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Multi-axis differential optical absorption spectroscopy (MAX-DOAS) and direct sun NO2 vertical column network data are used to investigate the accuracy of tropospheric NO2 column measurements of the GOME-2 instrument on the MetOp-A satellite platform and the OMI instrument on Aura. The study is based on 23 MAX-DOAS and 16 direct sun instruments at stations distributed worldwide. A method to quantify and correct for horizontal dilution effects in heterogeneous NO2 field conditions is proposed. After systematic application of this correction to urban sites, satellite measurements are found to present smaller biases compared to ground-based reference data in almost all cases. We investigate the seasonal dependence of the validation results as well as the impact of using different approaches to select satellite ground pixels in coincidence with ground-based data. In optimal comparison conditions (satellite pixels containing the station) the median bias between satellite tropospheric NO2 column measurements and the ensemble of MAX-DOAS and direct sun measurements is found to be significant and equal to −34 % for GOME-2A and −24 % for OMI. These biases are further reduced to −24 % and −18 % respectively, after application of the dilution correction. Comparisons with the QA4ECV satellite product for both GOME-2A and OMI are also performed, showing less scatter but also a slightly larger median tropospheric NO2 column bias with respect to the ensemble of MAX-DOAS and direct sun measurements.

This paper is an overview of the progress in sky radiometer technology and the development of the network called SKYNET. It is found that the technology has produced useful on-site calibration methods, retrieval algorithms, and data analyses from sky radiometer observations of aerosol, cloud, water vapor, and ozone. A formula was proposed for estimating the accuracy of the sky radiometer calibration constant F0 using the improved Langley (IL) method, which was found to be a good approximation to observed monthly mean uncertainty in F0, around 0.5 % to 2.4 % at the Tokyo and Rome sites and smaller values of around 0.3 % to 0.5 % at the mountain sites at Mt. Saraswati and Davos. A new cross IL (XIL) method was also developed to correct an underestimation by the IL method in cases with large aerosol retrieval errors. The root-mean-square difference (RMSD) in aerosol optical thickness (AOT) comparisons with other networks took values of less than 0.02 for λ≥500 nm and a larger value of about 0.03 for shorter wavelengths in city areas and smaller values of less than 0.01 in mountain comparisons. Accuracies of single-scattering albedo (SSA) and size distribution retrievals are affected by the propagation of errors in measurement, calibrations for direct solar and diffuse sky radiation, ground albedo, cloud screening, and the version of the analysis software called the Skyrad pack. SSA values from SKYNET were up to 0.07 larger than those from AERONET, and the major error sources were identified as an underestimation of solid viewing angle (SVA) and cloud contamination. Correction of these known error factors reduced the SSA difference to less than 0.03. Retrievals of other atmospheric constituents by the sky radiometer were also reviewed. Retrieval accuracies were found to be about 0.2 cm for precipitable water vapor amount and 13 DU (Dobson Unit) for column ozone amount. Retrieved cloud optical properties still showed large deviations from validation data, suggesting a need to study the causes of the differences. It is important that these recent studies on improvements presented in the present paper are introduced into the existing operational systems and future systems of the International SKYNET Data Center.

Spring 2020 broke sunshine duration records across western Europe. The Netherlands recorded the highest surface irradiance since 1928, exceeding the previous extreme of 2011 by 13 %, and the diffuse fraction of the irradiance measured a record low percentage (38 %). The coinciding irradiance extreme and a reduction in anthropogenic pollution due to COVID-19 measures triggered the hypothesis that cleaner-than-usual air contributed to the record. Based on analyses of ground-based and satellite observations and experiments with a radiative transfer model, we estimate a 1.3 % (2.3 W m$^{-2}$) increase in surface irradiance with respect to the 2010-2019 mean due to a low median aerosol optical depth, and a 17.6 % (30.7 W m$^{-2}$) increase due to several exceptionally dry days and a very low cloud fraction overall. Our analyses show that the reduced aerosols and contrails due to the COVID-19 measures are far less important in the irradiance record than the dry and particularly cloud-free weather., Comment: 21 pages, 12 figures, submitted to Communications Earth and Environment

