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M Blumthaler - One of the best experts on this subject based on the ideXlab platform.
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The influence of the spatial resolution of topographic input data on the accuracy of 3-D UV Actinic Flux and irradiance calculations
Atmospheric Chemistry and Physics, 2012Co-Authors: P. Weihs, M Blumthaler, Richard Kift, Gian Paolo Gobbi, J. E. Wagner, Federico Angelini, M. Fitzka, Harald E. Rieder, S. F. Schreier, Axel KreuterAbstract:Abstract. The aim of this study is to investigate the influence of the spatial resolution of a digital elevation map (DEM) on the three-dimensional (3-D) radiative transfer performance for both spectral ultraviolet (UV) irradiance and Actinic Flux at 305 nm. Model simulations were performed for clear sky conditions for three case studies: the first and second one using three sites in the Innsbruck area and the third one using three sites at the Sonnblick observatory and surrounding area. It was found that the DEM resolution may change the altitude at some locations by up to 500 m, resulting in changes in the sky obscured by the horizon of up to 15%. The geographical distribution of UV irradiance and Actinic Flux shows that with larger pixel size, uncertainties in UV irradiance and Actinic Flux determination of up to 100% are possible. These large changes in incident irradiance and Actinic Flux with changing pixel size are strongly connected to shading effects. The effect of the DEM pixel size on irradiance and Actinic Flux was studied at the six locations, and it was found that significant increases in irradiance and Actinic Flux with increasing DEM pixel size occurred at one valley location at high solar zenith angles in the Innsbruck area as well as for one steep valley location in the Sonnblick area. This increase in irradiance and Actinic Flux with increasing DEM resolution is most likely to be connected to shading effects affecting the reflections from the surroundings.
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Investigation of the 3-D Actinic Flux field in mountainous terrain
Atmospheric research, 2011Co-Authors: J. E. Wagner, M Blumthaler, Richard Kift, Gian Paolo Gobbi, Federico Angelini, M. Fitzka, Axel Kreuter, Harald E. Rieder, Stana Simic, Ann R. WebbAbstract:During three field campaigns spectral Actinic Flux was measured from 290–500 nm under clear sky conditions in Alpine terrain and the associated O3- and NO2-photolysis frequencies were calculated and the measurement products were then compared with 1-D- and 3-D-model calculations. To do this 3-D-radiative transfer model was adapted for Actinic Flux calculations in mountainous terrain and the maps of the Actinic Flux field at the surface, calculated with the 3-D-radiative transfer model, are given. The differences between the 3-D- and 1-D-model results for selected days during the campaigns are shown, together with the ratios of the modeled Actinic Flux values to the measurements. In many cases the 1-D-model overestimates Actinic Flux by more than the measurement uncertainty of 10%. The results of using a 3-D-model generally show significantly lower values, and can underestimate the Actinic Flux by up to 30%. This case study attempts to quantify the impact of snow cover in combination with topography on spectral Actinic Flux. The impact of snow cover on the Actinic Flux was ~ 25% in narrow snow covered valleys, but for snow free areas there were no significant changes due snow cover in the surrounding area and it is found that the effect snow-cover at distances over 5 km from the point of interest was below 5%. Overall the 3-D-model can calculate Actinic Flux to the same accuracy as the 1-D-model for single points, but gives a much more realistic view of the surface Actinic Flux field in mountains as topography and obstruction of the horizon are taken into account.
