Different techniques of data analysis have been successfully applied to Los Angeles County air monitoring data to delineate major source areas for carbon monoxide, sulfur dioxide, nitrogen oxides, and ozone; to develop contaminant transport patterns; and to demonstrate the progress of photochemical reactions in the Los Angeles atmosphere. The heavily industrialized South Coastal, Southwest Coastal, and East San Fernando Valley areas are most affected by contaminants derived from fuel combustion at stationary sources. Emissions from mobile sources are heavily concentrated in some of the above source areas and also in the Central area. Seasonal variations in weather affect the total contaminant emissions as well as the distribution, transport, and ultimate fate of the individual contaminants. More frequent surface inversions in winter, combined with greater quantities of emitted contaminants, result in winter time atmospheric concentrations of primary contaminants (carbon monoxide, nitric oxide, sulfur dioxide, and particulates) more than twice as high as comparable summer time concentrations. Stronger on-shore breezes of longer duration transport contaminated air parcels farther across the County during summer. Longer, more effective irradiation and low persistent inversions result in higher ozone concentrations in summer. Air monitoring data confirm the photochemical formation of ozone during transport of air parcels along the most common “pathways” of transport — the prevailing wind flows from coast to inland areas.