Observations of reactive bromine, iodine and chlorine species in the Arctic and Antarctic with Differential Optical Absorption Spectroscopy

A unique feature of the polar troposphere is the strong activity of halogen photochemistry, in which reactive halogen species (RHS), such as BrO, IO and ClO, are responsible for ozone depletion, the oxidation of elemental mercury and dimethyl sulphide. The typical abundances, sources, as well as the release and recycling mechanisms of these halogen species are far from being completely understood. Within this work, field observations were undertaken, in the Arctic (at Alert) and Antarctic (around Ross Island and in the Weddell Sea), using remote sensing and in-situ instruments based on Differential Optical Absorption Spectroscopy (DOAS) techniques such as MAX-DOAS, CE-DOAS and LP-DOAS. Chlorine monoxide was directly detected and quantified (up to 60 pptv, 20 pptv typical) in the Antarctic boundary layer for the first time, with IO and BrO being present simultaneously. The diurnal variation, correlating with insolation, point to the source being the oxidation of surface Cl- by OH or O3. In the Arctic, IO was observed for the first time on a regional scale and throughout the sunlit period during several years (2007 - 2014). The IO mixing ratios between 0 - 0.5 pptv are in agreement with an inorganic, oceanic source and resemble those at both Antarctic sites. The Antarctic IO mixing ratios are significantly lower than previous measurements at similar locations. IO was not detected (<1 pptv) originating from the snowpack nor within a penguin colony

The Role of Open Lead Interactions in Atmospheric Ozone Variability Between Arctic Coastal and Inland Sites

Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O3 \u3c 10 nmol mol-1) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions

Observations of bromine monoxide transport in the Arctic sustained on aerosol particles

The return of sunlight in the polar spring leads to the production of reactive halogen species from the surface snowpack, significantly altering the chemical composition of the Arctic near-surface atmosphere and the fate of long-range transported pollutants, including mercury. Recent work has shown the initial production of reactive bromine at the Arctic surface snowpack; however, we have limited knowledge of the vertical extent of this chemistry, as well as the lifetime and possible transport of reactive bromine aloft. Here, we present bromine monoxide (BrO) and aerosol particle measurements obtained during the March 2012 BRomine Ozone Mercury EXperiment (BROMEX) near Utqiaġvik (Barrow), AK. The airborne differential optical absorption spectroscopy (DOAS) measurements provided an unprecedented level of spatial resolution, over 2 orders of magnitude greater than satellite observations and with vertical resolution unable to be achieved by satellite methods, for BrO in the Arctic. This novel method provided quantitative identification of a BrO plume, between 500 m and 1 km aloft, moving at the speed of the air mass. Concurrent aerosol particle measurements suggest that this lofted reactive bromine plume was transported and maintained at elevated levels through heterogeneous reactions on colocated supermicron aerosol particles, independent of surface snowpack bromine chemistry. This chemical transport mechanism explains the large spatial extents often observed for reactive bromine chemistry, which impacts atmospheric composition and pollutant fate across the Arctic region, beyond areas of initial snowpack halogen production. The possibility of BrO enhancements disconnected from the surface potentially contributes to sustaining BrO in the free troposphere and must also be considered in the interpretation of satellite BrO column observations, particularly in the context of the rapidly changing Arctic sea ice and snowpack

Inference of cloud altitude and optical properties from MAX-DOAS measurements

Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) is a widely used technique for the detection of atmospheric trace gases, e.g. NO2, SO2, BrO, HCHO, but also for the oxygen collision complex O4. The atmospheric distribution of the latter is proportional to the square of the molecular oxygen concentration and thus well known. By comparing measured O4 differential slant column densities (dSCDs) from MAX-DOAS measurements with modeled ones, information on aerosol distributions and optical properties, as well as on clouds can be obtained using an algorithm based on optimal estimation. Here the ability of MAX-DOAS observations to detect cloud altitude and cloud optical properties of different cloud covers based on measurements of O4 will be discussed. The analysis uses measurements made by a shipborne instrument on two cruises of the German research vessel Polarstern to the Antarctic Weddell Sea from June to October 2013. During this time a broad range of cloud and aerosol conditions was encountered, in particular persistent low cloud cover with a high optical thickness. Aerosol and particle extinction profiles were retrieved with temporal resolutions of up to 15 minutes. For clouds at altitudes up to 2000 m the results show a very good agreement with co-located measurements of a commercial ceilometer and pictures from a cloud camera. Unless visibility was very poor due to fog, even rapid changes in cloud altitude or cover could be detected by MAX-DOAS. These results indicate that under homogeneous cloud cover an accurate retrieval of trace gas vertical profiles can be possible despite the strong influence of clouds on atmospheric light paths. We will discuss advantages and limitations of cloud detection with MAX-DOAS, implications for the subsequent retrieval of trace gas profiles and the possible use of external (ceilometer) data as a priori information for the profile retrieval algorithm

Cloud detection by inversion of MAX-DOAS measurements

Multi-Axis Differential Optical Absorption Spectroscopy (MAXDOAS)is a widely used technique for the detection of atmospheric trace gases, e.g. NO2, SO2, but also for the oxygen collision complex O4, whose atmospheric distribution is well known. By comparing measured O4 differential slant column densities (dSCDs) with modelled ones, information on aerosol distributions and optical properties can be gained. In combination with a radiative transfer model, an inversion of measured dSCDs allows the retrieval of vertical aerosol extinction profiles and properties. Here the ability of MAX-DOAS observations to detect cloud altitude and cloud optical properties of different cloud covers will be discussed. An accurate retrieval of these parameters is crucial for an interpretation of trace gas dSCDs and a subsequent retrieval of vertical profiles from MAX-DOAS measurements under cloudy conditions. The ability of MAX-DOAS to retrieve cloud layer height and optical properties will be demonstrated with a comparison to co-located measurements of a commercial Ceilometer during several cruises of the German research vessel Polarstern. Advantages, limitations and possible applications of the technique will be discussed

The role of open lead interactions in atmospheric ozone variability between Arctic coastal and inland sites

Abstract Boundary layer atmospheric ozone depletion events (ODEs) are commonly observed across polar sea ice regions following polar sunrise. During March-April 2005 in Alaska, the coastal site of Barrow and inland site of Atqasuk experienced ODEs (O3 < 10 nmol mol-1) concurrently for 31% of the observations, consistent with large spatial scale ozone depletion. However, 7% of the time ODEs were exclusively observed inland at Atqasuk. This phenomenon also occurred during one of nine flights during the BRomine, Ozone, and Mercury EXperiment (BROMEX), when atmospheric vertical profiles at both sites showed near-surface ozone depletion only at Atqasuk on 28 March 2012. Concurrent in-flight BrO measurements made using nadir scanning differential optical absorption spectroscopy (DOAS) showed the differences in ozone vertical profiles at these two sites could not be attributed to differences in locally occurring halogen chemistry. During both studies, backward air mass trajectories showed that the Barrow air masses observed had interacted with open sea ice leads, causing increased vertical mixing and recovery of ozone at Barrow and not Atqasuk, where the air masses only interacted with tundra and consolidated sea ice. These observations suggest that, while it is typical for coastal and inland sites to have similar ozone conditions, open leads may cause heterogeneity in the chemical composition of the springtime Arctic boundary layer over coastal and inland areas adjacent to sea ice regions