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Influence of clouds on the spectral Actinic Flux density in the lower troposphere (INSPECTRO): overview of the field campaigns
Atmospheric Chemistry and Physics, 2008Co-Authors: Stephan Thiel, A F Bais, M Blumthaler, Gian Paolo Gobbi, L. Ammannato, Birger Bohn, Brian J. Bandy, Ola Engelsen, Julian Gröbner, Evelyn JäkelAbstract:Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. The overall objective of the project INSPECTRO was the characterization of the three-dimensional Actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany) combining space-based, aircraft and ground-based measurements as well as simulations with the one-dimensional radiation transfer model UVSPEC and the three-dimensional radiation transfer model MYSTIC. During both campaigns the spectral Actinic Flux density was measured at several locations at ground level and in the air by up to four different aircraft. This allows the comparison of measured and simulated Actinic radiation profiles. In addition satellite data were used to complete the information of the three dimensional input data set for the simulation. A three-dimensional simulation of Actinic Flux density data under cloudy sky conditions requires a realistic simulation of the cloud field to be used as an input for the 3-D radiation transfer model calculations. Two different approaches were applied, to derive high- and low-resolution data sets, with a grid resolution of about 100 m and 1 km, respectively. The results of the measured and simulated radiation profiles as well as the results of the ground based measurements are presented in terms of photolysis rate profiles for ozone and nitrogen dioxide. During both campaigns all spectroradiometer systems agreed within ±10% if mandatory corrections e.g. stray light correction were applied. Stability changes of the systems were below 5% over the 4 week campaign periods and negligible over a few days. The J(O 1 D) data of the single monochromator systems can be evaluated for zenith angles less than 70°, which was satisfied by nearly all airborne measurements during both campaigns. The comparison of the airborne measurements with corresponding simulations is presented for the total, downward and upward Flux during selected clear sky periods of both campaigns. The compliance between the measured (from three aircraft) and simulated downward and total Flux profiles lies in the range of ±15%.
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NO 2 and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
Atmospheric Chemistry and Physics, 2005Co-Authors: C. Topaloglou, Stelios Kazadzis, M Blumthaler, B. Schallhart, Alkis Bais, D. BalisAbstract:Abstract. An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of Actinic Flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [J(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral Actinic Flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 12% (2σ) for J(NO2) and 6% (2σ) for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 12% (2σ) for the method that can be used with double Brewers and 20% for the method that can be used for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of Actinic Flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from Actinic Flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.
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Actinic Flux and O1D photolysis frequencies retrieved from spectral measurements of irradiance at Thessaloniki, Greece
Atmospheric Chemistry and Physics, 2004Co-Authors: S. Kazadzis, M Blumthaler, Andreas Kazantzidis, C. Topaloglou, A. F. Bais, D. Balis, B. SchallhartAbstract:The results of two methods retrieving Actinic Flux and ozone photolysis frequencies (JO1D), from measurements of irradiance with a Brewer MKIII spectroradiometer are investigated in this paper. The first method uses Actinic Flux retrieved from irradiance measurements by the use of known formulas while the second is an empirical method converting irradiance to JO1D through polynomials extracted from a study of synchronous Actinic Flux and irradiance measurements. When examining the Actinic Fluxes derived from the first method to those measured by an Actinic Flux spectrometer data agree within ±10% for solar zenith angles lower than 75° for the UV-B and the UV-A wavelength band. Also, the Actinic to global irradiance ratio derived, deviates within ±6% for solar zenith angles lower than 70° compared with cloudless sky calculations of the TUV model. For both cases the deviations are in the order of the magnitude of the measurement or model uncertainties. Values of JO1D calculated by the second method show a mean ratio of 0.99±0.10 (1?) and 0.98±0.06 for all data and for cloudless skies respectively when compared with values of JO1D derived by a Bentham Actinic Flux spectroradiometer. Finally, the agreement of the two methods is within ±5% comparing two years' data of JO1D retrieved from irradiance measurements at Thessaloniki, Greece. The use of such methods on extensive data sets of global irradiance can provide JO1D values with acceptable uncertainty, a parameter of particular importance for chemical process studies.
Sasha Madronich - One of the best experts on this subject based on the ideXlab platform.
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Improved modeling of cloudy-sky Actinic Flux using satellite cloud retrievals
Geophysical Research Letters, 2017Co-Authors: Young-hee Ryu, Samuel R. Hall, Alma Hodzic, Gael Descombes, Patrick Minnis, Douglas A. Spangenberg, Kirk Ullmann, Sasha MadronichAbstract:Clouds play a critical role in modulating tropospheric radiation and thus photochemistry. We develop a methodology for calculating the vertical distribution of tropospheric ultraviolet (300–420 nm) Actinic Fluxes using satellite cloud retrievals and a radiative transfer model. We demonstrate that our approach can accurately reproduce airborne-measured Actinic Fluxes from the 2013 Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign as a case study. The Actinic Flux is reduced below optically moderate-thick clouds inversely with cloud optical depth, and can be enhanced by a factor 2 above clouds. Inside clouds, the Actinic Flux can be enhanced by 2–3 times in the upper part of clouds or reduced by 90% in the lower parts of clouds. Our study suggests that the use of satellite-derived Actinic Fluxes as input to chemistry-transport models can improve the accuracy of photochemistry calculations.
