Organic nitrogen in Hawaiian rain and aerosol

Abstract

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.