Desiree Toom-Sauntry

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.

Correlations and emission ratios among bromoform, dibromochloromethane, and dibromomethane in the atmosphere

[1] Bromoform (CHBr 3 ), dibromochloromethane (CHBr 2 Cl), and dibromomethane (CH 2 Br 2 ) in the atmosphere were measured at various sites, including tropical islands, the Arctic, and the open Pacific Ocean. Up to 40 ppt of bromoform was observed along the coasts of tropical islands under a sea breeze. Polybromomethane concentrations were highly correlated among the coastal samples, and the ratios CH 2 Br 2 /CHBr 3 and CHBr 2 Cl/ CHBr 3 showed a clear tendency to decrease with increasing CHBr 3 concentration. These findings are consistent with the observations that polybromomethanes are emitted mostly from macroalgae whose growth is highly localized to coastal areas and that CHBr 3 has the shortest lifetime among these three compounds. The relationship between the concentration ratios CHBr 3 /CH 2 Br 2 and CHBr 2 Cl/CH 2 Br 2 suggested a large mixing/ dilution effect on bromomethane ratios in coastal regions and yielded a rough estimate of 9 for the molar emission ratio of CHBr 3 /CH 2 Br 2 and of 0.7 for that of CHBr 2 Cl/CH 2 Br 2 . Using these ratios and an global emission estimate for CH 2 Br 2 (61 Gg/yr (Br)) calculated from its background concentration, the global emission rates of CHBr 3 and CHBr 2 Cl were calculated to be approximately 820(±310) Gg/yr (Br) and 43(±16) Gg/yr (Br), respectively, assuming that the bromomethanes ratios measured in this study are global representative. The estimated CHBr 3 emission is consistent with that estimated in a very recent study by integrating the sea-to-air flux database. Thus the contribution of CHBr 3 and CHBr 2 Cl to inorganic Br in the atmosphere is likely to be more important than previously thought.

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.

Recent decline of methyl bromide in the troposphere

Abstract Atmospheric methyl bromide (CH 3 Br) measured at a remote ground station in the Arctic (mid-1996 to early 2002) and in the free troposphere at mid-latitude (early 1999 to early 2002) showed a steady annual average decrease of 4–6%. The trend was consistent with a simulation of the response to the phase-out schedule of anthropogenic emissions under the Montreal Protocol and its amendments, suggesting that a decrease in CH 3 Br abundance in the Northern Hemisphere of as much as 40% from the level of the early 1990s would be possible by the completion date of the program.

Atmospheric methyl iodide: High correlation with surface seawater temperature and its implications on the sea-to-air flux

Intensive measurements of atmospheric methyl iodide taken at high, middle, and low latitudes over a period of 3 years have provided evidence for its photochemical production in seawater and given new information that sea-to-air transport of CH3I is mainly controlled by surface seawater temperature (SST). These findings suggest a highly localized production and distribution of CH3I in the surface microlayer. As a result, the oceanic emission of CH3I is likely to be larger than previous estimates based on the classical two-layer model. Owing to the SST dependence of atmospheric CH3I concentration, its impact on tropospheric or stratospheric ozone depletion would be increased by El Nino or future global warming.

An Intensive Study of the Size and Composition of Submicron Atmospheric Aerosols at a Rural Site in Ontario, Canada

Atmospheric sampling was conducted at a rural site near Egbert, about 70 km north of Toronto, Ontario, Canada from March 27 to May 8, 2003 to characterize the physical and chemical properties of the ambient aerosol in near real-time. The instrumentation included a tapered element oscillating microbalance (TEOM), an ultrafine condensation particle counter (UCPC), a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), an aerosol mass spectrometer (AMS), and a particulate nitrate monitor (R&P 8400N) for aerosol measurements. Gas-phase non-methane hydrocarbon compounds (NMHCs) were measured by gas chromatograph-flame ionization detection (GC-FID). Filter samples were also collected for analysis of inorganic ions by ion chromatography (IC). Aerosol properties varied considerably depending upon meteorological conditions and airmass histories. For example, urban and industrial emissions advected from the south strongly influenced the site occasionally, resulting in higher particulate mass with the higher fractions of nitrate and organics. Cleaner northwesterly winds carried aerosols with relatively higher fractions of organics and sulfate. The AMS derived mass size distributions showed that the inorganic species in the particles with vacuum aerodynamic diameters between about 60 nm and 600 nm had mass modal vacuum aerodynamic diameters around 400–500 nm. The particulate organics often exhibited two modes at about 100 nm and 425 nm, more noticeable during fresh pollution events. The small organic mode was well correlated with gas-phase nonmethane hydrocarbons such as ethylbenzene, toluene, and propene, suggesting that the likely sources of small organic particles were combustion related emissions. The particulate nitrate exhibited a diurnal variation with higher concentrations during dark hours and minima in the afternoon. Particulate sulfate and organics showed evidence of photochemical processing with higher levels of sulfate and oxygenated organics in the afternoon. Reasonable agreement among all of the co-located measurements is found, provided the upper size limit of the AMS is considered.

