Hugo De Backer

Comparison of Models Used for UV Index Calculations

Eighteen radiative transfer models in use for calculation of UV index are compared with respect to their results for more than 100 cloud‐free atmospheres, which describe present, possible future and extreme conditions. The comparison includes six multiple‐scattering spectral models, eight fast spectral models and four empirical models. Averages of the results of the six participating multiple‐scattering spectral models are taken as a basis for assessment. The agreement among the multiple‐scattering models is within ±0.5 UV index values for more than 80% of chosen atmospheric parameters. The fast spectral models have very different agreement, between ±1 and up to 12 UV index values. The results of the empirical models agree reasonably well with the reference models but only for the atmospheres for which they have been developed. The data to describe the atmospheric conditions, which are used for the comparison, together with the individual results of all participating models and model descriptions are available on the Internet: http://www.meteo.physik.uni‐muenchen.de/strahlung/cost/.

Acute changes in pulse pressure in relation to constituents of particulate air pollution in elderly persons

An increased pulse pressure (difference between systolic and diastolic blood pressure) suggests aortic stiffening. The objective of this study was to examine the acute effects of both particulate matter (PM) mass and composition on blood pressure, among elderly persons. We carried out a panel study in persons living in elderly homes in Antwerp, Belgium. We recruited 88 non-smoking persons, 70% women with a mean age of 83 years (standard deviation: 5.2). Blood pressure was measured and a blood sample was collected on two time points, which were chosen so that there was an exposure contrast in ambient PM exposure. The elemental content of the collected indoor and outdoor PM(2.5) (particulate matter with an aerodynamic diameter <2.5 μm) mass concentration was measured. Oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) on outdoor PM(10) (particulate matter with an aerodynamic diameter <10 μm) were measured. Each interquartile range increase of 20.8 μg/m³ in 24-h mean outdoor PM(2.5) was associated with an increase in pulse pressure of 4.0 mm Hg (95% confidence interval: 1.8-6.2), in persons taking antihypertensive medication (n=57), but not in persons not using antihypertensive medication (n=31) (p for interaction: 0.02). Vanadium, iron and nickel contents of PM(2.5) were significantly associated with systolic blood pressure and pulse pressure, among persons on antihypertensive medication. Similar results were found for indoor concentrations. Of the oxy-PAHs, chrysene-5,6-dione and benzo[a]pyrene-3,6-dione were significantly associated with increases in systolic blood pressure and pulse pressure. In elderly, pulse pressure was positively associated with acute increases in outdoor and indoor air pollution, among persons taking antihypertensive medication. These results might form a mechanistic pathway linking air pollution as a trigger of cardiovascular events.

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

Comparison of ozone profiles obtained with Brewer‐Mast and Z‐ECC sensors during simultaneous ascents

Uccle is a European station with a long tradition in measuring ozone profiles with Brewer-Mast (BM) ozone sensors. In 1996 the use of model Z electrochemical concentration cell (Z-ECC) ozonesondes, manufactured by EN-SCI Corporation, was considered. To document the changeover from BM to Z-ECC ozonesondes, a program of double soundings was started in October 1996. With the use of an appropriate correction procedure, accounting for the loss of pump efficiency with decreasing pressure and temperature, it is possible to reduce the mean difference between ozone profiles obtained with both types of sondes below 3%, which is statistically insignificant over nearly the whole operational altitude range (from the ground to 32 km). It is concluded that with this new method the uncertainty, introduced in trend analyses by the changeover, is less than the internal uncertainty within the BM time series. The required corrections are discussed according to measurements of the pump efficiencies at different pressures and temperatures in a vacuum chamber.

