In-situ calibration of the water vapor channel for multi-filter rotating shadowband radiometer using collocated GPS, AERONET and meteorology data

The difficulty of in-situ calibration on the 940 nm channel of Multi-Filter Rotating Shadowband Radiometer (MFRSR) stems from the distinctive non-linear relationship between the amount of precipitable water vapor (PW) and its optical depth (i.e. curve of growth) compared to the counterpart of aerosols. Previous approaches, the modified Langley methods (MLM), require exact aerosol optical depth (AOD) values and a constant PW value at all points participating the regression. Instead, we propose a new method that substitutes the PW optical depth derived from collocated GPS zenith wet delay retrieval in conjunction with meteorology data and requires a constant AOD value at all points participating the regression. The main benefits of the new method include: (1) Aerosol stability is easier to fulfill than PW stability; (2) AOD stability could be inferred from adjacent channels (e.g. 672 and 870 nm) of MFRSR itself without measurements of a collocated AERONET sun photometer; and (3) When applicable, the time interval of GPS derived PW (i.e. 3 minutes) is more compatible with the MFRSR sampling interval (i.e. 3 minutes) than AERONET interpolated AOD (i.e. 15 minutes). Both MLM and the new method were applied to the MFRSR of USDA UV-B Monitoring and Research Program at the station in Billings, Oklahoma (active for 18 years so far) on July 28, 2015. The performances of the two methods are compared in order to assess their accuracy and the advantages and disadvantages.National Science Foundation (U.S.) (Grant S13-EAR1261833-S4)United States. National Aeronautics and Space Administration (Grant NNX14AQ03G

Validation of OMI erythemal doses with multi-sensor ground-based measurements in Thessaloniki, Greece

The aim of this study is to validate the Ozone Monitoring Instrument (OMI) erythemal dose rates using ground-based measurements in Thessaloniki, Greece. In the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki, a Yankee Environmental System UVB-1 radiometer measures the erythemal dose rates every minute, and a Norsk Institutt for Luftforskning (NILU) multi-filter radiometer provides multi-filter based irradiances that were used to derive erythemal dose rates for the period 2005–2014. Both these datasets were independently validated against collocated UV irradiance spectra from a Brewer MkIII spectrophotometer. Cloud detection was performed based on measurements of the global horizontal radiation from a Kipp & Zonen pyranometer and from NILU measurements in the visible range. The satellite versus ground observation validation was performed taking into account the effect of temporal averaging, limitations related to OMI quality control criteria, cloud conditions, the solar zenith angle and atmospheric aerosol loading. Aerosol optical depth was also retrieved using a collocated CIMEL sunphotometer in order to assess its impact on the comparisons. The effect of total ozone columns satellite versus ground-based differences on the erythemal dose comparisons was also investigated. Since most of the public awareness alerts are based on UV Index (UVI) classifications, an analysis and assessment of OMI capability for retrieving UVIs was also performed. An overestimation of the OMI erythemal product by 3–6% and 4–8% with respect to ground measurements is observed when examining overpass and noontime estimates respectively. The comparisons revealed a relatively small solar zenith angle dependence, with the OMI data showing a slight dependence on aerosol load, especially at high aerosol optical depth values. A mean underestimation of 2% in OMI total ozone columns under cloud-free conditions was found to lead to an overestimation in OMI erythemal doses of 1–5%.While OMI overestimated the erythemal dose rates over the range of cloudiness conditions examined, its UVIs were found to be reliable for the purpose of characterising the ambient UV radiation impact

Climatology of the solar ultraviolet radiation in the Eastern Mediterranean region

