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Comparisons between total ozone column (TOC) measurements from ground-based Dobson and Brewer spectrophotometers and from various satellite instruments generally reveal seasonally varying differences of a few percent. A large part of these differences has been attributed to the operationally used Bass and Paur ozone cross sections and the lack of accounting for varying stratospheric temperatures in the standard total ozone retrieval for Dobson. This paper demonstrates how the use of new ozone absorption cross sections from the University of Bremen (Weber et al., 2016), as recommended by the Absorption Cross Sections of Ozone (ACSO) committee; the application of appropriate slit functions, especially for the Dobson instrument (Bernhard et al., 2005); and the use of climatological values for the effective ozone layer temperature (Teff), e.g., from TEMIS (Tropospheric Emission Monitoring Internet Service), essentially eliminate these seasonally varying differences between Brewer and Dobson total ozone data (to generally less than ±0.5 %). For Hohenpeissenberg, the previous seasonal difference (close to 0 % in summer and up to 2.5 % in winter) is reduced to less than ±0.5 % year-round. Implementing this approach to the existing global network of Dobson spectrometers will reduce the overall uncertainty in their total ozone data from 3 % to 4 % previously to under 2 % at most locations. [ABSTRACT FROM AUTHOR]

Comparison of total ozone column (TOC) measurements from ground-based Dobson and Brewer spectrophotometers and from various satellite instruments generally reveals seasonally varying differences of a few percent. A large part of these differences has been attributed to the operationally used Bass & Paur ozone cross-sections and the lack of accounting for varying stratospheric temperatures in the standard total ozone retrieval for Dobson. This paper demonstrates how the use of new ozone absorption cross sections from the University of Bremen (Weber et al., 2016), as recommended by the committee on Absorption Cross-Sections of Ozone, the application of appropriate slit functions, especially for the Dobson instrument (Bernhard et al. 2005), and the use of climatological values for the effective ozone layer temperature (Teff), e.g. from TEMIS, essentially eliminate these seasonally varying differences between Dobson and Brewer total ozone data. Applying this approach to the existing global network of Dobson spectrometers will reduce the uncertainty of their total ozone data, from previously 3 to 4% to better than 2.0% at most locations. [ABSTRACT FROM AUTHOR]

The Brewer ozone spectrophotometer (the Brewer) is one of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW)'s standard ozone-monitoring instruments since the 1980s. The entire global Brewer ozone-monitoring network is operated and maintained via a hierarchical calibration chain, which started from world reference instruments that are independently calibrated via the primary calibration method (PCM) at a premium site (National Oceanic and Atmospheric Administration's (NOAA) Mauna Loa Observatory, Hawaii). These world reference instruments have been maintained by Environment and Climate Change Canada (ECCC) in Toronto for the last 4 decades. Their calibration is transferred to the travelling standard instrument and then to network (field) Brewer instruments at their monitoring sites (all via the calibration transfer method; CTM). Thus, the measurement accuracy for the entire global network is dependent on the calibration of world reference instruments. In 2003, to coordinate regional calibration needs, the Regional Brewer Calibration Center for Europe (RBCC-E) was formed in Izaña, Spain. From that point, RBCC-E began calibrating regional references also via PCM instead of CTM. The equivalency and consistency of world and regional references are then assured during international calibration campaigns. In practice, these two calibration methods have different physical requirements, e.g., the PCM requires a stable ozone field in the short term (i.e., half-day), while the CTM would benefit from larger changes in slant ozone conditions for the calibration periods. This difference dictates that the PCM can only be implemented on Brewer instruments at certain sites and even in certain months of the year. This work is the first effort to use long-term observation records from 11 Brewer instruments at four sites to reveal the challenges in performing the PCM. By utilizing a new calibration simulation model and reanalysis ozone data, this work also quantifies uncertainties in the PCM due to short-term ozone variability. The results are validated by real-world observations and used to provide scientific advice on where and when the PCM can be performed and how many days of observations are needed to achieve the calibration goal (i.e., ensure the calibration uncertainty is within a determined criterion, i.e., ≤5 R6 units; R6 is a measurement-derived double ratio in the actual Brewer processing algorithm). This work also suggests that even if the PCM cannot be used to deliver final calibration results for mid- or high-latitude sites, the statistics of the long-term PCM fitting results can still provide key information for field Brewer instruments as stability indicators (which would provide performance monitoring and data quality assurance). [ABSTRACT FROM AUTHOR]

