10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Developing a Large Surrogate Surface to Measure Dry Deposition of Atmospheric Aerosols
ALEXANDER JOHNSON, Cliff Davidson, Syracuse University
Abstract Number: 1047 Working Group: Remote/Regional Atmospheric Aerosol
Abstract Dry deposition of aerosols is a major input to urban and environmental systems and degrades natural surfaces as well as surfaces of infrastructure; surrogate surfaces are frequently used to measure dry deposition fluxes. Some of these surfaces are designed to minimize atmospheric turbulence near the leading edge and create a thin boundary layer of approximately constant thickness over the surface. These characteristics enable estimates of the lower limit of the dry deposition flux to rougher, more complex natural surfaces.
Typical collector geometries include the symmetric knife-edge surrogate surface and the frisbee-shaped surrogate surface- the latter is in the shape of an airfoil. An issue with many of these collectors is the small surface area; it is difficult to obtain reliable measurements during short dry periods, such as one, two, or three days of exposure. One category of aerosol species of interest is trace elements, but measuring deposition of trace elements can be especially difficult with small surrogate surfaces. It is often not possible to achieve a signal-to-noise ratio above unity; it is also easy to contaminate and invalidate a sample. Longer exposure times may be needed to minimize these issues, which may not be possible in climates with frequent precipitation. Therefore, there is interest in using larger surrogate surfaces to estimate dry deposition fluxes over short exposure periods.
Flat horizontal disks, 1.2 meters in diameter, were designed and used to obtain estimates of the dry deposition flux for three chemical species: sulfate, nitrate, and fluoride. The study had three objectives: to obtain a range of fluxes for each chemical species to determine if measurements were reproducible; to compare flux estimates onto the disks and onto airfoils to determine if the former can be used to estimate the lower limit of the flux to complex surfaces; and to model the boundary layer over the disks to compare with that over the airfoils.
Experiments lasting 2-6 days were conducted on the Syracuse University campus during 2016-2017. Fluxes of sulfate ranged from 71-180 µg m-2 day-1, fluxes of nitrate ranged from 120-250 µg m-2 day-1, and fluxes of fluoride ranged from 0.89-5.3 µg m-2 day-1: a value in each range is an average of the two measurements from both disks. Deposition velocities of sulfate ranged from 0.11-0.75 cm/s, deposition velocities of nitrate ranged from 0.20-1.1 cm/s, and deposition velocities of fluoride ranged from 4.2-5.9 cm/s. Signal-to-noise ratios were greater than 5 for these chemical species for most experiments. Fluxes onto the airfoils were simultaneously measured for sulfate and nitrate. The ratio of the average flux onto the disks and the average flux onto the airfoils was 0.79 for sulfate and was 0.60 for nitrate; fluxes were lower onto the disks. ANSYS FLUENT was used to model the boundary layer over the disks under laminar flow for three wind speeds: 50, 100, and 500 cm/s. The boundary layer grew in thickness with distance from the stagnation point. For all wind speeds, the boundary layer was generally thicker over the disks than over the airfoils; the airfoils were modeled in a previous study. These observations suggest that there is more resistance to deposition of submicron particles onto the disks, and hence the disks can be used to estimate a lower limit to dry deposition fluxes onto complex surfaces. Experiments with disks are underway to examine the dry deposition of other chemical species with very small concentrations, which would not be possible with the airfoils or other small surfaces.