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The Mauna Loa Observatory, Hawaii (3380 m altitude) is more than 3500 km away from nearest continental land and serves as an important benchmark climate observatory. This report describes results of a multi-wavelength study of the optical transmission through the atmosphere above Mauna Loa, its variation with time and its relation to geophysical parameters. The vertical optical thickness (τ = -lnT) from scattering and absorption of light by aerosols at MLO averages 0.020 at wavelength 500 nm, and 0.004 at 1000 nm, but varies from day to day by about ±25%. Air that has come from easterly and northerly directions is slightly more turbid (δτ ≃ 0.01) than air coming from SW directions; the increase in turbidity is probably due to continental aerosols from America reaching the islands. The optical extinction of the continental aerosol varies with wavelength as λ⁻². During clearest conditions, the aerosol optical thickness at 500 nm was only 0.010 to 0.015. Almost all the reported turbidity measurements were made during morning hours during subsidence conditions and are thought to be representative of “background” turbidities in the central Pacific. In the afternoon, the marine trade inversion breaks up and turbidity increases and fluctuates due to contamination aerosol brought to the observatory altitude by anabatic winds and convection. There was a slow decay in day-averaged aerosol extinction from March to June, 1976 that has been identified with dust veil caused from the explosive eruptions of Augustine volcano in the Gulf of Alaska in January-February, 1976; a Dust Veil Index of 4 has been assigned to the Augustine eruption on the basis of the optical measurements made at MLO, this compares with DVI = 1000 for Krakatoa (1883), DVI = 150 for Katmai (1912) and DVI = 800 for Agung (1963). The aerosol extinction spectrum for the Augustine dust veil is approximately given by τ = 0.0125 λ⁻¹ and an inversion, using the method of Box and Lo, indicates that the total mass of material in the stratosphere (March, 1970) was 10¹² g. Some preliminary extraterrestrial solar irradiance measurements were made during this study at twelve 10 nm-wide wavelength regions by using the Langley method to extrapolate through the atmosphere and an NBS primary standard tungsten lamp to refer the measurements of spectral intensity to SI electrical units. A check on the absolute accuracy of the lamp calibration was made (at one wavelength – 615.9 nm) by using a tunable dye laser and electrically-calibrated cavity radiometer at the World Radiation Center at Davos, Switzerland. The determinations of spectral intensity agree to within ±5% of those reported by Labs and Neckels. Error analysis of derived data shows that the extrapolation through the atmosphere introduces less than one-half percent uncertainty into the determinations of solar spectral irradiance. The solar spectral irradiance from March to August, 1976 remained constant to better than 0.3 percent at yellow to red wavelengths but there were indications that the sun may have brightened slightly over 5 months time by about 0.4 to 0.8%, at blue (400 nm) wavelengths. Atmospheric ozone and water vapor amounts were obtained at MLO by measuring optical absorption in the Chappuis band and in the ρ σ τ water band. Ozone amounts derived from Chappuis-band absorption were 23% lower than those derived with a Dobson spectrophotometer but correlated to within ±4% of the Dobson values. Column water vapor increased through the day; the morning values ranged from 0.5 to about 2.5 g cm⁻², although these values are somewhat uncertain because of poor information available on spectroscopic line parameters in the near infrared region. Evidence was found for a weak water vapor continuum between the ρ σ τ and φ bands with an absorption coefficient ≃ 0.08 g⁻¹ cm². Results of this study show that precision multi-wavelength spectrophotometry can be used to quantify and monitor important geophysical parameters that have bearing on climate.

Publication Date

1-17-1978

Keywords

Atmospheric turbidity, Hawaii

Handle

http://hdl.handle.net/11122/15498

Multi-wavelength turbidity at the Mauna Loa Observatory

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