Evidence of Orographic Precipitation Suppression by Air Pollution–Induced Aerosols in the Western United States

Abstract

Abstract Analyses of trends of the orographic winter precipitation enhancement factor Ro along the coastal mountain ranges of the west coast of the United States show a pattern of decreasing Ro during the last century by as much as −24% from the southern border to central California, to no decrease in northern California and Oregon, and to a renewed decrease of Ro (−14%) in Washington to the Seattle area east of the Puget Sound. Similar decreases also occurred well inland, over Arizona, New Mexico, Utah (this study), and the east slopes of the Colorado Rockies (already documented prior to this study). Both absolute precipitation amounts and Ro are affected by fluctuations in the atmospheric circulation patterns, such as those associated with the Pacific decadal oscillation and the Southern Oscillation index. However, these climatic fluctuations cannot explain the observed trends in Ro. Although the trends of aerosols are available only since 1988, aerosol measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) aerosol monitoring network show that negative trends in Ro are associated with elevated concentrations of fine aerosols [particulate matter less than 2.5 μm in diameter (PM 2.5)]. The PM 2.5 showed stability or some increase in the areas where their levels were elevated and decreasing trends of Ro were noted. Strong decreasing trends of the coarse aerosols (PM 10–PM 2.5) were noted, especially in the areas with elevated levels of PM 2.5. The decreasing trend in coarse aerosols (the coarse aerosols may act to initiate and enhance precipitation), in conjunction with the constancy and/or increases of the small aerosols (small aerosols suppress precipitation), can explain the continuing decreases in orographic precipitation during the last two decades despite the indicated improvement in the conventional air quality standards.