Ozone pollution affects human health, especially in urban areas on hot sunny days. Its basic photochemistry has been known for decades and yet it is still not possible to correctly predict the high ozone levels that are the greatest threat. The CalNex_SJV study in Bakersfield CA in May/June 2010 provided an opportunity to examine ozone photochemistry in an urban area surrounded by agriculture. The measurement suite included hydroxyl (OH), hydroperoxyl (HO₂), and OH reactivity, which are compared with the output of a photochemical box model. While the agreement is generally within combined uncertainties, measured HO₂far exceeds modeled HO₂in NO_x-rich plumes. OH production and loss do not balance as they should in the morning, and the ozone production calculated with measured HO₂is a decade greater than that calculated with modeled HO₂when NO levels are high. Calculated ozone production using measured HO₂is twice that using modeled HO₂, but this difference in calculated ozone production has minimal impact on the assessment of NO_x-sensitivity or VOC-sensitivity for midday ozone production. Evidence from this study indicates that this important discrepancy is not due to the HO₂measurement or to the sampling of transported plumes but instead to either emissions of unknown organic species that accompany the NO emissions or unknown photochemistry involving nitrogen oxides and hydrogen oxides, possibly the hypothesized reaction OH + NO + O₂→ HO₂+ NO₂.