Oxygenated volatile organic compounds dominate reactivity in central California

Central California has an ongoing ozone non-attainment problem in urban, agricultural and pristine natural areas. The wide range of land use in these areas includes major sources of natural and anthropogenic VOC emissions. In order to address the relative contributions of these VOC sources to ozone formation and assess control strategies across this diverse region, we simulated air quality for a present-day ozone episode and a future climate scenario. Our simulations suggest that oxygenated volatile organic compounds (VOCs) dominate the total VOC reactivity throughout the region. In the rural agricultural San Joaquin Valley, almost 90% of present-day total VOC reactivity is due to oxygenated compounds. Even in urban locations, we find that about half of the total reactivity is due to the secondary formation of oxygenated VOCs. Under future climate conditions, biogenic and oxygenated VOC reactivities increase while the relative importance of anthropogenic VOC reactivity decreases throughout most of the model domain. Biogenic VOC emissions will increase up to 30% over vegetated regions, typically replacing the reactivity of oxygenated and anthropogenic primary emissions. In regions with relatively low biogenic and anthropogenic emissions such as the San Joaquin Valley, the relative importance of oxygenated compounds increases an additional 5% in the future climate scenario. These results predict that VOC reactivity due to natural VOC emissions and secondary VOC production in the atmosphere will dominate those of anthropogenic VOC emissions even more in the future, indicating that anthropogenic VOC control may not be sufficient to suppress ozone production in this region. To better assess this issue with observations, a network of ground-based measurements including oxygenated VOCs is needed along the transect from urban to rural locations to further examine the relative emissions and chemical formation of oxygenated compounds.