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The presence of reactive bromine in polar regions is a widespread phenomenon that plays an important role in the photochemistry of the Arctic and Antarctic lower troposphere, including the destruction of ozone, the disturbance of radical cycles, and the oxidation of gaseous elemental mercury. The chemical mechanisms leading to the heterogeneous release of gaseous bromine compounds from saline surfaces are in principle well understood. There are, however, substantial uncertainties about the contribution of different potential sources to the release of reactive bromine, such as sea ice, brine, aerosols, and the snow surface, as well as about the seasonal and diurnal variation and the vertical distribution of reactive bromine. Here we use continuous long-term measurements of the vertical distribution of bromine monoxide (BrO) and aerosols at the two Antarctic sites Neumayer (NM) and Arrival Heights (AH), covering the periods of 2003–2021 and 2012–2021, respectively, to investigate how chemical and physical parameters affect the abundance of BrO. We find the strongest correlation between BrO and aerosol extinction (R=0.56 for NM and R=0.28 for AH during spring), suggesting that the heterogeneous release of Br2 from saline airborne particles (blowing snow and aerosols) is a dominant source for reactive bromine. Positive correlations between BrO and contact time of air masses, both with sea ice and the Antarctic ice sheet, suggest that reactive bromine is not only emitted by the sea ice surface but by the snowpack on the ice shelf and in the coastal regions of Antarctica. In addition, the open ocean appears to represent a source for reactive bromine during late summer and autumn when the sea ice extent is at its minimum. A source–receptor analysis based on back trajectories and sea ice maps shows that main source regions for BrO at NM are the Weddell Sea and the Filchner–Ronne Ice Shelf, as well as coastal polynyas where sea ice is newly formed. A strong morning peak in BrO frequently occurring during summer and that is particularly strong during autumn suggests a night-time build-up of Br2 by heterogeneous reaction of ozone on the saline snowpack in the vicinity of the measurement sites. We furthermore show that BrO can be sustained for at least 3 d while travelling across the Antarctic continent in the absence of any saline surfaces that could serve as a source for reactive bromine. [ABSTRACT FROM AUTHOR]

Polar stratospheric clouds (PSCs) are an important component of ozone chemistry in polar regions. Studying the ozone-depleting processes requires a precise description of PSCs on a long-term basis. Although satellite observations already yield high spatial coverage, continuous ground-based measurements covering long time periods can be a valuable complement. In this study, differential optical absorption spectroscopy (DOAS) instruments are used to investigate the occurrence of PSCs based on the so-called colour index (CI), i.e. the colour of the zenith sky. Defined as the ratio between the observed intensities of scattered sunlight at two wavelengths, it provides a method to detect PSCs during twilight even in the presence of tropospheric clouds. We present data from instruments at the German research station Neumayer, Antarctica (71 ∘ S, 8 ∘ W), as well as Kiruna, Sweden (68 ∘ N, 20 ∘ E), which have been in operation for more than 20 years. For a comprehensive interpretation of the measurement data, the well-established radiative transfer model McArtim is used and radiances of scattered sunlight are simulated at several wavelengths for different solar zenith angles and various atmospheric conditions. The aim is to improve and evaluate the potential of this method. It is then used to infer the seasonal cycle and the variability of PSC occurrence throughout the time series measured in both hemispheres. A good agreement is found to satellite retrievals with deviations particularly in spring. The unexpectedly high signal observed in the DOAS data during springtime suggests the influence of volcanic aerosol. This is also indicated by enhanced aerosol extinction as seen from OMPS (Ozone Mapping and Profiler Suite) data but is not captured by other PSC climatologies. The presented approach allows the detection of PSCs for various atmospheric conditions not only for individual case studies but over entire time series, which is a decisive advance compared to previous work on the PSC detection by ground-based instruments. Apart from the interannual variability, no significant trend is detected for either measurement station. The averaged PSC relative frequency amounts to about 37 % above the Neumayer station and about 18 % above Kiruna. [ABSTRACT FROM AUTHOR]

