Sampling and chemical interpretation of precipitation for mass balance studies

Abstract

From an analytical viewpoint, total dry and wet precipitation consists of three fractions: (1) dissolved materials in aqueous precipitation, (2) the water‐soluble component of dry precipitation, and (3) the water‐insoluble component of either wet or dry precipitation. Methods of precipitation collection and processing greatly affect the separation of these components. A literature survey shows that fraction 3 has typically been ignored and that samplers currently in use lead to a highly variable mixture of the three fractions. Major causes of interpretational ambiguity include (1) variable leaching of dry fallout, (2) particle formation and chemical repartitioning in aqueous precipitation, (3) height, texture, and composition of the collector surfaces, (4) relation of collector surfaces to natural environmental surfaces, (5) contamination by birds, insects, pollen, and organometallic release from local vegetation, (6) sample storage, and (7) failure to analyze insoluble particles. A literature survey also shows that most collectors currently in use are of insufficient size to collect samples large enough to support broad‐spectrum analysis on a weekly basis. The minimum satisfactory size of collectors is computed from (1) average rain chemistry, (2) sensitivity of standard chemical tests for chemical species of biological interest, and (3) volume required for each analysis. The computations show that collectors should have an area of at least 1200 cm2 in regions of average chemistry and as much as 8300 cm2 in cold climates with minimal aqueous precipitation. A design is given for a collector which combines large size with other desirable features.