Search

New remote sensing satellites for Earth Observation provide information to enhance situational awareness during exceptional socio-economic and natural events. Specifically, satellite-based observations of air pollutants, when combined with auxiliary (spaceborne and not) data, can provide new insights on societal and economic changes taking place around the world. Here we present a few applications of space-based atmospheric observations to support Finnish authorities. A recent history of such episodes includes: monitoring air quality changes in Helsinki during the COVID-19 pandemic; assessing methane emissions from the NordStream pipeline leakage over the Baltic Sea; assessing the environmental and social effects of the war in Ukraine as part of Finnish international cooperation projects. We integrate multiple data sources acquired by different spaceborne sensors including, e.g., the nitrogen dioxide (NO2) retrievals from the Copernicus Sentinel-5 Precursor/TROPOMI (TROPOspheric Monitoring Instrument), Sentinel-2 false color imagery, NASA VIIRS fire products as well as models and other environmental data.

We characterize major anthropogenic point sources in the South African Highveld region using Orbiting Carbon Observatory-3 (OCO-3) Snapshot Area Map (SAM) carbon dioxide (CO2) and Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI) nitrogen dioxide (NO2) observations. Altogether we analyze six OCO-3 SAMs. We estimate the emissions of six power stations (Kendal, Kriel, Matla, Majuba, Tutuka and Grootvlei) and the largest single emitter of greenhouse gas in the world, Secunda CTL synthetic fuel plant. We apply the cross-sectional flux method for the emission estimation, and we extend the method to fit several plumes at the same time. Overall, the satellite-based emission estimates are in good agreement (within the uncertainties) as compared to emission inventories, even for the cases where several plumes are mixed. We also discuss the advantages and challenges of the current measurement systems for greenhouse gas emission monitoring and reporting, and the applicability of different emission estimation approaches to future satellite missions such as the Copernicus CO2 Monitoring Mission (CO2M) and the Global Observing SATellite for Greenhouse gases and Water cycle (GOSAT-GW), including the joint analysis of CO2 and NO2 observations.

In recent decades, satellite instruments have been providing observations of air pollutants such as nitrogen dioxide (NO2) with global coverage. Since late 2018, the TROPOspheric Monitoring Instrument (TROPOMI) on-board the Copernicus Sentinel-5 Precursor satellite has produced NO2 vertical column densities (VCDs) with the best spatial resolution (currently 5.5x3.5 km at nadir). In order to compare satellite-based observations to traditional NO2 surface concentration measurements available from air quality (AQ) stations, methods based on chemical-transport model (CTM) simulations can be applied to convert satellite-based VCDs to surface concentrations. Such methods have been mostly developed and applied in regions with high NO2 concentrations at middle-low latitudes.In this paper, we test and adapt two methods to estimate surface-level NO2 concentrations from TROPOMI observations in Finland, which is characterised by low NO2 concentrations at a high latitude location. Satellite-based estimates show good correlation with co-located surface NO2 measurements from the Finnish AQ network, although the method lacking a correction for the level of NO2 mixing within the boundary layer is more prone to underestimation. New cut-off values accounting for the level of mixing were calculated to adapt the column-to-surface conversion to specific conditions in Finland. We also present an alternate approach to reduce the underestimation based on the linear relation between in situ measurements and surface-level estimates.We utilise chemical transport model simulations to correct our surface-level NO2 estimates to derive full annual mean concentration estimates compatible with annual limit values defined in AQ legislation. This correction is based on the temporal sampling differences between in situ station measurements and TROPOMI observations. The annual estimates tend to overestimate concentrations compared to ground-based measurements, but can be considered as an upper estimate of surface concentrations. They can therefore complement existing AQ station measurements, especially in regions where the surface networks are sparse. Despite the uncertainties, we find that the two Finnish AQ monitoring regions currently lacking surface stations have estimated mean annual concentrations well below existing limit values, suggesting no need for new ground-based monitoring stations.Overall, the results provide new information to complement traditional surface-based AQ measurements and to support national environmental authorities in air quality assessment and reporting.

In this report the current air quality in Finland has been assessed with air quality measurement data and satellite observations. The assessment of ambient air concentrations included following air impurities: NO2, NOx, PM10, PM2,5, SO2, CO, O3, benzo(a)pyrene, benzene, Pb, As, Cd ja Ni. For these pollutants air quality assessment thresholds are given in air quality legislation (2008/50/EY, 2004/107/EY). Assessment has been performed for air quality zones. The main data set included air quality measurements performed in Finland during 2015–2019. Satellite observations were used as an objective assessment tool in analysis of the spatial variation of NO2 and CO concentrations. Air quality measurements show that air quality has improved in Finland in many respects. Especially the need to monitor NO2 and PM10 with continuous measurements has decreased. Growing understanding of national benzo(a)pyrene concentrations has increased the monitoring needs. Efforts to decrease ozone levels still requires international actions. SO2, CO, benzene and heavy metal concentrations are on a low level in Finland outside industrial areas and other assessment methods than continuous monitoring can be used, and the number of continuous monitoring sites has already decreased. Satellite-based concentrations of nitrogen dioxide and carbon monoxide as well as their spatial variation in Finland were analyzed using observations from the TROPOsperic Monitoring Instrument (TROPOMI). The analysis of CO over Finland was carried out for the first time in this project. Results show that overall annual CO concentrations over Finland are low and spatial variability is small. Also, NO2 concentrations over Finland are rather low, but spatial patterns are more clearly visible. The highest NO2 concentrations are observed over the largest cities. By establishing a relationship between ground-based and satellite total column concentrations, surface concentrations of NO2 and CO were estimated from the satellite data for the zones. The satellite-based estimate for annual NO2 surface concentration over Helsinki metropolitan area is 28 μg/m3, and for the rest of Finland mostly between 10–15 μg/m3. For CO the differences between monitoring areas are small, with estimates varying between 160–164 μg/m3 or in other words about 0,16 mg/m3.

