Variation of Sodium and Chloride Concentrations with Rainfall Intensity in Hawaiian Trade Wind Showers

Abstract

The variation of sodium and chloride concentrations with rainfall intensity in Hawaii was investigated in 13 trade wind showers. The variation was found to be inversely dependent on rainfall intensity, although the ratio of Na to Cl appeared to be independent of the intensity. Several factors which may affect the dependence of concentration on rainfall intensity are discussed, and it is concluded that: (1) washout and evaporation, more than any process which may occur within the cloud, are the dominant factors in increasing the salinity of the small raindrops collected below the cloud base, and (2) exchange of gaseous and dissolved chlorine is a minor factor affecting the variation of chloride concentration with rainfall intensity. IT IS WELL ESTABLISHED that the concentration of various soluble substances in rain depends upon total rainfall amount.3 In general, concentration of such species as NOg-, SO/, Na+, and Cl-, and radioactivity, etc., decrease with increasing amount of rainfall, both for widespread rain systems and for thundershowers. An extensive review of the dependence of concentration on rainfall amount is given by Junge (1963) . Woodcock and Blanchard (1955) have shown that an inverse dependence of chlorine concentration in rain on rainfall intensity occurs in Hawaiian trade wind showers. This relationship was corroborated for sodium (and from this, for total salt) by Eriksson (1957) and Georgii and Weber (1961), and for total dissolved nitrogen (ammonia and nitrate) by Mordy (1953). In none of these investigations 1 Hawaii Institute of Geophysics Contribution No. 275. Manuscript received April 15, 1969. 2 Department of Chemistry and Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii 96822. 3 There is confusion in the literature concerning the meaning of "amount" and "intensity" when applied to rainfall. In general, "amount" has referred to the volume of rain collected, with no reference to the length of the collection period, whereas "intensity" has referred to the amount collected per unit time. These terms have been erroneously used interchangeably on many occasions. There is no necessary relation between rainfall "amount" and rainfall "intensity" as defined above. was an attempt made to collect numerous samples in a short time period so that the detailed variations which possibly occur in the chemical composition with rainfall intensity could be investigated within an individual rain shower. In fact, very few studies of this sort have ever been attempted, and in those only one or two showers were examined with generally a maximum of 5 or 6 samples collected in each shower (Gorham, 1958; Turner, 1955). Perhaps the most detailed study of the dependence of concentration on rainfall intensity was that of Gatz and Dingle (1966), who investigated the variation of radioactivity and pollen in the rain associated with two squall lines in Oklahoma. The purpose of our study was to investigate in detail the variation of the sodium and chloride concentrations with rainfall intensity in Hawiian trade wind showers in order to evaluate some of the major factors which influence these concentration variations. In addition, it was hoped, by studying changes in the Na/Cl ratio with varying rainfall intensity, that information could be obtained on the exchange of gaseous chlorine with dissolved chloride in the rain.

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Cite this paper

@inproceedings{Duce2008VariationOS, title={Variation of Sodium and Chloride Concentrations with Rainfall Intensity in Hawaiian Trade Wind Showers}, author={Robert A. Duce and Jarvis and MOYERS}, year={2008} }