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Richard C. Eagan, Peter V. Hobbs, and Lawrence F. Radke

Abstract

Airborne measurements of cloud condensation nuclei (CCN) were made in the vicinity of a planned burn of natural forest products. The concentrations of CCN, active at 0.5% supersaturation, reached a peak value of 5000 cm−3 about 38 km downwind of the fire, compared to concentrations of 250–500 cm−3 upwind. It is estimated that 6 × 1010 CCN were produced per gram of wood consumed.On another occasion the microstructure of small cumulus clouds were measured in the vicinity of a natural forest fire. The droplet size distributions were narrower and the concentrations of droplets higher in two clouds situated in the plume from the fire than in two clouds upwind of the plume.

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Richard C. Eagan, Peter V. Hobbs, and Lawrence F. Radke

Abstract

Airborne measurements of cloud condensation nuclei (CCN), total particle counts, the size distribution of particles, and the microstructure of warm clouds have been made in the vicinity of a large Kraft pulp and paper mill. The plume of CCN and total particles from the mill is well-defined at distances up to 40 km downwind and the concentrations of these particles reach values which exceed those in the ambient air by a factor of about 4. A large peak in the concentrations of CCN was measured in the plume at a distance downwind of the mill corresponding to a travel time of about 4 hr. The total particle count in the plume peaked after about 2 hr of travel time downwind. The particles in the plume were measured over the size range 0.3 to 8 μm; their concentrations were much higher than in the ambient air, particularly those with diameters <1 μm. At points downwind in the plume corresponding to travel times ≳6 min, a secondary peak appeared in the concentration of particles with diameters just in excess of 0.3 μm and it persisted for travel times up to about 4 hr. The rates of production of CCN and particles >0.3 μm in diameter in the plume is estimated to be about 0.05 cm−3 sec−1. It is suggested that this is due to gas-to-particle conversions.Clouds that form in the plume from the paper mill had a much broader spectrum of droplet sizes than those which form in the ambient air. Since the former clouds contain a significant number of droplets with diameters >30 μm, as well as a large number of smaller droplets, precipitable drops should form in them very efficiently by the coalescence mechanism. These observations support the suggestion made by Hobbs et al. that the effluents from paper mills are probably responsible for the anomalously high annual precipitation amounts which have occurred in the vicinity of such mills in Washington State in recent years.

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