THE CHEMICAL MASS-BALANCE METHOD FOR ESTIMATING ATMOSPHERIC PARTICLE SOURCES IN SOUTHERN CALIFORNIA

The greater Los Angeles metropolitan area (the South Coast Air Basin) historically has had a severe air pollution problem whose visible symp tom is a dense smoke-like haze mixed with fog (smog). More than 25 yea rs ago, Friedlander and his associates began attempts to characterize the detailed chemistry of smog particles, and to interpret these resul ts in terms of source emissions. Source apportionment was formalized u sing a ''model'', called the chemical mass balance (CMB). The method h as been applied periodically since 1968, with major attempts reported in 1975 and 1994, based on large field studies. Despite improvements i n particulate sampling and chemical characterization, data availabilit y and major emissions changes, the CMB results qualitatively remain si milar. Only broad source categories are identifiable, but they general ly make up more than 75% of the total mass concentration. These includ e primary motor vehicle emissions, suspended (earth) crustal material, and secondary species of ammonium nitrate and sulfate salts. The deco upling of source components in the ambient aerosols suggests that extr eme P(10)events are significantly influenced by factors other than sim ple mixing and dispersion. Carbonaceous material from combustion emiss ions and secondary atmospheric production also represent a significant fraction of southern California airborne particles. The carbon fracti on includes a complex mixture of soot or black carbon, oxygenated comp ounds, as well as traces of polycyclic aromatics. The last group has b een of particular interest because of its apparent common origins and its carcinogenic activity. The major contributor to primary carbonaceo us material is motor vehicles. The observations also indicate that a s ignificant fraction of carbon derives from contemporary sources such a s vegetation or cooking of meat. The CMB has been used to guide regula tory strategies for particulate emissions management. The method could be improved substantially with more extensive source profile developm ent, as well as improved knowledge about the origins of the organic fr action.