Factors affecting solar ultraviolet irradiance measured since 1990 at Thessaloniki, Greece
Factors affecting the solar spectral ultraviolet (UV) irradiance at Thessaloniki, Greece were investigated using measurements with single-and double-monochromator Brewer spectroradiometer, which started operating respectively in 1989 and 1993 and continue up to the present. The two data records were quality controlled, homogenized and finally merged into one dataset, which was used in the analysis. Subsets of these data corresponding to different solar zenith angles (SZAs) and to cloud-free skies were used to quantify the long-term changes in surface UV irradiance at different wavelengths, and the importance of the factors responsible for these changes is discussed. It is shown that the calculated UV changes vary with SZA due to the different atmospheric path of the photons and the dependence of the diffuse to direct irradiance ratio on the SZA. The effect of total ozone and aerosols on UV irradiance is examined and the corresponding radiation amplification factors (RAFs) at the various wavelengths are calculated. The observed changes in UV irradiance due to ozone are smaller than those expected for the changes in total ozone, suggesting that the influence of the ozone is masked by other factors. An important finding of this study is that the improvement in air quality at Thessaloniki, during the period under examination, is the main reason for the observed increase in solar UV irradiance.
Available from: Konstantinos Fragkos
- "The corresponding spectral range for B086 is 290–363 nm, with the same step and very similar spectral resolution. The UV dataset of both instruments was quality checked and re-evaluated 10 up to the end of 2005 (Garane et al., 2006) and has been used in different studies (Kazadzis et al., 2009;Kazantzidis et al., 2006Kazantzidis et al., , 2009Meleti et al., 2009). The estimated 1σ uncertainty of the measurements is about 5 % for B086 and ranges from 6.5 % near 305 nm to 5 % near 320 nm for B005 (Garane et al., 2006). "
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ABSTRACT: In this study, we discuss the short- and the long-term variability of spectral UV irradiance at Thessaloniki, Greece using a long, quality-controlled data set from two Brewer spectrophotometers. Long-term changes in spectral UV irradiance at 307.5, 324 and 350 nm for the period 1994–2014 are presented for different solar zenith angles and discussed in association to changes in total ozone column (TOC), aerosol optical depth (AOD) and cloudiness observed in the same period. Positive changes in annual mean anomalies of UV irradiance, ranging from 2 to 6 % per decade, have been detected both for clear- and all-sky conditions. The changes are generally greater for larger solar zenith angles and for shorter wavelengths. For clear skies, these changes are, in most cases, statistically significant at the 95 % confidence limit. Decreases in the aerosol load and weakening of the attenuation by clouds lead to increases in UV irradiance in the summer, of 7–9 % per decade for 64° solar zenith angle. The increasing TOC in winter counteracts the effect of decreasing AOD for this particular season, leading to small, statistically insignificant, negative long-term changes in irradiance at 307.5 nm. Annual mean UV irradiance levels are increasing from 1994 to 2006 and remain relatively stable thereafter, possibly due to the combined changes in the amount and optical properties of aerosols. However, no statistically significant corresponding turning point has been detected in the long-term changes of AOD. Trends in irradiance during the two sub-periods are not discussed, because the length of the two datasets is too short for deriving statistically significant estimates. The absence of signatures of changes in AOD in the short-term variability of irradiance in the UV-A may have been caused by changes in the single scattering albedo of aerosols, which may counteract the effects of changes in AOD on irradiance. The anti-correlation between the year-to-year variability of the irradiance at 307.5 nm and TOC is clear and becomes clearer as the AOD decreases.
- "Since 1984, the AOD at the six nominal wavelengths in the UV is also retrieved from the direct irradiance measurements (Meleti et al. 2009). "
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ABSTRACT: Thirty years of total ozone column (TOC) measurements conducted by a Brewer spectrophotometer, operating in Thessaloniki (40.6°) since March 1982, have been analyzed using the statistical extreme value theory for the identification of extreme TOC events. About 12 % of the total number of days with TOC measurements were identified as extreme-low and ∼15 % as extreme-high events. The influence of the extreme-low events on the annual mean TOC values is up to ∼18 DU, while the extreme-high events show lower impact (up to 12 DU). Removing the extreme events from the time series results in smoother year-to-year variability and reduction of the small long-term linear trend (−0.08 %/year) by a factor of 2. Furthermore, we examined the impact of the extreme events on the noon erythemal irradiance under clear skies, and we provide evidence that even under extreme-low TOC conditions, the UV radiation levels are determined to a great extent by the aerosol optical depth. Although the influence of aerosols is evident during all seasons, for spring and summer, the sensitivity of UV radiation is larger, probably due to the different nature of the aerosols over Thessaloniki during these seasons.
Available from: Stelios Kazadzis
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ABSTRACT: Spectral Ultraviolet (UV) measurements using a Brewer MKIII double spectroradiometer were used for the determination of the aerosol forcing efficiency (RFE) under cloud free conditions at Thessaloniki, Greece for the period 1998–2006. Using measured spectral UVA irradiance in combination with synchronous aerosol optical depth (AOD) measurements at 340 nm, we calculated the seasonal and the percent RFE changes with the help of radiative transfer model calculations used for cloud and aerosol free conditions reference. The calculated RFE for the 325–340 nm wavelength integral was found to be −0.71±0.30 W m−2/τs340 nm and corresponds to a mean calculated RFE% value of −15.2%±3.8% (2 σ) per unit of τs340 nm, for the whole period. This indicates a mean reduction of 15.2% of the 325–340 nm irradiance for a unit of aerosol optical depth slant column increase. Lower RFE% was found during summertime, which is a possible indication of lower absorbing aerosols. Mean AOD slant at 340 nm for the city of Thessaloniki were processed in combination with RFE% and a mean monthly UVA attenuation of ~10% for the whole period was revealed. The nine years' analysis results showed a reduction in RFE%, which provides a possible indication of the changes in the optical properties over the city area. If such changes are only due to changes in the aerosol absorbing properties, the above finding suggests a 2% per decade increase in UVA due to changes in the aerosol absorption properties, in addition to the calculated increase by 4.2%, which is attributed only to AOD decrease at Thessaloniki area over the 1998–2006 period.
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