ALBERT

Description: Episodic bromine enhancement occurs over first year sea ice zones during spring, with corresponding rapid decreases in surface ozone concentrations. The release of bromine via autocatalytic heterogeneous reactions on sea-salt surfaces, including young sea ice, snow pack and aerosols, is known as a 'bromine explosion'. With the return of sunlight in spring, the rate of bromine photolysis rapidly increases, and the rresulting bromide ions react with ozone to form bromine monoxide (BrO). A frequent stable inversion layer concentrates BrO near the surface and prevents replenishment of ozone from the overlying free troposphere, resulting in rapid ozone depletion.BrO measurements have been made at Arrival Heights using zenith-sky UV- visible differential optical absorption spectroscopy (DOAS) since 1995 and multi-axis (Max-) DOAS since 1998.

Description: An algorithm has been developed to retrieve altitude information at di8erent diurnal stages for trace gasspecies by combining direct-sun and zenith-sky UV-visible di8erential slant column density (DSCD) measurements.DSCDs are derived here using di8erential optical absorption spectroscopy. Combining the complementaryzenith-sky measurements (sensitive to the stratosphere) with direct-sun measurements (sensitive to thetroposphere) allows this vertical distinction. Trace gas species such as BrO and NO2 have vertical pro)leswith strong diurnal dependence. Information about the diurnal variation is simultaneously retrieved with thealtitude distribution of the trace gas. The retrieval is a formal optimal estimation pro)le retrieval, allowing acomplete assessment of information content and errors.

Description: Trends in NO2 derived from a 45 year integrationof a chemistry-climate model (CCM) run have beencompared with ground-based NO2 measurements at Lauder(45 S) and Arrival Heights (78 S). Observed trends in NO2at both sites exceed the modelled trends in N2O, the primarysource gas for stratospheric NO2. This suggests thatthe processes driving the NO2 trend are not solely dictatedby changes in N2O but are coupled to global atmosphericchange, either chemically or dynamically or both. If CCMsare to accurately estimate future changes in ozone, it is importantthat they comprehensively include all processes affectingNOx (NO+NO2) because NOx concentrations are animportant factor affecting ozone concentrations. Comparisonof measured and modelled NO2 trends is a sensitive test ofthe degree to which these processes are incorporated in theCCM used here. At Lauder the 19802000 CCM NO2 trends(4.2% per decade at sunrise, 3.8% per decade at sunset) arelower than the observed trends (6.5% per decade at sunrise,6.0% per decade at sunset) but not significantly different atthe 2 level. Large variability in both the model and measurementdata from Arrival Heights makes trend analysis ofthe data difficult. CCM predictions (20012019) of NO2 atLauder and Arrival Heights show significant reductions in therate of increase of NO2 compared with the previous 20 years(19802000). The model results indicate that the partitioningof oxides of nitrogen changes with time and is influenced byboth chemical forcing and circulation changes.

Description: Spectroscopic measurements of BrO using direct sun and zenith sky viewinggeometries are combined in an optimal estimation retrieval algorithm to obtaintropospheric and stratospheric columns of BrO. Twenty-two twilight periods areinvestigated over Arrival Heights, Antarctica (77.8 S, 166.7 E) during the polar springperiod of 2002. This paper presents the first tropospheric and stratospheric BrO columnretrievals from UV-visible ground-based measurements for a polar location. A directcomparison is made between stratospheric columns retrieved at 80 , 84 , and 88 solarzenith angles (SZA) from the spectroscopic measurements and those calculated by theSLIMCAT three-dimensional chemical transport model. The ground-based column BrOobservations are consistent with a SLIMCAT stratospheric Bry loading of 21.2 parts pertrillion at 20 km. SLIMCAT reproduces the observed sunrise column BrO increase butdoes not match the sunset observations, which display less variation. The significantwarming of the Antarctic polar stratosphere in 2002 led to highly variable stratosphericcolumns being observed. The observed column BrO decreased with the transition fromvortex to extravortex air on 21 September but did not change much following the return ofthe vortex on 12 October. For the tropospheric column, an almost normal distributionconsistent with a background of 0.3 ± 0.3 1013 molecules cm 2 is observed from theground (80 , 84 , and 88 for both sunrise and sunset). A statistically significant bromineexplosion event (at the 2s level) was detected at the end of October with a troposphericcolumn of 1.8 ± 0.1 1013 molecules cm 2. The measured tropospheric columns arecompared with the tropospheric Model of Atmospheric Transport and ChemistryMaxPlanck Institute for Chemistry version model. The tropospheric BrO sunrise columnobservations can only be explained with an additional bromine source other thandecomposition of CH3Br and downward transport of long-lived bromine from thestratosphere. A comparison with the spaceborne Global Ozone Monitoring Experiment(GOME) found the total columns observed from the ground to be 1625% smaller thanthe total columns observed by GOME for SZAs between 80 and 88 .

