Dry deposition

Gravitational sedimentation of particles during periods without precipitation. These particles include: aerosols, sea salts, particulate material, and adsorbed/reacted gases captured by vegetation. Impaction is the capture of particles moving horizontally in the airstream by the vegetation canopy. This filtering effect may accumulate large quantities of nutrients in some environments. Dry deposition appears to be roughly equivalent to wet deposition for sulfur and nitrate; however, dry deposition in the form of particulate matter is the dominant source of NH₄⁺. The principle form of gaseous sulfur is SO₂ and its concentrations in air ranged between 1 and 6 µg S/m³ across 17 forest ecosystems in the Integrated Forest Study (northeastern, southeastern, and northwestern North America, Canada, and Norway - Johnson and Lindberg, 1992). The dominant forms of gaseous nitrogen were NO₂ gas, nitric acid (HNO₃) vapor, and trace amounts of NH₃. Concentrations of NO₂ and HNO₃ vapor ranged between 1.9-15.1 and 0.8-2.8 µg N/m³, respectively, for sites in the Integrated Forest Study. The oxidation of NO₂ gas (to HNO₃) in the atmosphere is much more rapid that SO₂ gas (to H₂SO₄) resulting in longer-distance transport of SO₂ as a pollutant in the atmosphere. Examples of dry deposition inputs in the form of gaseous components and particulate matter are shown for a low deposition site in the Washington Cascades and a high deposition site in the Great Smokey Mountains, North Carolina (see Tables 1a & 1b).