MAX-DOAS and direct sun NO2 vertical column network data are used to investigate the accuracy of tropospheric NO2 column measurements of the GOME-2 instrument on the MetOP-A satellite platform and the OMI instrument on Aura. The study is based on 23 MAX-DOAS and 16 direct sun instruments at stations distributed worldwide. A method to quantify and correct for horizontal dilution effects in heterogeneous NO2 field conditions is proposed. After systematic application of this correction to urban sites, satellite measurements are found to present smaller biases compared to ground-based reference data in almost all cases. We investigate the seasonal dependence of the validation results, as well as the impact of using different approaches to select satellite ground pixels in coincidence with ground-based data. In optimal comparison conditions (satellite pixels containing the station) the median bias between satellite tropospheric NO2 column measurements and the ensemble of MAX-DOAS and direct sun measurements is found to be significant and equal to −36 % for GOME-2A and −20 % for OMI. These biases are further reduced to −24 % and −8 % respectively, after application of the dilution correction. Comparisons with the QA4ECV satellite product for both GOME-2A and OMI is also performed, showing less scatter but also a slightly larger median tropospheric NO2 column bias with respect to the ensemble of MAX-DOAS and direct sun measurements.

A comprehensive uncertainty analysis validates a Solar Spectral Irradiance Meter (SolarSIM) for accurately resolving the spectral and broadband direct normal irradiances (DNI), spectral aerosol optical depth (AOD), precipitable water vapour and atmospheric total column ozone amounts. The derivation of these parameters from four SolarSIMs were compared to reference instrumentation at the Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center (PMOD/WRC) in Davos, Switzerland in September 2015. The SolarSIMs are the first instruments to ever simultaneously participate in the 12th WMO International Pyrheliometer Comparison, Fourth Filter Radiometer Comparison, and First Spectroradiometer Comparison. The SolarSIMs’ DNI data were compared to the World Standard Group’s PMO2 absolute cavity radiometer, with World Radiometric Reference factors ranging from 0.999674 to 0.994610 for the best and the worst performing devices, respectively. In addition, the SolarSIMs’ spectral DNI data was compared against PMOD’s Precision Spectral Radiometer. The mean difference of the spectral DNI was found to be less than 5% for wavelengths above 400 nm. The SolarSIMs’ measurements of AOD data were compared against PMOD’s Precision Filter Radiometer triad. The median AOD differences and their standard deviations were found to be 0.0046 ± 0.0044, 0.0016 ± 0.0034, 0.0018 ± 0.0026, and 0.0041 ± 0.0022 for 368 nm, 412 nm, 500 nm, and 865 nm, respectively. The SolarSIMs’ measurements of precipitable water vapour were compared against PMOD’s Cimel CE318 sun photometer. The median difference and the corresponding standard deviation averaged 1 ± 0.2 mm for all SolarSIMs. Furthermore, the SolarSIMs’ measurements of total column atmospheric ozone were compared against PMOD’s Brewer MkIII spectrophotometer. The median difference and the corresponding standard deviation averaged 6 ± 7 DU for all SolarSIMs.

