The influence of cold-start vehicle emissions on air quality is investigated using a photochemical/diffusion model. Both the time and space distribution of cold starts are examined. A day from an October 1968 Los Angeles smog episode serves as a baseline for determining diffusion coefficients, nitrogen balance, and hydrocarbon reactivities. Vehicular and stationary sources for 1968, 1971, 1974, and 1980 are emission inputs, and pollutant concentrations at the ground are air quality outputs. Stagnant central basin conditions govern the time phasing studies. Emissions introduced during the starting process have the greatest effect of all on carbon monoxide peaks, the effect being to increase the peak CO concentration from 9 to 13 percent. The levels of ozone and nitrogen dioxide that build up later in the day are influenced less because chemical processes afford dilution time. If vehicle starts are decentralized geographically, only a slight increase in the pollutant loading is noted because of the low morning speeds. Based on the results of the study, three possible weighting schemes are proposed for combining cold-start and hot start driving cycles. (Author)