Alberto Redondas

SUSPEN intercomparison of ultraviolet spectroradiometers

Results from an intercomparison campaign of ultraviolet spectroradiometers that was organized at Nea Michaniona, Greece July, 1–13 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wave-length registration of the recorded spectra, for most of the spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy and in 1993 in Garmisch-Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.

Comparison of measured and modelled uv indices for the assessment of health risks

The World Health Organisation (WHO) and the World Meteorological Organisation (WMO) have jointly recommended that the UV Index (UVI) should be used to inform the public about possible health risks due to overexposure to solar radiation, especially skin damage. To test the current operational status of measuring and modelling techniques used in providing the public with UVI information, this article compares cloudless sky UVIs (measured using five instruments at four locations with different latitudes and climate) with the results of 13 models used in UVI forecasting schemes. For the models, only location, total ozone and solar zenith angle were provided as input parameters. In many cases the agreement is acceptable, i.e. less than 0.5 UVI. Larger differences may originate from instrumental errors and shortcomings in the models and their input parameters. A possible explanation for the differences between models is the treatment of the unknown input parameters, especially aerosols. Copyright

UV Index Experimental Values During the Years 2000 and 2001 from the Spanish Broadband UV-B Radiometric Network¶

Abstract An analysis is made of experimental ultraviolet erythemal solar radiation data measured during the years 2000 and 2001 by the Spanish UV-B radiation evaluation and prediction network. This network consists of 16 Robertson–Berger type pyranometers for evaluating solar erythemal radiation and five Brewer spectroradiometers for evaluating the stratospheric ozone. On the basis of these data the Ultraviolet Index (UVI) was evaluated for the measuring stations that are located either in coastal regions or in the more densely populated regions inland on the Iberian Peninsula. It has been checked that in most cases the maximum irradiance values corresponded to solar noon, although there were exceptions that could be explained by cloudiness. The maximum experimental values of the UVI were around 9 during the summer, though frequently passing this value at the inland measurement stations. The annual accumulated dose of irradiation on a horizontal plane has also been studied, as well as the evolution through the year in units of energy, standard erythemal doses and minimum erythemal doses, according to different phototypes.

Simulation of mineral dust effects on UV radiation levels

The role played by aerosols on UV radiative transfer in the atmosphere is very uncertain. This is especially true regarding mineral dust. To determine the sensitivity of the UV levels to the presence of this atmospheric specie, we have simulated the UV irradiance with different vertical distributions of mineral dust. We have used a discrete ordinates radiative transfer model to obtain the UV levels both at sea level and at 3000 m. We have computed the aerosol single-scattering albedo, the phase function, and the asymmetry factor by Mie scattering theory. The background aerosol profiles were taken from WCRP [1986] models, whereas the radiative properties of mineral dust have been calculated from the aerosol size distribution obtained during Saharan dust invasions at Tenerife island (28.5°N, 16.3°W). The values for aerosol optical depth assumed as input for the model calculations are 0.2 (at 550 nm) for clean background aerosols and 0.3 (at 550 nm) for the mineral dust component. From the results we can conclude that the dust vertical size distribution can affect the irradiance ratio F (with Saharan dust)/F (no Saharan dust) by 2-4%. In addition, we observe that to the same total optical depth the diffuse UV levels depend not only on the vertical dust distribution but also on the background aerosol vertical distribution. We have computed differences for the diffuse radiation fluxes of about 5% between a maritime and a continental model to the same mineral dust vertical distribution.

Testing the daytime oxidizing capacity of the troposphere: 1994 OH field campaign at the Izaña observatory, Tenerife

A field campaign was carried out during May 1994 at the Izana station, Tenerife. This campaign was part of the program Environment and Climate sponsored by the European Commission to study the influence of European emissions on the oxidizing capacity of a clean tropospheric environment. Daytime and also nighttime measurements were made, covering the OH as well as the NO3 chemistry. This paper presents the OH measurements taken with a multipass optical absorption spectrometer (MOAS) and discusses the daytime chemistry in a statistical and therefore more preliminary way. All relevant parameters influencing the OH concentration were monitored. From the data the two main contributions to the OH production can clearly be discerned and are given by the primary production following the ozone photolysis and the O(1D)-H2O reaction and by the catalytic reactions of NOx in the recycling process. The latter processes prove to contribute a dominant part to the OH concentration. The measurements of the nonmethane hydrocarbons (NMHC) especially of the biogenics, indicate a considerable influence of the NMHC on the absolute values of the OH concentration at Tenerife.

