Impacts of agricultural irrigation on ozone concentrations in the Central Valley of California and in the contiguous United States based on WRF-Chem simulations

•Irrigation reduces the air mixing layer and weakens vertical mixing of air masses.•Surface concentrations of primary pollutants increase within the irrigated areas.•Surface O3 decreases within irrigated areas, except in the northwestern U.S.•O3 increases in the Central Valley's unirrigated areas and along the east coast.•The model underestimates [O3] and [VOCs] at some sites, compared with observations. In this study, a realistic irrigation method is incorporated into a model called Weather Research and Forecasting with Chemistry (WRF-Chem) to determine the impacts of irrigation on ozone and other pollutants over the Central Valley of California and thereafter throughout the contiguous United States. In comparison with observations, model simulations at current configurations underestimate ozone (O3) and volatile organic compound (VOC) concentrations, especially during the elevated episode periods. The model results, however, are generally comparable with previous studies and the simulations adequately capture the spatial and temporal variability of these pollutant concentrations. In comparison with observations and model control runs, model simulations with irrigation runs are slightly improved. Our results show that irrigation increases primary pollutant concentrations in the irrigated areas under cloudless conditions, consistent with previous studies. In July and August, 2005, for instance, with clear skies, irrigation increases hourly surface [CO] up to 40ppb with an irrigated grid average of 16ppb or 8.3%; [VOC] up to 10ppb with an irrigated grid average of 4.6ppb or 21.4%; and [NOx] up to 4ppb with irrigated grid average of 0.72ppb or 12.6%, especially near urban areas during daytime. On the other hand, irrigation marginally decreases ground-level ozone concentrations ([O3]) over irrigated land: it decreases hourly [O3] by −0.14ppb or −0.45% and decreases daily 8h maximum average (DMA8) [O3] by −0.39ppb or −0.76%. In contrast, irrigation increases hourly [O3] up to 5ppb over the surrounding unirrigated areas of the San Joaquin Valley, during both daytime and nighttime. Furthermore, except for the Pacific Northwest, similar patterns of ozone variation are simulated by the model in other irrigated regions in the continental United States. The explanation is that irrigation results in cooler ground surfaces, which first decreases instability and turbulence, leading to a weakening of the vertical mixing of primary pollutants. Therefore, irrigation incre