Introduction 5848 2. Sources and Sinks of RONO 2 5849 2.1. OH-Initiated RONO 2 Formation 5850 2.2. NO 3 -Initiated RONO 2 Formation 5852 2.3. The Fate of RONO 2 Molecules 5852 3. Observations of Individual RONO 2 and ∑ANs 5853 3.1. Measurements of Individual RONO 2 5853 3.2. Measurements of ∑ANs 5854 4. RONO 2 and Local O x Production 5857 4.1. The Mechanistic Relationship between O x and ∑ANs 5857 4.2. Observed Relationships between O x and ∑ANs 5858 4.3.

Abstract. We describe the effects of nitrogen oxide (NO x ) and organic reactivity reductions on the frequency of high ozone days in California's San Joaquin Valley. We use sixteen years of observations of ozone, nitrogen oxides, and temperature at sites upwind, within, and downwind of three cities to assess the probability of exceeding the California 8-h average ozone standard of 70.4 ppb at each location. The comprehensive data records in the region and the steep decreases in emissions over the last decade are sufficient to constrain the relative import of NO x and organic reactivity reductions on the frequency of violations. We show that high ozone has a large component that is due to local production, as the probability of exceeding the state standard is lowest for each city at the upwind site, increases in the city center, is highest at downwind locations, and then decreases at the receptor city to the south. We see that reductions in organic reactivity have been very effective in the central and northern regions of the San Joaquin but less so in the southern portion of the Valley. We find evidence for two distinct categories of reactivity sources: one source that has decreased and dominates at moderate temperatures, and a second source that dominates at high temperatures, particularly in the southern San Joaquin, and has not changed over the last twelve years. We show that NO x reductions are already effective or are poised to become so in the southern and central Valley, where violations are most frequent, as conditions in these regions have or are transitioning to NO x -limited chemistry when temperatures are hottest and high ozone most probable.

Laboratory studies have established a number of chemical pathways by which nitrogen oxides (NO(x)) affect atmospheric organic aerosol (OA) production. However, these effects have not been directly observed in ambient OA. We report measurements of particulate organic nitrates in Bakersfield, California, the nighttime formation of which increases with NO(x) and is suppressed by high concentrations of organic molecules that rapidly react with nitrate radical (NO(3))--evidence that multigenerational chemistry is responsible for organic nitrate aerosol production. This class of molecules represents about a third of the nighttime increase in OA, suggesting that most nighttime secondary OA is due to the NO(3) product of anthropogenic NO(x) emissions. Consequently, reductions in NO(x) emissions should reduce the concentration of organic aerosol in Bakersfield and the surrounding region.

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