Impact of air mass histories on the chemical climate of Mount Mitchell, North Carolina

Abstract

Cloud water acidity and ionic content, as measured at the Mount Mitchell (35°44′05″N, 82°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 trajectories of cloud‐forming air masses which pass over areas of varying levels of pollutant emission. Regions of the United States which are emitters of high‐level pollutants, such as SOx and NOx 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 (May 15, 1993 to October 14, 1993) were computed using the hybrid singleparticle Lagrangian integrated trajectories (HY‐SPLIT) model. Three sectors, identified as the polluted sector, from 290° to 65° azimuth relative to the site, the continental sector, 240° to 290° azimuth, and the marine sector, 65° to 240° azimuth, were used to classify the cloud‐forming air masses. The polluted sector was associated with the lowest overall pH averages, with the marine sector following closely behind. The highest average pH values were received from air masses indicated as having crossed the continental and the marine sectors (in combination), with the largest portions of those air mass trajectories passing through the continental sector (exclusively continental sector air masses were also the most frequent). These observations are in agreement with findings in Colorado where aerosols produced by wind erosion were responsible for decreasing the precipitation acidity.