In this study, the tropospheric NO2 vertical column density (VCD) over an urban site in Guangzhou megacity in China is investigated by means of MAX-DOAS measurements during a campaign from late March 2015 to mid-March 2016. A MAX-DOAS system was deployed at the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences and operated there for about 1 year, during the spring and summer months. The tropospheric NO2 VCDs retrieved by the MAX-DOAS are presented and compared with space-borne observations from GOME-2/MetOp-A, GOME-2/MetOp-B and OMI/Aura satellite sensors. The comparisons reveal good agreement between satellite and MAX-DOAS observations over Guangzhou, with correlation coefficients ranging between 0.795 for GOME-2B and 0.996 for OMI. However, the tropospheric NO2 loadings are underestimated by the satellite sensors on average by 25.1, 10.3 and 5.7 %, respectively, for OMI, GOME-2A and GOME-2B. Our results indicate that GOME-2B retrievals are closer to those of the MAX-DOAS instrument due to the lower tropospheric NO2 concentrations during the days with valid GOME-2B observations. In addition, the effect of the main coincidence criteria is investigated, namely the cloud fraction (CF), the distance (d) between the satellite pixel center and the ground-based measurement site, as well as the time period within which the MAX-DOAS data are averaged around the satellite overpass time. The effect of CF and time window criteria is more profound on the selection of OMI overpass data, probably due to its smaller pixel size. The available data pairs are reduced to half and about one-third for CF ≤ 0.3 and CF ≤ 0.2, respectively, while, compared to larger CF thresholds, the correlation coefficient is improved to 0.996 from about 0.86, the slope value is very close to unity ( ∼ 0.98) and the mean satellite underestimation is reduced to about half (from ∼ 7 to ∼ 3.5 × 1015 molecules cm−2). On the other hand, the distance criterion affects mostly GOME-2B data selection, because GOME-2B pixels are quite evenly distributed among the different radii used in the sensitivity test. More specifically, the number of collocations is notably reduced when stricter radius limits are applied, the r value is improved from 0.795 (d ≤ 50 km) to 0.953 (d ≤ 20 km), and the absolute mean bias decreases about 6 times for d ≤ 30 km compared to the reference case (d ≤ 50 km).

One of the main issues arising from the comparison of ground-based and satellite measurements is the difference in spatial representativeness, which for locations with inhomogeneous spatial distribution of pollutants may lead to significant differences between the two data sets. In order to investigate the spatial variability of tropospheric NO2 within a sub-satellite pixel, a campaign which lasted for about 6 months was held in the greater area of Thessaloniki, Greece. Three multi-axial differential optical absorption spectroscopy (MAX-DOAS) systems performed measurements of tropospheric NO2 columns at different sites representative of urban, suburban and rural conditions. The direct comparison of these ground-based measurements with corresponding products from the Ozone Monitoring Instrument onboard NASA's Aura satellite (OMI/Aura) showed good agreement over the rural and suburban areas, while the comparison with the Global Ozone Monitoring