Photolysis frequency measurement techniques: results of a comparison within the ACCENT project

Abstract

An intercomparison of different radiometric techniques measuring atmospheric photolysis frequencies <i>j</i>(NO₂), <i>j</i>(HCHO) and <i>j</i>(O¹D) was carried out in a two-week field campaign in June 2005 at Jülich, Germany. Three double-monochromator based spectroradiometers (DM-SR), three single-monochromator based spectroradiometers with diode-array detectors (SM-SR) and seventeen filter radiometers (FR) (ten <i>j</i>(NO₂)-FR, seven <i>j</i>(O¹D)-FR) took part in this comparison. For <i>j</i>(NO₂), all spectroradiometer results agreed within &plusmn;3%. For <i>j</i>(HCHO), agreement was slightly poorer between &minus;8% and +4% of the DM-SR reference result. For the SM-SR deviations were explained by poorer spectral resolutions and lower accuracies caused by decreased sensitivities of the photodiode arrays in a wavelength range below 350 nm. For <i>j</i>(O¹D), the results were more complex within +8% and &minus;4% with increasing deviations towards larger solar zenith angles for the SM-SR. The direction and the magnitude of the deviations were dependent on the technique of background determination. All <i>j</i>(NO₂)-FR showed good linearity with single calibration factors being sufficient to convert from output voltages to <i>j</i>(NO₂). Measurements were feasible until sunset and comparison with previous calibrations showed good long-term stability. For the <i>j</i>(O¹D)-FR, conversion from output voltages to <i>j</i>(O¹D) needed calibration factors and correction functions considering the influences of total ozone column and elevation of the sun. All instruments showed good linearity at photolysis frequencies exceeding about 10% of maximum values. At larger solar zenith angles, the agreement was non-uniform with deviations explainable by insufficient correction functions. Comparison with previous calibrations for some <i>j</i>(O¹D)-FR indicated drifts of calibration factors.