During the elaboration of the present thesis, data of 5 years at least, were developed from measurements at nine stations of the National network for solar UV radiation measurements, which was established by the LAP during the midst of the decade of 2000. The aim of the thesis was the climatic study of variations in the solar radiation into several spectral ranges in UV and visible. Important part of the thesis is devoted to the methods and calibration procedures of the data, compiling measurements from other instruments and theoretical calculations. Among the others, appropriate methods were deployed in order to estimate several doses of the solar radiation of biological importance (cutaneous production of vitamin D, erythemal radiation). According to the findings of this scientific study, the levels of the solar radiation at the two spectral regions remain constant, with some annually variations during the years 2005-2010, always depending on the station. The higher levels of solar radiation were observed at the stations close to the equator, as expected. The geographical variation of the total ozone column was confirmed, while the maxima of the concentrations took place in April-May. The lowest ozone column values were occurred in Nicosia, while Xanthi experienced the highest values. This fact was confirmed by satellite estimates of ozone column. The climatology of the erythemal dose and the vitamin D dose is in accordance with the UV-B radiation climatology. However, the levels of the doses of vitamin D are slightly lower than those of CIE, mainly because of the differences between the action spectra. As concerning the atmospheric transmittance, during the years 2009-2010 it seems that the cloudless days at the greater area of Thessaloniki were decreased in number.Στην διδακτορική διατριβή αξιοποιούνται μετρήσεις τουλάχιστον μιας πενταετίας από τους εννέα σταθμούς του Δικτύου μέτρησης υπεριώδους ηλιακής ακτινοβολίας που εγκατέστησε το ΕΦΑ στα μέσα της δεκαετίας του 2000. Απώτερος στόχος ήταν η κλιματολογική μελέτη των μεταβολών της ηλιακής ακτινοβολίας σε διάφορες φασματικές ζώνες στην περιοχή του υπεριώδους και του ορατού. Σημαντικό μέρος της διατριβής αναφέρεται στις μεθόδους αξιολόγησης και βαθμονόμησης των μετρήσεων του Δικτύου με την χρήση συμπληρωματικών μετρήσεων άλλων οργάνων, και θεωρητικών υπολογισμών, όπως και ο υπολογισμός παραγώγων μεγεθών, όπως π.χ. της στήλης του όζοντος και διαφόρων βιολογικής σημασίας δόσεων της ηλιακής ακτινοβολίας (παραγωγή βιταμίνης D, ερυθηματογόνος ακτινοβολία). Με βάση τα ευρήματα της ερευνητικής εργασίας, παρατηρείται ότι στην περιοχή της Ανατολικής Μεσογείου τα επίπεδα της υπεριώδους ακτινοβολίας παραμένουν σταθερά, με μικρές διακυμάνσεις ανά έτος σε κάθε σταθμό. Οι Νοτιότεροι σταθμοί του Δικτύου σημειώνουν τις υψηλότερες τιμές επιπέδων ηλιακής ακτινοβολίας όπως και αναμένεται. Επίσης, επιβεβαιώθηκε η γεωγραφική κατανομή της ολικής στήλης όζοντος από σταθμό σε σταθμό, ενώ οι μέγιστες συγκεντρώσεις συμβαίνουν τους μήνες Απρίλιο-Μάιο. Οι χαμηλότερες τιμές παρατηρούνται στον σταθμό της Λευκωσίας ενώ οι υψηλότερες στον σταθμό της Ξάνθης, γεγονός που επιβεβαιώθηκε και από δορυφορικές μετρήσεις ολικής στήλης όζοντος. Η κλιματολογία της ερυθηματογόνου δόσης ακολουθεί αυτή της UV-B ακτινοβολίας, όπως και της δόσης βιταμίνης D. Ωστόσο τα επίπεδα βιταμίνης είναι ελαφρώς μικρότερα από αυτά της ερυθηματογόνου δόσης λόγω των διαφορών στα ενεργά φάσματα των δύο μεγεθών. Όσον αφορά την διαπερατότητα της ατμόσφαιρας, φαίνεται πως κατά την διετία 2009-2010 αυξήθηκε ελαφρά η νεφοκάλυψη στην ευρύτερη περιοχή της Θεσσαλονίκης

Modeling the relationship between photosynthetically active radiation and global horizontal irradiance using singular spectrum analysis

We report on the construction of generic models to calculate photosynthetically active radiation (PAR) from global horizontal irradiance (GHI), and vice versa. Our study took place at stations of the Greek UV network (UVNET) and the Hellenic solar energy network (HNSE) with measurements from NILU-UV multi-filter radiometers and CM pyranometers, chosen due to their long (~1 M record/site) high temporal resolution (~1 min) record that captures a broad range of atmospheric environments and cloudiness conditions. The uncertainty of the PAR measurements is quantified to be 6.5% while the uncertainty involved in GHI measurements is up to ~7% according to the manufacturer. We show how multi-linear regression and nonlinear neural network (NN) models, trained at a calibration site (Thessaloniki) can be made generic provided that the input–output time series are processed with multi-channel singular spectrum analysis (M-SSA). Without M-SSA, both linear and nonlinear models perform well only locally. M-SSA with 50 timelags is found to be sufficient for identification of trend, periodic and noise components in aerosol, cloud parameters and irradiance, and to construct regularized noise models of PAR from GHI irradiances. Reconstructed PAR and GHI time series capture ~95% of the variance of the cross-validated target measurements and have median absolute percentage errors <2%. The intra-site median absolute error of M-SSA processed models were ~8.2 +/- 1.7 W/m^2 for PAR and 9.2 +/- 4.2 W/m^2 for GHI. When applying the models trained at Thessaloniki to other stations, the average absolute mean bias between the model estimates and measured values was found to be ~1.2 W/m^2 for PAR and ~0.8 W/m^2 for GHI. For the models, percentage errors are well within the uncertainty of the measurements at all sites. Generic NN models were found to perform marginally better than their linear counterparts