Experiment-2 (GOME-2) onboard EUMETSAT's Meteorological Operational satellites' (MetOp-A and MetOp-B) observations is good only over the rural area. GOME-2A and GOME-2B sensors show an average underestimation of tropospheric NO2 over the urban area of about 10.51 ± 8.32 × 1015 and 10.21 ± 8.87 × 1015 molecules cm−2, respectively. The mean difference between ground-based and OMI observations is significantly lower (6.60 ± 5.71 × 1015 molecules cm−2). The differences found in the comparisons of MAX-DOAS data with the different satellite sensors can be attributed to the higher spatial resolution of OMI, as well as the different overpass times and NO2 retrieval algorithms of the satellites. OMI data were adjusted using factors calculated by an air quality modeling tool, consisting of the Weather Research and Forecasting (WRF) mesoscale meteorological model and the Comprehensive Air Quality Model with Extensions (CAMx) multiscale photochemical transport model. This approach resulted in significant improvement of the comparisons over the urban monitoring site. The average difference of OMI observations from MAX-DOAS measurements was reduced to −1.68 ± 5.01 × 1015 molecules cm−2.

The determination of aerosol properties, especially the aerosol optical depth (AOD) in the ultraviolet (UV) wavelength region, is of great importance for understanding the climatological variability of UV radiation. However, operational retrievals of AOD at the biologically most harmful wavelengths in the UVB are currently only made at very few places. This paper reports on the UVPFR (UV precision filter radiometer) sunphotometer, a stable and robust instrument that can be used for AOD retrievals at four UV wavelengths. Instrument characteristics and results of Langley calibrations at a high-altitude site were presented. It was shown that due to the relatively wide spectral response functions of the UVPFR, the calibration constants (V0) derived from Langley plot calibrations underestimate the true extraterrestrial signals. Accordingly, correction factors were introduced. In addition, the instrument's spectral response functions also result in an apparent air-mass-dependent decrease in ozone optical depth used in the AOD determinations. An adjusted formula for the calculation of AOD, with a correction term dependent on total column ozone amount and ozone air mass, was therefore introduced. Langley calibrations performed 13–14 months apart resulted in sensitivity changes of ≤ 1.1 %, indicating good instrument stability. Comparison with a high-accuracy standard precision filter radiometer, measuring AOD at 368–862 nm wavelengths, showed consistent results. Also, very good agreement was achieved by comparing the UVPFR with AOD at UVB wavelengths derived with a Brewer spectrophotometer, which was calibrated against the UVPFR at an earlier date. Mainly due to non-instrumental uncertainties connected with ozone optical depth, the total uncertainty of AOD in the UVB is higher than that reported from AOD instruments measuring in UVA and visible ranges. However, the precision can be high among instruments using harmonized algorithms for ozone and Rayleigh optical depth as well as for air mass terms. For 4 months of comparison measurements with the UVPFR and a Brewer, the root mean squared AOD differences were found

