Urea in rainwater and atmospheric aerosol

The measurement of urea (CO(NH2)2) in rainwater samples from predominantly marine-influenced locations in Bermuda, and Ireland, and in rains and aqueous aerosol extracts from a rural site at UEA, Norwich indicates that urea is not generally a major contributor to atmospheric water-soluble organic nitrogen. At UEA, where anthropogenic and natural sources of urea are expected to be most intense, urea accounts for <10% of rainwater dissolved organic nitrogen (DON), and <1% of the water-soluble fraction of aerosol organic nitrogen. The analysis of size-segregated aerosol samples indicates that the size distribution of urea is quite different from those of ammonium and nitrate. In the less anthropogenically impacted Atlantic sites, rainwater urea was below the detection limits of the colorimetric method used in this study, consistent with expected dilution processes or reaction of urea during transport. However, in a small set of rain samples collected in Tahiti, urea concentrations ranged from 1 to 8 μmol l-1, accounting for >40% of the DON measured in those samples. This may be a consequence of strong local sources, or it could possibly result from the partial breakdown of other DON compounds to urea during sample transport and storage. However, the similarity in urea concentrations observed in Pacific samples in this present study and in a previous one (Timperley et al., 1985, Canadian Journal of Fisheries and Aquatic Science 42, 1171–1177) suggests that this may reflect a difference in rain chemistry between Atlantic and Pacific rains, perhaps resulting from differences in levels of agricultural urea usage between Asia and the rest of the world.