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Effect of aerosols and NO 2 concentration on ultraviolet Actinic Flux near Mexico City during MILAGRO: measurements and model calculations
Atmospheric Chemistry and Physics, 2013Co-Authors: Gustavo G. Palancar, Barry Lefer, Samuel R. Hall, William J. Shaw, Chelsea A. Corr, Scott C. Herndon, James R. Slusser, Sasha MadronichAbstract:Abstract. Urban air pollution absorbs and scatters solar ultraviolet (UV) radiation, and thus has a potentially large effect on tropospheric photochemical rates. We present the first detailed comparison between Actinic Fluxes (AF) in the wavelength range 330–420 nm measured in highly polluted conditions and simulated with the Tropospheric Ultraviolet-Visible (TUV) model. Measurements were made during the MILAGRO campaign near Mexico City in March 2006, at a ground-based station near Mexico City (the T1 supersite) and from the NSF/NCAR C-130 aircraft. At the surface, measured AF values are typically smaller than the model by up to 25% in the morning, 10% at noon, and 40% in the afternoon, for pollution-free and cloud-free conditions. When measurements of PBL height, NO 2 concentration and aerosols optical properties are included in the model, the agreement improves to within ±10% in the morning and afternoon, and ±3% at noon. Based on daily averages, aerosols account for 68% and NO 2 for 25% of AF reductions observed at the surface. Several overpasses from the C-130 aircraft provided the opportunity to examine the AF perturbations aloft, and also show better agreement with the model when aerosol and NO 2 effects are included above and below the flight altitude. TUV model simulations show that the vertical structure of the Actinic Flux is sensitive to the choice of the aerosol single scattering albedo (SSA) at UV wavelengths. Typically, aerosols enhance AF above the PBL and reduce AF near the surface. However, for highly scattering aerosols (SSA > 0.95), enhancements can penetrate well into the PBL, while for strongly absorbing aerosols (SSA
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Ultraviolet Actinic Flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements
Atmospheric Chemistry and Physics, 2011Co-Authors: Gustavo G. Palancar, Richard E. Shetter, Samuel R. Hall, Beatriz M. Toselli, Sasha MadronichAbstract:Abstract. Ultraviolet (UV) Actinic Fluxes measured with two Scanning Actinic Flux Spectroradiometers (SAFS) aboard the NASA DC-8 aircraft are compared with the Tropospheric Ultraviolet-Visible (TUV) model. The observations from 17 days in July-August 2004 (INTEX-NA field campaign) span a wide range of latitudes (28° N–53° N), longitudes (45° W–140° W), altitudes (0.1–11.9 km), ozone columns (285–353 DU), and solar zenith angles (2°–85°). Both cloudy and cloud-free conditions were encountered. For cloud-free conditions, the ratio of observed to clear-sky-model Actinic Flux (integrated from 298 to 422 nm) was 1.01±0.04, i.e. in good agreement with observations. The agreement improved to 1.00±0.03 for the down-welling component under clear sky conditions. In the presence of clouds and depending on their position relative to the aircraft, the up-welling component was frequently enhanced (by as much as a factor of 8 relative to cloud-free values) while the down-welling component showed both reductions and enhancements of up to a few tens of percent. Including all conditions, the ratio of the observed Actinic Flux to the cloud-free model value was 1.1±0.3 for the total, or separately 1.0±0.2 for the down-welling and 1.5±0.8 for the up-welling components. The correlations between up-welling and down-welling deviations are well reproduced with sensitivity studies using the TUV model, and are understood qualitatively with a simple conceptual model. This analysis of Actinic Flux observations illustrates opportunities for future evaluations of photolysis rates in three-dimensional chemistry-transport models.
