Long-term, wintertime aerosol, cloud and precipitation measurements in the Northern Colorado Rocky Mountains, USA

At Storm Peak Laboratory (SPL) in the northern Colorado Rocky Mountains during the winters of 1983/1984 through 2003/2004, significant trends occurred of decreasing cloud droplet concentrations and initially increasing cloud and snow pH values then more recent decreasing values. The decrease in cloud droplet concentrations and a corresponding increase in mean droplet diameters are consistent with liquid water content trends in the long-term record. Decreased condensation nucleus concentrations, and most likely cloud-condensation nucleus concentrations as well, caused the decrease in droplet concentrations. An inverse relationship between cloud pH and condensation nucleus concentrations was identified. However, no relationship between condensation nucleus concentrations and precipitation rates was identified. Thus, the inverse relationship between aerosol concentration and precipitation rate reported by Borys et al. [Borys, R.D., Lowenthal, D.H., Cohn, S.A., Brown, W.O.J., 2003. Mountaintop and radar measurements of anthropogenic aerosol effects on snow growth and snowfall rate. Geophys. Res. Lett. 30, 1538] for SPL was not confirmed. This aerosol effect may be important for only a small subset of winter storms at SPL.