Comparison and validation of the aerosol optical depth obtained with the Langley plot method in the UV‐B from Brewer Ozone Spectrophotometer measurements

(1) The Aerosol Optical Depths (AODs) retrieved from Brewer Ozone Spectrophotometer measurements with a method previously developed (Cheymol and De Backer, 2003) are now validated by comparisons between AODs from six Brewer spectrophotometers and two CSEM SPM2000 sunphotometers: two Brewer spectrophotometers 016 and 178 at Uccle in Belgium; one Brewer spectrophotometer 128 and one sunphotometer CSEM SPM2000 at Norrkoping in Sweden; and three Brewer instruments 040, 072, 156 at Arosa and one CSEM SPM2000 sunphotometer at Davos in Switzerland. The comparison between AODs from Brewer spectrophotometer 128 at 320.1 nm and sunphotometer SPM2000 at 368 nm at Norrkoping shows that the AODs obtained from the Brewer measurements with the Langley Plot Method (LPM) are very accurate if the neutral density filter spectral transmittances are well known: with the measured values of these filters, the correlation coefficient, the slope, and the intercept of the regression line are 0.98, 0.85 ± 0.004, and 0.02 ± 0.0014, respectively. The bias observed is mainly owing to the wavelength difference between the two instruments. The comparison between AODs from different Brewer spectrophotometers confirm that AODs will be in very good agreement if they are measured with several Brewer instruments at the same place: At Uccle, the correlation coefficient, slope, and intercept of the regression line are 0.98, 1.02 ± 0.003, and 0.06 ± 0.001, respectively; at Arosa, the comparisons between the AODs from three Brewer spectrophotometers 040, 072, and 156 give a correlation coefficient, a slope, and an intercept of the regression line above 0.94, 0.98 and below 0.04, respectively.

Extreme events in total ozone over the Northern mid-latitudes: An analysis based on long-term data sets from five European ground-based stations

We apply methods from extreme value theory to identify extreme events in high (termed EHOs) and low (termed ELOs) total ozone and to describe the distribution tails (i.e. very high and very low values) of five long-term European ground-based total ozone time series. The influence of these extreme events on observed mean values, long-term trends and changes is analysed. The results show a decrease in EHOs and an increase in ELOs during the last decades, and establish that the observed downward trend in column ozone during the 1970–1990s is strongly dominated by changes in the frequency of extreme events. Furthermore, it is shown that clear ‘fingerprints’ of atmospheric dynamics (NAO, ENSO) and chemistry [ozone depleting substances (ODSs), polar vortex ozone loss] can be found in the frequency distribution of ozone extremes, even if no attribution is possible from standard metrics (e.g. annual mean values). The analysis complements earlier analysis for the world’s longest total ozone record at Arosa, Switzerland, confirming and revealing the strong influence of atmospheric dynamics on observed ozone changes. The results provide clear evidence that in addition to ODS, volcanic eruptions and strong/moderate ENSO and NAO events had significant influence on column ozone in the European sector.

Assessment of the Uccle ozone sounding time series quality using SAGEII data

Vertical ozone profiles measured with balloon sondes at Uccle are compared with version 5.96 ozone profiles retrieved by the SAGEII instrument to investigate instrumentally induced breaks and/or trends in the Uccle ozone sounding time series. This study covers the period 1985–1999, during which several changes took place in the ozone sounding procedure and processing method. These changes introduced breaks in the time series of ozone soundings, and a number of corrections have been developed that are not part of the standard processing method for ozone soundings. In this paper, we review the standard corrections applied to ozone soundings and describe additional corrections developed at Uccle. The improvement of the ozonesonde data set against SAGE II ozone measurements is shown. Jumps of up to 6% in the relative difference between SAGEII and ozonesondes are removed with corrections taking into account quality changes of the ozonesonde and the radiosonde. New corrections and refinements to the existing correction procedure reduce the drift between SAGEII and ozonesondes from −0.51% yr−1 to −0.07% yr−1 between 17 and 22 km with nonsignificant values at the 2σ level at all altitudes.

