Meteorological data has been collected at Storm Peak Laboratory (SPL) in northwestern Colorado by CCNY students almost every January since 1990. During a period of clear air in January 2001, a distinct east wind was recorded. This east wind was unusual because the prevailing winter weather pattern at SPL is dominated by westerly winds. Using NOAA-HYSPLIT 72-hour back-trajectory analyses, three distinct source regions were identified. At the beginning of the event, the air originated from the SW, during the middle of the event it originated from the E and at the end of the event it originated from the NW. Further, air originating from the SW and E had similar condensation nucleus (CN) concentrations but the air from the SW contained significantly higher trace gas concentrations (SO2, NO2). The CN and trace-gas emissions for the two source regions (the Phoenix and Denver metropolitan areas) are expected to be similar. Thus, the similar CN concentrations and different trace-gas concentrations may have been due to inefficient scavenging of CN by cloud and precipitation formation but efficient scavenging of trace-gases. To test this hypothesis, radar images of precipitation were superimposed on the corresponding air-parcel trajectories. It was found that the parcels originating from the E experienced approximately 12 hours of precipitation while those from the SW experienced no precipitation. Thus, cloud and precipitation scavenging may have reduced the concentrations of trace-gas but not the concentrations of CN arriving at SPL from the E source region.