Actinic radiation in the terrestrial atmosphere (original) (raw)
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Actinic Radiation and Photolysis Processes in the Lower Troposphere: Effect of Clouds and Aerosols
2002
Within the German Tropospheric Research Program (TFS) a series of projects were performed focussing on aspects of radiation transfer and the effects of UV-radiation on air chemistry. The individual projects covered laboratory investigations, instrument development for photolysis processes as well as field studies of actinic radiation and comparison to model calculations. One and three-dimensional models were tested against field campaign data. The results confirm the improvement of measurement technology achieved through deployment of new techniques like spectroradiometry that offer a wider range of investigations than was previously attainable using chemical actinometry or fixed wavelength filter radiometry. Reasonable agreement was also found between measurements and models for a few selected and well defined cloudy conditions. On the other hand, using simple stratiform geometry models yielded significant deviations between measurement and model in both directions particularly in the case of high zenith angles and with high aerosol load. Further tools both for experimental investigations and for model calculations were developed within the framework of the Troposphere Research Program (TFS) and deficiencies were identified demanding further investigations when broken clouds and more complex cloud layers prevail.
Applied Optics, 1999
A spectroradiometer has been developed for direct measurement of the solar actinic UV flux ͑scalar intensity͒ and determination of photolysis frequencies in the atmosphere. The instrument is based on a scanning double monochromator with an entrance optic that exhibits an isotropic angular response over a solid angle of 2 sr. Actinic flux spectra are measured at a resolution of 1 nm across a range of 280 -420 nm, which is relevant for most tropospheric photolysis processes. The photolysis frequencies are derived from the measured radiation spectra by use of published absorption cross sections and quantum yields. The advantage of this technique compared with the traditional chemical actinometry is its versatility. It is possible to determine the photolysis frequency for any photochemical reaction of interest provided that the respective molecular photodissociation parameters are known and the absorption cross section falls within a wavelength range that is accessible by the spectroradiometer. The instrument and the calibration procedures are described in detail, and problems specific to measurement of the actinic radiation are discussed. An error analysis is presented together with a discussion of the spectral requirements of the instrument for accurate measurements of important tropospheric photolysis frequencies ͑ J O 1 D , J NO 2 , J HCHO ͒. An example of measurements from previous atmospheric chemistry field campaigns are presented and discussed.
Journal of Geophysical Research, 2002
1] The relationship between photolysis frequencies derived from spectroscopic measurements of actinic fluxes and irradiances was determined during a coordinated measurement campaign (International Photolysis Frequency Measurement and Modeling Intercomparison campaign (IPMMI)). When differences in viewing geometries are taken into account, the measurements are in close agreement. An empirical relationship, which is useful for high sun (noon) conditions or for daily integrals, was found to convert irradiance data to photolysis frequencies. For low-sun conditions (large solar zenith angle), model calculations were shown to improve the accuracy. However, the input parameters to the model are site specific and the conversion depends on diffuse/ direct ratios. During cloudy conditions, significant improvements in the conversion can be achieved by assuming the radiation field to comprise entirely diffuse isotropic radiation when the UVA transmission by cloud is less than 0.8. Changing cloud conditions remain the greatest limitation, but they tend to bias the results away from the clear-sky case in a systematic way. Furthermore, although the cloud effects on the photolysis rates of nitrogen dioxide (J(NO 2 )) are rather large, they are much smaller for ozone photolysis (J(O 3 ! O( 1 D))), which is of prime importance in tropospheric chemistry. The study shows the potential for deriving historical and geographical differences in actinic fluxes from the extensive records of ground-based measurements of spectral irradiance.
Journal of Geophysical Research, 2002
1] Airborne measurements of the spectrally resolved actinic flux (280 -420 nm) between the ground and 12 km altitude have been made using a new calibrated dual-channel spectroradiometer. The measurements were made as part of the Photochemical Activity and Ultraviolet Radiation/Altitude Dependence of the Tropospheric Ozone Photolysis (PAUR/ATOP) measurement campaign in Greece during June 1996. Flights were made over the Agean Sea under cloudless conditions for various aerosol loads and solar zenith angles. The spectral actinic flux measurements are compared with radiative transfer model simulations based on the multistream discrete ordinate radiative transfer (DISORT) algorithm. All input to the radiative transfer model was provided by independent measurements performed simultaneously on the nearby island of Agios Efstratios. For altitudes between 3000 -12,000 m the agreement between the measurements and the model simulations is within Ϯ5% for wavelengths larger than 310 nm and within Ϯ10% at 295-310 nm. For the lowest flight altitude, 108 m, the model underestimates the measured actinic flux systematically by about 12%. This may be partly explained by uncertainties in the aerosol optical properties and the surface albedo. Flights on days with small and large amounts of aerosols showed that under otherwise identical conditions the actinic flux increased by up to 10% when the aerosol amount was larger.