The Brewer ozone spectrophotometer (the Brewer) is one of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) standard ozone monitoring instruments since the 1980s. The entire global Brewer ozone monitoring network is operated and maintained via a hierarchical calibration chain, which started from world reference instruments that are independently calibrated via the primary calibration method (PCM) at a premium site (National Oceanic and Atmospheric Administration’s (NOAA) Mauna Loa Observatory, Hawaii). These world reference instruments have been maintained by Environment and Climate Change Canada (ECCC) in Toronto for the last four decades. Their calibration is transferred to the travelling standard instrument and then to network (field) Brewer instruments at their monitoring sites (all via the calibration transfer method; CTM). Thus, the measurement accuracy for the entire global network is dependent on the calibration of world reference instruments. In 2003, to coordinate regional calibration needs, the Regional Brewer Calibration Center-Europe (RBCC-E) was formed in Izaña, Spain. From that point, RBCC-E began calibrating regional references also via PCM, instead of CTM. The equivalency and consistency of world and regional references are then assured during international calibration campaigns. In practice, these two calibration methods have different physical requirements, e.g., the PCM requires a stable short-term ozone field, while CTM would benefit from larger changes in slant ozone conditions for the calibration periods. This difference dictates that the PCM can only be implemented on Brewers at certain sites and even in certain months of the year. This work is the first effort to use long-term observation records from 11 Brewers at four sites to reveal the challenges in performing PCM. By utilizing a new calibration simulation model and reanalysis ozone data, this work also quantifies uncertainties in the PCM due to short-term ozone variability. The results are validated by real-world observations and used to provide scientific advice on where and when the PCM can be performed and how many days of observations are needed to achieve the calibration goal (i.e., ensure the calibration uncertainty is within a determined criterion, i.e., ≤5 R6 unit; R6 is a measurement derived double ratio in the actual Brewer processing algorithm). This work also suggests that even if the PCM cannot be used to deliver final calibration results for mid- or high-latitude sites, the statistics of the long-term PCM fitting results can still provide key information for field Brewers as stability indicators (which would provide performance monitoring and data quality assurance). [ABSTRACT FROM AUTHOR]

This study presents total column ozone (TCO) retrieval from a new system, called Koherent, developed at PMOD/WRC. The instrument is based on a small, cost effective, robust, low-maintenance and state-of-the-art technology array spectroradiometer. It consists of a BTS-2048-UV-S-F array spectroradiometer from Gigahertz-Optik GmbH, coupled with an optical fiber to a lens-based telescope mounted on a sun tracker for measuring direct UV irradiance in the ultraviolet wavelength band between 305 nm to 345 nm. Two different algorithms are developed for retrieving TCO from these spectral measurements: 1) TCO retrieved by a minimal least squares fit algorithm (LSF) and 2) a Custom Double Ratio (CDR) technique using four specifically selected wavelengths from the spectral measurements. The double ratio technique is analogous to the retrieval algorithm applied for the Dobson and the Brewer but adopted and optimized here for TCO retrieval with Koherent. The instrument was calibrated in two different ways: a) absolute calibration of the spectra using the portable reference for ultraviolet radiation QASUME for the LSF retrieval and b) relative calibration of the extraterrestrial constant (ETC) of the CDR retrieval, by minimizing the slope between air mass and the relative differences of TCO from QASUME and Koherent. This adjustment of the ETC allows the instrument to be calibrated with standard TCO reference instruments during calibration campaigns, such as a double monochromator Brewer. A two-year comparison in Davos, Switzerland, between Koherent and the Brewer 156 (double monochromator) shows that TCO derived from Koherent and the Brewer 156 agree in average over the entire period within less than 0.7 % for all retrievals in terms of offset. The performance in terms of slant path depends on the selected retrieval and the applied corrections. The stray light corrected LSF retrieval exhibits a smaller slant path dependency than the CDR retrieval and performs almost as for a double monochromator system. The slant path dependency of the CDR is comparable to the slant path dependency of a single Brewer monochromator. The combination of both retrievals leads to performance with an offset close to zero compared to Brewer 156, a seasonal amplitude of the relative difference of 0.08 % and a slant path dependency of maximum 1.64 %, which is similar as other standard TCO instruments such as single Brewer or Dobson. Applying the double ratio technique by selecting the wavelengths and slit functions from Brewer and Dobson, respectively, allow to determine the effective ozone temperature within 3 K on daily averages. With the improved TCO retrieval, Koherent serves as a new low maintenance instrument to operationally monitor TCO at remote sites. The presented TCO retrieval may be applied to other array based spectroradiometers providing direct spectral measurements in the ultraviolet. [ABSTRACT FROM AUTHOR]

