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The spatial and temporal distribution of precipitation as well as its quantity is highly variable. The coefficient of variation is a function of total annual amount. The smaller the total precipitation, the greater the variability. Sampling procedures are very deficient. The small orifices of gages cause sizeable errors. Differences in amounts collected even by closely spaced gages can be very large, especially in summer. Time series of neighboring gages as little as 5 km apart may have correlations of less than 0.5. In rolling and mountainous terrain differences in single rain events can be tremendous. This will affect the dilution of aerosols that are washed out.
Essential parameters are generally not measured. Principal among these are drop or flake diameters. These will affect both the dilution of the nuclei material and the efficiency of washout of aerosols. The number and nature of the cloud condensation nuclei are only observed in special projects. Drop sizes will change during single rain events, and the washout process will be strongly affected by the duration of precipitation events. Very few records exist of electrical charges on precipitation elements. These may have a fairly substantial influence on aerosol collection.
All of these characteristics have an influence on precipitation chemistry. They will affect the choice of sampling equipment and the sampling frequency. Notable differences exist between liquid and frozen precipitation, and information must be available about various forms of precipitation for proper interpretation. The chemical nature of the condensation nuclei for cloud formation, ultimately ending up in liquid rain, is different from the freezing nuclei involved in solid precipitation. And in many instances rain may start out as snow, melting on its way down. In all instances, whatever chemical material is dissolved or suspended in precipitation is a mixture of both the nuclei necessary for condensation or sublimation and aerosol washed out below the cloud level, and gases incorporated into precipitation either in or below the cloud.
The possibility of further chemical action among various components of advective and intercepted material makes it advisable to do any analysis as promptly as possible, preferably during the precipitation event.
precipitation gages, precipitation sampling, precipitation variability, atmospheric scavenging
Professor emeritus, Institute for Physical Science and Technology, University of Maryland, College Park, Md.