We have used total and diffuse UV irradiance measurements from a multi-filter rotating shadow-band radiometer (UVMFR) in order to investigate aerosol absorption in the UV range for a 5-year period in Athens, Greece. This dataset was used as input to a radiative transfer model and the single scattering albedo (SSA) at 368 and 332 nm was calculated. Retrievals from a collocated CIMEL sun photometer were used to evaluate the products and study the absorption spectral behavior of retrieved SSA values. The UVMFR SSA, together with synchronous, CIMEL-derived retrievals of SSA at 440 nm, had a mean of 0.90, 0.87 and 0.83, with lowest values (higher absorption) encountered at the shorter wavelengths. In addition, noticeable diurnal variation of the SSA in all wavelengths is shown, with amplitudes up to 0.05. Strong SSA wavelength dependence is revealed for cases of low Ångström exponents, accompanied by a SSA decrease with decreasing extinction optical depth, suggesting varying influence under different aerosol composition. However, part of this dependence for low aerosol optical depths is masked by the enhanced SSA retrieval uncertainty. Dust and brown carbon UV absorbing properties were also investigated to explain seasonal patterns.

The main aim of this work is to evaluate the NASA EOS AURA Ozone Monitoring Instrument (OMI) UV irradiance estimates through ground-based measurements performed by a NILU-UV multichannel radiometer (NILU-UV) operating in Thessaloniki, Greece, for the time period between January 2005 and December 2014. NILU-UV multi-filter radiometers can provide measurements at 5 UV wavelength bands with full width at half maximum (FWHM) of 10 nm approximately and a time analysis of 1 min. An additional channel measuring the Photosynthetically Active Radiation (PAR) is also incorporated to the instrument and is used for the stringent characterization of the cloud free instances. The OMI instrument estimates solar UV irradiances at four wavelengths close to those of the NILU-UV in Thessaloniki. Clear and all-sky overpass-time, as well as solar local-noon time, UV estimates are provided by the NASA Aura Data Validation Center. Spectra measured from a collocated MKIII Brewer spectrophotometer with serial number 086 (Brewer #086) were utilized for the whole period (2005–2014) in order to estimate the NILU-UV irradiances at the OMI wavelength irradiances and therefore provide a direct comparison and validation to the NILU UV measurements provided by OMI. For the nominal comparisons, using un-flagged OMI data within a 50 km radius from Thessaloniki, the linear determination coefficient, R2, ranges between 0.91 and 0.97 for the 305 nm and between 0.75 and 0.92 for 380 nm depending on the choice of overpass or local-noon time data and the cloudiness flags. The best agreement is found for the clear-sky overpass-time comparisons as well as the both PAR- and satellite algorithm-deduced clear-sky overpass and local-noon comparisons for all wavelengths. The OMI irradiances were found to overestimate the NILU-UV observations in Thessaloniki between ∼4.5% and 13.5% for the 305 nm and between ∼1.5% and ∼10.0% for the 310 nm wavelength depending on the choice of time [overpass vs local noon] and cloudiness restrictions [only satellite-deduced or both PAR- and satellite-deduced]. For the 324 nm and 380 nm wavelengths, the satellite-deduced local-noon time clear skies comparisons show a satellite under-estimation of −3.75% and −4.15% respectively whereas the overpass-time comparisons range between −1.55% and −1.90% for the same wavelengths. When imposing the PAR-deduced clear-sky assessment as well, the comparisons show a satellite over-estimation of 2.50% and 2.75% for the local-noon and 3.70% and 4.10% for the overpass-time cases for the 324 nm and 380 nm wavelengths. The effect of a stricter radius of collocation selection (10 km and 25 km vs 50 km) appears to have a negligible impact on the comparisons however results in a smaller statistical sample. When flagging restrictions are applied to the quality indicators of the OMI data, the amount of co-locations decreases appreciatively but the correlations improve for most cases (R2 > 0.9 in most cases and wavelengths). The effect of the temporal averaging window of the ground-based measurements, of around 10 and 60 min to the overpass time, was also investigated. The differences of the time averaging span result in a more obvious improvement of the agreement for the all-sky cases; the R2 factor improves to 0.94, 0.90, 0.87 and 0.85 from the original 0.91, 0.87, 0.82 and 0.78 for the local-noon comparisons and to 0.96, 0.94, 0.92 and 0.91 from the original 0.93, 0.90, 0.80 and 0.83 for the overpass-time comparisons for the 305 nm, 310 nm, 324 nm and 380 nm wavelengths respectively. Hence, it is shown that this type of temporal averaging can implicitly compensate for the changes of cloud position and optical properties when comparing UV measurements from ground and space. For the detection of the seasonal characteristics of the satellite irradiances it was shown that the 380 nm wavelength can be safely studied. The standard deviation of the strict cloud free overpass-time comparisons of ±12% may form a dependable measure of the seasonality imposed by the satellite retrieval algorithm assumptions. Additionally, the aerosol load at 340 nm and the possible solar zenith angle (SZA) effect on the comparisons was considered. No marked aerosol dependence was found with the ratio of satellite-to-ground irradiances remaining 1.1 ± 1.3 for the all-sky comparisons and 1 ± 0.1 for the clear-sky comparisons, while the SZA effect becomes appreciable only for angles higher than 70° with ratios 1.1 ± 1.0 for all-skies and 1.0 ± 0.1 for the clear-sky cases below that threshold.

