Spektrale aktinische Flussdichten und Photolysefrequenzen – Untersuchungen in der atmosphärischen Grenzschicht und der freien Troposphäre

Solar UV radiation is driving atmospheric photochemistry because the photolysis of atmospheric trace gases yields highly reactive atoms or radicals. Thus, trace gas concentrations as well as accurate photolysis frequencies are needed to understand atmospheric photochemical processes. Especially under varying cloud conditions, measurements can often not be replaced by radiative transfer calculations with sufficient accuracy. In this work, airborne measurements of the separate upwelling and downwelling components of the actinic flux densities (280–650 nm) were performed with CCD-spectroradiometers. For accurate UV measurements a thorough treatment of stray light was applied for the single monochromator based array spectrometers. Moreover, the angular sensitivities of the optical receivers were determined to analyze their influence under various atmospheric conditions using radiative transfer calculations of realistic atmospheric radiance distributions. Corresponding correction factors in the range of 5% were derived. The overall performance was tested on the ground by in-field comparisons with a doublemonochromator reference system and found to have maximum deviations of 7%. Measurements of the spectral actinic flux density were performed aboard Zeppelin NT in the atmospheric boundary layer during the PEGASOS campaign 2012/13 over different parts of Europe. Moreover the research aircraft HALO was used during the NARVAL campaign 2013/14 for measurements in the upper troposphere and the lower stratosphere over the Atlantic Ocean. Typical Zeppelin flight heights ranged from 100m to 900m and flights were therefore always performed below possible cloud layers. Thus the measurements were influenced by potentially overlaying clouds and a small upwelling part of radiation. Radiative transfer calculations of the downwelling component under the assumption of clearsky conditions showed good agreement with the maximum values of the measurements. The upwelling component of the spectral actinic flux density was unexpectedly greater than the model results. The reason for this is unknown and requires further reasearch. Owing to the typical flight heights of HALO in the range 8–14 km, the measurements were affected by a high upwelling part of radiation, especially when flying over clouds. The measured downwelling components of j(O$^{1}$D) and j(NO$_{2}$) for all flights and various cloud conditions showed only small deviations of 4–5% compared to clearsky model calculations. Cloud-microphysical properties of underlying clouds were retrieved for a certain time period of a HALO-flight using spectral radiance measurements performed by the Leipzig Institute for Meteorology and were used as additional input parameters for radiative transfer calculations of spectral actinic flux densities. The deviations between model and measurements of up to 40% for the upwelling component can partly be attributed to the geometrical receiving characteristics of the radiance optic. Comparisons of measured photolysis frequencies and model values of regional and global chemistry transport models showed good agreements with small underestimations of j(NO$_{2}$) by the models in the range of 20%. For the PEGASOS campaign the regional EURAD-IM model was found to overestimate j(O$^{1}$D) significantly due to a low and constant ozone column in the model. For the NARVAL campaign good agreement for j(O$^{1}$D) with the global MOZART model, that uses variable, modelled ozone columns, was obtained

Spektrale aktinische Flussdichten und Photolysefrequenzen - Untersuchungen in der atmosphärischen Grenzschicht und der freien Troposphäre

