Ann-Lise Norman

Sources of aerosol sulphate at Alert : Apportionment using stable isotopes

From July 1993 to September 1994, seasonal variations in the sources of SO42− aerosols in the Arctic lower atmosphere at Alert, Canada, (82°30′N, 62°20′W) were investigated using the sulphur isotope abundance of as little as 10 μg of sulphur analyzed by combustion-flow isotope-ratio mass spectrometry. In conjunction with air mass trajectories and in parallel with measurements of aerosol composition, the sulphur isotope composition was used to discern sources of aerosol SO42−. Total SO42− is composed of sea-salt SO42−, marine biogenic, and nonmarine SO42−. From June through September the fraction of biogenic SO42− in the non-sea-salt (nss) component ranged from 0.09 to 0.40 with an average of 0.31 ± 0.11. Summertime nonmarine SO42− is likely anthropogenic in origin since it is isotopically indistinguishable from SO42− in the polluted winter/spring period of arctic haze (δ34S = +5‰). In summer there was no significant difference in isotope composition of aerosol sulphate between air which recently traversed Eurasia and the Arctic Ocean and air arriving from North America. In contrast to summer and late winter/spring, δ34S values for nonmarine SO42− in fall and early winter were often less than +5‰. These isotopically light samples were divisible into two groups: (1) those associated with air mass trajectories potentially affected by North American soils and/or smelters and (2) three weekly samples between October and December which could be attributed to fractionated sea-salt aerosol formed on refrozen Arctic Ocean leads. For the latter the ratio of SO42−/Na was estimated to be a factor of 3.6 lower than in bulk seawater. From November to May, nonmarine aerosol SO42− was apportioned into 10 aerosol components using positive matrix factor analysis of 18 aerosol ions and trace elements [Sirois and Barrie, this issue]. In turn, a multiple linear regression of δ34S values against the scores of the components was used to predict the isotope composition of six components. It was concluded that the main mass of anthropogenic SO42− had a δ34S value near +5‰ and that biogenic SO42− had a δ34S of +16 ± 3.9‰. Reasonable agreement between model results and sulphur isotope measurements at Alert show that SO42− apportionment using positive matrix factor analysis is a reasonable approach which gives realistic results.

Stable carbon isotope composition of nonmethane hydrocarbons in emissions from transportation related sources and atmospheric observations in an urban atmosphere

Abstract The stable carbon isotope ratios of nonmethane hydrocarbons (NMHC) emitted by traffic related sources are presented. Six sets of samples were collected in the greater Toronto area at locations heavily impacted by engine exhaust, fuel losses and fuel evaporation. Furthermore, two series of measurements were made in a suburban area. The stable isotope ratios of alkanes and arenes in the emission studies are on average −27.7±1.7‰ (relative to Vienna Peedee belemnite, VPDB), fully compatible with the average composition of crude oils. On average alkenes are enriched by 2‰ in 13 C relative to alkanes and arenes. The differences between measurements at locations impacted predominantly by specific source types, e.g. tailpipe emissions, fuel evaporation, and losses, seldom are statistically significant and only occasionally exceed 2‰. Ethyne emitted from engine exhaust is enriched in 13 C by several tens of per mil relative to other NMHC studied. The ambient measurements at a suburban location in the greater Toronto area showed that the ambient stable carbon isotope ratios are close to the source composition, but not completely identical. On average ambient NMHC are enriched in 13 C relative to the average of the sources by a few per mil. Furthermore, for most NMHC the ambient measurements exhibit a higher overall variability than the source compositions. The small, but nevertheless often significant differences between source composition and ambient observations can be explained by the isotope fractionation associated with the reaction of NMHC with OH-radicals; the most important atmospheric loss process for NMHC. Overall the results demonstrate that for the metropolitan region the variability of the stable carbon isotope ratios of NMHC emitted from traffic related sources is small.

Influence of transport and ocean ice extent on biogenic aerosol sulfur in the Arctic atmosphere

The recent decline in sea ice cover in the Arctic Ocean could affect the regional radiative forcing via changes in sea ice-atmosphere exchange of dimethyl sulfide (DMS) and biogenic aerosols formed from its atmospheric oxidation, such as methanesulfonic acid (MSA). This study examines relationships between changes in total sea ice extent north of 70 degrees N and atmospheric MSA measurement at Alert, Nunavut, during 1980-2009; at Barrow, Alaska, during 1997-2008; and at Ny-Alesund, Svalbard, for 1991-2004. During the 1980-1989 and 1990-1997 periods, summer (July-August) and June MSA concentrations at Alert decreased. In general, MSA concentrations increased at all locations since 2000 with respect to 1990 values, specifically during June and summer at Alert and in summer at Barrow and Ny-Alesund. Our results show variability in MSA at all sites is related to changes in the source strengths of DMS, possibly linked to changes in sea ice extent as well as to changes in atmospheric transport patterns. Since 2000, a late spring increase in atmospheric MSA at the three sites coincides with the northward migration of the marginal ice edge zone where high DMS emissions from ocean to atmosphere have previously been reported. Significant negative correlations are found between sea ice extent and MSA concentrations at the three sites during the spring and June. These results suggest that a decrease in seasonal ice cover influencing other mechanisms of DMS production could lead to higher atmospheric MSA concentrations.

