THE CHEMICAL-COMPOSITION OF INTERCEPTED CLOUDWATER IN THE SIERRA-NEVADA

The chemical composition of cloudwater in the Sierra Nevada is dominated by NO3-, SO42-, and NH4+. Cloudwater pH is determined largely by the balance between the concentrations of these three species, although inputs of formic and acetic acid also are believed to be important, particularly when anthropogenic inputs are small. Cloudwater samples collected in Sequoia National Park (SNP) exhibited pH values ranging from 3.9 to 6.5; Yosemite National Park (YNP) cloudwater samples had pH values ranging from 3.8 to 5.2. Samples collected at YNP were more acidic than those collected at SNP. The difference in pH between the two regions appears to be due to relatively small differences in inputs of NO3-, SO42-, and NH4+. In the absence of inputs of NH3, cloudwater pH values in the Sierra may fall below 3. Over 250 h of cloud interception were observed during a 12 month period at a cloud monitoring site at 1856 m elevaton in SNP. Estimates of cloudwater deposition of NO3-, SO42-, and NH4+ indicate that cloud interception contributes significantly to regional acid deposition for closed forest canopies. Cloud interception may be the dominant deposition mechanism for isolated conifers and ridgetop canopies, where wind speeds are higher and cloudy air parcels can impact directly on foliar surfaces. © 1990.