Cloud and fog processing of atmospheric organic compounds

Over the last twenty years, it has become increasingly evident that clouds and fogs play an important role as processors of inorganic aerosol particles and trace gases. They both promote new particle mass formation (e.g., via rapid aqueous oxidation of sulfur dioxide to sulfate) and promote particle removal (e.g., via nucleation scavenging followed by direct drop deposition or drop incorporation into precipitation). Up until now, however, we know relatively little about fog and cloud processing of organic aerosol particles and trace gases. While a handful of compounds have received moderate attention (e.g., low molecular weight carboxylic acids), they form only a fraction of the multitude of organic compounds known to be present in the atmosphere.

Cloud chemistry research efforts at Colorado State University and elsewhere are being increasingly directed to understanding processing of carbonaceous aerosol particles by fogs and clouds. We are continuing to evaluate several methods for extraction of organic compounds from fog samples, including liquid-liquid extraction, solid phase extraction, and solid phase microextraction (SPME) using small fibers which adsorb organic solutes from the aqueous sample matrix. Both liquid-liquid extraction using dichloromethane and SPME have proven useful in examining concentrations of a wide range of organic compounds. SPME, in particular, is interesting because it shows high reproducibility and provides a simple means of compound extraction without intensive labor or the use of organic solvents. Samples are desorbed from the fiber that is inserted directly into the GC-MS sample injection port.

We will present some of our findings regarding the wide variety of organic compounds (including alkanes, organic acids and diacids, polyaromatic hydrocarbons (PAH), and substituted phenols) present in authentic fog samples collected using specially designed stainless steel samplers. We will present findings concerning partitioning of individual compounds between dissolved and insoluble phases within the droplets. The discussion will also focus on the identification and quantification of specific organic compounds shown by other investigators to serve as tracers for various combustion source types. Evaluation of the efficiency with which these compounds are scavenged by clouds and fogs will help us better understand interactions between fogs/clouds and various carbonaceous aerosol types in the future.