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M Blumthaler – One of the best experts on this subject based on the ideXlab platform.

  • 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, 2012
    Co-Authors: P. Weihs, M Blumthaler, Richard Kift, Gian Paolo Gobbi, J. E. Wagner, Federico Angelini, M. Fitzka, Harald E. Rieder, S. F. Schreier, Axel Kreuter
    Abstract:

    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.

  • Investigation of the 3-D Actinic Flux field in mountainous terrain
    Atmospheric research, 2011
    Co-Authors: J. E. Wagner, M Blumthaler, Richard Kift, Gian Paolo Gobbi, Federico Angelini, M. Fitzka, Axel Kreuter, Harald E. Rieder, Stana Simic, Ann R. Webb
    Abstract:

    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.

  • Influence of clouds on the spectral Actinic Flux density in the lower troposphere (INSPECTRO): overview of the field campaigns
    Atmospheric Chemistry and Physics, 2008
    Co-Authors: Stephan Thiel, M Blumthaler, A F Bais, Gian Paolo Gobbi, L. Ammannato, Birger Bohn, Brian J. Bandy, Ola Engelsen, Julian Gröbner, Evelyn Jäkel
    Abstract:

    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 dioxdioxide. 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%.

Sasha Madronich – One of the best experts on this subject based on the ideXlab platform.

  • Improved modeling of cloudy-sky Actinic Flux using satellite cloud retrievals
    Geophysical Research Letters, 2017
    Co-Authors: Young-hee Ryu, Samuel R. Hall, Alma Hodzic, Gael Descombes, Patrick Minnis, Douglas A. Spangenberg, Kirk Ullmann, Sasha Madronich
    Abstract:

    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.

  • Effect of aerosols and NO 2 concentration on ultraviolet Actinic Flux near Mexico City during MILAGRO: measurements and model calculations
    Atmospheric Chemistry and Physics, 2013
    Co-Authors: Gustavo G. Palancar, Barry Lefer, Samuel R. Hall, William J. Shaw, Chelsea A. Corr, Scott C. Herndon, James R. Slusser, Sasha Madronich
    Abstract:

    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 altialtitude. 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

  • Ultraviolet Actinic Flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements
    Atmospheric Chemistry and Physics, 2011
    Co-Authors: Gustavo G. Palancar, Richard E. Shetter, Samuel R. Hall, Beatriz M. Toselli, Sasha Madronich
    Abstract:

    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.

B. Schallhart – One of the best experts on this subject based on the ideXlab platform.

  • NO 2 and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
    Atmospheric Chemistry and Physics, 2005
    Co-Authors: C. Topaloglou, Stelios Kazadzis, M Blumthaler, B. Schallhart, Alkis Bais, D. Balis
    Abstract:

    Abstract. An empirical approach for the retrieval of nitrogen dioxdioxide (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.

  • NO2 and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
    Atmospheric Chemistry and Physics, 2005
    Co-Authors: C. Topaloglou, B. Schallhart, S. Kazadzis, A. F. Bais, M. Blumthaler, D. Balis
    Abstract:

    An empirical approach for the retrieval of nitrogen dioxdioxide (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.

  • NO<sub>2</sub> and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece
    , 2005
    Co-Authors: C. Topaloglou, B. Schallhart, S. Kazadzis, A. F. Bais, M. Blumthaler, D. Balis
    Abstract:

    Abstract. An empirical approach for the retrieval of nitrogen dioxdioxide (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.

Peter G Duynkerke – One of the best experts on this subject based on the ideXlab platform.

  • surface and tethered balloon observations of Actinic Flux effects of arctic stratus surface albedo and solar zenith angle
    Journal of Geophysical Research, 2001
    Co-Authors: Stephan R De Roode, W Boot, Peter G Duynkerke, Jeroen C H Van Der Hage
    Abstract:

    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 radiradiometer, 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.

  • Surface and tethered‐balloon observations of Actinic Flux: Effects of arctic stratus, surface albedo, and solar zenith angle
    Journal of Geophysical Research: Atmospheres, 2001
    Co-Authors: Stephan R De Roode, Peter G Duynkerke, W Boot, J. C. H. Van Der Hage
    Abstract:

    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 radiradiometer, 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.

  • Actinic Fluxes in broken cloud fields
    Journal of Geophysical Research: Atmospheres, 1997
    Co-Authors: A. Los, M. Van Weele, Peter G Duynkerke
    Abstract:

    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.

Thomas Trautmann – One of the best experts on this subject based on the ideXlab platform.

  • An efficient method to increase vertical resolution of Actinic Flux calculations in clouds
    , 2015
    Co-Authors: Merlinde J. Kay, Michael A. Box, Thomas Trautmann
    Abstract:

    [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 inteinterpolation between the original data points. INDEX TERMS: 0320 Atmospheric Composition and Structure

  • 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, 2011
    Co-Authors: Anke Kniffka, Thomas Trautmann
    Abstract:

    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.

  • Airborne system for fast measurements of upwelling and downwelling spectral Actinic Flux densities
    Applied optics, 2005
    Co-Authors: Evelyn Jäkel, Manfred Wendisch, Anke Kniffka, Thomas Trautmann
    Abstract:

    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.