We present a comparison between satellite-based TROPOMI (TROPOspheric Monitoring Instrument) NO2 products and ground-based observations in Helsinki (Finland). TROPOMI NO2 total (summed) columns are compared with the measurements performed by the Pandora spectrometer between April and September 2018. The mean relative and absolute bias between the TROPOMI and Pandora NO2 total columns is about 10 % and 0.12×1015 molec. cm −2 respectively. The dispersion of these differences (estimated as their standard deviation) is 2.2×1015 molec. cm −2 . We find high correlation ( r = 0.68 ) between satellite- and ground-based data, but also that TROPOMI total columns underestimate ground-based observations for relatively large Pandora NO2 total columns, corresponding to episodes of relatively elevated pollution. This is expected because of the relatively large size of the TROPOMI ground pixel ( 3.5×7 km) and the a priori used in the retrieval compared to the relatively small field-of-view of the Pandora instrument. On the other hand, TROPOMI slightly overestimates (within the retrieval uncertainties) relatively small NO2 total columns. Replacing the coarse a priori NO2 profiles with high-resolution profiles from the CAMS chemical transport model improves the agreement between TROPOMI and Pandora total columns for episodes of NO2 enhancement. When only the low values of NO2 total columns or the whole dataset are taken into account, the mean bias slightly increases. The change in bias remains mostly within the uncertainties. We also analyse the consistency between satellite-based data and in situ NO2 surface concentrations measured at the Helsinki–Kumpula air quality station (located a few metres from the Pandora spectrometer). We find similar day-to-day variability between TROPOMI, Pandora and in situ measurements, with NO2 enhancements observed during the same days. Both satellite- and ground-based data show a similar weekly cycle, with lower NO2 levels during the weekend compared to the weekdays as a result of reduced emissions from traffic and industrial activities (as expected in urban sites). The TROPOMI NO2 maps reveal also spatial features, such as the main traffic ways and the airport area, as well as the effect of the prevailing south-west wind patterns. This is one of the first works in which TROPOMI NO2 retrievals are validated against ground-based observations and the results provide an early evaluation of their applicability for monitoring pollution levels in urban sites. Overall, TROPOMI retrievals are valuable to complement the ground-based air quality data (available with high temporal resolution) for describing the spatio-temporal variability of NO2 , even in a relatively small city like Helsinki.

The 6-month-long 2014–2015 Holuhraun eruption was the largest in Iceland for 200 years, emitting huge quantities of sulfur dioxide (SO2) into the troposphere, at times overwhelming European anthropogenic emissions. Weather, terrain and latitude made continuous ground-based or UV satellite sensor measurements challenging. Infrared Atmospheric Sounding Interferometer (IASI) data are used to derive the first time series of daily SO2 mass present in the atmosphere and its vertical distribution over the entire eruption period. A new optimal estimation scheme is used to calculate daily SO2 fluxes and average e-folding time every 12 h. For the 6 months studied, the SO2 flux was observed to be up to 200 kt day−1 and the minimum total SO2 erupted mass was 4.4±0.8 Tg. The average SO2 e-folding time was 2.4±0.6 days. Where comparisons are possible, these results broadly agree with ground-based near-source measurements, independent remote-sensing data and values obtained from model simulations from a previous paper. The results highlight the importance of using high-resolution time series data to accurately estimate volcanic SO2 emissions. The SO2 mass missed due to thermal contrast is estimated to be of the order of 3 % of the total emission when compared to measurements by the Ozone Monitoring Instrument. A statistical correction for cloud based on the AVHRR cloud-CCI data set suggested that the SO2 mass missed due to cloud cover could be significant, up to a factor of 2 for the plume within the first kilometre from the vent. Applying this correction results in a total erupted mass of 6.7±0.4 Tg and little change in average e-folding time. The data set derived can be used for comparisons to other ground- and satellite-based measurements and to petrological estimates of the SO2 flux. It could also be used to initialise climate model simulations, helping to better quantify the environmental and climatic impacts of future Icelandic fissure eruptions and simulations of past large-scale flood lava eruptions.

The increase in atmospheric greenhouse gas concentrations of CO2 and CH4, due to human activities, is the main driver of the observed increase in surface temperature by more than 1 °C since the pre-industrial era. At the 2015 United Nations Climate Change Conference held in Paris, most nations agreed to reduce greenhouse gas emissions to limit the increase in global surface temperature to 1.5 °C. Satellite remote sensing of CO2 and CH4 is now well established thanks to missions such as NASA’s OCO-2 and the Japanese GOSAT missions, which have allowed us to build a long-term record of atmospheric GHG concentrations from space. They also give us a first glimpse into CO2 and CH4 enhancements related to anthropogenic emission, which helps to pave the way towards the future missions aimed at a Monitoring & Verification Support (MVS) capacity for the global stock take of the Paris agreement. China plays an important role for the global carbon budget as the largest source of anthropogenic carbon emissions but also as a region of increased carbon sequestration as a result of several reforestation projects. Over the last 10 years, a series of projects on mitigation of carbon emission has been started in China, including the development of the first Chinese greenhouse gas monitoring satellite mission, TanSat, which was successfully launched on 22 December 2016. Here, we summarise the results of a collaborative project between European and Chinese teams under the framework of the Dragon-4 programme of ESA and the Ministry of Science and Technology (MOST) to characterize and evaluate the datasets from the TanSat mission by retrieval intercomparisons and ground-based validation and to apply model comparisons and surface flux inversion methods to TanSat and other CO2 missions, with a focus on China.

Solar ultraviolet (UV) radiation has a broad range of effects concerning life on Earth. Because of its high photon energy, UV radiation influences human health, terrestrial and aquatic ecosystems, air quality, and materials in various ways. The Sentinel 5 Precursor (S5P) mission on a sun-synchronous orbit with an ascending node equatorial crossing at 13:30, which in conjunction with a wide swath of 2600 km provides near-global daily coverage. S5P’s TROPOMI instrument measures radiation backscattered from the Earth–atmosphere system and provides observations of atmospheric composition with the best spatial resolution presently. Among other things, TROPOMI measurements are used for calculating the UV radiation reaching the Earth's surface over the sunlit part of the globe. This UV-radiation product is processed at the Finnish Meteorological Institute Copernicus Collaborative Ground segment. The product was released via FinHUB in summer 2020. The TROPOMI L2 UV product contains 36 UV parameters in total, including irradiances at four different wavelengths and dose rates for erythemal and vitamin D synthesis action spectra. All parameters are calculated for overpass time, for solar noon time, and for theoretical clear-sky conditions with no clouds or aerosols. Daily doses and accumulated irradiances are also calculated by integrating over the sunlit part of the day. In addition to UV parameters, quality flags related to the UV product and processing are generated.Validation with ground based instruments have shown that the agreement is very good, typically within 10%. The S5P is the first Copernicus mission dedicated to atmospheric observations, and it will be complemented by Sentinel 4 with geostationary orbit and Sentinel 5 on Sun-synchronous morning orbit with planned launches in the coming years. It is expected that surface UV-radiation products from these instruments will continue the present time series. The TROPOMI surface UV radiation product responds to the increasing need for information regarding the tropospheric chemistry and biologically active wavelengths of the solar spectrum reaching the surface. In this presentation we introduce the TROPOMI UV radiation product and future developments, discuss about the quality of the product and demonstrate the usefulness of the satellite UV-data by showing resent applications including among others the exceptionally high UV-radiation conditions in mid latitudes due to persistent Antarctic ozone hole in December 2020 and modeling of seasonal cycle of COVID-19. By combining the TROPOMI UV data with observations of trace gases from the same instrument, there is also a potential for new kind of applications, where satellite data can be used in novel ways to study photochemical processes in the troposphere.