Total column ozone (TCO) is commonly measured by Brewer and Dobson spectroradiometers. Both types of instruments are using four wavelengths in the ultraviolet radiation range to derive TCO. For the calibration and quality assurance of the measured TCO both instrument types require periodic field comparisons with a reference instrument. This study presents traceable TCO retrievals from direct solar spectral irradiance measurements with the portable UV reference instrument QASUME. TCO is retrieved by a spectral fitting technique derived by a minimal least square fit algorithm using spectral measurements in the wavelength range between 305 nm and 345 nm. The retrieval is based on an atmospheric model accounting for different atmospheric parameters such as effective ozone temperature, aerosol optical depth, Rayleigh scattering, SO2, ground air pressure, ozone absorption cross sections and top-of-atmosphere solar spectrum. Traceability means, that the QASUME instrument is fully characterized and calibrated in the laboratory to SI standards (International System of Units). The TCO retrieval method from this instrument is independent from any reference instrument and does not require periodic in situ field calibration. The results show that TCO from QASUME can be retrieved with a relative standard uncertainty of less than 0.8 %, when accounting for all possible uncertainties from the measurements and the retrieval model, such as different cross sections, different reference solar spectra, uncertainties from effective ozone temperature or other atmospheric parameters. The long-term comparison of QASUME TCO with a Brewer and a Dobson in Davos, Switzerland, reveals, that all three instruments are consistent within 1 % when using the ozone absorption cross section from the University of Bremen. From the results and method presented here, other absolute SI calibrated cost effective solar spectroradiometers, such as array spectroradiometers, may be applied for traceable TCO monitoring. [ABSTRACT FROM AUTHOR]

In the 2019/2020 measurement campaign at Hohenpeißenberg (Germany) and Davos (Switzerland) we compared the well-established Dobson and Brewer spectrometers (single and double monochromator Brewer) with newer BTS array spectroradiometer based systems in terms of total ozone column (TOC) determination. The aim of this study is to validate the BTS performance in a longer-term TOC analysis over more than one year with seasonal and weather influences. Two different BTS setups have been used. A fibre coupled entrance optic version by PMOD/WRC called Koherent and a diffusor optic which proved to be simpler in terms of calibration from Gigahertz-Optik GmbH called BTS Solar. The array-spectrometer based BTS systems have been traceable calibrated to National Metrology Institutes (NMI) and the used TOC retrieval algorithms are based on spectral measurements in the range of 305 nm and 350 nm instead of single wavelength measurements as for Brewer or Dobson. The two BTS based systems, however, used fundamentally different retrieval algorithms for the TOC assessment, whereby the retrieval of the BTS solar turned out to achieve significantly smaller seasonal drifts. The intercomparison showed a deviation of the BTS Solar to Brewers of <0.1% with an expanded standard deviation of <1.5% within the whole measurement campaign. Koherent showed a deviation of 1.7% with an expanded standard deviation of 2.7% mostly given by a significant seasonal drift. Resulting, the BTS Solar performance is comparable to Brewers at the comparison in Hohenpeißenberg. The slant path slope is in-between double monochromator and single monochromator Brewer. Koherent shows a strong seasonal variation in Davos due to the sensitivity of its ozone retrieval algorithm to stratospheric temperature similar to the Dobson results. [ABSTRACT FROM AUTHOR]

For spectroradiometers the characterisation of their wavelength scale and spectral bandwidth underpins substantially the quality of measured data. This characterisation can be performed using metrology- grade tuneable monochromatic sources, which are currently available only in a few laboratories world-wide. Yet in numerous applications only the in-field calibration is a feasible solution. We have designed and developed a Tuneable and Portable radiation Source (TuPS) in the wavelength range from 300 nm to 350 nm for the in-field characterization of Dobson and Brewer spectrometers wavelength scale and slit-function with uncertainties better than 0.02 nm in wavelength and with the bandwidth of emitted radiation smaller than 0.1 nm FWHM. The TuPS is designed such that only minor modifications of its optical system extends/shifts its spectral range towards visible and near-infrared spectral regions and thus expand its application for characterisation of any spectroradiometers in the relevant spectral region of interest. [ABSTRACT FROM AUTHOR]