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Non-mass dependent photodissociation rates of ozone isotopologues from ab-initio absorption cross sections and experimental Actinic Flux
The EGU General Assembly, 2010Co-Authors: Steve Alexandre Ndengué, Rémy Jost, Fabien Gatti, Reinhard Schinke, Sasha MadronichAbstract:The absorption cross sections (XSs) of eighteen isotopologues of the ozone molecule have been calculated in the range of the Chappuis-Huggins-Hartley bands: 15000-55000 cm-1 with special emphasis to those of atmospheric interest: symmetric 16O3, 16O17O16O, and 16O18O16O and asymmetric 17O16O2 and 18O16O2. We have used the MCTDH code which is based on the time propagation of the X(0,0,0) ground state initial wavepacket on the excited state PESs. The XSs have been obtained as the Fourier transform of the autocorrelation function of this wavepacket. The calculations have been performed only for zero total angular momentum and the rotational structure has been modeled numerically. The isotopologue dependence of the overall XSs has been characterized differently in each of the three bands: in the Chappuis band (15000-27000 cm-1) and in the Hartley band (33000-55000 cm-1), the XSs are weakly structured and the isotopologue dependence is globally weak. In contrast, in the Huggins band (27000 to 33000 cm-1) the different XSs are highly structured and their peaks are significantly shifted from those of the 16O3 absolute XS which has been chosen as reference. The Hartley band of each isotopologue can be approximated by a bell shape envelop modeled by a modified Gaussian depending on only four parameters: amplitude, centre, width and asymmetry. The isotopologue dependence of the Hartley band resumes only into tiny differences between these parameters. The dependence of the Chappuis band is also weak. The isotopologue shifts of peaks in the Huggins bands induce a significant dependence of the photodissociation rates because these rates are the integral of the product of the XS by the Actinic Flux. Below 30 km, the Actinic Flux displays a tremendous attenuation in the range of the Hartley band because the solar Flux is strongly absorbed by the stratospheric ozone, almost exclusively by the 16O3 isotopologue. This implies two consequences: a) the Actinic Flux reproduces (negatively and using a log scale because of the Beer-Lambert law) the XS of the 16O3 isotopologue, including its well pronounced vibronic structures in the range of the Huggins band; b) below typically 30 km, all the ozone isotopologues absorb (and are photodissociated) mostly in the range of their Huggins and the Chappuis bands. The J photodissociation rates of various ozone isotopologues may differ significantly because the dips in the Actinic Flux (these dips correspond to the vibronic structures of the 16O3 absolute XS) may not coincide with the maxima of the absolute XS of the various minor isotopologues. Quantitatively, the asymmetric isotopologues, 17O16O2 and 18O16O2, are more efficiently photodissociated than their symmetric partners 16O17O16O, and 16O18O16O. These differences between photodissociation rates reflect on the photodissociation oxygen isotope budget because the symmetric isotopologue 16O17O16O, (resp. 16O18O16O) leads only to one dissociation channel, 16O17O+ 16O (resp. 16O18O + 16O) and then only to 16O oxygen while the asymmetric isotopologue 17O16O2 (resp. 18O16O2) leads to two (almost equivalent) channels: either 16O17O + 16O or 16O16O + 17O (resp. 16O18O+ 16O or 16O16O + 18O) and then to about the same amount of 16O and 17O oxygen (resp. of 16O and 18O). The J rates of various isotopologues, function of the altitude but averaged over other variables, will be presented.
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Actinic Flux and photolysis in water droplets: Mie calculations and geometrical optics limit
Atmospheric Chemistry and Physics, 2004Co-Authors: Bernhard Mayer, Sasha MadronichAbstract:Abstract. Photolysis of water-soluble components inside cloud droplets by ultraviolet/visible radiation may play an important role in atmospheric chemistry. Two earlier studies have suggested that the Actinic Flux and hence the photolysis frequency within spherical droplets is enhanced relative to that in the surrounding air, but have given different values for this enhancement. Here, we reconcile these discrepancies by noting slight errors in both studies that, when corrected, lead to consistent results. Madronich (1987) examined the geometric (large droplet) limit and concluded that refraction leads to an enhancement factor, averaged over all incident directions, of 1.56. However, the physically relevant quantity is the enhancement of the average Actinic Flux (rather than the average enhancement factor) which we show here to be 1.26 in the geometric limit. Ruggaber et al. (1997) used Mie theory to derive energy density enhancements slightly larger than 2 for typical droplet sizes, and applied these directly to the calculation of photolysis rates. However, the physically relevant quantity is the Actinic Flux (rather than the energy density) which is obtained by dividing the energy density by the refractive index of water, 1.33. Thus, the Mie-predicted enhancement for typical cloud droplet sizes is in the range 1.5, only coincidentally in agreement with the value originally given by Madronich. We also investigated the influence of resonances in the Actinic Flux enhancement. These narrow spikes which are resolved only by very high resolution calculations are orders of magnitude higher than the intermediate values but contribute only little to the Actinic Flux enhancement when averaged over droplet size distributions. Finally, a table is provided which may be used to obtain the Actinic Flux enhancement for the photolysis of any dissolved species.
