquin Valley in order to better understand VOC sources and sinks in the region. The flights covered ~6000 km of transects between Bakersfield, Visalia and Fresno, and were conducted in June 2021 as part of the RECAP-CA (Re-Evaluating the Chemistry of Air Pollutants in CAlifornia) campaign. By flying slowly (50-60 m/s) at a low altitude of ~ 400 m above ground, we were able to map VOC sources and sinks with ~ 3-5 km spatial resolution using Airborne Eddy Covariance with a wavelet analysis approach. NOx , CO, CH4, and CO2 fluxes were also measured.

Our results show that citrus and other tree crops are an underestimated source of mono- and sesquiterpenes, and that industrial processing of these fruits locally emits large amounts of terpenoids. Our emission measurements indicate that citrus is the largest agricultural source of OH reactivity in the San Joaquin Valley, followed by dairy and cattle operations. Deposition of OVOCs, currently not considered in inventories, was important over the oak woodlands on the eastern edges of the valley. The emission of biogenic reactive VOCs, including crops, dairy and cattle operations, and oak shrublands, was consistently temperature dependent. These findings highlight the importance of the interrelation between future climate change-related temperature increases and air quality, with implications for air quality regulation strategies in the San Joaquin Valley and in agricultural production regions worldwide.