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 con-5 centrations 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 pho-tochemical 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 10 to the daytime accumulation of ozone at the surface in Sacramento. Ozone production in the rural Mountain Counties is currently NOx-limited and will decrease in response to NOx 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 im-15 pact of NOx emission reductions on the frequency of exceeding the federal 8-hour ozone standard at an individual site will depend on the balance between reduced titra-tion and the sign and magnitude of production changes. We further show that HNO 3 production, which depends on the product of OH and NO2 mixing ratios, is a constant at high NOx , suggesting that NOx must be reduced below a threshold before nitrate 20 aerosol can be expected to decrease.