The stable carbon isotope composition of atmospheric PAHs

Abstract A method for compound-specific stable carbon isotope analysis of atmospheric polycyclic aromatic hydrocarbons (PAHs) in carbonaceous aerosols is described. Atmospheric aerosol samples containing PAHs (C-10 to C-20) were collected on filters using a high-volume sampling technique, solvent extracted, taken through a cleanup procedure, separated by gas chromatography, oxidized to CO 2 on-line, and introduced into an isotope ratio mass spectrometer for analysis. The method can be used to determine the isotope composition of a few nanograms of PAHs. This technique was used to analyse and compare the isotope composition of atmospheric PAHs from standards, as well as two samples from urban and rural locations. Isotopic variability in atmospheric PAHs is greater than measurement uncertainties which makes this a potentially useful technique for source identification when used together with concentration measurements.

Sulphur isotope fractionation during sulphur mineralization: results of an incubation–extraction experiment with a Black Forest soil

Laboratory incubation–extraction experiments were used to study sulphur isotope fractionation during sulphur mineralization in Oh and Ah horizons of a Black Forest soil. Changes in δ34S values for sulphate extracted every three days with deionized-distilled water over a three week incubation period were small (<1.5‰). A second experiment used the addition of dilute ammonium sulphate solution, enriched in 34S relative to soil sulphur, to demonstrate unequivocally that sulphate adsorption and desorption during the incubation were negligible. Sulphur isotope fractionation during mineralization of carbon-bonded sulphur was shown to be a two-step process. The first and slower step was the formation of soluble organic sulphate from carbon-bonded sulphur, accompanied by a kinetic isotope effect, k32/k34=1.0040±0.0008. The faster step, identified with the hydrolysis of organic sulphate favored 34S in the product with a k32/k34=0.9967±0.0003. Leaching the soils led to a loss of isotopically light organic sulphate from the organic sulphur pool and is likely the process responsible for progressively heavier δ34S values for organic sulphur with depth in undisturbed forest soils in the Black Forest region.

Modelling of hydrogen and oxygen isotope compositions for local precipitation

Stable isotope compositions of hydrogen and oxygen for continental condensates are determined foremost by equilibrium and kinetic isotope fractionation during evaporation at the oceanic source regions. Subsequently they are modified by a series of in-cloud processes, which include condensation and possible admixture of vapour from evaporation and transpiration over the continents. The effects of vapour admixture from evaporation and transpiration on the isotope compositions of hydrogen (δ2H) and oxygen (δ18O) for local precipitation are discussed in this paper. Using a modified Rayleigh fractionation model, the effects are described for both constant and stepwise evaporation and transpiration fluxes. Further, deuterium (d)-excess values are employed to estimate the evaporation ratio and the slopes of δ–T polynomial regression curves are used to estimate the transpiration ratio. Finally, the influence of sea surface temperature on isotope compositions in condensates is modelled and its effect on d-excess and regression equations is examined.

Spatial patterns and seasonal variation of snowpack sulphate isotopes of the Prince of Wales Icefield, Ellesmere Island, Canada

Abstract Ion-chemistry and sulphate-isotope values for snow samples from the Prince of Wales Icefield, Ellesmere Island, Canada, show distinct seasonal trends and spatial patterns. Sixteen surface snow samples from two transects, and 30 samples from five depth profiles, representing fall 2003 to spring 2004 accumulation, have been analyzed. Surface snow samples show decreasing SO4 2– and Na+ concentrations along with decreasing δ34S values with distance inland and increased elevation. These trends follow an expected pattern of decreasing sea-salt aerosol impact with greater vertical and horizontal distance from sea-water sources. Depth-profile total sulphate and non-sea-salt sulphate increase in concentration with height in the snowpack, and these results, combined with δ34S values that are more positive in fall snow, are consistent with increased amounts of anthropogenic sulphate in surface spring snow. Sulphate apportionment was performed on surface snow assuming an isotopically light sulphate source (anthropogenic plus volcanic) mixed with isotopically heavier sulphate from dimethylsulphide (DMS) oxidation and sea water. Isotopically light sulphate in surface snow was apparent at all elevations at a reasonably uniform concentration. DMS sulphate, however, decreased exponentially with altitude, reflecting an ocean level source with oxidation occurring during transport and deposition. The significance to multi-year deposition studies is that sulphate from DMS oxidation may be related to sea-ice conditions in the region.

