Effects of Non-Uniform Cloud Drop Composition on Pollutant Transformation and Removal in Winter CloudsEPA Grant Number: R823979
Title: Effects of Non-Uniform Cloud Drop Composition on Pollutant Transformation and Removal in Winter Clouds
Investigators: Collett Jr., Jeffrey L.
Institution: Colorado State University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 30, 1998
Project Amount: $339,597
RFA: Exploratory Research - Chemistry and Physics of Air (1995) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals
The purpose of this project is to characterize how variations in cloud drop composition as a function of drop size influence removal of individual chemical species through precipitation scavenging and influence particle production by in-cloud sulfur oxidation. Because large drops are more efficiently incorporated into precipitation, species enriched in large drops should be scavenged more efficiently by precipitation than will species enriched in small cloud drops. Sulfate production in clouds is influenced by chemical heterogeneity among cloud drop populations because of the nonlinear dependence of the rates of several key oxidation pathways on droplet composition.
The project focuses on field studies sampling winter clouds and precipitation at a high elevation laboratory in the Rocky Mountains of northern Colorado. Measurements of cloud drop chemistry as a function of drop size will enable prediction of how various sulfur oxidation pathways are enhanced (or suppressed) relative to rates predicted from the average cloud composition. Measurements of drop sizes captured by snow crystals will be used to determine which drop sizes are most important for transferring solute to precipitation for removal by wet deposition. By combining the size-resolved cloud drop composition with the drop sizes captured by snow crystals, it is possible to evaluate which chemical species experience enhanced (or reduced) removal by precipitation scavenging as a result of their distribution across the cloud drop size spectrum. Two intermediate goals for technology development are also included in the project. One is design of a cloud collector capable of sampling three independent portions of the drop size spectrum for chemical analysis from a supercooled cloud. The second is development of an image processing method for automated sizing of drops captured by snow crystals.
Information about the influence of chemical heterogeneity among winter cloud drop populations on in-cloud sulfate production will aid understanding of true rates of this important pathway for atmospheric aerosol production and will provide a much needed data set for testing of models designed to simulate chemically heterogeneous clouds. Relative precipitation scavenging efficiencies for different chemical species will provide a basis for evaluating whether model simulations of wet deposition need to include information about solute distributions with drop size. The cloud collector developed as part of this project will provide technology for the atmospheric chemistry community to obtain information about the size-dependence of cloud drop chemistry under demanding winter conditions, while the techniques developed for image analysis of snow crystals will be of benefit to future experimental studies of precipitation growth mechanisms.