Cloud Condensation Nuclei from Industrial Sources and Their Apparent Influence on Precipitation in Washington State

Measurements of the concentrations of cloud condensation nuclei (CCN) in the air in Washington State have shown that pulp and paper mills, and certain other industries, are prolific sources of CCN. The rate of production of CCN from large paper mills can be as high as 1019 sec−1. Direct observations show that clouds often form downwind of these mills and, in some cases, these clouds produce precipitable particles very efficiently. A comparison of precipitation and streamflow records in Washington for the period 1929–46 with those for 1947–66 shows that a number of areas have had a mean annual precipitation during the second period more than 30% greater than that in the first period. In nearly all cases these areas are in the vicinity of large industrial sources of CCN and it is suggested that the higher precipitation in recent years is a consequence of the increased numbers of very efficient CCN emitted into the atmosphere by these sources.