Organic nitrogen in Hawaiian rain and aerosol

Water-soluble organic nitrogen (ON) is an important component of fixed nitrogen in clean marine aerosol and rainwater collected at a site on the windward coast of Oahu, Hawaii. Aerosol material associated with the predominant trade winds carries 3.3 ± 2.0 nmol ON m-3, which makes up roughly one third of the total nitrogen in aerosol (11 ± 4 nmol N m-3). The inorganic nitrogen (65% as nitrate) is predominantly found in coarse-mode aerosol, consistent with displacement reactions of sea-salt chloride. In contrast, most of the ON is found in fine particle (submicrometer) aerosol, and may be associated with gas-to-particle conversions and with long-range transport in the atmosphere. At times, aerosol ON also appears to have a local, anthropogenic source, and when meteorological conditions are favorable, large pulses of ON from these local sources can dominate the total fixed nitrogen in the sampled aerosol (30-50 nmol ON m-3, contributing about 80% of the total aerosol nitrogen). About one fifth of rainwater dissolved nitrogen at this site is organic nitrogen. The average rainwater concentration of dissolved ON was 2.8 µmol N L-1, and of inorganic nitrogen (nitrate plus ammonium) was 15 µmol N L-1. In both rainwater and aerosol, urea was a major component of the ON, contributing about half of the ON and about 15% of total nitrogen. This quantitative importance of urea as a component of ON has not previously been seen in continental locations.