A PM₁₀ monitoring network was established throughout the South Coast Air Basin (SOCAB) in the greater Los Angeles region during the calendar year 1986. Annual average PM₁₀ mass concentrations within the Los Angeles metropolitan ranged from 47.0 mum^-3 along the coast to 87.4 mum^-3 at Rubidoux, the furthest inland monitoring station. Measurements made at San Nicolas Island suggest that regional background aerosol contributes between 28 to 44 percent of the PM₁₀ aerosol at monitoring sites in the SOCAB over the long term average. Five major aerosol components (carbonaceous material, NO₃^-, SO₄^--, NH₄⁺, and soil-related material) account for greater the 80 percent of the annual average PM₁₀ mass at all on-land monitoring stations. Peak 24-h average mass concentrations of nearly 300 mug m^-3 were observed at inland locations, with lower peak values ( ~ 130-150 mug m^-3) measured along the coast. Peak-day aerosol composition was characterized by increased NO₃-</sup>ion and associated ammonium ion levels, as compared to the annual average. There appears to be only a weak dependence of PM₁₀ mass concentration on season of the year. This lack of a pronounced seasonal dependence results from the complex and contradictory seasonal variations in the major chemical components (carbonaceous material, nitrate, sulfate, ammonium ion and crustal material). At most sites within the Los Angeles metropolitan area, PM₁₀ mass concentrations exceeded both the annual and 24-h average federal and state of California PM₁₀ regulatory standards.