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Solar actinic radiation in the UV/VIS range perpetuates atmospheric photochemistry by inducing photolysis processes which form reactive radical species. Photolysis frequencies are rate constants that quantify the rates of photolysis reactions and therefore constitute important parameters for quantitative analyses. Photolysis frequencies are usually calculated from modelled or measured solar spectral actinic flux densities. Suitable measurements techniques are available but measurement accuracy can suffer from non-ideal 2π or 4π solid angle reception characteristics of the usually employed 2π optical receivers, or receiver combinations. These imperfections, i.e. deviations from an angle-independent response, should be compensated by corrections of the measured data. In this work, the relative angular sensitivities of four commonly used 2π quartz receivers were determined in the laboratory in a range 280 - 660 nm. Based on this information, the influence of the non-ideal responses on measured spectral actinic flux densities for ground-based and airborne applications was investigated for a wide range of atmospheric conditions. Spectral radiance distributions and contributions of direct, diffuse downward and diffuse upward spectral actinic flux densities were calculated with a radiative transfer model to derive the corrections. The intention was to determine the ranges of possible corrections under realistic measurement conditions and to derive simple parametrizations with reasonable uncertainties. For ground-based 2π measurements of downward spectral actinic flux densities, corrections typically range <10% dependent on wavelength and solar zenith angle, with 2 - 8% uncertainties covering all atmospheric conditions. Corrections for 4π airborne measurements were determined for the platforms Zeppelin NT (New Technology) and HALO (High Altitude and Long Range Research Aircraft) in altitude ranges 0.05 - 2 km and 0.2 -15 km, respectively. Total, downward and upward spectral actinic flux densities were treated separately. In addition to various atmospheric conditions, different ground albedos and small (<5°) aircraft attitude variations were considered in the uncertainties, as well as aircraft headings with respect to the sun in the case of HALO. Corrections for total and downward spectral actinic flux densities again typically range < 10% dependent on wavelength, solar zenith angle and altitude, with 2 - 10% uncertainties covering all atmospheric conditions for solar zenith angles below 80°. For upward spectral actinic flux densities corrections were more variable and significantly greater, up to about -50% at low altitudes and low ground albedos. A parametrization for corrections and uncertainties was derived using uncorrected ratios of upward/downward spectral actinic flux densities as input, applicable independent of atmospheric conditions for a given wavelength, solar zenith angle and altitude. The use was limited to conditions with solar zenith angles <80° when direct sun radiation cannot strike upward and downward looking receivers simultaneously. Examples of research flights with the Zeppelin and HALO are discussed, as well as other approaches described in the literature. [ABSTRACT FROM AUTHOR]

The properties and performance of CCD array spectroradiometers for the measurement of atmospheric spectral actinic flux densities and photolysis frequencies were investigated. These instruments are widely used in atmospheric research and are suitable for aircraft applications because of high time resolutions and high sensitivities in the UV range. The laboratory characterization included instrument-specific properties like wavelength accuracy, dark signals, dark noise and signal-to-noise ratios. Spectral sensitivities were derived from measurements with spectral irradiance standards. The calibration procedure is described in detail and a straightforward method to minimize the influence of stray light on spectral sensitivities is introduced. Detection limits around 1×1010cm-2 s-1 nm-1 were derived for spectral actinic flux densities in a 300 nm range (1 s integration time). As a prerequisite for the determination of stray light under field conditions, atmospheric cutoff wavelengths were defined using radiative transfer calculations as a function of solar zenith angles and ozone columns. The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromator reference spectroradiometer. Spectral actinic flux densities were compared as well as photolysis frequencies j(NO2) and j(O¹D), representing UV-A and UV-B ranges, respectively. The spectra expectedly revealed an increased daytime level of residual noise below atmospheric cutoff wavelengths that is caused by stray light. The influence of instrument noise and stray light induced noise was found to be insignificant for j(NO2) and rather limited for j(O¹D), resulting in estimated detection limits of 5×10-7 s-1 and 1×10-7 s-1, respectively. For j(O¹D) the detection limit could be further reduced by setting spectral actinic flux densities below cutoff wavelengths to zero. The accuracies of photolysis frequencies were determined from linear regressions with reference instrument data. The agreement was typically within ±5 %. Optical receiver aspects were widely excluded in this work and will be treated in a separate paper in particular with regard to airborne applications. Overall, the investigated instruments are clearly suitable for high quality photolysis frequency measurements with high time resolution as required for airborne applications. An example of data from a flight on the research aircraft HALO is presented. [ABSTRACT FROM AUTHOR]

Doses of erythemally weighted irradiances derived from polysulphone (PS) and electronic ultraviolet (EUV) dosimeters have been compared with measurements obtained using a reference spectroradiometer. PS dosimeters showed mean absolute deviations of 26% with a maximum deviation of 44%, the calibrated EUV dosimeters showed mean absolute deviations of 15% (maximum 33%) around noon during several test days in the northern hemisphere autumn. In the case of EUV dosimeters, measurements with various cut-off filters showed that part of the deviation from the CIE erythema action spectrum was due to a small, but significant sensitivity to visible radiation that varies between devices and which may be avoided by careful preselection. Usually the method of calibrating UV sensors by direct comparison to a reference instrument leads to reliable results. However, in some circumstances the quality of measurements made with simple sensors may be over-estimated. In the extreme case, a simple pyranometer can be used as a UV instrument, providing acceptable results for cloudless skies, but very poor results under cloudy conditions. It is concluded that while UV dosimeters are useful for their design purpose, namely to estimate personal UV exposures, they should not be regarded as an inexpensive replacement for meteorological grade instruments. [ABSTRACT FROM AUTHOR]

Gaseous nitrous acid (HONO) is an important precursor of tropospheric hydroxyl radicals (OH). OH is responsible for atmospheric self-cleansing and controls the concentrations of greenhouse gases like methane and ozone. Due to tack of measurements, vertical distributions of HONO and its sources in the troposphere remain unclear. Here, we present a set of observations of HONO and its budget made onboard a Zeppelin airship. In a sunlit layer separated from Earth's surface processes by temperature inversion, we found high HONO concentrations providing evidence for a strong gas-phase source of HONO consuming nitrogen oxides and potentially hydrogen oxide radicals. The observed properties of this production process suggest that the generally assumed impact of HONO on the abundance of OH in the troposphere is substantially overestimated. [ABSTRACT FROM AUTHOR]

Ye et al. have determined a maximum nitrous acid (HONO) yield of 3% for the reaction HO2·H2O + NO2, which is much lower than the yield used in our work. This finding, however, does not affect our main result that HONO in the investigated Po Valley region is mainly from a gas-phase source that consumes nitrogen oxides. [ABSTRACT FROM AUTHOR]