The effect of changes in the spectral distribution of the incident solar radiation on the direct normal responsivity of the LI-COR photodiode pyranometer is examined using DNI spectral data from a PMOD spectroradiometer and the generic spectral response of a LI-COR pyranometer. The direct normal responsivity is found to vary over the day under all weather conditions at Payerne, Switzerland in a systematic manner that can be modeled as a function of air mass or solar zenith angle. This suggests that scattering of other atmospheric constituents plays as a subsidiary role to Rayleigh scattering that preferentially scatters more blue light as the path through the atmosphere increases. The SMARTS2 model is used to examine the effect on the full spectral response range of the photodiode based pyranometer and to refine the estimated response changes. The use of this information is discussed relative to improving adjustments made to Rotating Shadowband Irradiometers measurements. Similar methodology can be used to estimate the spectral effect on the performance of solar modules.

This paper presents the new photometer CE318-T, able to perform daytime and night-time photometric measurements using the sun and the moon as light source. Therefore, this new device permits a complete cycle of diurnal aerosol and water vapour measurements valuable to enhance atmospheric monitoring to be extracted. In this study we have found significantly higher precision of triplets when comparing the CE318-T master instrument and the Cimel AErosol RObotic NETwork (AERONET) master (CE318-AERONET) triplets as a result of the new CE318-T tracking system. Regarding the instrument calibration, two new methodologies to transfer the calibration from a reference instrument using only daytime measurements (Sun Ratio and Sun-Moon gain factor techniques) are presented and discussed. These methods allow the reduction of the previous complexities inherent to nocturnal calibration. A quantitative estimation of CE318-T AOD uncertainty by means of error propagation theory during daytime revealed AOD uncertainties (uDAOD) for Langley-calibrated instruments similar to the expected values for other reference instruments (0.002–0.009). We have also found uDAOD values similar to the values reported in sun photometry for field instruments ( ∼ 0.015). In the case of the night-time period, the CE318-T-estimated standard combined uncertainty (uNAOD) is dependent not only on the calibration technique but also on illumination conditions and the instrumental noise. These values range from 0.011–0.018 for Lunar Langley-calibrated instruments to 0.012–0.021 for instruments calibrated using the Sun Ratio technique. In the case of moon-calibrated instruments using the Sun-Moon gain factor method and sun-calibrated using the Langley technique, we found uNAOD ranging from 0.016 to 0.017 (up to 0.019 in 440 nm channel), not dependent on any lunar irradiance model.A subsequent performance evaluation including CE318-T and collocated measurements from independent reference instruments has served to assess the CE318-T performance as well as to confirm its estimated uncertainty. Daytime AOD evaluation, performed at Izaña station from March to June 2014, encompassed measurements from a reference CE318-T, a CE318-AERONET master instrument, a Precision Filter Radiometer (PFR) and a Precision Spectroradiometer (PSR) prototype, reporting low AOD discrepancies between the four instruments (up to 0.006). The nocturnal AOD evaluation was performed using CE318-T- and star-photometer-collocated measurements and also by means of a day/night coherence transition test using the CE318-T master instrument and the CE318 daytime data from the CE318-AERONET master instrument. Results showed low discrepancies with the star photometer at 870 and 500 nm channels ( ≤ 0.013) and differences with AERONET daytime data (1 h after and before sunset and sunrise) in agreement with the estimated uNAOD values at all illumination conditions in the case of channels within the visible spectral range, and only for high moon's illumination conditions in the case of near-infrared channels.Precipitable water vapour (PWV) validation showed a good agreement between CE318-T and Global Navigation Satellite System (GNSS) PWV values for all illumination conditions, within the expected precision for sun photometry.Finally, two case studies have been included to highlight the ability of the new CE318-T to capture the diurnal cycle of aerosols and water vapour as well as short-term atmospheric variations, critical for climate studies.

The World Optical Depth Research Calibration Center (WORCC) is a section within the World Radiation Center at Physikalisches-Meteorologisches Observatorium (PMOD/WRC), Davos, Switzerland, established after the recommendations of the World Meteorological Organization for calibration of aerosol optical depth (AOD)-related Sun photometers. WORCC is mandated to develop new methods for instrument calibration, to initiate homogenization activities among different AOD networks and to run a network (GAW-PFR) of Sun photometers. In this work we describe the calibration hierarchy and methods used under WORCC and the basic procedures, tests and processing techniques in order to ensure the quality assurance and quality control of the AOD-retrieved data.

This study introduces the measurement of the total ozone column using Differential Optical Absorption Spectroscopy (DOAS) analysis of direct-sun spectra recorded by the Phaethon system. This methodology is based on the analysis of spectra relative to a reference spectrum that has been recorded by the same instrument. The slant column density of ozone associated with the reference spectrum is derived by Langley extrapolation. Total ozone data derived by Phaethon over two years in Thessaloniki are compared with those of a collocated, well-maintained and calibrated, Brewer spectrophotometer. When the retrieval of total ozone is based on the absorption cross sections of (Paur and Bass, 1984) at 228 K, Phaethon shows an average overestimation of 1.85 ± 1.86%. Taking into account the effect of the day-to-day variability of stratospheric temperature on total ozone derived by both systems, the bias is reduced to 0.94 ± 1.26%. The sensitivity of the total ozone retrieval to changes in temperature is larger for Phaethon than for Brewer.