The TROPOspheric Monitoring Instrument (TROPOMI) onboard the Sentinel-5 Precursor (S5P) satellite was launched on 13 October 2017 to provide the atmospheric composition for atmosphere and climate research. The S5P is a sun-synchronous polar-orbiting satellite providing global daily coverage. The TROPOMI swath is 2600 km wide, and the ground resolution for most data products is 7.2 x 3.5 km2 (5.6 x 3.5 km2 since 6 August 2019) at nadir. The Finnish Meteorological Institute (FMI) is responsible for the development and processing of the TROPOMI Surface Ultraviolet (UV) Radiation Product which includes 36 UV parameters in total. Ground-based data from 25 sites located in arctic, subarctic, temperate, equatorial and antarctic areas were used for validation of TROPOMI overpass irradiance at 305, 310, 324 and 380 nm, overpass erythemally weighted dose rate/UV index and erythemally weighted daily dose for the period from 1 January 2018 to 31 August 2019. The validation results showed that for most sites 60-80 % of TROPOMI data was within ±20 % from ground-based data for snow free surface conditions. The median relative differences to ground-based measurements of TROPOMI snow free surface daily doses were within ±10 % and ±5 % at two thirds and at half of the sites, respectively. At several sites more than 90 % of clear sky TROPOMI data were within ±20 % from ground-based measurements. Generally median relative differences between TROPOMI data and ground-based measurements were a little biased towards negative values, but at high latitudes where non-homogeneous topography and albedo/snow conditions occurred, the negative bias was exceptionally high, from -30 % to -65 %. Positive biases of 10-15 % were also found for mountainous sites due to challenging topography. The TROPOMI Surface UV Radiation Product includes quality flags to detect increased uncertainties in the data due to heterogeneous surface albedo and rough terrain which can be used to filter the data retrieved under challenging conditions. [ABSTRACT FROM AUTHOR]

We demonstrate a Monte Carlo model to calculate the uncertainties of total ozone column, TOC, derived from ground-based directional solar spectral irradiance measurements. The model takes into account effects that correlations in the spectral irradiance data may have on the results. The model is tested with spectral data measured with three different spectroradiometers at an intercomparison campaign of the research project Traceability for atmospheric total column ozone at Izaña, Tenerife on 17 September 2016. The TOC values derived at noon have expanded uncertainties of 1.3% for a high-end scanning spectroradiometer, 1.3% for a high-end array spectroradiometer, and 3.3% for a roughly adopted instrument based on commercially available components and an array spectroradiometer. The level of TOC measured with reference Brewer spectrophotometer #183 is of the order of 282 DU during the analysed day and in agreement with the results of the two former instruments. [ABSTRACT FROM AUTHOR]

The performance of the cosine error correction method used at the Finnish Meteorological Institute (FMI) for correcting spectral UV measurements of the Brewer spectroradiometer (Brewer) was studied. An instrument specific cosine error correction has to be applied due to the non ideal angular response of the Brewer. The correction depends on the actual sky radiation distribution, which can change even during one spectral scan due to rapid changes in cloudiness. The method has been developed to take into account such changes and derive a correction coefficient for each measured wavelength. Measurements of five Brewers were corrected using the method and the results were compared to a travel reference spectroradiometer (QASUME). Measurements were performed during the RBCC-E (Regional Brewer Calibration Center - Europe) X Campaign held at El Arenosillo, Huelva (37° N, 7° W), Spain, in 2015. In addition, results of site audits of FMI's Brewers in Sodankylä (67° N, 23° W) and Jokioinen (61° N, 24° W) during 2002-2014 were studied. The results showed that the spectral cosine error correction varied between 4 to 14 %, and the differences between the QASUME and the Brewers diminished by up to 10 %. The study showed that the method, originally developed for measurements made at high latitudes, can be used at midlatitudes as well. It also showed that the method is applicable to other Brewers as far as required input parameters, i.e., total ozone, information from aerosols, albedo, instrument specific angular response and slit function, are known. [ABSTRACT FROM AUTHOR]

A high resolution extraterrestrial solar spectrum has been determined from ground-based measurements of direct solar spectral irradiance over the range 300nm to 500nm using the Langley-plot technique. The measurements were obtained at the Iza\na Atmospheric Research Center from AEMET, Tenerife, Spain during the period 12 to 24 September 2016. This solar spectrum (QASUMEFTS) was combined from medium resolution (bandpass of 0.86nm) measurements of the QASUME spectroradiometer in the range 300nm to 500nm and high resolution measurements (0.025nm) from a fourier transform spectroradiometer over the range 305nm to 380nm. The KittPeak solar atlas was used to extend this high resolution solar spectrum to 500nm. The expanded uncertainties of this solar spectrum are 2% between 310nm and 500nm and 4% at 300nm. The comparison of this solar spectrum with solar spectra measured in space (top of the atmosphere) gives very good agreements in some cases, while in some other cases discrepancies of up to 5% could be observed. The QASUMEFTS solar spectrum represents a benchmark dataset with uncertainties lower than anything previously published. The metrological traceability of the measurements to the International System of Units (SI) is assured by an unbroken chain of calibrations leading to the primary spectral irradiance standard of the Physikalisch-Technische Bundesanstalt in Germany. [ABSTRACT FROM AUTHOR]