Comparison of Sun photometer calibration by use of the Langley technique and the standard lamp

Abstract

Asix-channel Sun photometer has been calibrated by means of two different methods: Langley plots and standard irradiance lamps. A 4-month calibration campaign was carried out at a high mountain site, Jungfraujoch (3580 m above sea level), in the Swiss Alps. Calibration constants V(0)(λ) determined on clear and stable days by means of a refined Langley-plot technique scatter by less than 0.25% (rms) for wavelengths outside of strong gaseous absorption bands. Inside the 0.94-µm water-vapor absorption band, the V(0)(λ) values retrieved by means of modified Langley plots scatter by 1.0% (rms). Repeated calibrations of the Sun photometer by means of irradiance standard lamps were performed at the World Radiation Center in Davos. The comparison of both methods ranges from perfect agreement to a deviation of 4.9% for the different channels. A discussion of the errors introduced by both methods shows that the Langley-plot calibration, when performed under very clear atmospheric conditions, is superior. However, by means of the standard-lamp calibrations a temporal degradation of the instrument's response up to 4.6% per year was found, implying that a single calibration campaign as done here is not sufficient. Thus we recommend the use of a combination of both methods for maintaining an accurate calibration.

Particle size distributions of Mount Etna's aerosol plume constrained by Sun photometry

“…This would have led to an assessment of the temporal stability of the V0 values over a much longer period of time making the measurements more robust [Tomasi et al, 1997]. A standard irradiance lamp, of precisely known spectral characteristics, can also be used to repeat calibrate a Sun photometer, although both methods used in conjunction is argued to be the most preferable [Schmid and Wehrli, 1995]. As the instrument used in this study had not previously been calibrated in the field or laboratory (although a lamp calibration has been undertaken (E. Rollin, personal As with previous work using the Langley calibration at altitudes greater than 3000 m (Etna, Mauna Loa, Jungfraujoch), the errors on an individual V0 value are calculated to be < 0.1%.…”

“…As the instrument used in this study had not previously been calibrated in the field or laboratory (although a lamp calibration has been undertaken (E. Rollin, personal As with previous work using the Langley calibration at altitudes greater than 3000 m (Etna, Mauna Loa, Jungfraujoch), the errors on an individual V0 value are calculated to be < 0.1%. As the instrument was calibrated and measured aerosols in the same day a significant error in Sun photometry, instrument drift estimated to be as much as 5% a year [Schmid and Wehrli, 1995], is removed. The Langley …”

Columnar aerosol characterization over Scandinavia and Svalbard

“…The Sun photometers listed in Table 2 are instruments operating routinely in the sites above. They are extensively described elsewhere [4][5][6][7]. All of them perform direct sun observations, from which the optical thickness of the atmosphere is derived.…”

Improved Langley and Ratio Langley Methods for Improving Sky-Radiometer Accuracy

“…Measure the direct sunlight from sunrise to southern middle time to estimate the atmospheric mass to 0, that is, to estimate radiance outside the solar atmosphere. A calibration method has been proposed to determine the calibration coefficient of the sky-radiometer so that the difference between this estimated value and the stable solar atmospheric radiance and irradiance model [5] called socalled solar constant is 0, which is widely used [7], [8]. In addition, a method of reducing the influence of noise in observation has been proposed [9].…”