We present the results of two projects completed for the Finnish Ministry of the Environment that assessed the capability of satellites in supporting traditional in situ air quality (AQ) measurements. These projects analysed the correlation of co-located NO2 measurements from the TROPOspheric Monitoring Instrument (TROPOMI, measuring in molec./cm2) and traditional air quality stations (measuring in µg/m3) in Finland and Europe in 2018 and 2019, and used the results to estimate annual mean ground-level NO2 concentrations in Finland’s 14 different AQ monitoring regions.We find that the correlation is dependent on the location of the AQ station, with city stations having a higher correlation than rural background stations. This is expected, as the variability of NO2 levels in Finnish rural areas is usually within TROPOMI’s random measurement error. We also find that the estimated annual mean regional ground level NO2 concentrations compare well to the in situ measurements, as the associated uncertainties provide reliable upper estimates for ground level concentrations. These estimates were used to establish that annual NO2 concentrations were below the EU limit in two AQ monitoring regions with no active ground stations.We also analyse TROPOMI’s and the Ozone Monitoring Instrument’s (OMI) ability to study the spatial distribution of NO2 over Finland using gridded maps. Oversampled TROPOMI measurements are able to distinguish relatively small sources such as roads, airports and refineries, and the difference in concentrations between weekdays and weekends. TROPOMI is also able to detect emissions from different sources of NO2 such as cities, mining sites and industrial areas. Long time series measurements from OMI show decreasing NO2 levels over Finland between 2005 and 2018.The studies were conducted on behalf of the Finnish Ministry of the Environment, and showcase how satellite measurements can be used to supplement traditional air quality measurements in areas with poor ground station coverage. Launched in 2017, TROPOMI is currently the highest-resolution air quality sensing satellite, and its societal uses are only beginning to be realised. Future Sentinel missions, especially the geosynchronous Sentinel-4, will further extend satellite air quality monitoring capabilities and enable continuous daytime observations in cloud-free conditions.

We evaluate the satellite-based TROPOMI (TROPOspheric Monitoring Instrument) NO2 products against ground-based observations in Helsinki (Finland). TROPOMI NO2 total (summed) columns are compared with the measurements performed by the Pandora spectrometer during April–September 2018. The mean relative and absolute bias between the TROPOMI and Pandora NO2 total columns is about 10% and 0.12 × 1015 molec. cm-2 respectively. We find high correlation (r = 0.68) between satellite- and ground-based data, but also that TROPOMI total columns underestimate ground-based observations for relatively large Pandora NO2 total columns, corresponding to episodes of relatively elevated pollution. This is expected because of the relatively large size of the TROPOMI ground pixel (3.5 × 7 km) and the a priori used in the retrieval compared to the relatively small field-of-view of the Pandora instrument. On the other hand, TROPOMI slightly overestimates relatively small NO2 total columns. Replacing the coarse a priori NO2 profiles with high-resolution profiles from the CAMS chemical transport model improves the agreement between TROPOMI and Pandora total columns for episodes of NO2 enhancement, but the overall bias remains the same (within the uncertainties).In order to evaluate the capability of TROPOMI observations for monitoring urban air quality, we also analyse the consistency between satellite-based data and NO2 surface concentrations from the Kumpula air quality station in Helsinki. We find similar day-to-day variability between TROPOMI and in situ measurements, with NO2 enhancements observed during the same days. Both satellite- and ground-based data show a similar weekly cycle, with lower NO2 levels during the weekend compared to the weekdays as a result of reduced emissions from traffic and industrial activities (as expected in urban sites).Several applications have been already carried on to support informed decision making and Finnish society in general. We developed a simple web platform to inform environmental authorities at municipal level about the use of satellite observations for air quality monitoring. We assisted the Finnish authorities during the first period of the COVID-19 pandemic in assessing the effect of the lockdown on air quality. We supported the Finnish Ministry of Environment in compiling the periodic national air pollution assessment report to the EU. We participated in several international cooperation projects for assessing the major air pollution sources and the available air quality monitoring systems over several developing countries and for providing recommendations on strengthening air quality monitoring. We collaborated with the department of Social Science at the Univ. of Helsinki for the assessment of the environmental impacts of the energy and extracting sector in Yakutia (Russia).Reference: Ialongo, I., Virta, H., Eskes, H., Hovila, J., and Douros, J.: Comparison of TROPOMI/Sentinel-5 Precursor NO2 observations with ground-based measurements in Helsinki, Atmos. Meas. Tech., 13, 205–218, https://doi.org/10.5194/amt-13-205-2020, 2020.