Die solare UV-Strahlung beeinflusst in entscheidendem Maße die Chemie der Atmosphäre, da durch die Photolyse atmosphärischer Spurengase hoch reaktive Atome und Radikale gebildet werden. Zum Verständnis atmosphärenchemischer Prozesse sind daher, neben der Messung von Spurengasen, genaue Bestimmungen der Photolysefrequenzen notwendig. Dabei können, insbesondere wegen des variablen Einflusses von Bewölkung, Messungen häufig nicht mit ausreichender Genauigkeit durch Strahlungstransferrechnungen ersetzt werden. Zur separaten Messung der abwärts- und aufwärtsgerichteten spektralen aktinischen Flussdichten (280–650 nm) auf Flugzeugen wurden im Rahmen dieser Arbeit CCD-Spektralradiometer mit Einzelmonochromatoren eingesetzt. Akkurate Messungen mit diesen Geräten erfordern im UV-B-Bereich eine sorgfältige Streulichtkorrektur bei Kalibrations- und Feldmessungen. Außerdem wurde die geometrische Empfindlichkeit der aktinischen 2pi-Eingangsoptiken bestimmt und der Einfluss der Abweichungen von der idealen Empfindlichkeit auf Basis von Berechnungen realistischer Strahldichteverteilungen für verschiedene atmosphärische Bedingungen ermittelt. Entsprechende Korrekturfaktoren, die im Bereich von 5% liegen, wurden abgeleitet. Messvergleiche mit einem Doppelmonochromator-Spektralradiometer am Boden ergaben im Rahmen der Unsicherheiten beider Geräte gute Übereinstimmungen mit maximalen Abweichungen von ca. 7%. Messungen der spektralen aktinischen Flussdichte wurden während der PEGASOS-Kampagne 2012/13 auf einem Zeppelin NT in der atmosphärischen Grenzschicht über dem europäischen Festland durchgeführt. Darüber hinaus erfolgten auf dem Forschungsflugzeug HALO während der NARVAL-Kampagne 2013/14 Messungen im Bereich der oberen Troposphäre und der unteren Stratosphäre über dem Atlantik. Zeppelinflüge fanden bodennah im Höhenbereich 100–900 m und stets unterhalb gegebenenfalls vorhandener Bewölkung statt. Die Messungen waren somit von möglicher überliegender Bewölkung und einem geringen aufwärtsgerichteten Strahlungsanteil beeinflusst. Strahlungstransferrechnungen der abwärtsgerichteten Komponente unter der Annahme wolkenfreier Bedingungen zeigten gute Übereinstimmungen mit den gemessenen Maximalwerten. Die aufwärtsgerichtete Komponente der spektralen aktinischen Flussdichte lag unerwartet deutlich über den Modellrechnungen. Die Ursachen hierfür sind unklar und erfordern weitere Untersuchungen. Typische Flughöhen von HALO lagen im Bereich 8–14 km, sodass die Messungen von einem hohen aufwärtsgerichteten Strahlungsanteil, insbesondere bei überflogener Bewölkung, geprägt waren. Wolkenfreie Strahlungstransferrechnungen zeigten für alle Flüge geringe Abweichungen von 4–5% zu den gemessenen abwärtsgerichteten Komponenten von j(O1D) und j(NO2), unabhängig von unterliegender Bewölkung. Wolkenmikrophysikalische Parameter unterliegender Wolken wurden für einen ausgewählten Zeitraum eines HALO-Fluges mittels spektraler Strahldichtemessungen des Leipziger Instituts für Meteorologie bestimmt und als zusätzliche Eingabeparameter für Strahlungstransferrechnungen der spektralen aktinischen Flussdichte genutzt. Die Abweichungen zwischen Modell und Messungen betrugen hier für die aufwärtsgerichtete Komponente bis zu 40% und können vermutlich zum Teil auf die Empfangscharakteristik der Strahldichteoptik zurückgeführt werden. Vergleiche gemessener Photolysefrequenzen mit Modellwerten regionaler und globaler Chemietransportmodelle ergaben gute Übereinstimmungen mit leichten Unterschätzungen von j(NO2) im Bereich von 20% durch die Modelle. Für die PEGASOS-Kampagne zeigte das regionale EURAD-IM Modell eine deutliche Überschätzung für j(O1D) aufgrund einer zu niedrigen, konstanten Gesamtozonsäule im Modell. Für die NARVAL-Kampagne ergab sich für j(O1D) dagegen eine gute Übereinstimmung mit dem globalen MOZART Modell, das variable, modellierte Ozonsäulen verwendet

Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

The properties and performance of charge-coupled device (CCD) array spectroradiometers for the measurement of atmospheric spectral actinic flux densities (280–650 nm) 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 the wavelength accuracy, dark signal, dark noise and signal-to-noise ratio (SNR). 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. From instrument dark noise, minimum detection limits  ≈  1  ×  1010 cm−2 s−1 nm−1 were derived for spectral actinic flux densities at wavelengths around 300 nm (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 the solar zenith angle (SZA) and total ozone column (TOC). The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight on HALO (High Altitude and Long Range Research Aircraft) as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromator-based, highly sensitive 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 increased daytime levels of stray-light-induced signals and noise below atmospheric cutoff wavelengths. 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 and 1  ×  10−7 s−1, respectively, derived from nighttime measurements on the ground (0.3 s integration time, 10 s averages). For j(O1D) the detection limit could be further reduced by setting spectral actinic flux densities to zero below atmospheric cutoff wavelengths. The accuracies of photolysis frequencies were determined from linear regressions with data from the double-monochromator reference instrument. The agreement was typically within ±5 %. Because optical-receiver aspects are not specific for the CCD spectroradiometers, they were widely excluded in this work and will be treated in a separate paper, in particular with regard to airborne applications

Optical receiver characterisations and corrections for ground-based and airborne measurements of spectral actinic flux densities

Solar actinic radiation in the ultraviolet and visible range (UV/VIS) 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 measurement 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 for 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 &lt;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 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 (&lt;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 &lt;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 &lt;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.</p

Calibration and evaluation of CCD spectroradiometers for airborne measurements of spectral actinic flux densities

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