B. Schallhart - One of the best experts on this subject based on the ideXlab platform.
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NO 2 and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
Atmospheric Chemistry and Physics, 2005Co-Authors: C. Topaloglou, Stelios Kazadzis, M Blumthaler, B. Schallhart, Alkis Bais, D. BalisAbstract:Abstract. An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of Actinic Flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [J(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral Actinic Flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 12% (2σ) for J(NO2) and 6% (2σ) for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 12% (2σ) for the method that can be used with double Brewers and 20% for the method that can be used for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of Actinic Flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from Actinic Flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.
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NO2 and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
Atmospheric Chemistry and Physics, 2005Co-Authors: C. Topaloglou, B. Schallhart, S. Kazadzis, A. F. Bais, M. Blumthaler, D. BalisAbstract:An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of Actinic Flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [J(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral Actinic Flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 12% (2?) for J(NO2) and 6% (2?) for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 12% (2?) for the method that can be used with double Brewers and 20% for the method that can be used for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of Actinic Flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from Actinic Flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.
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NO<sub>2</sub> and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
2005Co-Authors: C. Topaloglou, B. Schallhart, S. Kazadzis, A. F. Bais, M. Blumthaler, D. BalisAbstract:Abstract. An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of Actinic Flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral Actinic Flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 6% for J(NO2) and 3% for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 6% for double and 10% for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of Actinic Flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from Actinic Flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.
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NO2 and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
Atmospheric Chemistry and Physics Discussions, 2005Co-Authors: C. Topaloglou, B. Schallhart, S. Kazadzis, A. F. Bais, M. Blumthaler, D. BalisAbstract:An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of Actinic Flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral Actinic Flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 6% for J(NO2) and 3% for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 6% for double and 10% for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of Actinic Flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from Actinic Flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.
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Actinic Flux and O1D photolysis frequencies retrieved from spectral measurements of irradiance at Thessaloniki, Greece
Atmospheric Chemistry and Physics, 2004Co-Authors: S. Kazadzis, M Blumthaler, Andreas Kazantzidis, C. Topaloglou, A. F. Bais, D. Balis, B. SchallhartAbstract:The results of two methods retrieving Actinic Flux and ozone photolysis frequencies (JO1D), from measurements of irradiance with a Brewer MKIII spectroradiometer are investigated in this paper. The first method uses Actinic Flux retrieved from irradiance measurements by the use of known formulas while the second is an empirical method converting irradiance to JO1D through polynomials extracted from a study of synchronous Actinic Flux and irradiance measurements. When examining the Actinic Fluxes derived from the first method to those measured by an Actinic Flux spectrometer data agree within ±10% for solar zenith angles lower than 75° for the UV-B and the UV-A wavelength band. Also, the Actinic to global irradiance ratio derived, deviates within ±6% for solar zenith angles lower than 70° compared with cloudless sky calculations of the TUV model. For both cases the deviations are in the order of the magnitude of the measurement or model uncertainties. Values of JO1D calculated by the second method show a mean ratio of 0.99±0.10 (1?) and 0.98±0.06 for all data and for cloudless skies respectively when compared with values of JO1D derived by a Bentham Actinic Flux spectroradiometer. Finally, the agreement of the two methods is within ±5% comparing two years' data of JO1D retrieved from irradiance measurements at Thessaloniki, Greece. The use of such methods on extensive data sets of global irradiance can provide JO1D values with acceptable uncertainty, a parameter of particular importance for chemical process studies.
Peter G Duynkerke - One of the best experts on this subject based on the ideXlab platform.
