Interactions of atmospheric particulate and trace gases with clouds at Cheeka Peak, WA

Abstract

Measurements of various aerosol, trace gas, and cloud water constituents were made during two intensive observation periods at the Cheeka Peak Observatory (CPO) on the Washington state coast. These measurements were a part of the second and third Cloud and Aerosol Chemistry Experiments (CAChE 2 and CAChE 3) conducted from April 19 to May 18, 1994 and August 30 to September 23, 1994, respectively. Gas phase hydrogen peroxide was measured continuously during these two time periods. Cloud water samples were analyzed for major ion constituents and aqueous hydrogen peroxide. The cloud water samples collected during CAChE 2 represented bulk collection, whereas the cloud water samples collected during CAChE 3 were segregated into two size fractions, with mean droplet diameter of approximately 12 $\mu$m and 13 $\mu$m. Major ion concentrations (e.g., Na$\sp+,$ NH$\sb4\sp+,$ H$\sp+,$ NO$\sb3\sp-,$ SO$\sb4\sp=,$ etc.) were higher than previously observed at Cheeka Peak, but comparable to other West coast locations. Despite the small difference in collected droplet size, size dependence was observed for several species. Seasalt ions and NO$\sb3\sp-$ were more concentrated in the larger drops, whereas peroxide was more concentrated in the smaller drops. Size dependence was not observed for MSA, non seasalt sulfate (nss-SO$\sb4\sp=)$ and NH$\sb4\sp+,$ and was only weakly observed for H$\sp+.$Cloud-free gas phase H$\rm\sb2O\sb2$ and organic peroxide average concentrations were on the order of a few hundred pptv and did not vary much by wind sector; slightly higher concentrations were observed from the continental sector. In general, these concentrations were lower than those measured at east coast sites. Average cloudwater H$\rm\sb2O\sb2$ concentrations for both CAChE experiments were approximately 35 $\mu$M and indicated conditions at Cheeka Peak were never oxidant-limited.. Evidence for sulfate production via H$\rm\sb2O\sb2$ oxidation was observed but was not conclusive.Finally, a preliminary comparison was made between predictions of a size-resolved cloud chemical model and limited observations of size-resolved cloud chemistry for cases upon which the model was initialized. Some agreement was obtained and discrepancies could be attributed to an unrealistic treatment of particulate nitrate in the model.