The weekend effect within and downwind of Sacramento: Part 2. Observational evidence for chemical and dynamical contributions

Abstract. Observations of day-of-week patterns and diurnal profiles of ozone, volatile organic compounds and nitrogen oxides are examined to assess the chemical and dynamical factors governing the daytime ozone accumulation and the distribution of chemically related species in Central California. Isoprene observations show that urban OH concentrations are higher on the weekend whereas rural OH concentrations are lower on the weekend, confirming that NO_x concentrations have a direct effect on the rate of photochemical ozone production and that the transition from NO_x-saturated (VOC-limited) to NO_x-limited chemistry occurs between the city and the downwind rural counties. We quantify the extent to which mixing of ozone and its precursors from aloft contributes to the daytime accumulation of ozone at the surface in Sacramento. Ozone production in the rural Mountain Counties is currently NO_x-limited and will decrease in response to NO_x emission reductions in the Sacramento Valley. However, NO_x emissions reductions of at least 50% (from weekday levels) are necessary to bring about a significant decrease in accumulation of ozone at the surface in the Sacramento Valley. The impact of NO_x emission reductions on the frequency of exceeding the federal 8-hour ozone standard at an individual site will depend on the balance between reduced titration and the sign and magnitude of production changes. We further show that HNO₃ production, which depends on the product of OH and NO₂ mixing ratios, is a constant at high NO_x, suggesting that NO_x must be reduced below a threshold before nitrate aerosol can be expected to decrease.