Monitoring the chemical climate of the Mt. Mitchell State Park for evaluation of its impact on forest decline

Abstract

During the last decade, a new type of forest decline has become increasingly apparent, especially at high elevations, in north America and western Europe. One of the causes for this decline could be anthropogenic air pollutants, the deposition of which is facilitated by natural causes at high elevations such as strong windfield, direct cloud capture by the forest canopy, high frequency of frosting and riming etc. At Mt. Mitchell, North Carolina (35°44'05"N, 82°17"15'W) which is the highest peak (2,038 m MSL) in the eastern United States, the decline of red spruce and Fraser fir forest is noticeable above the cloud-base which is frequently observed around 1585 m MSL. In order to quantitatively assess the chemical climate of Mt. Mitchell State Park, we erected a 16.5 m tall aluminium walk-up tower and instrumented it with an electronic weather station, cloud water collectors and ozone analyzers. Our observations during summer 1986 confirm that Mt. Mitchell has a high frequency of being immersed in clouds. During early morning hours, cloudiness is maximum in frequency and results in short duration cloud events ranging from 2-8 h. Although the average pH of precipitation was about 4.4, the pH of cloud water ranged between 2.2—5.4. Short-duration cloud events (8 h or less) were more acidic, in general, than the long duration ones during which the cloud water was most acidic at the onset and dissipative stages. Both types of event were characterized by highly variable liquid water content (LWC) which was lower for the short events. The pH of the cloud water was demonstrated to be a function of wind direction at the site. For the average wind direction of 210°, the pH was 3.4. During early summer episodes, neutralization of the acidic anions (SO⁼₄ and NO^-₃) was not as effective as in the later summer episodes. During all cloud episodes, ozone concentration registered a dramatic decrease. The average minimum ozone concentration of about 63 ppb during the whole summer of 1986 was above the background level (50 ppb) in the United States. Diurnal ozone variation revealed a nocturnal maximum of about 75 ppb after sunset. This is contrary to conventional midday maximum in ozone concentration reported in the literature. Episodic excursions in ozone concentration equal to or exceeding 100 ppb level were found on 5 occasions. The maximum cloud water deposition rate, which is estimated using a micrometeorological model, is found to be 1.30 mm h^-1. Ionic deposition on the forest canopy is found to be similar to that reported for the Mt. Moosilauke (New Hampshire) forest. Ionic deposition due to direct cloud capture is found to be 2 to 5 times the deposition due to precipitation, and the evaporation associated with wind speeds as high as 15-18 m s^-1 realized during some episodes is pointed out to be of great significance regarding the canopy exposure.