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Atmospheric chemistry and physics 23(13), 7799 - 7822 (2023). doi:10.5194/acp-23-7799-2023, Published by EGU, Katlenburg-Lindau

Every polar spring, phenomena called bromine explosions occur over sea ice. These bromine explosions comprise photochemical heterogeneous chain reactions that release bromine molecules, Br2, to the troposphere and lead to tropospheric plumes of bromine monoxide, BrO. This autocatalytic mechanism depletes ozone, O3, in the boundary layer and troposphere and thereby changes the oxidizing capacity of the atmosphere. The phenomenon also leads to accelerated deposition of metals (e.g., Hg). In this study, we present a 22-year (1996 to 2017) consolidated and consistent tropospheric BrO dataset north of 70∘ N, derived from four different ultraviolet–visible (UV–VIS) satellite instruments (GOME, SCIAMACHY, GOME-2A and GOME-2B). The retrieval data products from the different sensors are compared during periods of overlap and show good agreement (correlations of 0.82–0.98 between the sensors). From our merged time series of tropospheric BrO vertical column densities (VCDs), we infer changes in the bromine explosions and thus an increase in the extent and magnitude of tropospheric BrO plumes during the period of Arctic warming. We determined an increasing trend of about 1.5 % of the tropospheric BrO VCDs per year during polar springs, while the size of the areas where enhanced tropospheric BrO VCDs can be found has increased about 896 km2 yr−1. We infer from comparisons and correlations with sea ice age data that the reported changes in the extent and magnitude of tropospheric BrO VCDs are moderately related to the increase in first-year ice extent in the Arctic north of 70∘ N, both temporally and spatially, with a correlation coefficient of 0.32. However, the BrO plumes and thus bromine explosions show significant variability, which also depends, apart from sea ice, on meteorological conditions.

Hydroperoxyl (HO2) and organic peroxy (RO2) radicals have an unpaired spin and are highly reactive free radicals. Measurements of the sum of HO2 and RO2 provide unique information about the chemical processing in an air mass. This paper describes the experimental features and capabilities of the Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). This is an instrument designed to make measurements on aircraft from the boundary layer to the lower stratosphere. PeRCEAS combines the amplified conversion of peroxy radicals to nitrogen dioxide (NO2) with the sensitive detection of NO2 using cavity ring-down spectroscopy (CRDS) at 408 nm. PeRCEAS is a dual-channel instrument, with two identical reactor–detector lines working out of phase with one another at a constant and defined pressure lower than ambient at the aircraft altitude. The suitability of PeRCEAS for airborne measurements in the free troposphere was evaluated by extensive characterisation and calibration under atmospherically representative conditions in the laboratory. The use of alternating modes of the two instrumental channels successfully captures short-term variations in the sum of peroxy radicals, defined as RO2∗ (RO2∗=HO2+∑RO2+OH+∑RO, with R being an organic chain) in ambient air. For a 60 s measurement, the RO2∗ detection limit is 2σ) NO2 detectable mixing ratio

In this study we report on airborne imaging DOAS measurements of NO2 from two flights performed in Bucharest during the AROMAT campaign (Airborne ROmanian Measurements of Aerosols and Trace gases) in September 2014. These measurements were performed with the Airborne imaging Differential Optical Absorption Spectroscopy (DOAS) instrument for Measurements of Atmospheric Pollution (AirMAP) and provide nearly gapless maps of column densities of NO2 below the aircraft with a high spatial resolution of better than 100 m. The air mass factors, which are needed to convert the measured differential slant column densities (dSCDs) to vertical column densities (VCDs), have a strong dependence on the surface reflectance, which has to be accounted for in the retrieval. This is especially important for measurements above urban areas, where the surface properties vary strongly. As the instrument is not radiometrically calibrated, we have developed a method to derive the surface reflectance from intensities measured by AirMAP. This method is based on radiative transfer calculation with SCIATRAN and a reference area for which the surface reflectance is known. While surface properties are clearly apparent in the NO2 dSCD results, this effect is successfully corrected for in the VCD results. Furthermore, we investigate the influence of aerosols on the retrieval for a variety of aerosol profiles that were measured in the context of the AROMAT campaigns. The results of two research flights are presented, which reveal distinct horizontal distribution patterns and strong spatial gradients of NO2 across the city. Pollution levels range from background values in the outskirts located upwind of the city to about 4 × 1016 molec cm−2 in the polluted city center. Validation against two co-located mobile car-DOAS measurements yields good agreement between the datasets, with correlation coefficients of R = 0.94 and R = 0.85, respectively. Estimations on the NOx emission rate of Bucharest for the two flights yield emission rates of 15.1 ± 9.4 and 13.6 ± 8.4 mol s−1, respectively.

SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) on-board the ESA Envisat satellite platform provided daily solar spectra in the wavelength region from 0.24 μm to 2.4 μm. The instrument was operating for nearly a decade from August 2002 to April 2012. This document reports on the validation of the SCIAMACHY solar spectral irradiances (SSI) in its data version 8. Like other UV satellite sounders, the SCIAMACHY spectrometer suffers from optical degradation due to polymerisation of optical surfaces from the harsh UV radiation in space (e.g. Bramstedt et al., 2009; DeLand et al., 2012; Krijger et al., 2014). In data version 8 of SCIAMACHY a physical model of the scanner unit was implemented to provide a degradation correction (Bramstedt et al., 2009; Krijger et al., 2014). Comparisons with several other established solar reference spectra show agreement to within 4 % in most parts of the visible and near infrared from 350 to 1400 nm.

Presentación realizada en: 14th WMO-GAW Brewers Users Group Meeting celebrada en Tenerife del 24 al 28 de marzo de 2014.

Hydroperoxyl (HO2) and organic peroxy (RO2, where R stands for any organic group) radicals are highly reactive molecules produced in the oxidation of many compounds in the troposphere. They participate in the catalytic cycle producing or destroying ozone (O3) in the troposphere. Thus, HO2 and RO2 measurements provide unique information about the chemical processing of an air mass. Over the last decades, the understanding of the role of HO2 and RO2 in the chemical processes in the planetary boundary layer (PBL) has improved through ground-based in-situ measurements. However, the number of unequivocal measurements of peroxy radicals in the free troposphere is still quite limited. Measurements from airborne platforms offer a unique opportunity to measure HO2 and RO2 together with other relevant trace gases to test and improve the understanding of their chemistry in the free troposphere. During this doctoral study, an extensive set of airborne RO2* (RO2* = HO2 + ∑RO2, where RO2 represents the organic peroxy radicals reacting with NO to produce NO2) measurements in the PBL and free troposphere was acquired, analysed and interpreted. The RO2* measurements were made using the Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS) developed at the Institut für Umweltphysik (IUP) of the University of Bremen. PeRCEAS has successfully deployed onboard the High Altitude LOng range research aircraft (HALO) in three research campaigns: the Oxidation Mechanism Observations (OMO) Asia and the Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales (EMeRGe) field missions in Europe and Asia. The PeRCEAS instrument was characterised and calibrated under atmospherically representative conditions in the laboratory to assure data quality, reproducibility, accuracy and to define optimal operating conditions for the airborne measurements. PeRCEAS successfully measured RO2* in 33 HALO flights. RO2* mixing ratios of up to 120 pmole mole-1 were measured in air masses having different origins, chemical compositions and physical conditions in Europe and Asia. The RO2* measurements, the simultaneous measurements of other relevant trace gases, aerosol concentration, photolysis frequencies and other meteorological parameters were synergistically analysed to identify the chemical processes controlling the amount of RO2*. From the analysis, it was found that RO2* is primarily produced following the photolysis of ozone (O3), formaldehyde (HCHO), glyoxal (CHOCHO), and nitrous acid (HONO) in the air masses investigated. The estimate for the contribution of O3 photolysis to RO2* production rate is > 40 % in the PBL and < 40% in the free troposphere. This reduction is explained by the decrease in the water vapour concentration ([H2O]) as a function of altitude. Subsequently, the RO2* mixing ratios in the air masses measured during the EMeRGe in Asia and Europe campaigns were calculated assuming a photostationary steady-state (PSS) for RO2*. The RO2* production from precursor photolysis, the loss through HO2 – HO2, RO2 – RO2 and HO2 – RO2 reactions, the hydroxyl radical (OH) and organic oxy-radicals (RO) loss during the radical interconversion, and HO2 uptake on aerosol were considered for the calculation of RO2*. The calculations were constrained by the simultaneous measurements of photolysis frequencies, trace gas concentrations and aerosol particle number concentrations onboard HALO. Case studies confirmed the validity of the PSS assumption for air masses having different chemical compositions under different physical conditions. The RO2* calculated are generally in excellent agreement with the RO2* measurements. An experimental budget analysis was performed to