Over the last few decades, differential optical absorption spectroscopy (DOAS) has been used as a common technique to simultaneously measure abundances of a variety of atmospheric trace gases. Exploiting the unique differential absorption cross section of trace-gas molecules, mixing ratios can be derived by measuring the optical density along a defined light path and by applying the Beer–Lambert law. Active long-path (LP-DOAS) instruments can detect trace gases along a light path of a few hundred metres up to 20 km , with sensitivities for mixing ratios down to ppbv and pptv levels, depending on the trace-gas species. To achieve high measurement accuracy and low detection limits, it is crucial to reduce instrumental artefacts that lead to systematic structures in the residual spectra of the analysis. Spectral residual structures can be introduced by most components of a LP-DOAS measurement system, namely by the light source, in the transmission of the measurement signal between the system components or at the level of spectrometer and detector. This article focuses on recent improvements by the first application of a new type of light source and consequent changes to the optical setup to improve measurement accuracy. Most state-of-the-art LP-DOAS instruments are based on fibre optics and use xenon arc lamps or light-emitting diodes (LEDs) as light sources. Here we present the application of a laser-driven light source (LDLS), which significantly improves the measurement quality compared to conventional light sources. In addition, the lifetime of LDLS is about an order of magnitude higher than of typical Xe arc lamps. The small and very stable plasma discharge spot of the LDLS allows the application of a modified fibre configuration. This enables a better light coupling with higher light throughput, higher transmission homogeneity, and a better suppression of light from disturbing wavelength regions. Furthermore, the mode-mixing properties of the optical fibre are enhanced by an improved mechanical treatment. The combined effects lead to spectral residual structures in the range of 5-10×10-5 root mean square (rms; in units of optical density). This represents a reduction of detection limits of typical trace-gas species by a factor of 3–4 compared to previous setups. High temporal stability and reduced operational complexity of this new setup allow the operation of low-maintenance, automated LP-DOAS systems, as demonstrated here by more than 2 years of continuous observations in Antarctica. [ABSTRACT FROM AUTHOR]

The role of Reactive Halogen Species (RHS - IO, BrO, ClO, etc.) in the lower polartroposphere has been subject of intense research for over three decades. Elevated BrO mixingratios have been identified as the cause of Ozone Depletion Events (ODEs) and thecoincidental oxidation of gaseous elemental mercury frequently occurring in polar springtimein Arctic and Antarctic. An autocatalytic release mechanism from saline surfaces (sea ice oraerosols), the so-called bromine explosion, has been identified as the likely cause of the rapidincrease of inorganic bromine mixing ratios over large areas. Laboratory studies and recentfield observations suggest however, that other release pathways might exist. The role ofhalogens like iodine and chlorine species remain unclear – for a large part due to a lack ofobservations. From January 2016 until August 2018, we operated an automated Long-Path DifferentialOptical Absorption Spectroscopy (LP-DOAS) instrument at the German research stationNeumayer III (NMIII) in coastal Antarctica. Over the 31 months observation period atemporal coverage of about 60% was reached with interruptions mainly due toblowing snow during strong storms. The instrument’s coverage of the UV and visiblespectral range allows the detection of a wide range of trace gases including ClO,BrO, OClO, IO, O3 and NO2 at temporal resolutions of 5-30 minutes. Co-locatedinstruments (e.g. MAX-DOAS, ozone monitor, CPC, SMPS, nephelometer) and extensivemeteorological observations at NMIII station allow detailed interpretations of the dataset. In addition to the well-known springtime events, our analysis unexpectedly revealshalogen activity throughout large parts of the year, which seems to be modulated by thesemi-annual oscillation over the Southern Ocean and the associated occurrence and strengthof cyclones that transport salty aerosols to snow surfaces on the ice shelf at NMIII.Most of the non-springtime events occur during surface temperature inversions inshallow layers close to the ground and seem to be the result of local chemistryrather than transport. BrO mixing ratios frequently exceed 20 ppt and peak up tounprecedented 110 ppt. Furthermore, ClO mixing ratios up to 90 ppt could be detectedand the influence of inter-halogen reactions on ozone mixing ratios was observed.The source mechanism for reactive chlorine, however, remains unclear. IO most ofthe time was below the detection limit of 1-2 ppt and sporadically reached 3-4ppt. [ABSTRACT FROM AUTHOR]