This work focuses on studying signatures of individual ships in TROPOMI NO2 observations. Information on ships, their location and NOx emissions, are obtained from the Ship Traffic Emission Assessment Model (STEAM, Jalkanen et al., 2009). For this work altogether 33 large container ships are selected that operated between Europe and Asia between May-October in 2018 and/or 2019. TROPOMI NO2 data is sampled over the Mediterranean along each ships route provided by STEAM, allowing a maximum of 15 minutes temporal difference between the satellite observation and the ship location. Each of the matching TROPOMI NO2 scene is analysed using the ships route information for the past 2 hours from the satellite observation. For each container ship multiple matching observations are found where a signature of the ships emissions is visible in the TROPOMI NO2 data. These signatures are seen both under sun glint and non-glint conditions, but under glint the signature is often more clear (Fig.1). Over the Mediterranean there aren't any significant differences at which month these signatures were observed, but only for few cases the match and a clear signature are obtained in consecutive days. However, there are also multiple cases when it was not possible to connect a plume to a specific ship, especially near the Strait of Gibraltar or locations where the shipping lane goes close to the coast. In this work also comparisons between the STEAM NOx emissions and TROPOMI NO2 were carried out. In addition, meteorological conditions were analysed using ERA5 data. Preliminary results indicate that the R2 value between STEAM NOx and TROPOMI NO2 is about 0.15-0.2 when signatures from all individual ships are combined, but for glint cases and high planetary boundary layer conditions R2 is somewhat higher. These results are sensitive to the sampling method, and therefore more testing on modified sampling will be carried out. Next steps include also more detailed analysis of meteorological conditions. This work is funded by the SCIPPER project (H2020 grant agreement Nr. 814893) Figure 1. An example of a ship plume signature in TROPOMI NO2 data, under glint (left) and non-glint (right) conditions. Black dot indicate the ship location within 15 min. the TROPOMI overpass, and the dotted line the ships route 2h prior to the overpass. Reference: Jalkanen, J-P., Brink, A., Kalli, J., Pettersson, H., Kukkonen, J. and Stipa, T.: A modelling system for the exhaust emissions of marine traffic and its application in the Baltic Sea area", Atmos. Chem. Phys., 9, 9209‐9223, 2009.

Measurements of solar ultraviolet radiation (UVR) performed between January and June 2020 at 10 Arctic and subarctic locations are compared with historical observations. Differences between 2020 and prior years are also assessed with total ozone column and UVR data from satellites. Erythemal (sunburning) UVR is quantified with the UV Index (UVI) derived from these measurements. UVI data show unprecedently large anomalies, occurring mostly between early March and mid-April 2020. For several days, UVIs observed in 2020 exceeded measurements of previous years by up to 140%. Historical means were surpassed by more than six standard deviations at several locations in the Arctic. In northern Canada, the average UVI for March was about 75% larger than usual. UVIs in April 2020 were elevated on average by about 25% at all sites. However, absolute anomalies remained below 3.0 UVI units because the enhancements occurred during times when the solar elevation was still low.

We present the results of a project for the Finnish Ministry of the Environment that aimed to assess the potential of satellite measurements in complementing traditional in situ air quality measurements. Co-located NO2 measurements from the TROPOspheric Monitoring Instrument (TROPOMI) and several traditional air quality stations (measuring in µg/m3) in Finland and Europe between April 2018 and June 2019 are compared to determine their correlation. We find that the correlation of individual air quality stations with TROPOMI is dependent on the location of the station, but is more reliable when all stations in a city centre are studied as a group. This is expected due to the spatial averaging of satellite measurements. We also find that NO2 measurements between different cities in Finland and Europe in general correlate well.We also analyse TROPOMI’s and the Ozone Monitoring Instrument’s (OMI) ability to study the spatial distribution of NO2 over Finland and the Helsinki metropolitan area using gridded maps. Oversampled TROPOMI measurements are able to distinguish relatively small sources such as roads and airports, and the difference in concentrations between weekdays and weekends. TROPOMI is also able to detect emissions from different sources of NO2 such as cities, mining sites and industrial areas. Long time series measurements from OMI show decreasing NO2 levels over Finland between 2005 and 2018.Finally, we convert air quality station measurements to vertical column densities using boundary layer height data, and study the effect that this has on their correlation with TROPOMI measurements.This study was conducted on behalf of the Finnish Ministry of the Environment, and showcases how satellite measurements can be used reliably alongside traditional air quality measurements to provide a better picture of current pollution levels. Launched in 2017, TROPOMI is currently the highest-resolution air quality sensing satellite, and its societal uses are only beginning to be realized. Future Sentinel missions, especially the geosynchronous Sentinel-4, will provide an even more comprehensive view of the daily air quality situation.

In this study, we present the result of the application of space-based sulfur dioxide (SO2) observations to evaluate the efficacy of cleantech solutions in reducing air polluting emissions from metal smelting. We analyze the Ozone Monitoring Instrument (OMI) satellite-based SO2 observations over Tsumeb (Namibia) and Bor (Serbia) copper smelters, where two sulfur-capture plants, designed to transform gaseous SO2 emissions into sulfuric acid, were implemented in 2015. We observe a reduction in the annual SO2 emissions by up to 90% after 2015 at both smelters, as a result of the implementation of the sulfuric acid plants. The OMI-based emission estimates are mostly in line with those reported at facility-level and reproduce the same year-to-year variability. This variability is driven by the changes in the copper production, the sulfur-to-copper ratio and by the technology employed to reduce the SO2 emissions. OMI observations are directly used by the company operating the sulfuric acid plants to confirm the efficacy of the employed technology using independent satellite-based observations. The results demonstrate how satellite-based observations are able to detect relative changes in SO2 emissions and can be used to verify and complete existing emission informations. The approach presented here can be applied to other sources on global scale to support cleantech companies as well as decision-makers involved in environmental policies and sustainable development.

Observations taken over the last few decades indicate that dramatic changes are occurring in the ArcticBoreal Zone (ABZ), which are having significant impacts on ABZ inhabitants, infrastructure, flora and fauna, and economies. While suitable for detecting overall change, the current capability is inadequate for systematic monitoring and for improving processbased and largescale understanding of the integrated components of the ABZ, which includes the cryosphere, biosphere, hydrosphere, and atmosphere. Such knowledge will lead to improvements in Earth system models, enabling more accurate prediction of future changes and development of informed adaptation and mitigation strategies. In Duncan et al. (2020), we review the strengths and limitations of current spacebased observational capabilities for several important ABZ components and make recommendations for improving upon these current capabilities. We recommend an interdisciplinary and stepwise approach to develop a comprehensive ABZ Observing Network (ABZON), beginning with an initial focus on observing networks designed to gain processbased understanding for individual ABZ components and systems that can then serve as the building blocks for a comprehensive ABZON.