Potential for meteorological bias in lidar ozone data sets resulting from the restricted frequency of measurement due to cloud cover

Lidar-based ozone data sets may be meteorologically biased since the measurements are performed in the absence of precipitation and fog and the vertical profiles are truncated at the cloud base. This problem was studied for the routine ozone lidar facility at Bilthoven, Netherlands (52°07′N, 5°12′E). We used the long-term data record of balloon soundings at Uccle, Belgium (50°48′N, 4°21′E) between 1969 and 1992 to examine the average ozone concentrations as a function of altitude and season at different amounts of cloud cover. Since Uccle and Bilthoven are only 160 km apart and to a large extent have the same climatology, the Uccle data are assumed to be representative of the atmospheric conditions at Bilthoven. The data set of ozone profiles was arranged into several subsets according to increasing amounts of cloud cover. When compared to the total set, the subset for clear sky conditions showed significantly lower ozone partial pressures (absolute differences down to −2.37 mPa and percentage differences down to −43.6%) around the tropopause in spring and early summer and higher values (up to +1.10 mPa and +39.2%) in the lower troposphere in summer. These phenomena are explained by differences in the average origin of air masses and photochemical ozone production. The results have led to the definition of the monitoring strategy for the tropospheric ozone lidar facility at Bilthoven such that a meteorological bias in the set of ozone profiles is avoided.

Retrieval of atmospheric water vapor columns from FT visible solar absorption spectra and evaluation of spectroscopic databases

The absorption of solar light by atmospheric water vapor in the visible spectral region is analyzed by means of ground-based absorption Fourier transform spectroscopy, performed at high resolution in Brussels during summer 2001. Several microwindows between 14,000 and 18,000 cm -1 , in which water vapor lines are well isolated from solar lines and other atmospheric trace gases absorptions, are examined. They are demonstrated to be adequate for the retrieval of the total water vapor column. Based on the retrievals, a detailed analysis of the water vapor line parameters published in the HITRAN database and recently reinvestigated by different groups is performed. The analysis focuses on the one hand on the comparison of the retrieved water vapor columns with in situ measurements, performed at the same time as the spectroscopic measurements and at the same location, and on the other hand on the quality of the spectral fits. It is shown that the discrepancies between the line lists affect significantly the results. In particular it is shown that the weaker lines, not measured in earlier laboratory experiments, do contribute at large zenith angles and need to be taken into account in order to better simulate the atmospheric spectra. The importance of the pressure broadening parameters is also highlighted.

Time series of daily erythemal UV doses at Uccle, Belgium

The relation of the daily doses of erythemal UV at Uccle with total solar irradiation, total ozone column and the aerosol optical depth in the UV are investigated together with trends in these atmospheric parameters for the period 1990 to March 2007. It is shown that the trend in UV radiation is negative in January, August and December and positive the rest of the year. The correlations with total solar radiation, total ozone and aerosol optical depth at 320 nm are in the range of 0.90 to 0.99, −0.2 to −0.5 and −0.0 to −0.3, respectively. However the UV daily dose changes with up to 1.5% for a 1% change in total solar radiation, with −0.8% for a 1% change in aerosol optical depth and with only −0.1% for a 1% change in ozone, indicating the importance, on the short time-scale, of changes in aerosol properties for the UV irradiance reaching the surface.The relation of the daily doses of erythemal UV at Uccle with total solar irradiation, total ozone column and the aerosol optical depth in the UV are investigated together with trends in these atmospheric parameters for the period 1990 to March 2007. It is shown that the trend in UV radiation is negative in January, August and December and positive the rest of the year. The correlations with total solar radiation, total ozone and aerosol optical depth at 320 nm are in the range of 0.90 to 0.99, −0.2 to −0.5 and −0.0 to −0.3, respectively. However the UV daily dose changes with up to 1.5% for a 1% change in total solar radiation, with −0.8% for a 1% change in aerosol optical depth and with only −0.1% for a 1% change in ozone, indicating the importance, on the short time-scale, of changes in aerosol properties for the UV irradiance reaching the surface.