HR: 0800h
AN: A31B-0831    [Abstracts]
TI: Comparison of measured and modeled Aerosol Optical Thickness in Marine Atmospheres
AU: * Halthore, R N
EM: halthore@nrl.navy.mil
AU: Caffrey, P F
EM: peter.caffrey@nrl.navy.mil
AB: Direct solar irradiance measurements in the shortwave (340 nm - 1 μm) at two remote Pacific Ocean locations in the northern hemisphere reveal a minimum in aerosol optical thickness (AOT) in the solar spectrum of magnitude ~ 0.02 at 550 nm and an Å ngström exponent (a measure of the slope of AOT versus wavelength curve: b in τ ~ aλ ^b where τ is AOT and λ is wavelength) of about -1 similar to the measurements at continental sites reported previously. This minimum indicates the presence of background aerosol at both continental and marine sites. While detailed aerosol physical and chemical characterization under background conditions is necessary to understand the origin and transport of these aerosols, we are able to deduce the possible nature of these aerosols with the aid of a comprehensive Lagrangian boundary-layer microphysical aerosol model in conjunction with a mesoscale model and trajectory analysis. First we obtain a measure of the marine boundary layer AOT by taking the difference between values measured at sea-level and at altitude (free tropospheric aerosol) on Mauna Loa, Hawaii. We obtain a `flat' wavelength dependence (|b| < 0.5) indicating that the particles are large, likely consisting of sea-salt. This then is compared with AOT calculated by using sea-salt and sulfate aerosols in the model and with meteorology input from a mesoscale model to provide constraints, under certain conditions, to within a factor of 5 of a hybrid Monahan-Smith formulation of the sea salt flux. We perform sensitivity studies to ascertain the role of uncertainties in the model assumptions and inputs, such as the uncertainty in sulfate and sea-salt flux formulations or the adequacy of the Lagrangian time span. Finally we note that this procedure of calculating AOT as a function of wavelength from basic principles, despite the potentially large modeling uncertainties, can be effective in constraining aerosol flux in the marine boundary layer.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0360 Radiation: transmission and scattering
DE: 1640 Remote sensing (1855)
DE: 3311 Clouds and aerosols
DE: 4801 Aerosols (0305, 4906)
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005