IMPACT OF AIR-MASS HISTORIES ON THE CHEMICAL CLIMATE OF MOUNT MITCHELL, NORTH-CAROLINA

Cloud water acidity and ionic content, as measured at the Mount Mitche ll (35 degrees 44'05 '' N, 82 degrees 17'15 '' W, 2038 m msl (highest peak in the eastern United States)) State Park observing site, using a passive cloud water collector, are directly influenced by the traject ories of cloud-forming air masses which pass over areas of varying lev els of pollutant emission. Regions of the United States which are emit ters of high-level pollutants, such as SO, and NO,, will thus serve to reduce observed pH levels in cloud water samples and raise the levels of acidifying ions, such as sulfate and nitrate. The 48-hour backward trajectories for all 39 cloud events during the 1993 field season (Ma y 15, 1993 to October 14, 1993) were computed using the hybrid single- particle Lagrangian integrated trajectories (HY-SPLIT) model. Three se ctors, identified as the polluted sector, from 290 degrees to 65 degre es azimuth relative to the site, the continental sector, 240 degrees t o 290 degrees azimuth, and the marine sector, 65 degrees to 240 degree s azimuth, were used to classify the cloud-forming air masses. The pol luted sector was associated with the lowest overall pH averages, with the marine sector following closely behind. The highest average pH val ues were received from air masses indicated as having crossed the cont inental and the marine sectors (in combination), with the largest port ions of those air mass trajectories passing through the continental se ctor (exclusively continental sector air masses were also the most fre quent). These observations are in agreement with findings in Colorado where aerosols produced by wind erosion were responsible for decreasin g the precipitation acidity.