This paper analyzes the possible effects of increased urban vegetation on the ozone air quality in California's South Coast Air Basin (SoCAB). Air quality impacts are accounted for through mesoscale meteorological and photochemical modeling of a late-August period. The simulations indicate that the net effect of increased urban vegetation is a decrease in ozone concentrations if the additional vegetation (trees) are low emitters. Hydrocarbon-emitting tree species have negative impacts on air quality. Episode-specific simulations in this study suggest that trees emitting roughly more than 2 μg g-1 h-1 of isoprene (micrograms of isoprene per gram dry-leaf mass per hour) and 1μg g-1 h-1 of monoterpenes should not be introduced in the SoCAB.