estimate the main loss processes of RO2* by introducing the RO2* measurements in the PSS equation. Except for the measurements inside pollution plumes with NO > 800 pmole mole-1 or aerosol particle number concentration > 800 particles cm-3, the HO2 – RO2 and HO2 – HO2 were the dominant RO2* loss process during both EMeRGe Asia and Europe. The RO2* losses through HO2 uptake on aerosol were higher in the pollution outflows measured in Asia than in Europe. This is attributed to the higher aerosol concentrations observed in the air masses probed during EMeRGe in Asia. The contribution from the HO2 uptake on aerosol increases up to 60 % for an assumed aerosol uptake coefficient of 0.24 inside pollution plumes in Asia, where the aerosol particle number concentration is > 1000 particles cm-3. In Europe, the OH – NOx reactions were the dominant RO2* loss process in the pollution outflow. This finding is explained by the EMeRGe in Europe measurements being typically closer to anthropogenic emissions sources than in Asia, except for the case study of Taipei and Manila.

The lack knowledge of aerosol optical properties is one of the sources of uncertainty in assessment and projections of the evolution of climate change and the phenomenon of Arctic Amplification. The spatial and temporal change of microphysical, chemical and optical properties of aerosols in the Arctic and the induced effects through direct and indirect radiative forcing of aerosols remain an open question. The cause of this gap in our understanding and therefore in the global aerosol optical thickness (AOT) maps is associated with the difficulty of retrieving aerosol properties over bright surfaces covered with snow and ice. Decoupling a strong surface signal from that of aerosol in the measured top-of-atmosphere reflectance is challenging and still hampered due to remaining unresolved issues in state-of-the-art algorithms. Despite the promising performance of previously-developed methods and ongoing research, there is no published long-term AOT product over polar regions (over land and ocean) to be used for climate studies. In this work, to extend our knowledge about the open issues and improve the existing algorithms, first we focus on the two major obstacles in the retrieval of AOT over snow/ice surfaces: i) cloud identification, and ii) surface properties; Second, we apply the outcome of studying the two mentioned prerequisites to improve the previously-developed aerosol retrieval algorithm called AEROSNOW and create a long-term data record for aerosol optical thickness over the Arctic circle. In the framework of this work, a new cloud identification algorithm called the AATSR/SLSTR Cloud Identification Algorithm (ASCIA) has been developed to screen cloudy scenes in observations of Advanced Along-Track Scanning Radiometer (AATSR) on-board ENVISAT as well as its successor Sea and Land Surface Temperature Radiometer (SLSTR) on-board Sentinel-3. The cloud detection results are verified by comparing them with available cloud products over the Arctic. Furthermore, the cloud product from ASCIA is validated using the ground-based measurements SYNOP, resulting in a promising agreement. In general, ASCIA shows an improved performance in comparison with other algorithms applied to AATSR measurements over snow/ice. For the study of snow surface properties, the reflectance is simulated in a snow–atmosphere system, using the SCIATRAN radiative transfer model, and the results are compared with those of airborne measurements. A sensitivity study is conducted to highlight the importance of having a priori knowledge about snow morphology (size and shape) and atmospheric parameters to minimise the difference between simulated and real world reflectance. The absolute difference between the modelled results and measurements in off-glint regions is generally small and promising. In the final step, we apply the outcome of previous steps in the AEROSNOW algorithm as far as possible within the scope of this work and retrieve AOT over the Arctic circle for the 2002-2012 period with the spatial resolution of 1 km2. The retrieved AOT is validated using ground-based measurements AErosol RObotic NETwork (AERONET). The results of validation are promising and show the successful performance of the algorithm especially during haze episodes. However, in some cases large differences exist between the retrieved AOT and AERONET measurements for which more statistical and physical analysis are necessary to better understand the cause. Nevertheless, the long-term data record and validation produced hold significant value as are the first attempt to better understand the role of aerosols in the Arctic Amplification over land and ocean on the full Arctic scale.

