During the winter, Storm Peak Laboratory (SPL), at the crest of the north-south oriented Park Range in the northern Colorado Rockies, is frequently enveloped in clouds. The clouds form in moist, stable Pacific air masses which are forced to rise over the barrier. As a result, SPL was almost continuously immersed in cloud between 1700 MST on 19 January 1998 and 1100 MST on 23 January 1998. During a 30 hour period, within this cloud episode, cloud water pH values reduced dramatically from an average of 4.5 to 3.8. Similarly, cloud droplet number-concentrations increased, mean droplet diameters decreased, anion concentrations increased and cloud liquid water contents remained essentially constant. Surprisingly, in-cloud condensation nucleus concentrations did not increase significantly. These changes in cloud properties are expected when a polluted air mass flows through the cloud enveloping SPL. We are in the process of constructing 48 hour back-trajectories to determine the source(s) of the air mass(es) that led to this apparent pollution event. We expect to determine if the event was natural or human-caused
Symposium on Interdisciplinary Issues in Atmospheric Chemistry