The budget of biologically active ultraviolet radiation in the Earth-atmosphere system
Journal of Geophysical Research, 1988
This study applies the concept of a budget to describe the interaction of solar ultraviolet (UV) radiation with the Earth-atmosphere system. The wavelength ranges of interest are the biologically relevant UV-B between 280 and 320 nm and the UV-A from 320 to 400 nm. The Nimbus 7 solar backscattered ultraviolet (SBUV) instrument provides measurements of total column ozone and information concerning cloud cover which, in combination with a simple model of radiation transfer, define the fractions of incident solar irradiance absorbed in the atmosphere, reflected to space, and absorbed at the ground. Results for the month of July quantify the contribution of fractional cloud cover and cloud optical thickness to the radiation budget's three components. Scattering within a thick cloud layer makes the downward radiation field at the cloud base more isotropic than is the case for clear skies. For small solar zenith angles, typical of summer midday conditions, the effective path length of this diffuse irradiance through tropospheric ozone is greater than that under clear-sky conditions. The result is an enhanced absorption of UV-B radiation in the troposphere during cloud-covered conditions. Major changes in global cloud cover or cloud optical thicknesses could alter the ultraviolet radiation received by the biosphere by an amount comparable to that predicted for long-term trends in ozone.
Geophysical Research Letters, 1995
The solar actinic flux has been measured directly and spectrally resolved between 280 nm and 330 nm by means of a spectroradiometer. From these measurements the photolysis frequency J(O(1)D) of ozone is calculated using ozone absorption cross-sections and O((1)D) quantum yields from the literature. Different published wavelength dependencies of the O((1)D) quantum yield data are used to compare the spectroradiometric J(O(1)D) values to J(O(1)D) data measured simultaneously with a chemical actinometer. This field comparison supports laboratory studies that have measured O((1)D) quantum yields of about 0.2 to 0.3 between 315 nm and 320 nm. Implications for tropospheric photochemistry are discussed.
Atmospheric Chemistry and Physics, 2011
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 clearsky-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 downwelling 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.
Atmospheric Environment, 1997
We have developed a detailed model of solar radiation in the atmosphere as it is affected by atmospheric constituents, aerosols, clouds and the surface characteristics of the earth. Such a model is the foundation for studying global change and atmospheric chemistry under natural and disturbed conditions, The model includes radiative transfer processes for solar ultraviolet and visible wavelengths (290-700 mn) under different environmental conditions. It calculates the optical properties of aerosols and cloud droplets as well as the direct, diffuse, net, and actinic fluxes for different wavelengths, altitudes, and zenith angles at a relatively high computational speed and accuracy. It only takes about three and a half minutes to calculate all the optical properties and radiative fluxes in a cloudy air (including all the properties and fluxes in 100 sub-layers inside a cloud), and about 20 seconds in a clear sky and clean air condition at a SUN SPARCStation lo/50 (with single SPARC CPU running at about 50 MHz). We show that local environmental conditions, particularly in the lower atmosphere, can greatly alter the actinic flux throughout the atmosphere. This feature is especially apparent in the wavelengths with weak or no 0, absorption, as multiple scattering dominates the atmospheric radiative transfer. Compared to the actinic flux under clear sky and clean air conditions, for example, the actinic flux around 400 nm at zero zenith angle decreases by a factor of 5 at the earth's surface while increasing by more than 100% at the top of the atmosphere when a one-km altostratus cloud is added to the middle troposphere. According to our calculations, the radiation field outside a cloud is mainly controlled by the total liquid water content of the cloud; however, the actinic flux inside a cloud is very sensitive to the macro structure of the cloud. Readers may acquire the computer model from the authors.
Journal of Geophysical Research, 2003
1] Ground-based spectral measurements of actinic flux density (300-660 nm wavelength) and downward UV irradiance (300-324 nm) under cloudless conditions have been compared with the results of one-dimensional radiative transfer calculations employing concurrent airborne vertical profile measurements of aerosol particle size distributions. Good agreement (within ±10%) between measured and calculated spectra was found. The remaining differences were explained by uncertainties inherent in the aerosol particle microphysical input data and the column ozone content. A respective sensitivity analysis of the calculated spectra, which was based on the observed variability of microphysical properties, has shown that the particle number concentration is the most crucial input uncertainty for both the actinic flux density and the UV irradiance. For the wavelength range investigated, the uncertainty of the column ozone content is of minor importance for both spectral quantities. INDEX TERMS: 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; KEYWORDS: actinic flux, UV irradiance, aerosol radiative forcing, radiative transfer simulations Citation: Früh, B., E. Eckstein, T. Trautmann, M. Wendisch, M. Fiebig, and U. Feister, Ground-based measured and calculated spectra of actinic flux density and downward UV irradiance in cloudless conditions and their sensitivity to aerosol microphysical properties,