Bromine plays a crucial role in polar atmospheric composition. During springtime, photochemistry converts bromine compounds originating from ice and snow into gaseous reactive bromine radicals (Br), which deplete ozone in the boundary layer, forming bromine oxides (BrO). Due to the autocatalytic nature of the reaction mechanism, it has been called bromine explosion. The strong relationship of bromine photochemistry and release from its sources to ozone depletion events (ODEs) was discovered in the late 1980s. Since then, and because of the importance of tropospheric ozone (the primary source of hydroxyl radical (OH), the major oxidizing agent of the atmosphere), many studies focused on the mechanisms which release bromine into the troposphere and the driving parameters which enhance BrO production and therefore ozone depletion. Arctic Amplification (AA) is the phenomenon that surface temperature in high latitudes increases more rapidly than at lower latitudes. One of the most profound consequences of AA is the significant changes in sea ice conditions. Sea ice extent, age and thickness are drastically changing. Inevitably, all aspects of the Arctic ecosystem are expected to be affected by Arctic Amplification. Remote sensing from satellites can be extremely useful for studying the Arctic region. By the end of the 1970s, sea ice concentration was successfully monitored by satellite sensors. Since 1995, we also have the ability to study atmospheric composition worldwide with data retrieved from nadir radiance spectra from a series of European satellite sensors: GOME on ERS-2 (1995 - 2003), SCIAMACHY on ENVISAT (2002 - 2012), GOME-2A on MetOpA (2007 - today) and GOME-2B on MetOpB (2012 - today). The focus of this thesis is twofold: firstly, to create the first consolidated and consistent long-term (1996 to 2017) tropospheric BrO dataset for the Arctic region and the Hudson Bay (a well-known hotspot for bromine explosion events), retrieved from the four ultraviolet-visible (UV-VIS) sensors mentioned above, in order to assess the changes and the impact that AA has on tropospheric BrO. Since the different satellite instruments have different instrumental attributes and the fact that BrO is a weak absorber in the UV spectral region, many sensitivity retrieval tests have been performed in order to identify the proper fitting settings for each instrument and to derive a high-quality BrO dataset with high consistency between the instruments. Vertical column densities (VCD) of tropospheric BrO are extracted from total (geometric) VCDs, using a climatology of stratospheric BrO VCDs from a chemical transport model. The BrO time-series (geometric and tropospheric VCDs) show remarkable agreement during the overlapping periods between the sensors (the best agreement being between SCIAMACHY and GOME-2A with a correlation coefficient squared of 0.97). Additionally, the agreement is verified by studying daily and monthly maps of geometric and tropospheric BrO VCD. This agreement allows us to create a merged tropospheric BrO VCD dataset, the basis for deriving geophysical conclusions on the impact of AA on the Arctic BrO atmospheric composition. By studying the trends of tropospheric BrO VCDs, we infer an increase of around 1% per year. The increase is significant