Aerosol distributions and cloud interactions at a mountaintop laboratory

The first measurements of ultrafine (3–10 nm diameter) condensation nuclei concentrations and particle size distributions at Storm Peak Laboratory (3210 MSL) are presented. In‐cloud average hourly average ultrafine concentrations peaked at 1608 cm−3 in the 1300 hour, MST. Under clear conditions in April and May 2001, average hourly average ultrafine concentrations reached 8947 cm−3 during the 1400 hour, MST. The highest observed 5‐minute average ultrafine concentration was 32000 cm−3. A comparison of clear‐air and interstitial aerosol size distributions indicates that particles larger than 0.5 μm were efficiently removed by snowing, mixed‐phase clouds. Particles with diameters between 0.03–0.04 and 0.2 μm appeared to serve as cloud condensation nuclei. The observed loss of particles smaller than 16 nm in‐cloud may be explained by diffusion to cloud drops.