International audience; Global Ozone Monitoring by Occultation of Stars (GOMOS) on board Envisat has performed about 440 000 night-time occultations during 2002–2012. Self-calibrating measurement principle, good vertical resolution, excellent pointing accuracy and the wide vertical range from the troposphere up to the lower thermosphere make GOMOS profiles interesting for different analyses. The GOMOS ozone data are of high quality in the stratosphere and the mesosphere, but the current operational retrieval algorithm (IPF v.6) is not optimized for retrievals in the upper troposphere–lower stratosphere (UTLS). In particular, validation of GOMOS profiles against ozonesonde data has revealed a substantial positive bias (up to 100 %) in the UTLS region. The retrievals in the UTLS are challenging because of low signal-to-noise ratio and the presence of clouds. In this work, we discuss the reasons for the systematic uncertainties in the UTLS with the IPF v.6 algorithm or its modifications based on simultaneous retrievals of several constituents using the full visible wavelength range. The main reason is high sensitivity of the UTLS retrieval algorithms to an assumed aerosol extinction model. We have developed a new aerosol–insensitive ozone profile inversion algorithm for GOMOS data in the UTLS using a DOAS-type method at visible wavelengths. The method uses minimal assumptions about the atmospheric profiles. The ozone retrievals in the whole altitude range from the troposphere to the lower thermosphere are performed in two steps, as in the operational algorithm: spectral inversion followed by the vertical inversion. The horizontal column ozone densities retrieved in the spectral inversion follow V6 profiles in the middle atmosphere and follow the triplet ozone profiles in the UTLS. The vertical inversion is performed as in IPF v6 with the Tikhonov-type regularization according to the target resolution. The validation of new retrieved ozone profiles with ozonesondes show dramatic reduction of GOMOS ozone biases in the UTLS. The new GOMOS ozone profiles are also in a very good agreement with measurements by MIPAS, ACE-FTS and OSIRIS satellite instruments in the UTLS. It is also shown that the known geophysical phenomena in the UTLS ozone are well reproduced with the new GOMOS data.

We present the comparison of satellite-based OMI (Ozone Monitoring Instrument) NO2 products with ground-based observations in Helsinki. OMI NO2 total columns, available from NASA's standard product (SP) and KNMI DOMINO product, are compared with the measurements performed by the Pandora spectrometer in Helsinki in 2012. The relative difference between Pandora no. 21 and OMI SP total columns is 4 and −6 % for clear-sky and all-sky conditions, respectively. DOMINO NO2 retrievals showed slightly lower total columns with median differences about −5 and −14 % for clear-sky and all-sky conditions, respectively. Large differences often correspond to cloudy fall–winter days with solar zenith angles above 65°. Nevertheless, the differences remain within the retrieval uncertainties. The average difference values are likely the result of different factors partly canceling each other: the overestimation of the stratospheric columns causes a positive bias partly compensated by the limited spatial representativeness of the relatively coarse OMI pixel for sharp NO2 gradients. The comparison between Pandora and the new version (V3) of OMI NO2 retrievals shows a larger negative difference (about −30 %) than the current version (V2.1) because the revised spectral fitting procedure reduces the overestimation of the stratospheric column. The weekly and seasonal cycles from OMI, Pandora and NO2 surface concentrations are also compared. Both satellite- and ground-based data show a similar weekly cycle, with lower NO2 levels during the weekend compared to the weekdays as a result of reduced emissions from traffic and industrial activities. The seasonal cycle also shows a similar behavior, even though the results are affected by the fact that most of the data are available during spring–summer because of cloud cover in other seasons. This is one of few works in which OMI NO2 retrievals are evaluated in a urban site at high latitudes (60° N). Despite the city of Helsinki having relatively small pollution sources, OMI retrievals have proved to be able to describe air quality features and variability similar to surface observations. This adds confidence in using satellite observations for air quality monitoring also at high latitudes.

NASA's carbon dioxide mission, Orbiting Carbon Observatory-2, has been operating for three full years (2015–2017). Here, we provide a global (60° S–60° N) view of the XCO2 anomalies along with their annual variations and seasonal patterns. We show that the XCO2 anomaly patterns are robust and consistent from year-to-year. We compare these anomalies to fluxes from anthropogenic, biospheric and biomass burning and to model-simulated local concentration enhancements. We find that, despite the simplicity of the method, the anomalies describe the spatio-temporal variability of XCO2 (including anthropogenic emissions and seasonal variability related to vegetation and biomass burning) consistently with more complex model-based approaches. We see, for example, that positive anomalies correspond to fossil fuel combustion over the major industrial areas (e.g., China, eastern USA, central Europe, India, and the Highveld region in South Africa), shown as large positive XCO2 enhancements in the model simulations. Also, we find corresponding positive anomalies and fluxes over biomass burning areas during different fire seasons. On the other hand, the largest negative anomalies correspond to the growing season in the northern middle latitudes, characterized by negative XCO2 enhancements from simulations and high SIF values (indicating the occurrence of photosynthesis). Finally, we show how XCO2 anomalies facilitate the detection of anthropogenic signatures for several local scale case studies, both in the Northern and Southern Hemisphere. The results demonstrate the potential of satellite-based XCO2 observations for understanding the role of man-made and natural contributions to the atmospheric CO2 levels.

This paper shows the results of the comparison of satellite SO2 observations from OMI (Ozone Monitoring Instrument) and OMPS (Ozone Mapping Profiler Suite) with ground-based measurements during the Icelandic Holuhraun fissure eruption in September 2014. The volcanic plume reached Finland on several days during the month of September. The SO2 total columns from the Brewer direct sun (DS) measurements in Sodankylä (67.42° N, 26.59° E), northern Finland, are compared to the satellite data. The operational satellite SO2 products are evaluated for high latitude conditions (e.g. large solar zenith angle, SZA). The results show that the best agreement can be found for lowest SZAs, close-to-nadir satellite pixels, cloud fraction below 0.3 and small distance between the station and the centre of the pixel. Under good retrieval conditions, the difference between satellite data and Brewer measurements remains mostly below the uncertainty on the satellite SO2 retrievals (up to about 2 DU at high latitudes). The satellite products assuming a priori profile with SO2 predominantly in the planetary boundary layer give total column values with the best agreement with the ground-based data. The analysis of the SO2 surface concentrations at four air quality stations in northern Finland shows that the volcanic plume coming from Iceland was located very close to the surface. This is connected to the fact that this was a fissure eruption and most of the SO2 was emitted into the troposphere. This is an exceptional case because the SO2 volcanic emissions directly affect the air quality levels at surface in an otherwise pristine environment like northern Finland. The time evolution of the SO2 concentrations peaks during the same days when large SO2 total column values are measured by the Brewer in Sodankylä and enhanced SO2 signal is visible over northern Finland from the satellite maps. Thus, the satellite retrievals were able to detect the spatiotemporal evolution of the volcanic plume as compared to the surface observations. Furthermore, direct-broadcast SO2 satellite data (from both OMI and OMPS instruments) are compared for the first time against ground-based observations.

