Trends in air pollution have shown improvements due to new laws and regulations as well as improved technologies. These results show decreasing numbers of non-attainment days (days in which state air pollution maximums are exceeded) in several air basins for ozone and particulate matter greater than ten microns in size (PM10). Ozone levels remain a concern to forest and rangeland resources within the Sierra Nevada mountains and east of the San Joaquin Valley and South Coast air basins. Wide variations in air quality are found throughout these air basins. The primary source of ozone that drifts east into the Sierra Nevada mountains has been linked to the agricultural activity in the Sacramento and San Joaquin valleys via westerly air flows. (from the 2003 Forest and Rangeland Assessment)