Total ozone column retrieval from UV-MFRSR irradiance measurements: Evaluation at Mauna Loa station

The USDA UV-B Monitoring and Research Program (UVMRP) comprises of 36 climatological sites along with 4 long-duration research sites, in 27 states, one Canadian province, and the south island of New Zealand. Each station is equipped with an Ultraviolet multi-filter rotating shadowband radiometer (UV-MFRSR) which can provide response-weighted irradiances at 7 wavelengths (300, 305.5, 311.4, 317.6, 325.4, and 368 nm) with a nominal full width at half maximun of 2 nm. These UV irradiance data from the long term monitoring station at Mauna Loa, Hawaii, are used as input to a retrieval algorithm in order to derive high time frequency total ozone columns. The sensitivity of the algorithm to the different wavelength inputs is tested and the uncertainty of the retrievals is assessed based on error propagation methods. For the validation of the method, collocated hourly ozone data from the Dobson Network of the Global Monitoring Division (GMD) of the Earth System Radiation Laboratory (ESRL) under the jurisdiction of the US National Oceanic & Atmospheric Administration (NOAA) for the period 2010-2015 were used

Short- and long-term variability of spectral solar UV irradiance at Thessaloniki, Greece: Effects of changes in aerosols, total ozone and clouds

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 with 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. 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

NILU-UV multi-filter radiometer total ozone columns: comparison with satellite observations over Thessaloniki, Greece

This study aims to construct and validate a neural network (NN) model for the production of high frequency (~1 min) ground-based estimates of total ozone column (TOC) at a mid-latitude UV and ozone monitoring station in the Laboratory of Atmospheric Physics of the Aristotle University of Thessaloniki (LAP/AUTh) for the years 2005–2014. In the first stage of model development, ~30,000 records of coincident solar UV spectral irradiance measurements from a Norsk Institutt for Luftforskning (NILU)-UV multi-filter radiometer and TOC measurements from a co-located Brewer spectroradiometer are used to train a NN to learn the nonlinear functional relation between the irradiances and TOC. The model is then subjected to sensitivity analysis and validation. Close agreement is obtained (R2 = 0.94, RMSE = 8.21 DU and bias = −0.15 DU relative to the Brewer) for the training data in the correlation of NN estimates on Brewer derived TOC with 95% of the coincident data differing by less than 13 DU. In the second stage of development, a long time series (≥1 million records) of high frequency (~1 min) NILU-UV ground-based measurements are presented as inputs to the NN model to generate high frequency TOC estimates. The advantage of the NN model is that it is not site dependent and is applicable to any NILU input data lying within the range of the training data. GOME/ERS-2, SCIAMACHY/Envisat, OMI/Aura and GOME2/MetOp-A TOC records are then used to perform a precise cross-validation analysis and comparison with the NILU TOC estimates over Thessaloniki. All 4 satellite TOC dataset are retrieved using the GOME Direct Fitting algorithm, version 3 (GODFIT_v3), for reasons of consistency. The NILU TOC estimates within ±30 min of the overpass times agree well with the satellite TOC retrievals with coefficient of determination in the range 0.88 ≤ R2 ≤ 0.90 for all sky conditions and 0.95 ≤ R2 ≤ 0.96 for clear sky conditions. The mean fractional differences are found to be −0.67% ± 2.15%, −1.44% ± 2.25%, −2.09% ± 2.06% and −0.85% ± 2.19% for GOME, SCIAMACHY, OMI and GOME2 respectively for the clear sky cases. The near constant standard deviation (~±2.2%) across the array of sensors testifies directly to the stability of both the GODFIT_v3 algorithm and the NN model for providing coherent and robust TOC records. Furthermore, the high Pearson product moment correlation coefficients (0.94 ≤ R ≤ 0.98) testify to the strength of the linear relationship between the satellite algorithm retrievals of TOC and ground-based estimates, while biases of less than 5 DU suggest that systematic errors are low. This novel methodology contributes to the ongoing assessment of the quality and consistency of ground and space-based measurements of total ozone columns

CIE, Vitamin D and DNA damage: A synergetic study in Thessaloniki, Greece

The present study aims to validate different approachesfor the estimation of three photobiological effectivedoses: the erythemal UV, the Vitamin D and that forDNA damage, using high temporal resolution surfacebasedmeasurements of solar UV from 2005-2015. Datafrom a UV spectrophotometer, a multi-filter radiometer,and a UV radiation pyranometer that are located inThessaloniki, Greece are used together with empiricalrelations, algorithms and models in order to calculatethe desired quantities. In addition to the surface-baseddose retrievals, OMI/Aura and the combinedSCIAMACHY/Envisat and GOME/MetopA satelliteproducts are also used in order to assess the accuracy ofeach method for deriving the photobiological doses