Primary marine aerosol-cloud interactions off the coast of California

Primary marine aerosol (PMA)-cloud interactions off the coast of California were investigated using observations of marine aerosol, cloud condensation nuclei (CCN), and stratocumulus clouds during the Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) and the Stratocumulus Observations of Los-Angeles Emissions Derived Aerosol-Droplets (SOLEDAD) studies. Based on recently reported measurements of PMA size distributions, a constrained lognormal-mode-fitting procedure was devised to isolate PMA number size distributions from total aerosol size distributions and applied to E-PEACE measurements. During the 12 day E-PEACE cruise on the R/V Point Sur, PMA typically contributed less than 15% of total particle concentrations. PMA number concentrations averaged 12 cm^(−3) during a relatively calmer period (average wind speed 12 m/s^1) lasting 8 days, and 71 cm^(−3) during a period of higher wind speeds (average 16 m/s^1) lasting 5 days. On average, PMA contributed less than 10% of total CCN at supersaturations up to 0.9% during the calmer period; however, during the higher wind speed period, PMA comprised 5–63% of CCN (average 16–28%) at supersaturations less than 0.3%. Sea salt was measured directly in the dried residuals of cloud droplets during the SOLEDAD study. The mass fractions of sea salt in the residuals averaged 12 to 24% during three cloud events. Comparing the marine stratocumulus clouds sampled in the two campaigns, measured peak supersaturations were 0.2 ± 0.04% during E-PEACE and 0.05–0.1% during SOLEDAD. The available measurements show that cloud droplet number concentrations increased with >100 nm particles in E-PEACE but decreased in the three SOLEDAD cloud events.

Latitudinal distribution of atmospheric methyl bromide: Measurements and modeling

The global distribution of atmospheric methyl bromide (CH 3 Br) obtained from extensive new measurements of atmospheric CH 3 Br from latitude 82.5°N to 69.1°S, showed a small decrease from mid- to high-latitudes, a gradient between the northern and southern hemispheres with a ratio of 1.2 to 1.3, and occasional high concentrations in the tropics. The observed data and modeled distributions of industrial CH 3 Br were used to apportion CH 3 Br between natural and industrial components for both hemisphere. We obtained an estimated man-made contribution of 4.3 pptv and 2.3 pptv in the northern and southern hemispheres, respectively and a natural (non-industrial) background concentration in both hemispheres of 6 pptv with a slight increase in the tropics.

Size distribution and estimated optical properties of carbonate, water soluble organic carbon, and sulfate in aerosols at a remote high altitude site in western China

Measurements at the GAW station in western China reveal the levels and size distributions of chemical components in aerosols. The results indicate similarly high levels of three components, water-soluble organic carbon (WSOC), Ca2++CO3=, and NH4++SO4=. Both WSOC and SO4= show a dominant accumulation mode, with a geometric mean mode diameter Dg of 0.41 and 0.34 µm and geometric standard deviation σsgof 0.31 and 0.33, respectively. This mode makes up >70% of the total mass of both species. In comparison, Ca2+and CO3= show a prominent coarse mode with Dg of 2.98 and 1.76 µm and σsg of 0.29 and 0.22, respectively, that accounts for >60% of the mass. Based on these characteristics, estimates of the volume scatter coefficient β, direct back scatter coefficient βπ, and mass scattering efficiency ϕ for each component were made, assuming external mixing and optical and growth characteristics of corresponding pure chemical compounds. The results show that NH4++SO4= has the largest β, βπ, and ϕ (median 1760×10−8 m−1, 79×10−8 m−1 sr−1, and 6.9 m² g−1, respectively). It is followed by WSOC with 1470×10−8 m−1, 30×10−8 m−1 sr−1, and 5.3 m² g−1 for β, βπ, and ϕ, respectively. Ca2++CO3= has only about 10% of the β and ϕ values but 20% of the βϕ of NH4++SO4= respectively. For both NH4++SO4= and WSOC, the mass scattering efficiency ϕ is inversely related to their Dg between the size range of 0.2–0.45 µm.

Cloud partitioning of isocyanic acid (HNCO) and evidence of secondary source of HNCO in ambient air

Although isocyanic acid (HNCO) may cause a variety of health issues via protein carbamylation and has been proposed as a key compound in smoke-related health issues, our understanding of the atmospheric sources and fate of this toxic compound is currently incomplete. To address these issues, a field study was conducted at Mount Soledad, La Jolla, CA, to investigate partitioning of HNCO to clouds and fogs using an Acetate Chemical Ionization Mass Spectrometer coupled to a ground-based counterflow virtual impactor. The first field evidence of cloud partitioning of HNCO is presented, demonstrating that HNCO is dissolved in cloudwater more efficiently than expected based on the effective Henrys law solubility. The measurements also indicate evidence for a secondary, photochemical source of HNCO in ambient air at this site.