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surface and tethered balloon observations of Actinic Flux effects of arctic stratus surface albedo and solar zenith angle
Journal of Geophysical Research, 2001Co-Authors: Stephan R De Roode, W Boot, Peter G Duynkerke, Jeroen C H Van Der HageAbstract:As part of the First ISCCP Regional Experiment (FIRE III) Arctic Cloud Experiment Actinic Flux measurements were made above the Arctic Sea ice during May 1998. The Actinic Flux, which is also referred to as the 4π radiative Flux, is the relevant radiative parameter needed to determine photodissociation rates. It is shown that for a plane-parallel cloud the change in the net irradiance as a function of the optical depth is proportional to the magnitude of the Actinic Flux. Continuous Actinic Flux measurements were made just above the snow-covered ice surface by a UV-A and a visible 4π radiometer (wavelengths ∼365 and ∼550 nm, respectively). In addition, vertical profiles of the Actinic Flux through low arctic stratus clouds were obtained by means of a visible 4π radiometer suspended under a tethered balloon. The cloud thermodynamic and microphysical structure was determined from observations made by the National Center for Atmospheric Research C-130 aircraft. In addition, the phase and liquid water path of the cloud was assessed from microwave radiometer, lidar, and radar data. During clear-sky conditions the diurnal variation of the magnitude of Actinic Flux was controlled mainly by Rayleigh scattering and surface reflection. Above a stratus cloud layer the Actinic Flux was found to be almost the same as during clear-sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. In the arctic stratus clouds the Actinic Flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the Actinic Flux increased significantly with height. The vertical profiles that were observed in arctic stratus differed from those measured in Atlantic stratocumulus; in the latter the Actinic Flux was found to increase gradually from cloud base to cloud top. A delta-Eddington model is utilized to illustrate that the exact shape of the vertical profile is very sensitive to the solar zenith angle. During the arctic experiments the solar zenith angle was generally much larger than during the observations in Atlantic stratocumulus.
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Surface and tethered‐balloon observations of Actinic Flux: Effects of arctic stratus, surface albedo, and solar zenith angle
Journal of Geophysical Research: Atmospheres, 2001Co-Authors: Stephan R De Roode, W Boot, Peter G Duynkerke, J. C. H. Van Der HageAbstract:As part of the First ISCCP Regional Experiment (FIRE III) Arctic Cloud Experiment Actinic Flux measurements were made above the Arctic Sea ice during May 1998. The Actinic Flux, which is also referred to as the 4π radiative Flux, is the relevant radiative parameter needed to determine photodissociation rates. It is shown that for a plane-parallel cloud the change in the net irradiance as a function of the optical depth is proportional to the magnitude of the Actinic Flux. Continuous Actinic Flux measurements were made just above the snow-covered ice surface by a UV-A and a visible 4π radiometer (wavelengths ∼365 and ∼550 nm, respectively). In addition, vertical profiles of the Actinic Flux through low arctic stratus clouds were obtained by means of a visible 4π radiometer suspended under a tethered balloon. The cloud thermodynamic and microphysical structure was determined from observations made by the National Center for Atmospheric Research C-130 aircraft. In addition, the phase and liquid water path of the cloud was assessed from microwave radiometer, lidar, and radar data. During clear-sky conditions the diurnal variation of the magnitude of Actinic Flux was controlled mainly by Rayleigh scattering and surface reflection. Above a stratus cloud layer the Actinic Flux was found to be almost the same as during clear-sky conditions. This could be attributed to the fact that the effective albedo of the arctic sea ice and the cloud is only slightly higher than the ground albedo alone. In the arctic stratus clouds the Actinic Flux was found to be nearly constant with height, except in a shallow layer near the cloud top where the Actinic Flux increased significantly with height. The vertical profiles that were observed in arctic stratus differed from those measured in Atlantic stratocumulus; in the latter the Actinic Flux was found to increase gradually from cloud base to cloud top. A delta-Eddington model is utilized to illustrate that the exact shape of the vertical profile is very sensitive to the solar zenith angle. During the arctic experiments the solar zenith angle was generally much larger than during the observations in Atlantic stratocumulus.
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Actinic Fluxes in broken cloud fields
Journal of Geophysical Research: Atmospheres, 1997Co-Authors: A. Los, M. Van Weele, Peter G DuynkerkeAbstract:Photochemical processes in the atmosphere are driven by solar ultraviolet radiation. The photodissociation rate coefficients of atmospheric species are determined by the Actinic Flux, which is defined as 4π times the mean ultraviolet intensity. Because of the presence of clouds the Actinic Flux can change drastically throughout the atmosphere. Therefore clouds have large effects on photodissociation rate coefficients. At cloud top, photodissociation rate coefficients can be 300% higher than in clear sky conditions. We use Monte Carlo simulations to investigate the reflectance, the transmittance, and the Actinic Flux for cloud fields at various degrees of cloudiness. Scattering processes in the clouds are due to cloud particles only. We do not take absorption of radiation into account. The atmosphere outside the clouds is assumed to be completely transparent. The simulated reflectance and transmittance of plane-parallel cloud fields and in broken cloud field conditions reproduce the results of previous model studies within statistical uncertainties. The results of Actinic Flux calculations for plane-parallel cloud fields agree with the results obtained with a doubling-adding algorithm. Horizontal and vertical Actinic Flux profiles in broken cloud fields are studied for various solar zenith angles and for different cloud optical thicknesses. The aim of the present model study is to obtain insight into the effect of broken cloud fields on the Actinic Flux.
