SEASONAL AND SPATIAL VARIATION OF THE BACTERIAL MUTAGENICITY OF FINE ORGANIC AEROSOL IN SOUTHERN CALIFORNIA

The bacterial mutagenicity of a set of 1993 urban articulate air pollu tion samples is examined using the Salmonella typhimurium TM677 forwar d mutation assay. Ambient line particulate samples were collected for 24 hr every sixth day throughout 1993 at four urban sites, including L ong Beach, central Los Angeles, Azusa, and Rubidoux, California, and a t an upwind background sire on San Nicolas Island. Long Beach and cent ral Los Angeles are congested urban areas where air quality is dominat ed by fresh emissions from air pollution sources; Azusa and Rubidolux are located Farther downwind and receive transported air pollutants pl us increased quantities of the products of atmospheric chemical reacti ons. Pine aerosol samples from Long Beach and Los Angeles show a prono unced seasonal variation in bacterial mutagenicity per cubic meter of ambient air, with maximum in the winter and a minimum in the summer. T he downwind smog receptor site at Rubidoux shows peak mutagenicity (wi th postmitochondrial supernatant but no peak without postmitochondrial supernatant) during the September-October periods when direct transpo rt from upwind sources can be expected. At most sites the mutagenicity per microgram of organic carbon from the aerosol is not obviously hig her during the summer photochemical smog period than during the colder months. Significant spatial variation in bacterial mutagenicity is ob served: mutagenicity per cubic meter of ambient air, on average, is mo re than an order of magnitude lower at San Nicolas Island than within the urban area The highest mutagenicity values per microgram of organi cs supplied to the assay are found at the most congested urban sites a t central Los Angeles and Long Beach. The highest annual average value s of mutagenicity per cubic meter of air sampled occur at central Los Angeles. These findings stress the importance of proximity to sources of direct emissions of bacterial mutagens and imply that if important mutagen-forming atmospheric reactions occur, they likely occur in the winter and spring seasons as well as the photochemically more active s ummer and early fall periods.