Synchronized sun-photometric measurements from the AERONET-CIMEL and GAW-PFR aerosol networks are used to compare retrievals of the aerosol optical depth, effective radius and volume concentration during a high temporal resolution measurement campaign at the Athens site in the Mediterranean Basin from 14–22 July 2009. During this period, direct sun AOD retrievals from both instruments exhibited small differences in the range 0.01–0.02 despite the presence of a strong dust event. In addition to AERONET-CIMEL inversion data, an independent inversion method was applied that involves expanding the particle size distribution in terms of measurement kernels so as to estimate bulk particle parameters from a linear-estimated combination of the input optical data. AOD measurements obtained from both CIMEL and PFR instruments using this method also showed reasonable agreement. For low aerosol loads (AOD < 0.2), measurements of the effective radius by the PFR were found to be −20% to +30% different from CIMEL values for both direct sun data and inversion data. At higher loads (AOD > 0.4), measurements of the effective radius by the PFR are consistently 20% lower than CIMEL for both direct sun and inversion data. Volume concentrations at low aerosol loads from the PFR are up to 80% higher than the CIMEL for direct sun data, but inversion data suggests that volume concentrations from the PFR are up to 20% lower than the CIMEL under these same conditions. At higher loads, the percentage difference in volume concentrations from the PFR and CIMEL is systematically negative with inversion data predicting differences 30% lower than those obtained from direct sun data. An assessment of the effect of errors in the AOD retrieval on the estimation of PFR bulk parameters was made using Monte Carlo simulations and demonstrated that it is possible to estimate the effective radius with an uncertainty below 60% and the volume concentration with an uncertainty below 65% even when AOD < 0.2 and when the input errors are as high as 10%. Highlights – A comparison of high temporal resolution synchronous CIMEL and PFR direct sun AOD measurement retrievals – Calculation of bulk aerosol microphysics parameters using a linear estimation inversion technique – A comparison of retrieved aerosol volume concentrations and effective radii from CIMEL and PFR inversions – An analysis of the sensitivity of PFR retrievals to random errors on the optical input data

Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. The overall objective of the project INSPECTRO was the characterization of the three-dimensional actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany) combining space-based, aircraft and ground-based measurements as well as simulations with the one-dimensional radiation transfer model UVSPEC and the three-dimensional radiation transfer model MYSTIC. During both campaigns the spectral actinic flux density was measured at several locations at ground level and in the air by up to four different aircraft. This allows the comparison of measured and simulated actinic radiation profiles. In addition satellite data were used to complete the information of the three dimensional input data set for the simulation. A three-dimensional simulation of actinic flux density data under cloudy sky conditions requires a realistic simulation of the cloud field to be used as an input for the 3-D radiation transfer model calculations. Two different approaches were applied, to derive high- and low-resolution data sets, with a grid resolution of about 100 m and 1 km, respectively. The results of the measured and simulated radiation profiles as well as the results of the ground based measurements are presented in terms of photolysis rate profiles for ozone and nitrogen dioxide. During both campaigns all spectroradiometer systems agreed within ±10% if mandatory corrections e.g. stray light correction were applied. Stability changes of the systems were below 5% over the 4 week campaign periods and negligible over a few days. The J(O1D) data of the single monochromator systems can be evaluated for zenith angles less than 70°, which was satisfied by nearly all airborne measurements during both campaigns. The comparison of the airborne measurements with corresponding simulations is presented for the total, downward and upward flux during selected clear sky periods of both campaigns. The compliance between the measured (from three aircraft) and simulated downward and total flux profiles lies in the range of ±15%. © Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License.

Spectral measurements of the aerosol optical depth (AOD) and the Ångström coefficient were conducted at Thessaloniki, Greece (40.5° N, 22.9° E) between January 1997 and December 2005 with a Brewer MKIII double-monochromator spectroradiometer. The dataset was compared with collocated measurements of a second spectroradiometer (Brewer MKII) and a CIMEL sun-photometer, showing correlations of 0.93 and 0.98, respectively. A seasonal variation of the AOD was observed at Thessaloniki, with AOD values at 340 nm of 0.52 and 0.28 for August and December respectively. Back trajectories of air masses for up to 4 days were used to assess the influence of long-range transport from various regions to the aerosol load over Thessaloniki. It is shown that part of the observed seasonality can be attributed to air masses with high AOD originating from North-Eastern and Eastern directions during summertime. The analysis of the long-term record (9 years) of AOD showed a downward tendency. A similar decreasing tendency was found in the record of the PM$_{10}$ aerosol measurements, which are conducted near the surface at 4 air-quality monitoring stations in the area of the city of Thessaloniki.