This overview paper highlights the successes of the Ozone Monitoring Instrument (OMI) spanning more than 12 years of the OMI data record. Data from OMI has been used in a wide range of applications. Due to its unprecedented spatial resolution, in combination with daily global coverage, OMI plays a unique role in measuring trace gases important for the ozone layer, air quality and climate change, including new research findings using these satellite data. Due to the operational Very Fast Delivery (VFD) (direct readout) and Near Real Time (NRT) availability of the data, OMI also plays an important role in the development of operational services in the atmospheric chemistry domain.

Satellite-based data are very important for air-quality applications in the Baltic Sea region, because they provide information on air pollution over the sea and where ground-based and aircraft measurements are not available. Both the emissions from urban sites over land and ships over sea, contribute to tropospheric NO2 levels. Tropospheric NO2 monitoring at high latitudes using satellite data is challenging because of the reduced light hours in winter and the weak signal due to the low Sun, which make the retrieval complex. This work presents a characterization of tropospheric NO2 columns based on case-study analysis in the Baltic Sea region, using the Ozone Monitoring Instrument (OMI) tropospheric NO2 standard product. Previous works have focused on larger seas and lower latitudes. The results of this paper showed that, despite the regional area of interest, it is possible to distinguish the signal from the main coastal cities and from the ships by averaging the data over a~seasonal time range. The summertime NO2 emission and lifetime values (E' = (1.5 ± 0.6) mol s−1 and τ = (3 ± 1) h, respectively) in Helsinki were estimated from the decay of the signal with distance from the city center. These results agree within the uncertainties with the emissions from the existing database. For comparison, the results for the cities of Saint Petersburg and Stockholm are also shown. The method developed for megacities was successfully applied to smaller-scale sources, in both size and intensity, which are located at high latitudes (~60° N). The same methodology could be applied to similar-scale cities elsewhere, as long as they are relatively isolated from other sources. Transport by the wind plays an important role in the Baltic Sea region. The NO2 spatial distribution is mainly determined by the contribution of westerly winds, which dominate the wind patterns during summer. The comparison between the ship emissions from model calculations and OMI NO2 tropospheric columns supports the applicability of satellite data for ship emission monitoring. In particular, both the ship emission data and the OMI observations showed similar year-to-year variability, with a drop in the year 2009, corresponding to the effect of the financial crisis.

[1] The link between stratospheric ozone decline and ultraviolet (UV) radiation increase at the Earth's surface is well established. In the Northern Hemisphere extratropics, stratospheric ozone is accumulated from autumn to spring as a result of transport from its source region in the tropics. The amount of accumulated ozone varies from year to year due to natural dynamical variability and chemical destruction by natural and anthropogenic substances. Observational and modeling studies show that these total ozone anomalies persist in the extratropics from spring to summer. Here we analyze time series of ground-based UV measurements and satellite retrievals of total ozone and UV radiation and demonstrate that there is a strong link between springtime total ozone and summer UV anomalies in the Northern Hemisphere extratropics. In some regions, the interannual variability in springtime ozone abundance explains 20–40% of the summer UV variability, and this relation can be used for improving seasonal UV forecasts. Using chemistry transport models, we estimate the influence of polar chemical ozone loss on the summer UV north of 35°N. We estimate that the massive Arctic ozone depletion 2011 increased the March–August cumulative erythemal clear-sky UV dose in the Northern Hemisphere extratropics by 3–4% compared to the climatology, with about 75% of the increase accumulated after the breakup of the polar vortex. This result strongly suggests that the effect of seasonal ozone anomaly persistence should be included in the assessment of the impacts of polar ozone losses.

At Sodankyla (67°N), spectra of solar ultraviolet radiation (UVR) have been measured with a Brewer spectroradiometer since 1990. The time series is one of the longest in the European Arctic region. In this work, the time series 1990-2014 was homogenized, and the data were corrected with respect to known error sources using laboratory characterizations and theoretical approaches. Methods for cosine correction, temperature correction and determination of long-term changes in spectral responsivity were applied. Bad measurements were identified by using various quality assurance tools including comparisons with reconstructed UV dose rates, synchronous broadband UV dose rates, global radiation and clear sky model calculations. We calculated daily maximum UV indices from the spectral time series. The daily maxima reached on average a value of 5 in midsummer, whereas the maximum UV index value of 6 was measured only twice: in 2011 and in 2013. We calculated the relative spectral changes in measured UV irradiance...

Anthropogenic CO2 emissions from fossil fuel combustion have large impacts on climate. In order to monitor the increasing CO2 concentrations in the atmosphere, accurate spaceborne observations—as available from the Orbiting Carbon Observatory-2 (OCO-2)—are needed. This work provides the first direct observation of anthropogenic CO2 from OCO-2 over the main pollution regions: eastern USA, central Europe, and East Asia. This is achieved by deseasonalizing and detrending OCO-2 CO2 observations to derive CO2 anomalies. Several small isolated emission areas (such as large cities) are detectable from the anomaly maps. The spatial distribution of the CO2 anomaly matches the features observed in the maps of the Ozone Monitoring Instrument NO2 tropospheric columns, used as an indicator of atmospheric pollution. The results of a cluster analysis confirm the spatial correlation between CO2 and NO2 data over areas with different amounts of pollution. We found positive correlation between CO2 anomalies and emission inventories. The results demonstrate the power of spaceborne data for monitoring anthropogenic CO2 emissions.