Description: Spectroscopic measurements of BrO using direct sun and zenith-sky viewinggeometries are combined in an optimal estimation retrieval algorithm to obtaintropospheric and stratospheric columns of BrO. Seventy-two twilight periods areinvestigated over Lauder, New Zealand (45.0 S, 169.7 E), between March 2001 and April2003. A direct comparison between tropospheric and stratospheric columns retrieved at80 , 84 , and 87 solar zenith angles (SZAs) from the spectroscopic measurements andthose calculated by the three-dimensional chemical transport model SLIMCAT showsgood agreement. The stratospheric Bry loading of 21 pptv from the SLIMCAT calculationsis consistent with the ground-based measurements. The seasonal and diurnal variationof the stratospheric BrO columns evident from the ground-based measurement retrievals iswell described by the SLIMCAT model. The tropospheric column retrievals illustratea high variability with a mean value of 0.2 pptv if the troposphere is assumed to be wellmixed. An upper limit of 0.9 pptv is established for the ubiquitous BrO troposphericcolumn at 80 under cloud free conditions.

Description: As noted in the WMO/UNEP Scientific Assessment of Ozone Depletion: 2006, halogenated very short-lived substances (VSLS) contribute to the atmospheric budget of halogens and thereby lead to substantial decreases in ozone and increases in surface UV radiation in the tropics and mid-latitudes. Halogenated VSLS are primarily of natural origin; oceanic emissions constitute the largest source providing 90-95% of the total global flux to the atmosphere. Macro algae in the ocean appear to be an important source of polyhalogenated VSLS. Oxidation of halogenated VSLS in the atmosphere (i.e. photolysis and reactions with OH) produces halogen oxide radicals (e.g. ClO, BrO, IO) which have been suggested as the main component of gas-phase halogens. Countries with long coastlines and little land suitable for forestation are investigating the possibility of industrial scale marine kelp farming as a means of carbon sequestration. This marine analogy of the Kyoto Protocol forest has been thought as a means to contribute to climate change mitigation. Knowledge of how natural emissions of VSLS will respond to both the drivers of climate change (e.g. changes in CO2 and land use) and to the consequences of climate change (e.g. changes in sea surface temperature and wind stress) is very limited. As a result, it is imperative that observational studies are performed to quantify the contributions of these natural VSLS to halogen loading in the troposphere and, subsequently, in the stratosphere. For this, transport and degradation processes of the source gases and product gases need to be studied and quantified. A key question surfacing from the WMO Assessment is to what extent halogenated VSLS contribute to atmospheric Bry and Iy. During a field campaign conducted during the spring of 2009, measurements of BrO and IO were made along the coastline of the South Island of New Zealand using a portable Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) spectrometer with the aim of determining coastal sites where high active halogen release could be observed. The selected sites had high biomass concentration of marine algae that would be exposed by low tides. Local macro algae type, tidal height, sunlight, temperature, and wind speed were recorded and correlated to the resulting data in order to better understand the environmental factors that modulate the emissions of halogen oxides from the marine environment to the troposphere. Results of this multi-disciplinary approach to studying brominated VSLS and their atmospheric implications are presented. As well, the chemical processes taking place and producing these halogen oxides are discussed in a thorough manner. This study contributes to a better understanding of the origin of bromine and iodine in the lowermost atmosphere (i.e. marine boundary layer). Particularly, the role that natural emissions of halogenated VSLS from the ocean may play in the halogen budget of the lower atmosphere is addressed by quantitatively understanding key links in this chain so that its potential future impacts on atmospheric chemistry, surface UV radiation, and the biosphere can be thoroughly assessed.