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A class of trajectories found in 2005 but not much in evidence in the limited sampling <br />campaign in 2004 was from the south, originating in the Oklahoma/Texas area. The <br />highest CCN concentrations observed in 2005 at Stanley were associated with such <br />trajectories. Examples are 8 and 9 July. The concentration of CCN active at 1% <br />-3 <br />supersaturation on 9 July exceeded 2000 cm, the upper limit of the range of the <br />instrument. A significant forest fire started in the afternoon on 9 July in the northern <br />Black Hills of South Dakota, but this was near the end of the sampling period at Stanley <br />and smoke from the fire could not have reached Stanley during the sampling period. <br />These were, however, precipitation-free trajectories arriving from regions with higher <br />population densities and associated human activities compared to trajectories arriving <br />from the northwest or north. <br />Precipitation may be a factor in determining CCN concentrations, as was suggested in our <br />analysis of the 2004 observations. (Detwiler et al., 2005) The trajectories for relatively <br />clean days, such as 3, 5, 26, 27, and 28 June, were associated with precipitation one and <br />two days before arriving at Stanley. These also were trajectories generally from the <br />northwest and north. There was no precipitation along the back-trajectories from the <br />south arriving at Stanley on 8 and 9 July when concentrations were high. The 14 July <br />CCN concentrations were low, and were associated with a trajectory originating from the <br />south but looping around and arriving at Stanley from the northwest, with precipitation <br />overnight before arriving from the northwest. Thus the 14 July trajectory is difficult to <br />classify in our simplified analysis scheme. The only day with a clear consistent trajectory <br />from the south, and with precipitation within a day of its arrival, is 17 June. <br />Concentrations of CCN on 17 June were intermediate between those on the days with <br />lowest and highest concentrations. <br />It is difficult to separate out the effects on CCN concentration of trajectory origin and <br />precipitation within this 40-day data set. <br />Conclusions: <br />Sampling of CCN concentrations was conducted in Bowman in 2004 for 14 days in <br />August, and in 2005 in Stanley for 40 days in June and July. Different instruments were <br />used in the two years. Significant variability in concentrations averaged over periods of <br />~1/2 hr to ~3 hours was observed. In the 2004 14 day sample, variability between lowest <br />and highest concentrations active at 0.4% supersaturation was a factor of 6. In 2005, <br />variability defined in the same way but determined with a 40 day sample was a factor of <br />10 at 4 supersaturations ranging from 0.3% to 1.0%.Concentrations at 0.4% <br />supersaturation in 2004 were low compared to those observed at 0.3% and 0.5% in 2005, <br />and it suspected that the supersaturation calibration and/or the sample volume <br />determination for the instrument used in 2004 may be in error. Average concentrations of <br />-3 <br />CCN active at 0.3% in 2005 ranged from ~100 to ~1200 cm. At 1% supersaturation the <br />-3-3 <br />range was ~300 cm to greater than 2000 cm. <br />13 <br />