The Precision and Accuracy of Volz Sunphotometry

Nels S. Laulainen Depts. of Astronomy, Geophysics and Physics, University of Washington, Scattle 98195

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Benjamin J. Taylor Depts. of Astronomy, Geophysics and Physics, University of Washington, Scattle 98195

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Abstract

In this paper, we report the results of a preliminary investigation to determine the precision and accuracy of Volz sunphotometry. We find that errors may be introduced into turbidities obtained from such photometry through the measured secant of the zenith angle, the instantaneous meter deflection, and the meter deflection outside the atmosphere. By fitting the Beer-Lambert law to sunphotometer measurements made over a range of air masses on a number of days, we find that day-to-day variation in the outside-atmosphere meter deflection is the most important of these sources of error; the amount of the error would be as small as 3%-and as large as 67%-of sea-level Rayleigh optical depth if the photometers we consider were used to make observations from the latitude of Seattle. 11 the results are characteristic of the Volz sunphotometry, much existing Volz turbidity data are of dubious value, and the standard Volz technique should not be used at clean or relatively clean cites. One may, however, escape this problem at such sites by observing over a range of air masses and fitting the Beer-Lambert law to the results.

Abstract

In this paper, we report the results of a preliminary investigation to determine the precision and accuracy of Volz sunphotometry. We find that errors may be introduced into turbidities obtained from such photometry through the measured secant of the zenith angle, the instantaneous meter deflection, and the meter deflection outside the atmosphere. By fitting the Beer-Lambert law to sunphotometer measurements made over a range of air masses on a number of days, we find that day-to-day variation in the outside-atmosphere meter deflection is the most important of these sources of error; the amount of the error would be as small as 3%-and as large as 67%-of sea-level Rayleigh optical depth if the photometers we consider were used to make observations from the latitude of Seattle. 11 the results are characteristic of the Volz sunphotometry, much existing Volz turbidity data are of dubious value, and the standard Volz technique should not be used at clean or relatively clean cites. One may, however, escape this problem at such sites by observing over a range of air masses and fitting the Beer-Lambert law to the results.

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