Global Ozone Monitoring by Occultation of Stars (GOMOS) on board Envisat has performed about 440 000 night-time occultations during 2002–2012. Self-calibrating measurement principle, good vertical resolution, excellent pointing accuracy and the wide vertical range from the troposphere up to the lower thermosphere make GOMOS profiles interesting for different analyses. The GOMOS ozone data are of high quality in the stratosphere and the mesosphere, but the current operational retrieval algorithm (IPF v.6) is not optimized for retrievals in the upper troposphere–lower stratosphere (UTLS). In particular, validation of GOMOS profiles against ozonesonde data has revealed a substantial positive bias (up to 100 %) in the UTLS region. The retrievals in the UTLS are challenging because of low signal-to-noise ratio and the presence of clouds. In this work, we discuss the reasons for the systematic uncertainties in the UTLS with the IPF v.6 algorithm or its modifications based on simultaneous retrievals of several constituents using the full visible wavelength range. The main reason is high sensitivity of the UTLS retrieval algorithms to an assumed aerosol extinction model. We have developed a new aerosol–insensitive ozone profile inversion algorithm for GOMOS data in the UTLS using a DOAS-type method at visible wavelengths. The method uses minimal assumptions about the atmospheric profiles. The ozone retrievals in the whole altitude range from the troposphere to the lower thermosphere are performed in two steps, as in the operational algorithm: spectral inversion followed by the vertical inversion. The horizontal column ozone densities retrieved in the spectral inversion follow V6 profiles in the middle atmosphere and follow the triplet ozone profiles in the UTLS. The vertical inversion is performed as in IPF v6 with the Tikhonov-type regularization according to the target resolution. The validation of new retrieved ozone profiles with ozonesondes show dramatic reduction of GOMOS ozone biases in the UTLS. The new GOMOS ozone profiles are also in a very good agreement with measurements by MIPAS, ACE-FTS and OSIRIS satellite instruments in the UTLS. It is also shown that the known geophysical phenomena in the UTLS ozone are well reproduced with the new GOMOS data.

We present the comparison of satellite-based OMI (Ozone Monitoring Instrument) NO2 products with ground-based observations in Helsinki. OMI NO2 total columns, available from standard product (SP) and DOMINO algorithm, are compared with the measurements performed by the Pandora spectrometer in Helsinki in 2012. The relative difference between Pandora #21 and OMI SP retrievals is 4 % and −6 % for clear sky and all sky conditions, respectively. DOMINO NO2 retrievals showed slightly lower total columns with median differences about −5 % and −14 % for clear sky and all sky conditions, respectively. Large differences often correspond to cloudy autumn-winter days with solar zenith angles above 65°. Nevertheless, the differences remain within the retrieval uncertainties. Furthermore, the weekly and seasonal cycles from OMI, Pandora and NO2 surface concentrations are compared. Both satellite- and ground-based data show a similar weekly cycle, with lower NO2 levels during the weekend compared to the weekdays as result of reduced emissions from traffic and industrial activities. Also the seasonal cycle shows a similar behaviour, even though the results are affected by the fact that most of the data are available during spring-summer because of cloud cover in other seasons. This is one of few works in which OMI NO2 retrievals are evaluated in a urban site at high latitudes (60° N). Despite the city of Helsinki having relatively small pollution sources, OMI retrievals have proved to be able to describe air quality features and variability similar to surface observations. This adds confidence in using satellite observations for air quality monitoring also at high latitudes.

A large zonal asymmetry of ozone has been observed over Antarctica during winter-spring, when the ozone hole develops. It is caused by a planetary wave-driven displacement of the polar vortex. The total ozone data by OMI (Ozone Monitoring Instrument) and the ozone profiles by MLS (Microwave Limb Sounder) and GOMOS (Global Ozone Monitoring by Occultation of Stars) were analysed to characterize the ozone zonal asymmetry and the wave activity during Antarctic spring. Both total ozone and profile data have shown a persistent zonal asymmetry over the last years, which is usually observed from September to mid-December. The largest amplitudes of planetary waves at 65° S (the perturbations can achieve up to 50% of zonal mean values) is observed in October. The wave activity is dominated by the quasi-stationary wave 1 component, while the wave 2 is mainly an eastward travelling wave. Wave numbers 1 and 2 generally explain more than the 90% of the ozone longitudinal variations. Both GOMOS and MLS ozone profile data show that ozone zonal asymmetry covers the whole stratosphere and extends up to the altitudes of 60–65 km. The wave amplitudes in ozone mixing ratio decay with altitude, with maxima (up to 50%) below 30 km. The characterization of the ozone zonal asymmetry has become important in the climate research. The inclusion of the polar zonal asymmetry in the climate models is essential for an accurate estimation of the future temperature trends. This information might also be important for retrieval algorithms that rely on ozone a priori information.

In the summer 2010 extensive wildfires in the western parts of Russia emitted massive amounts of smoke and aerosols into the atmosphere. These smoke plumes also drifted to Finland over 1000 km away from the fires. The smoke plumes were detected in Kuopio (Eastern Finland) with a wide range of instruments on two specific days: July 29 and August 8. The plumes were studied with several spaceborne instruments: MODIS, OMI, AIRS and CALIOP. Furthermore, a ground-based remote sensing instrument (Cimel) was also used in the analysis. Our results show that ground-based and spaceborne instruments were in good agreement on the Aerosol optical depth (AOD 550 ) values during the episode (July–mid August). The correlation coefficient between MODIS and AERONET measurements done in Kuopio was 0.98 and the mean difference was 0.005 (AERONET AOD being larger). Spaceborne measurements of carbon monoxide provided a clear indicator for biomass burning aerosols. Particle mass (PM 2.5 ) and AERONET AOD 550 measurements were also in good agreement with a correlation coefficient of 0.87. Single scattering albedo (SSA 675 ) values derived from AERONET measurements and calculated from in-situ absorption and scattering measurements had similar values close 0.9. On average, the daily in-situ SSA values were 0.02 smaller than the corresponding AERONET values. CALIOP provided vertical profiles of the smoke plumes for the two most polluted days in Kuopio. In July the plume was located close to the surface (mainly below 2 km) while in August the plume had two elevated layers: one between 1 and 2 km and the other between 2.5 and 4 km.

The amount of surface UV radiation increased during the last 30 years because of the effect of the ozone decrease (from 1979 to 1998) combined with the cloud-aerosol reflectivity changes. Long term changes in solar UV radiation affect the global biogeochemistry, climate and their interactions. During the last decades, satellite UV estimates became available for monitoring the UV level at surface from TOMS (Total Ozone Monitoring System), OMI (Ozone Monitoring Instrument) and GOME-2 (Global Ozone Monitoring Experiment-2) instruments. The combined TOMS-OMI dataset of erythemal UV products was applied for the first time to estimate the long-term UV changes at the global scale. The analysis of the uncertainty related to the different input information, is presented. OMI and GOME-2 products were compared in order to analyse the differences in the global UV distribution and their effect on the linear trend estimation. The results showed that the differences in the inputs (mainly surface albedo and aerosol information) used in the retrieval affect significantly the UV change calculation, pointing out the importance of using a consistent dataset when calculating the long term UV changes. The areas where these differences played a major role were located using global maps of monthly UV changes. Despite the uncertainties, significant positive UV changes (ranging from 0 to 6 %/decade) were observed, with higher values on the Southern Hemisphere at mid-latitudes during spring-summer, where the largest ozone decrease was observed.