Evidence of sea ice source in aerosol sulfate loading and size distribution in the Canadian High Arctic from isotopic analysis

The influence of frost flowers and seawater brine on ion chemistry in snow, snowpack, ice cores, and aerosols is detected when a lower sulfate to sodium ratio than in seawater is present in polar regions. This evidence can be masked when large amounts of non-sea-salt sulfate are present from other sources such as biogenic and anthropogenic sulfate. Frost flower δ34S values were measured for the first time in frost flower sulfates and did not differ significantly from the sea salt δ34S values of +21‰. A method using stable isotopes is introduced to determine the limit of contributions from sea salt and sea ice sources (including frost flowers and brine) on sulfate concentrations in aerosol samples from Alert, Canada. Knowledge of the range of values of δ34Snss and the SO4/Na ratio found in sea ice sources (i.e., frost flowers) is used to quantitatively constrain the contributions from frost flowers and sea salt in the Arctic aerosol mass during the onset of winter in 2007 and 2008, allowing for quantification of non-sea-salt sulfate amounts during times when frost flowers are present. Frost flower and/or brine influence was found predominantly in the coarse-mode aerosols (>0.95 µm). This method to determine the contributions from sea salt and sea ice sources can be carried over to future studies with snow and ice cores.

Marine aerosol source regions to Prince of Wales Icefield, Ellesmere Island, and influence from the tropical Pacific, 1979–2001

Using a coastal ice core collected from Prince of Wales (POW) Icefield on Ellesmere Island, we investigate source regions of sea ice-modulated chemical species (methanesulfonic acid (MSA) and chloride (Cl−)) to POW Icefield and the influence of large-scale atmospheric variability on the transport of these marine aerosols (1979–2001). Our key findings are (1) MSA in the POW Icefield core is derived primarily from productivity in the sea ice zone of Baffin Bay and the Labrador Sea, with influence from waters within the North Water (NOW) polynya, (2) sea ice formation processes within the NOW polynya may be a significant source of sea-salt aerosols to the POW core site, in addition to offshore open water source regions primarily in Hudson Bay, and (3) the tropical Pacific influences the source and transport of marine aerosols to POW Icefield through its remote control on regional winds and sea ice variability. Regression analyses during times of MSA deposition reveal sea level pressure (SLP) anomalies favorable for opening of the NOW polynya and subsequent oceanic dimethyl sulfide production. Regression analyses during times of Cl− deposition reveal SLP anomalies that indicate a broader oceanic region of sea-salt sources to the core site. These results are supported by Scanning Multichannel Microwave Radiometer- and Special Sensor Microwave/Imager-based sea ice reconstructions and air mass transport density analyses and suggest that the marine biogenic record may capture local polynya variability, while sea-salt transport to the site from larger offshore source regions in Baffin Bay is likely. Regression analyses show a link to tropical dynamics via an atmospheric Rossby wave.

Early atmospheric detection of carbon dioxide from carbon capture and storage sites.

ABSTRACT The early atmospheric detection of carbon dioxide (CO2) leaks from carbon capture and storage (CCS) sites is important both to inform remediation efforts and to build and maintain public support for CCS in mitigating greenhouse gas emissions. A gas analysis system was developed to assess the origin of plumes of air enriched in CO2, as to whether CO2 is from a CCS site or from the oxidation of carbon compounds. The system measured CO2 and O2 concentrations for different plume samples relative to background air and calculated the gas differential concentration ratio (GDCR = −ΔO2/ΔCO2). The experimental results were in good agreement with theoretical calculations that placed GDCR values for a CO2 leak at 0.21, compared with GDCR values of 1–1.8 for the combustion of carbon compounds. Although some combustion plume samples deviated in GDCR from theoretical, the very low GDCR values associated with plumes from CO2 leaks provided confidence that this technology holds promise in providing a tool for the early detection of CO2 leaks from CCS sites.  Implications: This work contributes to the development of a cost-effective technology for the early detection of leaks from sites where CO2 has been injected into the subsurface to enhance oil recovery or to permanently store the gas as a strategy for mitigating climate change. Such technology will be important in building public confidence regarding the safety and security of carbon capture and storage sites.