The GATORG-GCMM global- through urban-scale nested air pollution/weather forecast model was applied to study ozone layers aloft, ozone in national parks, and weather during the August 3--6, 1990, SARMAP field campaign in northern and central California. Predictions of meteorological variables and mixing ratios of 20 gases were compared with observations. With nesting, the normalized gross error in predicted near-surface Kelvin temperatures was 1.02% and that in near-surface ozone above 50 ppbv was 22.5%. Statistics from outer nested domains indicated that the coarser the grid spacing, the greater the underprediction of ozone. In the absence of nesting, statistics deteriorated but not a lot. The model-simulated observed nighttime ozone layers aloft and daytime ozone mixed layers in the San Joaquin Valley and San Francisco Bay Area. It also simulated observed daytime and nighttime ozone layers aloft over the San Francisco Bay near Hayward. The formation mechanism of these layers is discussed. The model was used to estimate that about 47--57% of peak daytime ozone in Sequoia and Yosemite National Parks during SARMAP was produced by anthropogenic gases, 13--3% was produced by biogenic hydrocarbons, and the rest (about 40%) was background. ¿ 2001 American Geophysical Union