during polar spring, reaching 1.5% per year. A similar increase can be observed for the Hudson Bay, where tropospheric BrO VCD have increased around 0.9% per year for the spring period and 2.3% per year for the winter period. However, the increasing trend is not monotonic, and variability on the tropospheric BrO VCD appear (e.g. 2016 and 2017 are lower than 2015 for the Arctic region), as many parameters influence bromine release and subsequent BrO formation. Secondly, the link of observed tropospheric BrO VCDs to possible bromine sources and favoring weather conditions was investigated. The primary source of inorganic bromine release in the Arctic atmosphere is sea ice, especially first year ice, which is rich in sea salts. Therefore, a long-term sea ice age dataset from NSIDC was compared to the tropospheric BrO VCD dataset. The increase of first year ice extent due to the changes in the Arctic climate is in general agreement with the observed increase in tropospheric BrO, with a moderate daily correlation coefficient of +0.32, but a strong yearly one of +0.62. Also, the increase of the occurrences of first year ice over some regions in the Arctic (i.e. northeast of Greenland) is correlated with the increase of tropospheric BrO VCD in these regions. Apart from the bromine sources, driving mechanisms, like air temperature, wind speed, boundary layer height, and cyclone activity, can contribute to the Amplification, transport, vertical uplifting and recycling of tropospheric BrO plumes. Consequently, similar comparisons have been performed between tropospheric BrO and meteorological data. We infer that the parameter with the most substantial influence on the formation of bromine explosion events is air temperature, with a correlation coefficient of -0.54 (anti-correlation) for the Arctic during spring and -0.78 for Hudson Bay spring. Furthermore, the spatial agreement and correlation between trend maps of tropospheric BrO VCD and air temperature verify the anti-correlation between the two quantities. However, the bromine release and the formation, transport, and re-cycling of BrO plumes are complex and dynamic phenomena. They depend on many geophysical parameters, and there are also complex relationships between these parameters. Therefore, single individual linear comparisons between tropospheric BrO VCDs and key parameters of BrO formation cannot fully represent the actual relationship between them. This leads us to the effort of employing, for the first time, an artificial intelligence neural network in order to model and predict Arctic tropospheric BrO VCDs using the parameters contributing to tropospheric BrO formation. By training the neural network only with one year of data, it can reproduce accurately (both spatially and in magnitude) many BrO plumes which occurred in other years. This ability of the neural network to efficiently model some bromine explosion events allows us to distinguish them between those occurring at the surface and those at higher altitudes. From studying the effect of each of the individual key parameters on the magnitude of modeled tropospheric BrO VCD, we conclude that air temperature and mean sea level pressure (which can describe the boundary conditions under which bromine is released) have the highest impact.