This paper shows the validation results of the satellite SO2 observations from OMI (Ozone Monitoring Instrument) and OMPS (Ozone Mapping Profiler Suite) during the Icelandic Holuhraun fissure eruption in September 2014. The volcanic plume reached Finland on several days during the month of September. The SO2 total columns from the Brewer direct sun (DS) measurements in Sodankylä (67.42° N, 26.59° E), northern Finland, are compared to the satellite data. Challenging retrieval conditions at high latitudes (like large solar zenith angle, SZA) are considered in the comparison. The results show that the best agreement can be found for small SZAs, close-to-nadir satellite pixels, cloud fraction below 0.3 and small distance between the station and the centre of the pixel. Under good retrieval conditions, the difference between satellite data and Brewer measurements remains mostly below the uncertainty on the satellite SO2 retrievals (up to about 2 DU at high latitudes). The satellite products assuming a priori profile with SO2 predominantly in the planetary boundary layer give total column values close to the ground-based data, suggesting that the volcanic SO2 plume was located at particularly low altitudes. This is connected to the fact that this was a fissure eruption and most of the SO2 was emitted into the troposphere. The analysis of the SO2 surface concentrations at four air quality stations in northern Finland supports the hypothesis that the volcanic plume coming from Iceland was located very close to the surface. The time evolution of the SO2 concentrations peaks during the same days when large SO2 total column values are measured by the Brewer in Sodankylä and enhanced SO2 signal is visible over northern Finland from the satellite maps. This is an exceptional case because the SO2 volcanic emission directly affect the air quality levels at surface in an otherwise pristine environment like northern Finland. OMI and OMPS SO2 retrievals from direct-broadcast measurements are validated for the first time in this paper.

The 2014–2015 Bárðarbunga-Veiðivötn fissure eruption at Holuhraun produced about 1.5 km3 of lava, making it the largest eruption in Iceland in more than 200 years. Over the course of the eruption, daily volcanic sulfur dioxide (SO2) emissions exceeded daily SO2 emissions from all anthropogenic sources in Europe in 2010 by at least a factor of 3. We present surface air quality observations from across Northern Europe together with satellite remote sensing data and model simulations of volcanic SO2 for September 2014. We show that volcanic SO2 was transported in the lowermost troposphere over long distances and detected by air quality monitoring stations up to 2750 km away from the source. Using retrievals from the Ozone Monitoring Instrument (OMI) and the Infrared Atmospheric Sounding Interferometer (IASI), we calculate an average daily SO2 mass burden of 99 ± 49 kilotons (kt) of SO2 from OMI and 61 ± 18 kt of SO2 from IASI for September 2014. This volcanic burden is at least a factor of 2 greater than the average SO2 mass burden between 2007 and 2009 due to anthropogenic emissions from the whole of Europe. Combining the observational data with model simulations using the United Kingdom Met Office's Numerical Atmospheric-dispersion Modelling Environment model, we are able to constrain SO2 emission rates to up to 120 kilotons per day (kt/d) during early September 2014, followed by a decrease to 20–60 kt/d between 6 and 22 September 2014, followed by a renewed increase to 60–120 kt/d until the end of September 2014. Based on these fluxes, we estimate that the eruption emitted a total of 2.0 ± 0.6 Tg of SO2 during September 2014, in good agreement with ground-based remote sensing and petrological estimates. Although satellite-derived and model-simulated vertical column densities of SO2 agree well, the model simulations are biased low by up to a factor of 8 when compared to surface observations of volcanic SO2 on 6–7 September 2014 in Ireland. These biases are mainly due to relatively small horizontal and vertical positional errors in the simulations of the volcanic plume occurring over transport distances of thousands of kilometers. Although the volcanic air pollution episodes were transient and lava-dominated volcanic eruptions are sporadic events, the observations suggest that (i) during an eruption, volcanic SO2 measurements should be assimilated for near real-time air quality forecasting and (ii) existing air quality monitoring networks should be retained or extended to monitor SO2 and other volcanic pollutants., School of Earth and Environment (University of Leeds) UK Natural Environment Research Council (NERC) NE/I015612/1 Met Office Academic Partnership NERC NE/M021130/1 COMET

The aerosol Single Scattering Albedo (SSA) and Absorbing Aerosol Optical Depth (AAOD) at 320.1 nm are derived at Rome site by the comparison between Brewer and modelled spectra. The UVSPEC radiative transfer model is used to calculate the UV irradiances for different SSA values, taking into account as input data total ozone and Aerosol Optical Depth (AOD) obtained from Brewer spectral measurements. The accuracy in determining SSA depends on the aerosol amount and on Solar Zenith Angle (SZA) value: SSA uncertainty increases when AOD and SZA decrease. The monthly mean values of SSA and AAOD during the period January 2005–June 2008 are analysed, showing a monthly and seasonal variability. It is found that the SSA and AAOD averages are 0.80±0.08 and 0.056±0.028, respectively. AAOD retrievals are also used to quantify the error in the Ozone Monitoring Instrument (OMI) surface UV products due to absorbing aerosols, not included in the current OMI UV algorithm. OMI and Brewer UV irradiances at 324.1 nm and Erythemal Dose Rates (EDRs) under clear sky conditions, are compared as a function of AAOD. Three methods are considered to investigate on the applicability of an absorbing aerosol correction on OMI UV data at Rome site. Depending on the correction methodology, the bias value decreases from 18% to 2% for spectral irradiance at 324.1 nm and from 25% to 8% for EDR.

[1] Several validation studies of surface UV irradiance based on the Ozone Monitoring Instrument (OMI) satellite data have shown a high correlation with ground-based measurements but a positive bias in many locations. The main part of the bias can be attributed to the boundary layer aerosol absorption that is not accounted for in the current satellite UV algorithms. To correct for this shortfall, a post-correction procedure was applied, based on global climatological fields of aerosol absorption optical depth. These fields were obtained by using global aerosol optical depth and aerosol single scattering albedo data assembled by combining global aerosol model data and ground-based aerosol measurements from AERONET. The resulting improvements in the satellite-based surface UV irradiance were evaluated by comparing satellite and ground-based spectral irradiances at various European UV monitoring sites. The results generally showed a significantly reduced bias by 5–20%, a lower variability, and an unchanged, high correlation coefficient.