Nitrogen deposition to the eastern Atlantic Ocean: The importance of south-easterly flow

• Published in: Tellus B: Chemical and Physical Meteorology
• Main Authors: Spokes, Lucinda J., Yeatman, Stuart G., Cornell, Sarah E., Jickells, Tim D.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2000
• Subjects: Arctic; Mace; Phytoplankton
• Online Access: https://ueaeprints.uea.ac.uk/id/eprint/32310/; https://doi.org/10.3402/tellusb.v52i1.16080

Converting measured concentrations into fluxes and using estimates of biological productivity in the coastal waters of the eastern Atlantic Ocean enables us to determine the role of the atmosphere as a source of biologically essential species, including nitrate and ammonium, to the marine biota. To understand the effects of the atmosphere as a source of nitrogen capable of promoting new production, we need to know both the seasonality of the input as well as the effects of extreme high deposition events which, while small in overall annual budget terms, maybe able to extend, or even promote, phytoplankton growth under nutrient depleted summer conditions. Aerosols and rainwater were collected at both Mace Head and at sea aboard RRS Challenger. Temporal patterns have been interpreted using airmass back trajectories which give the predicted air path prior to arrival at the sampling site. Low levels of both nitrate and ammonium are seen associated with marine westerly flow across the Atlantic and northerly air originating in the Arctic region. As expected, marine derived sodium, chloride, magnesium and seasalt sulphate are high during these periods. High concentration nitrate and ammonium events are seen associated with south-easterly flow where the airmass passes over the UK and northern Europe prior to arrival on the west coast of Ireland. In the polluted atmosphere, nitrate exists as nitric acid and as fine mode (< 1 µm diameter) ammonium nitrate aerosol. In the coastal zone, nitric acid reacts with coarse mode seasalt aerosols to form coarse mode (> 1 µm diameter) sodium nitrate: HNO3 (g) + NaCl(s) ? NaNO3(s) + HCl(g). This seasalt displacement reaction not only enhances dry nitrate deposition through more efficient gravitational settling of large particles, but also increases the efficiency of precipitational scavenging via inertial impaction. By looking at the size distribution of nitrate, we can see evidence for the seasalt displacement reaction. Under the polluted south-easterly flow, ~40-60% of the ...