Relationships of precipitation chemistry, atmospheric circulation, and elevation at two sites on the Colorado front range

The acidity and conductivity of precipitation at two sites located east of the Continental Divide in the Front Range of Colorado is examined for temporal trends, differences, and relationships to atmospheric circulation patterns for a 14 yr period (1984–1997). The elevationally lower station, Sugarloaf, is about 20 km west of Boulder, CO, and closer to the Denver/Boulder urban corridor than the higher site, Niwot, which lies 11.4 km northwest of Sugarloaf, and 1000 m higher in elevation. Key findings of this study are that Sugarloaf precipitation has the higher pH (less acidic) and lower conductivity of the two sites, and that different circulation patterns are associated with different inter-site precipitation characteristics: chemistry, amount, occurrence, and seasonal differences. Circulation indices account for about one-fourth to one-third of the variability in precipitation chemistry, and synoptic scale circulation patterns are clearly different for extremely low versus extremely high pH conditions at these two sites.