The concentration and speciation of iodine have been determined in wet and dry deposition at a coastal site over a 15‐month period. Deposition fluxes in rain (2.7 μmol m −2 yr −1 ) and aerosol (3.6–6.5 μmol m −2 yr −1 ) are the major routes for removal of iodine from the marine atmosphere onto the Earth's surface, with only a minor contribution from direct deposition of methyl iodide (0.003–0.17 μmol m −2 yr −1 ). Iodate (IO 3 − ) is often considered to be the only species of iodine that is permanently removed to the aerosol phase, and IO 3 − may therefore be expected to be the dominant form of iodine in precipitation. However, iodide (I − ) was found to constitute a significant fraction (5–100%) of iodine in both rain and aerosol. This implies that the rates of iodate formation and iodide volatilization (through reaction with hypohalous acids) are relatively slow. A third pool of aerosol iodine (nonvolatile organic compounds) may also contribute to removal of iodine from the atmosphere in dry or wet deposition.
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.
In some places the boundary between land and sea is in the form of abrupt and often spectacular cliffs but, elsewhere, the boundary can take the form of a complex environment of intertidal sediments. These environments include shingle banks, sandy beaches, mud flats, saltmarsh and mangrove (or mangal) communities. In some cases one or other of these environments will occur, in others they will be associated with one another. For example, on many North Sea and North American East Coast shorelines, mud flats grade into saltmarshes behind the shelter of shingle spits and sand dunes. In general, saltmarshes and mangroves occupy similar ecological niches with mangroves at lower latitudes (winter temperatures greater than 10°C) and saltmarshes at high latitudes, though in some locations both communities coexist (Chapman, 1977).
Reactive nitrogen (N(r)) compounds have different fates in the atmosphere due to differences in the governing processes of physical transport, deposition and chemical transformation. N(r) compounds addressed here include reduced nitrogen (NH(x): ammonia (NH(3)) and its reaction product ammonium (NH(4)(+))), oxidized nitrogen (NO(y): nitrogen monoxide (NO) + nitrogen dioxide (NO(2)) and their reaction products) as well as organic nitrogen compounds (organic N). Pollution abatement strategies need to take into account the differences in the governing processes of these compounds when assessing their impact on ecosystem services, biodiversity, human health and climate. NO(x) (NO+NO(2)) emitted from traffic affects human health in urban areas where the presence of buildings increases the residence time in streets. In urban areas this leads to enhanced exposure of the population to NO(x) concentrations. NO(x) emissions generally have little impact on nearby ecosystems because of the small dry deposition rates of NO(x). These compounds need to be converted into nitric acid (HNO(3)) before removal through deposition is efficient. HNO(3) sticks quickly to any surface and is thereby either dry deposited or incorporated into aerosols as nitrate (NO(3)(-)). In contrast to NO(x) compounds, NH(3) has potentially high impacts on ecosystems near the main agricultural sources of NH(3) because of its large ground-level concentrations along with large dry deposition rates. Aerosol phase NH(4)(+) and NO(3)(-) contribute significantly to background PM(2.5) and PM(10) (mass of aerosols with an aerodynamic diameter of less than 2.5 and 10 mu m, respectively) with an impact on radiation balance as well as potentially on human health. Little is known quantitatively and qualitatively about organic N in the atmosphere, other than that it contributes a significant fraction of wet-deposited N, and is present in both gaseous and particulate forms. Further studies are needed to characterise the sources, air chemistry and removal rates of organic N emissions.
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