Chemical deposition to a high elevation red spruce forest

Abstract

A preliminary analysis of O3, SO2, SO inf4 sup2− , and total NO inf3 sup− deposition to the red spruce forest on the summit of Whitetop Mountain, Virginia, illustrates uncertainties in analysis methodologies, establishes the relative importance of three deposition pathways, and suggests areas for further research. Results are presented here for an analysis of the dry, wet (precipitation), and cloud water deposition pathways for the four chemical species during a 26-day period in April and May 1986. Dry and cloud water depositions are estimated using available models along with air and cloud water chemistry measurements made at the summit. For water soluble species, depositions by precipitation and cloud interception are found to be comparable in magnitude, while dry deposition appears to be about an order of magnitude less. High levels of atmospheric O3 lead to a large estimate of 03 deposition (on a mass flux basis) when compared to the estimated deposition of gaseous SO2. This is in spite of the fact that computed SO2 dry deposition velocities exceed those for O3. Model uncertainties are large for both dry deposition velocity and cloud water flux computations, and some bias in computations probably exists because of the application of the models to a complex terrain situation. Field evaluation of the cloud water deposition model is of greatest priority because of the apparent relative importance of that deposition pathway.