ALBERT

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.

Description: NIWA operates a network of zenith-sky viewing DOAS (Differential Optical Absorption Spec- troscopy) instruments to measure NO2 and BrO. The longest existing time series (1981 present) of NO2 has been measured at Lauder (45oS), New Zealand and the trend of this long- term data set has been studied extensively. Here we present a summary of stratospheric NO2 trends observed at several Northern and Southern Hemisphere stations (including Lauder) and an update of our understanding of the observed hemispheric asymmetry. These trends provide an important anchor for the interpretation of NO2 trends measured by satellites.BrO observations are currently made by NIWA at two Southern Hemisphere sites, Lauder and Arrival Heights (78oS) with each data set spanning more than 15 years. The zenith sky BrO observations are complemented with direct sun observations at Lauder since 2001and with MAX-DOAS (Multi-axis Differential Optical Absorption Spectroscopy) observations at Arrival Heights (78oS) since 1998. A retrieval technique to separate the tropospheric and stratospheric partial columns of BrO was developed for the combination of zenith sky and direct sun measurements with the zenith sky observations providing predominantly the information on the stratospheric partial column and the direct sun observations providing the tropospheric contribution. This retrieval has now been applied to Lauder BrO UV-visible measurements for the whole time period (2001 - present) and the updated results including an upper limit of BrO in the troposphere and the stratospheric bromine loading will be presented.The retrieval method has now also been extended so that it can be applied to zenith sky data only. Furthermore, an independent retrieval algorithm has been developed including a forward model capable of dealing with multiple scattering (Monte Carlo radiative transfer model) to enable us to retrieve altitude information in the boundary layer and lower troposphere. This retrieval method has been applied to MAX-DOAS measurements made at Arrival Heights for the last 12 years with the aim to investigate bromine explosion events observed in McMurdo Sound during Antarctic springtime and the results of this investigation will be presented.

Description: The 2006 WMO/UNEP Scientific Assessment of Ozone Depletion identified halogenated very short-lived substances (VSLS) as contributors to the atmospheric budget of halogens. As well, it raised a question regarding the extent of the contribution of halogenated VSLS to atmospheric Bry and Iy. Traditionally, scientists have been more concerned in determining the anthropogenic budget of halogenated compounds while nature is the major producer of such species. In order to have a complete atmospheric budget of halogenated VSLS, it is important to have a better understanding of what species are biogenically produced as well as their respective degradation pathways. Oceanic emissions of halocarbons may be a new link between climate change and the composition of the global atmosphere. The rates of halocarbon emissions are sensitive to sea-surface temperatures (SSTs), nutrient supply and upwelling; all of which are to be affected by climate change. Therefore, increases in SSTs will increase emission rates. On the one hand, seaweed has been identified as a major producer of biogenic polyhalogenated VSLS. Marine macroalgae (kelp) and phytoplankton emit halogen containing gases into the marine boundary layer, constituting 90 to 95% of the total global flux of volatile halocarbons to the atmosphere. On the other hand, the possibility of industrial scale marine kelp farming as a means of carbon sequestration (i.e. marine analogy of the Kyoto Protocol forest) is being pondered by countries with long coastlines and little land suitable for forestation. Would a Kyoto Protocol forest analog be the right strategy for climate change mitigation?With the use of a portable Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) spec- trometer, studies have been performed in the coast of New Zealand in order to determine the presence of BrO and IO during the spring and summer months of the Southern Hemisphere. MAX-DOAS uses scattered sunlight received from multiple viewing directions. The spatial distribution of various trace gases close to the instrument can be derived by combining several viewing geometries. Ground based MAX-DOAS is highly sensitive to absorbers in the lowest few kilometers of the atmosphere. The selected sites had high biomass concentration of marine algae that would be exposed by low tides and therefore, stressed in order to liberate the species of interest. In order to better understand the environmental factors that modulate the emissions of halogen oxides from the marine environment to the troposphere, results have been correlated to local macro algae type, ozone concentration, tidal height, incident sunlight, temperature and wind speed and direction.

Description: We will present a newly developed algorithm for the retrieval of tropospheric trace gas profiles from MAX-DOAS measurements. A Monte Carlo radiative transfer model, NIMO (NIWA Monte Carlo model) is used to calculate the weighting functions and forward model DSCDs (Differential Slant Column Densities). NIMO uses the local estimation technique to substantially speed up the determination of DSCDs for any given set of measurement ge- ometries, enabling use of the model online rather than using pre-calculated lookup tables. The optimal estimation method is used to retrieve profiles for either single or multiple scan sequences or over prescribed time intervals. This inversion method is used to derive NO2 profiles from MAX-DOAS measurements made during the CINDI campaign at Cabauw, Netherlands, in June/July 2009. BrO profiles retrieved from sea-ice MAX-DOAS measure- ments, made during two Antarctic springtime campaigns in 2006 and 2007, are also presented.