Compensating for the Effects of Stray Light in Single-Monochromator Brewer Spectrophotometer Ozone Retrieval

Abstract Spectrometers are designed to isolate particular wavebands and suppress light from wavelengths outside the band of interest. However, a small amount of undesired light will always enter the detector, not through the designed optical path, but through random scattering from the instrument optical components, housing, and dust particles. Every spectrophotometer has stray light coming from outside the nominal measurement waveband. For Dobson spectrophotometers and single monochromator Brewer spectrophotometers, which are basic instruments in the World Meteorological Organization (WMO) ozone and ultraviolet (UV) monitoring network, the error introduced by stray light is substantial when the ozone slant path becomes very large because of high solar zenith angles and a thick ozone layer. These are common conditions during Arctic spring. To study the issue, a long ozone slant path Intercomparison/Calibration campaign for Nordic Brewers and Dobsons was held at Sodankylä 8–24 March 2011 and a follow-up campaign to extend calibrations to shorter ozone slant paths took place at Izaña observatory, Tenerife, between 28 October and 18 November 2011. These campaigns were part of the Committee on Earth Observation Satellites (CEOS) Intercalibration of Ground-based Spectrometers and Lidars project funded by the European Space Agency (ESA), intended to permit the homogenization of ozone data from the European ozone ground-truthing network. During the active intercomparison periods, measurements were taken only when good conditions for sun or moon observations existed. Laboratory measurements using calibration lamps and helium-cadmium (HeCd) lasers were an essential part of both campaigns. The campaigns produced a high-quality database of total ozone and UV measurements and an accurate, up-to-date calibration and characterization of participating Brewers and Dobsons against the European standard instruments from the Regional Dobson Calibration Centre-Europe (RDCC-E) and the Regional Brewer Calibration Centre-Europe (RBCC-E). In the present work we focus on single monochromator Brewers and present a physics-based method to compensate for the stray-light effects in ozone retrieval using laboratory characterizations and radiative transfer modelling. The method was tested with independent data from the campaign.

Wavelength calibration of Brewer spectrophotometer using a tuneable pulsed laser and implications to the Brewer ozone retrieval

In this contribution we present the wavelength calibration of the travelling reference Brewer spectrometer of the Regional Brewer Calibration Center for Europe (RBCCE) at PTB in Braunschweig, Germany. The wavelength calibration is needed for the calculation of the ozone absorption coefficients used by the Brewer ozone algorithm. In order to validate the standard procedure for determining Brewer’s wavelength scale, a calibration has been performed by using a tunable laser source at PTB in the framework of the EMRP project ENV59 ATMOZ “Traceability for the total column ozone”. Here we compare these results to those of the standard procedure for the wavelength calibration of the Brewer instrument. Such a comparison allows validating the standard methodology used for measuring the ozone absorption coefficient with respect to several assumptions. The results of the laser-based calibrations reproduces those obtained by the standard operational methodology and shows that there is an underestimation of 0.8 % of the ozone absorption coefficients due to the use of the parametrized slit functions.

Optical calibration facility at the Izaña Atmospheric Research Center

During the last years a new optical calibration facility has been developed and deployed at the Izana Observatory for the calibration and characterization of the radiation measuring instruments within research activities. These activities require a traceable Quality Assurance & Quality Control system. This new facility allows the absolute, spectral and cosine response calibration. At present there are six set-ups running at the laboratory for radiance calibration, angular response determination, spectral response characterization, slit function determination and absolute irradiance calibration (vertical and horizontal set-ups). Each method and procedure has required the development of the corresponding protocol. These systems, as well as some examples of instrument calibrations, are presented in this work.

Calibrating six years of multiband UV measurements at Ushuaia and Marambio for model and satellite comparisons

An Antarctic UV-monitoring network established in 1999 as a Spanish-Finnish-Argentinian co-operation consists of multiband filter radiometers located at Belgrano, Marambio, and Ushuaia. To provide with quality controlled and assured calibrated groundbased Antarctic UV data, bi-weekly lamp tests were used on every site and visits of travelling reference instruments on two of the sites. Along the six years of operation, the sensitivity in some of the instrument channels was found to drift up to 61%. In both stations, always the same channels showed the best stability or worst instability. The rigorous quality assurance programme ensured that reliable time series of solar data could be produced, however. The most recent Antarctic ozone depletion period of 2005/2006 was studied by comparing OMI satellite-based erythemally weighted daily doses with the measured polynomial corrected data for August 2005-March 2006 for Ushuaia and Marambio. The root mean square (RMS) of difference between the groundbased and satellite-retrieved daily doses was on monthly basis smaller for Ushuaia (19 - 28 %) than for Marambio (17-58 %), possibly due to e.g. bigger heterogeneity of the ground albedo, and variability of the cloudiness. Our final task of combining the polynomial corrected lamp calibration factors and the traveling reference calibration factors, to produce the final calibrated Antartic UV data, is discussed, too.

UV measurements at Marambio and Ushuaia during 2000–2010

Kaisa Lakkala1,2, Alberto Redondas3, Outi Meinander1, Laura Thölix1, Britta Hamari1, Antonio Fernando Almansa3, Virgilio Carreno3, Guillermo Deferrari4,5, Hector Ochoa6, Germar Bernhard7, Ricardo Sanchez8, and Gerardus de Leeuw1 1Finnish Meteorological Institute, Climate Research, Helsinki, Finland 2Finnish Meteorological Institute, Arctic Research, Sodankylä, Finland 3Izaña Atmospheric Research Center, Agencia Estatal de Meteorología, Tenerife, Spain 4Centro Austral de Investigaciones Cientificas (CADIC/CONICET), Ushuaia, Argentina 5Universidad Nacional de Tierra del Fuefo, Ushuaia, Argentina 6Dirección Nacional del Antártico-Instituto Antártico Argentino, Buenos Aires, Argentina 7Biospherical Instruments, Inc., San Diego, U.S.A 8Servicio Meteorológico Nacional, Argentina Correspondence to: Kaisa Lakkala (kaisa.lakkala@fmi.fi)