Global warming is already impacting the modern ocean. To recognize changes and predict the future ocean, ocean color (OC) remote sensing aims at quantifying and monitoring optically active oceanic constituents such as phytoplankton, as well as the underwater light field, on synoptic scales. The underwater light field is typically characterized by the diffuse attenuation coefficient (Kd), which describes how fast the downwelling light diminishes with depth. Phytoplankton diversity is typically assessed by grouping phytoplankton with respect to their function in biogeochemical cycles, which are described as phytoplankton functional types (PFT). Typically, OC sensors record radiometric measurements at several bands, 10-20 nm wide, in the visible and near-infrared. However, direct information of OC variables in the ultraviolet (UV) spectral range is still widely lacking. Also, the coarse spectral resolution of traditional OC sensors hampers the retrieval of information on biodiversity, e.g., represented through PFT, and ecosystems. While OC sensors have weaknesses in spectral resolution, a suite of atmospheric satellite sensors measures radiation at high spectral resolution (about 0.5 nm) and in spectral ranges (UV) not provided by typical OC sensors. These sensors are designed for the retrieval of atmospheric trace gases using Differential Optical Absorption Spectroscopy (DOAS) and have been utilized for OC retrievals by an adaptation of this method to the ocean domain. PFT, light attenuation and availability, and chlorophyll-a fluorescence have been successfully derived from radiances recorded by the atmospheric sensor SCIAMACHY between 2002 and 2012. So far, only little experience has been gathered with other SCIAMACHY-like sensors, limited to fluorescence retrievals at red wavelengths. This thesis focuses on retrievals of OC variables in the UV to green spectral range from multiple atmospheric sensors, namely SCIAMACHY, GOME-2, OMI, and TROPOMI. By comparison of OC retrievals from multiple sensors, insight can be gained on retrieval robustness and sources of uncertainty. Merging of data sets from multiple sensors is desirable for creating long time series necessary for observing climate change impacts. Three hypotheses are investigated: (1) OC variables in the blue-green spectral range, such as PFT and the diffuse attenuation coefficient, can be successfully retrieved from multiple SCIAMACHY-like sensors; (2) OC variables from multiple SCIAMACHY-like sensors can be merged to create long time series; and (3) UV-visible bands of SCIAMACHY-like sensors can be exploited for the retrieval of novel OC products in the UV. The thesis comprises three studies. Two studies concentrate on the diffuse attenuation coefficient with the second study focusing on novel Kd products in the UV. The third study utilizes knowledge from the first two studies to retrieve two key PFT, diatoms and coccolithophores, in the Southern Ocean. Based on experience gathered with the SCIAMACHY sensor, the retrieval of OC variables was adapted to the other atmospheric sensors and further improved to provide complementary and novel OC information. The mean Kd in the blue spectral range, 390 to around 425 nm, was successfully derived from measurements taken by GOME-2, OMI, TROPOMI (as well as SCIAMACHY). Coccolithophores were successfully retrieved from GOME-2 data in the Southern Ocean, however, diatom retrievals were found to be more challenging and resulting data quality was deemed insufficient. Intersensor biases were found between the derived Kd data sets as well as for the derived PFT data in comparison to the established SCIAMACHY PFT time series prior to this work. Complex instrument-specific corrections would be necessary to remove these biases. Biases were successfully reduced, but unfortunately merging of the multiple data sets is still not possible at this stage. The UV-blue spectral range was exploited for deriving two novel Kd products in the UV, 312.5 to 338.5 nm and 356.5 to 390 nm. Comparison with field measurements and sensitivity analysis of the retrievals showed promising results. Findings from the third study on Southern Ocean PFT support the Great Calcite Belt hypothesis, which suggests that coccolithophores cause a wide band of enhanced water reflectivity around Antarctica, and indicate that total chlorophyll-a concentration is not an adequate variable for predicting coccolithophore abundance. Spectral-based PFT retrievals, as used in this thesis, are found to be more suitable. Future applications of the data sets derived and characterized in this thesis include: the estimation of the oceanic heat budget and the UV dose rate on marine organisms, the investigation of the sources of colored dissolved organic matter and of phytoplankton photoprotective pigments, as well as residence times of particulate inorganic carbon in the upper ocean.

This thesis presents MAX-DOAS measurements of the air pollutants NO2 and SO2 in the marine environment of the North and Baltic Sea and the surrounding coastal areas. As the first part, the results from more than three years of MAX-DOAS measurements on the island Neuwerk in the German Bight are presented. As the second part, a novel application of an "onion peeling" like approach to MAX-DOAS measurements of shipping emissions on Neuwerk was developed, aiming at investigating the strong horizontal inhomogeneities in NO2 over a shipping lane. In the third part, expanding the research area from the German North Sea coast to the entire greater North Sea area and the German part of the Baltic Sea, shipborne MAX-DOAS measurements for air quality assessment have been performed for the first time on a regular basis in this area on routinely conducted survey cruises in cooperation with the German Federal Maritime and Hydrographic Agency (Bundesamt für Seeschifffahrt und Hydrographie, BSH).