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  • Author or Editor: Antony D. Clarke x
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Antony D. Clarke and Vladimir N. Kapustin

Abstract

The Shoreline Environment Aerosol Study (SEAS) was carried out in Hawaii on the southeast coast of Oahu in an area exposed to relatively steady onshore flow. This location provided favorable opportunities to test and evaluate new instrumentation designed to improve measurements of marine aerosol and its physical, chemical, and optical properties, including the remote sensing (lidar) of coastal aerosol fields. Data acquired from the diverse instruments deployed before and during SEAS demonstrate that environmental and meteorological features actively influence aerosol measurements at this location. Both a ceilometer and a visibility sensor were operated continuously and found to be effective autonomous tools for characterizing mixed-layer aerosol conditions. These instruments also helped demonstrate that changes in physical and optical properties measured during SEAS were also linked to volcanic aerosol transported across the Pacific in the free troposphere from Japan and later entrained into the marine boundary layer. This and various local effects of wind direction, wind speed, tides, and sampling altitude were superimposed on the variations in offshore marine aerosol. Characterization of these site-specific effects provides a context and guidance for the interpretation of coastal aerosol data and evaluation of SEAS data.

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Antony D. Clarke, Norman C. Ahlquist, Steven Howell, and Ken Moore

Abstract

Modification of a commercial Met One 237A optical sensor to accept custom electronics consisting of a single logarithmic amplifier providing 256 size bins over the 0.3–14-μm diameter range is described. Configuration of the optical particle spectrometer for airborne aerosol measurements is found to be effective for both miniature remote control aircraft and large research aircraft (NASA P-3B). The instrument is rugged, of low cost, uses low power, and is easily integrated into various platforms. The high size resolution and the 1.6 l min–1 sample rate provide excellent count statistics and high sensitivity for ambient out-of-cloud aircraft measurements and for other diverse applications. It can be readily configured for isokinetic or subisokinetic aircraft sampling. Initial comparison with other optical particle counters over the Sea of Japan reveals it to be an effective instrument for in situ aircraft measurements.

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N. C. Parikh Sharma, John E. Barnes, Trevor B. Kaplan, and Antony D. Clarke

Abstract

A bistatic lidar configuration of a wide-angle camera (100°) and vertically pointed laser (532 nm) was used to profile aerosols at a coastal site. Aerosol profiles were measured on two evenings from the surface through the boundary layer. The site, on the eastern tip of the Big Island of Hawaii, is influenced by both marine boundary layer aerosols and breaking waves. Two nephelometers, located at 7 and 25 m above sea level, were compared directly with the 0.5-m-altitude resolution of the camera lidar (clidar). At 7 m, changes in aerosol were tracked quite well by the clidar. At 25 m the aerosol was fairly constant and a useful comparison could only be made with averaged values. The clidar results showed a steep gradient (decreasing with altitude) in the aerosol extinction from 7 to about 35 m. The gradient continued to 200 m at a lower rate. This demonstrated the use of the clidar in characterizing the environment for the in situ aerosol sampling. Both a measured and a NASA Aerosol Robotic Network (AERONET)-derived aerosol phase function, representing similar marine conditions but from different locations, were used to convert the single-angle clidar scatter to extinction. The measured function gave the best fit to the near-surface nephelometer data. The extinction/backscatter ratio, derived by comparing the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth to the integrated clidar profile, was higher than the long-term average value from the AERONET aerosol phase function.

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Sarah J. Masonis, Theodore L. Anderson, David S. Covert, Vladimir Kapustin, Antony D. Clarke, Steven Howell, and Kenneth Moore

Abstract

Ground-based aerosol optical measurements made at near-ambient relative humidity (RH) under clean marine sampling conditions are presented and compared to 1) almost identical optical measurements made at a polluted continental site and 2) optical properties calculated from measured size distributions and Mie theory. The use of Mie theory (which assumes homogeneous spheres) is justified based on the fact that the sea-salt aerosol was measured in a hydrated state. This study focuses on the extinction-to-backscatter ratio S, an optical property required to interpret remote measurements by elastic backscatter lidar. For clean marine conditions, S is found to be 25.4 ± 3.5 sr at 532 nm (central value ± 95% confidence uncertainty). Other optical properties reported include single-scattering albedo, wavelength dependence of scattering, fraction of scattering due to submicrometer particles, and hemispheric-backscatter fraction, as well as the extensive properties (e.g., scattering coefficient) upon which these intensive properties are based. In addition, correlation scale lengths are examined via the autocorrelation function. Except during deliberate drying experiments that lowered the measurement RH below 43%, S exhibited little variation with RH. A subtle but clearly detectable change in optical properties was observed at the onset of volcanically influenced sampling conditions.

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Vladimir N. Kapustin, Antony D. Clarke, Steven G. Howell, Cameron S. McNaughton, Vera L. Brekhovskikh, and Jingchuan Zhou

Abstract

Topography-induced steady-state accelerated wind flow in the Alenuihaha Channel between the islands of Hawaii and Maui provides about 100 km of fetch with winds that can nearly double over trade wind speed. Here ship- and aircraft-based observations of meteorological parameters and aerosols in Hawaii’s orographic natural “wind tunnel” are used for the study of sea salt aerosol (SSA) production, evolution, and related optical effects under clean oceanic conditions. There are certain advantages of channel measurements, such as a broad and uniform upstream area usually filled with background aerosol, stationary flow, and known fetch, but also some difficulties, like vigorous entrainment and persistent presence of organized structures (rolls). It is found that marine boundary layer (MBL) rolls are a common occurrence near the Hawaiian Islands even when cloud streets are not visible in satellite imagery. The presence of rolls tends to enhance the variability of ambient aerosol concentration and probably affects production of primary sea salt aerosol and entrainment from above. The possibility of channel measurements of the size-dependent flux of SSA is explored using a concentration buildup method as surface wind speeds range from 7 to 11 m s−1. Production of SSA particles with dry diameter as small as 0.18 μm was observed. General agreement with reported SSA fluxes was found.

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