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A Photoelectric Detector Suspended under a Balloon for Actinic Flux Measurements
Journal of Atmospheric and Oceanic Technology, 1994Co-Authors: J. C. H. Van Der Hage, H. Van Dop, W Boot, Peter G Duynkerke, J. Vilà-guerau De ArellanoAbstract:Abstract An instrument to measure ultraviolet Actinic Flux in and outside clouds was constructed and calibrated. Characteristics of the instrument axe: 1) it is equipped with gallium phosphide photodiodes, 2) its isotropic directional response is almost perfect due to the special design of the optical diffusers, and 3) it is a lightweight construction (150 g) allowing the use of a balloon or a kit as an observation platform. Ground-level observations and vertical profiles, measured with this UV photometer in clear air and in stratocumulus during the Atlantic Stratocumulus Transition Experiment in the Azores, confirm that the Madronich radiation transfer model is valid for clouds.
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tethered balloon measurements of Actinic Flux in a cloud capped marine boundary layer
Journal of Geophysical Research, 1994Co-Authors: Jordi Vilà-guerau De Arellano, Peter G Duynkerke, Michiel Van WeeleAbstract:As part of the Atlantic Stratocumulus Experiment (ASTEX), more than 34 tethered-balloon soundings were carried out on Santa Maria Island, Azores. The main purpose of the soundings was to measure vertical profiles of Actinic Flux under different meteorological conditions. In addition, vertical profiles of temperature, relative humidity, and wind speed and direction were measured. The Actinic Flux was measured with a photoelectrical detector with a spectral response that ranges from 330 nm to 390 nm. It is in this region where the photodissociation of nitrogen dioxide, the main precursor of ozone in the troposphere, takes place. Several vertical profiles of Actinic Flux under clear and cloudy conditions are presented. These profiles show the different behavior of the Actinic Flux below, in, and above clouds. These observations are complemented with detailed measurements of cloud characteristics; relevant cloud properties for this study are the cloud-base and cloud-top heights and the cloud optical depth. A slightly increasing tendency for the Actinic Flux in the whole atmospheric boundary layer was observed under clear sky conditions. Under the presence of clouds and compared with clear sky values measured at the same solar zenith angle and at the surface, lower values were observed below cloud (ratio ranging from 0.39 to 0.92) and higher at the top of the cloud (ratio ranging from 1.68 to 2.32). In the cloud an increase in the values of Actinic Flux with height was measured. The Actinic Flux measurements are evaluated against values obtained from a multilayer delta-Eddington model. An excellent agreement is found for the soundings made under total overcast conditions.
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An efficient method to increase vertical resolution of Actinic Flux calculations in clouds
2015Co-Authors: Merlinde J. Kay, Michael A. Box, Thomas TrautmannAbstract:[1] The errors associated with using different vertical resolutions for broadband Actinic Flux calculations have been investigated for the case of a boundary layer cloud. Results are presented for 10, 20, and 40 layers in the lowest kilometer of the atmosphere, with comparisons made against a benchmark of 100 layers. A more accurate picture of Actinic Flux profiles is obtained by increasing the number of vertical layers. We also derive an expression for the average value of Actinic Flux within a computational layer. This result, when used to quadratically interpolate the coarser layerings, improved the accuracy in the 10-layer case by a factor of five, when compared with a simple linear interpolation between the original data points. INDEX TERMS: 0320 Atmospheric Composition and Structure
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Combining the independent pixel and point-spread function approaches to simulate the Actinic radiation field in moderately inhomogeneous 3D cloudy media
Journal of Quantitative Spectroscopy and Radiative Transfer, 2011Co-Authors: Anke Kniffka, Thomas TrautmannAbstract:A fast method is presented for gaining 3D Actinic Flux density fields, Fact, in clouds employing the Independent Pixel Approximation (IPA) with a parameterized horizontal photon transport to imitate radiative smoothing effects. For 3D clouds the IPA is an efficient method to simulate radiative transfer, but it suffers from the neglect of horizontal photon Fluxes leading to significant errors (up to locally 30% in the present study). Consequently, the resulting Actinic Flux density fields exhibit an unrealistically rough and rugged structure. In this study, the radiative smoothing is approximated by applying a physically based smoothing algorithm to the calculated IPA Actinic Flux field.
