Acidic cloud episodes in the Northern Colorado Rockies: Inadvertent weather modification?

During the winter months, Storm Peak Laboratory (3210 m MSL) in the northern Colorado Rockies is frequently enveloped by precipitating supercooled clouds. Occasionally during these one-to-two day cloud episodes, cloud water pH values, which average 5 or greater, reduce to 4 or less for periods of 12 to 24 hours. The corresponding snowfall pH values, which are typically larger than the cloud values, likewise reduce. The acidic episodes are characterized by increased concentrations of small droplets, reduced cloud liquid water contents, reduced precipitation rates and increased aerosol particle concentrations in the sub-cloud inflow air. The increased particle concentrations are consistent with the increased concentrations of small droplets which may lead to the reduction in precipitation rates by a reduction in snow crystal riming. To understand the increased particle concentrations and reduced liquid water contents, the meteorological histories of the air parcels arriving at SPL were studied using 48 hour back-trajectories and the corresponding radar images of regions of precipitation. It was found that the parcels arriving at SPL during the acidic episodes had encountered the least precipitation suggesting a reduction in the removal of aerosol particles and reduced water vapor contents. Hence, meteorological conditions upwind of SPL may be as significant to the formation of the acidic cloud episodes as human-produced aerosol particles.