Evidence of high deposition of ultrafine particles at Mauna Loa Observatory

Few measurements exist for the dry deposition of ultrafine particles (diameters less than about 10 nm) under realistic field conditions. Recently refined screen-diffusion techniques have sufficient sensitivity to measure radon (222Rn) progeny under low-level outdoor conditions. Vertical gradients of ultrafine radon progeny at Mauna Loa Observatory were measured for a range of atmospheric conditions. Gradients were generally stronger, for example, than previously reported for the reactive vapor HNO3 under comparable meteorological conditions. Gradient analysis indicated a mean deposition velocity for unstable conditions and moderate winds of 5.2 ± 0.9 cm s−1 at a reference height of 18 m. Estimated deposition velocities for lower reference heights were higher for many daytime conditions. These results are higher than suggested by some models and references, and indicate that the aerodynamic part of deposition might be greater than sometimes assumed. At Mauna Loa Observatory, dry deposition could be more important than previously thought for reduction of the concentration of certain reactive trace gases and ultrafine particles sampled near ground.