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Airborne system for fast measurements of upwelling and downwelling spectral Actinic Flux densities
Applied optics, 2005Co-Authors: Evelyn Jäkel, Manfred Wendisch, Anke Kniffka, Thomas TrautmannAbstract:An airborne system for fast measurements of spectral Actinic Flux densities in the wavelength range 305-700 nm is introduced. The system is called the Actinic Flux Density Meter (AFDM). The AFDM utilizes the diode array technique and measures downwelling and upwelling spectral Actinic Flux densities separately with a time resolution of less than 1 s. For airborne measurements this means a spatial resolution of approximately 60 m, assuming an average aircraft velocity of 60 m/s. Thus the AFDM resolves fast changes in the Actinic radiation field, which are of special importance for conditions of inhomogeneous clouds or surface reflection. Laboratory characterization measurements of the AFDM are presented, and a method to correct the nonideal angular response of the optical inlets is introduced. Furthermore, exemplar field data sampled simultaneously with spectral irradiance measurements are shown. The horizontal variability of the measured spectra of Actinic Flux density is quantified, and profile measurements for overcast situations are presented. Finally, the effects of clouds on the spectral Actinic Flux density are discussed.
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A new Actinic Flux 4π-spectroradiometer: instrument design and application to clear sky and broken cloud conditions
Atmospheric Chemistry and Physics, 2003Co-Authors: Evi Eckstein, Dieter Perner, Christoph Brühl, Thomas TrautmannAbstract:A new 4 p -spectroradiometer was developed for measuring Actinic Flux especially under cloudy conditions based on a fixed grating imaging spectrograph and a CCD-detector leading to a simultaneous measurement of the spectrum. The new instrument incorporates a novel optical head with a 4 p -field of view independent of angle of incidence. Comparisons with the Actinic Flux spectroradiometer of the Institute of Atmospheric Chemistry of Forschungszentrum Julich showed a very good agreement within the limit of the uncertainties of the two instruments. Our spectroradiometer was applied to investigate the effects of broken clouds on the Actinic Flux and photolysis frequencies on the ground during the BERLIOZ campaign. Reductions as well as enhancements compared to the clear sky case were seen, both effects are larger in the UV-A than the UV-B spectral region. Furthermore the new instrument was used for simultaneous measurements in different altitudes on a tower to study the transmission and attenuation of Actinic Flux in low clouds. A correlation of attenuation with the simultaneously measured liquid water content of the cloud was found.
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three dimensional solar radiation effects on the Actinic Flux field in a biomass burning plume
Journal of Geophysical Research, 2003Co-Authors: Jörg Trentmann, Thomas Trautmann, Barbara Früh, Olivier Boucher, Meinrat O. AndreaeAbstract:[1] Three-dimensional (3-D) solar radiative transfer models describe radiative transfer under inhomogeneous atmospheric conditions more accurately than the commonly used one-dimensional (1-D) radiative transfer models that assume horizontal homogeneity of the atmosphere. Here results of 3-D radiative transfer simulations for a biomass-burning plume are presented and compared with local one-dimensional (l-1-D) simulations, i.e., 1-D simulations in every column of the model domain. The spatial distribution of the aerosol particles was derived from a 3-D atmospheric transport simulation. We studied the impact of 3-D radiative effects on the Actinic Flux within the plume center. The differences in the Actinic Flux between results from the 3-D and the l-1-D simulations are considerable, ranging from � 40% to more than +200%, depending on the wavelength, solar zenith angle, and the absorbing properties of the aerosol. The reason for this discrepancy is the neglect of horizontal photon transport in the 1-D simulation. These large 3-D effects on the Actinic Flux have the potential to influence significantly the in-plume photochemistry. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution— urban and regional (0305); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; KEYWORDS: biomass burning plume, Actinic Flux, 3-D solar radiative transfer simulations, aerosol absorption
