The composition and deposition of organic carbon in precipitation

Abstract

Wet-only precipitation was collected at Hubbard Brook, New Hampshire and Ithaca, New York from June 1976 through May 1977. Samples were analyzed for dissolved organic carbon (DOC), particulate organic carbon (POC), carboxylic acids, aldehydes, carbohydrates, tannidlignin, primary amines and phenols. DOC was partitioned into molecular weight ranges via ultrafiltration at nominal cut-off values of 1000; 5000; 20,000; 50,000 and 100,000 daltons. Annual total organic carbon (TOC) concentrations and depositions were 1.28 mg I-1 and 14.0 kg ha-1 at Hubbard Brook, and 2.37 mg 1-1 and 24.2 kg ha-1 at Ithaca. DOC comprised 84% and 80% of these totals, respectively. The majority of annual deposition occurred in summer (June-September), when both concentrations and rainfall amounts were higher. Although volume-weighted, annual mean TOC concentrations at Ithaca were more than 1.8 times higher than at Hubbard Brook; the physical/chemical profiles were similar at both sites. Particulate plus dissolved macromolecular (> 1000 MW) organics accounted for 51% and 63% of TOC at Hubbard Brook and Ithaca, respectively. Carboxylic acids, aldehydes, carbohydrates (chiefly polysaccharides) and tannidlignin comprised most of the remaining carbon, and were present in similar amounts (as % of TOC). It is suggested that a major fraction of TOC in precipitation at these two sites in the northeastern U.S. was derived from airborne particulate matter, such as soil dust and plant material.