In-cloud scavenging and deposition of sulfates and nitrates: Case studies and parameterization

Scavenging of sulfates and nitrates-two most common ions leading the cloudwater acidity-was investigated during field studies atop a site in Mt. Mitchell (35°44′05″N, 82°17′15″W) State Park where the highest peak (2038 m MSL) of the eastern U.S. is located. Experiments were conducted during the growing seasons (15 May-30 September) of 1986 and 1987 using an instrumented meteorological tower (16.5 m tall) and a passive cloudwater collector. A cloud episode that occurred on 12 October 1987, was also comprehensively investigated. Clouds were frequently observed in which the Fraser fir and red spruce stands stayed immersed 28% and 41% of the time during the 1986 and 1987 seasons, respectively. Rate of cloudwater deposition on the forest canopy was determined using an inferential cloud deposition model. It was found by analysing nine short duration (lasting 8 h or less) and 16 long duration cloud events that the ionic concentration (SO₄^2- and NO₃^-) is inversely proportional to the rate (I_c) of cloudwater deposition (in mm h^-1) and can be expressed by the following relationship: [SO₄^2-] = aI_c^-b or [NO₃^-] = aI_c^-b. Theoretical arguments leading to these relationships are presented. The b values for predicting NO₃^2- concentration are found in the range of 0.14-1.24 (mean = 0.48) for short duration and 0.062-0.63 (mean = 0.27) for long duration cloud events, respectively. The corresponding b values for predicting NO₃^- concentrations are 0.19-1.16 (mean = 0.49) and 0.072-0.59 (mean = 0.27), respectively. When the b parameter was between 0.2 and 0.6, the correlation coefficients between measured and predicted ionic concentrations were found to exceed 0.7. The parameter a is shown to represent the maximum ionic flux for a given cloud event. The ratio of the a parameter for SO₄^2- to NO₃^- varied between 1.75 and 6.95, indicating that the SO₄^2- contributes to the total ionic concentration substantially more than the NO₃^- leading to the conclusion that the cloudwater acidity is primarily due to the presence of sulfuric acid which has been demonstrated to cause foliar injury and growth retardation in red spruce trees. The above parameterization is similar to the one that is frequently used to relate ionic concentration in precipitation to the rainfall rate. In order to understand physico-chemical processes leading to the proposed parameterization schemes, meteorological and chemical variables are comprehensively analysed for one short duration and two long duration cloud events. The concentrations of principal ions (SO₄^2-, NO₃^-, H⁺ and NH₄⁺) during the short duration cloud events were found to be much higher than those during the long duration ones, especially at colder temperatures. Such short cloud events have a potential of causing foliar narcosis in red spruce stands because of unusually acidic cloudwater to which these stands stay exposed intermittently during each growing season.