Shenandoah Cloud and Photochemistry Experiment (SCAPE): Overview

Abstract

The Shenandoah Cloud and Photochemistry Experiment (SCAPE) investigated the chemistry of the atmosphere at a high-elevation forested site in central Virginia with particular focus on the factors controlling concentrations of ozone, carbonyl compounds, and carboxylic acids. Carbonyl compounds and carboxylic acids are ubiquitous chemical constituents of the global troposphere, but many aspects of their biogeochemical cycling are poorly understood. Carbonyls are generated from the oxidation of hydrocarbons; formaldehyde (HCHO) produced via this pathway is an important precursor for OH. Thus the carbonyls are intermediates in and tracers of tropospheric photochemistry [e.g., Carlier et al., 1988]. Carboxylic acids are major natural sources of atmospheric acidity thereby partially controlling pH-dependent chemical transformations in clouds [e.g., Keene and Galloway, 1988]. The aqueous-phase oxidation of HCHO in clouds has been hypothesized as a significant source of formic acid (HCOOH) and the aqueous-phase oxidation of HCOO− a potentially important in-cloud sink for OH [Chameides and Davis, 1983; Jacob, 1986]. Under many conditions, clouds should be a net sink for tropospheric HCOOH [Jacob and Wofsy, 1988]. Specific research objectives were (1) to assess the relative importance of NOx and hydrocarbons in limiting regional ozone production, (2) to investigate sources and sinks for carbonyl compounds and caiboxylic acids, and (3) to examine the droplet-size dependence of solute concentrations and the thermodynamics of phase partitioning for carbonyl compounds and carboxylic acids in clouds.