Local and transported pollution over San Diego, California

Ten air-quality samplings flights were performed over the San Diego metropolitan area during July 2003. One of the objectives of these flights was to assess the contribution of the offshore pollution sources (civilian and military vessels) to the air quality in the region. The flights, which took place during the mid-day hours in the lower levels of the boundary layer, originated offshore and proceeded eastwards through a series of concentric arcs covering an area of approximately 600 km2. The data from the continuous gas analyzers were translated to image plots that enabled the determination of the impact of the various sources on air quality. The results revealed that the offshore sources could often be detected (at least during five of the 10 flights) from their SO2 plume. However, the area covered by these plumes and the concentrations were significantly lower than those measured over other parts of the domain due to sources south of the region. It was further observed that vehicular traffic contributed in a major source way to the formation of ozone. During most flights the pollution cloud, originating over the high traffic area in the vicinity of the downtown area, could be tracked ‘downwind’ until the completion of the photochemical processing. By then, most of the NOX had been converted to NOZ, and peak O3 levels caused by the local emissions had started to ebb. Also, during most flights boundary layer peak levels of O3 exceeded 100 ppbv, and in one case were above 140 ppbv.