The first multi-instrument nocturnal aerosol optical depth (AOD) intercomparison campaign was held at the high-mountain Izaña Observatory (Tenerife, Spain) in June 2017, involving 2-min synchronous measurements from two different types of lunar photometers (Cimel CE318-T and Moon Precision Filter Radiometer, LunarPFR) and one stellar photometer. The Robotic Lunar Observatory (ROLO) model developed by the U.S. Geological Survey (USGS) was compared with the open-access ROLO Implementation for Moon photometry Observation (RIMO) model. Results showed rather small differences at Izaña over a 2-month time period covering June and July, 2017 (±0.01 in terms of AOD calculated by means of a day/night/day coherence test analysis and ± 2% in terms of lunar irradiance). The RIMO model has been used in this field campaign to retrieve AOD from lunar photometric measurements. No evidence of significant differences with the Moon's phase angle was found when comparing raw signals of the six Cimel photometers involved in this field campaign. The raw signal comparison of the participating lunar photometers (Cimel and LunarPFR) performed at coincident wavelengths showed consistent measurements and AOD differences within their combined uncertainties at 870 nm and 675 nm. Slightly larger AOD deviations were observed at 500 nm, pointing to some unexpected instrumental variations during the measurement period. Lunar irradiances retrieved using RIMO for phase angles varying between 0° and 75° (full Moon to near quarter Moon) were compared to the irradiance variations retrieved by Cimel and LunarPFR photometers. Our results showed a relative agreement within ± 3.5% between the RIMO model and the photometer-based lunar irradiances. The AOD retrieved by performing a Langley-plot calibration each night showed a remarkable agreement (better than 0.01) between the lunar photometers. However, when applying the Lunar-Langley calibration using RIMO, AOD differences of up to 0.015 (0.040 for 500 nm) were found, with differences increasing with the Moon's phase angle. These differences are thought to be partly due to the uncertainties in the irradiance models, as well as instrumental deficiencies yet to be fully understood. High AOD variability in stellar measurements was detected during the campaign. Nevertheless, the observed AOD differences in the Cimel/stellar comparison were within the expected combined uncertainties of these two photometric techniques. Our results indicate that lunar photometry is a more reliable technique, especially for low aerosol loading conditions. The uncertainty analysis performed in this paper shows that the combined standard AOD uncertainty in lunar photometry is dependent on the calibration technique (up to 0.014 for Langley-plot with illumination-based correction, 0.012–0.022 for Lunar-Langley calibration, and up to 0.1 for the Sun-Moon Gain Factor method). This analysis also corroborates that the uncertainty of the lunar irradiance model used for AOD calculation is within the 5–10% expected range. This campaign has allowed us to quantify the important technical difficulties that still exist when routinely monitoring aerosol optical properties at night-time. The small AOD differences observed between the three types of photometers involved in the campaign are only detectable under pristine sky conditions such as those found in this field campaign. Longer campaigns are necessary to understand the observed discrepancies between instruments as well as to provide more conclusive results about the uncertainty involved in the lunar irradiance models. This work has been developed within the framework of the activities of the World Meteorological Organization (WMO) Commission for Instruments and Methods of Observations (CIMO) Izaña Testbed for Aerosols and Water Vapour Remote Sensing Instruments. AERONET sun photometers at Izaña have been calibrated within the AERONET Europe TNA, supported by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 654109 (ACTRIS‒2). CE318-T linearity check has been performed as part of the ESA-funded project “Lunar spectral irradiance measurement and modelling for absolute calibration of EO optical sensors” under ESA contract number: 4000121576/17/NL/AF/hh. LunarPFR has been performing measurements since 2014 in Norway thanks to Svalbard Science Forum funded project, 2014–2016. The authors would like to thank AERONET team for their support and also to NASA’s Navigation and Ancillary Information Facility (NAIF) at the Jet Propulsion Laboratory to help the implementation of the “SPICE” ancillary information system used in this study. We also thank Izaña's ITs for their work to implement the RIMO model in the free-access server. Special thanks should be given to Tom Stone, who has kindly provided us with the USGS/ROLO irradiance values used in the model comparison analysis. This work has also received funding from the European Union’s Horizon 2020 research and innovation programme and from Marie Skłodowska-Curie Individual Fellowships (IF) ACE-GFAT (grant agreement no. 659398). The authors are grateful to Spanish MINECO (CTM2015-66742-R) and Junta de Castilla y León (VA100P17).