S.-M. Li

Possible sources and preferred pathways for biogenic and non-sea-salt sulfur for the high Arctic

Sulfate is a major constituent observed in Arctic haze. Sulfur sources include anthropogenic, biogenic, and other natural sources. Previous studies have examined the concentrations and temporal variability of the concentrations of methanesulfonic acid (MSA) and sulfate (SO4=) at Alert, Northwest Territories, Canada. A receptor modeling method called the potential source contribution function (PSCF) combines the concentration data for these species measured in 7-day samples continuously collected between 1980 and 1991 with meteorological information in the form of air parcel back trajectories into conditional probability maps indicating the possible source areas and/or the preferred pathways that give rise to the observed high-concentration samples. After examination of the time series for MSA and SO4=, the data were segregated into time periods representing the spring, summer, and winter months and the PSCF analyses performed based on criterion values of the annual average species concentration. The potential source contribution method has been found to be effective in identifying possible source locations and the preferred pathways of MSA and SO4= in samples collected at Alert. Two concentration peaks are typically observed in the time series for MSA. The time series for SO4= is quite different from the series for MSA. The SO4= series only has peaks in the winter caused primarily by anthropogenic emissions. It was found that different regions of the North Atlantic Ocean contribute to the observed MSA concentrations during these different periods in agreement with prior hypotheses. Sources areas for sulfate during the summer and MSA during the winter can only be observed by changing the criterion value to the average during the period.

A case study of gas-to-particle conversion in an eastern Canadian forest

Aerosol and trace gas measurements were made at Kejimkujik National Park, Nova Scotia, Canada, during the summer of 1996. A case study from July 7-8 provides evidence of nucleation and condensation of products related to the oxidation of different biogenic emissions. Particles from 5 nm to 50 nm in diameter evolved during the afternoon and early evening associated with variations in isoprene. Late in the evening the α- and β-pinene mixing ratios and the aerosol particle volume increased. Soon after, there was a sharp increase in RO 2 H/H 2 O 2 that persisted until about 0100 LT. The initial increases in the pinenes and aerosols were strong and influenced by changes in winds. After 2200 LT, and into the early morning, the winds were relatively steady, and the α-and β-pinene mixing ratios continually decreased. The decay of α-pinene is explained through reaction with O 3 . However, the addition of OH radicals from the reaction of terpenes with O 3 is necessary to explain the observed rate of decay of β-pinene. During the same time, the aerosol volume increased with the decrease in α- and β-pinene. The volume increase was distributed 40:60 between particles in a mode centered at 80-90 nm and particles > 150 nm. The fine particle mass concentrations of the measured inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, and calcium) and organic ions (oxalate, formate, acetate, pyruvate, propionate) account for 25-30% of the total aerosol volume during the period (2.7 μm 3 cm -3 ) indicating that the aerosol volume increase was due to unidentified species. Assuming that the increase in the aerosol was the result of products from the oxidation of α- and β-pinene, an aerosol mass yield of 13% is estimated. The concentrations of cloud condensation nuclei active at 0.2% supersaturation were enhanced by the appearance of the 80-90 nm mode pointing to at least some of these forest-generated particles as being able to serve as nuclei for cloud droplets at common atmospheric supersaturations.

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.

The chemistry of biogenic hydrocarbons at a rural site in eastern Canada

An intensive field study was undertaken in southern Nova Scotia, on the east coast of Canada, for several weeks during the summer of 1996 as part of the North American Research Strategy for Tropospheric Ozone - Canada East ( NARSTO-CE ) 1996 field measurement campaign. Clean air conditions prevailed during most of the study period, which allowed an examination of biogenic hydrocarbon chemistry with minimal influence from anthropogenic pollutants. Low NO x mixing ratios during the study had an impact on the ratio of isoprenes oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) to isoprene. The effects include changes to the fate of isoprene peroxy radicals and to the concentration of OH compared to conditions of higher [NO x ]. Comparison to other studies indicate that there is a relationship between the ratio (MVK+MACR)/isoprene and the mixing ratio of NO x . Biogenic hydrocarbons were the dominant reactive volatile organic compound (VOC) precursors to ozone production in this region, although the net ozone production rate predicted by a box-model simulation of the measurement data was only < 1 ppbv h -1 . The evidence confirms that ozone production at this site is very NO x -sensitive. Model simulations indicated that the ozonolysis of biogenic hydrocarbons is an important source of the hydroxyl radical at this site and that OH was, in fact, the dominant oxidant during the nighttime under the observed low NO x conditions. Although the OH source did affect the nighttime mixing ratios of biogenic hydrocarbons, it could not fully explain the rapid nocturnal decay of isoprene observed on most evenings.

Seasonal and geographic variations of methanesulfonic acid in the arctic troposphere

Measurements in the Arctic troposphere over several years show that MSA concentrations in the atmospheric boundary layer, 0.08-6.1 parts per trillion (ppt, molar mixing ration), are lower that those over mid-latitude oceans. The seasonal cycle of MSA at Alert, Canada (82.5°N, 62.3°W), has two peaks of 6 ppt in March–April and July–August and minima of 0.3 ppt for the rest of the year. At Dye 3 (65°N, 44°W) on the Greenland Ice Sheet, a similar seasonal MSA cycle is observed although the concentrations are much lower with a maximum of 1 ppt. Around Barrow, Alaska (71.3°N, 156.8°W), MSA is between 1.0 and 25 ppt in July, higher than 1.5 ± 1.0 ppt in March–April. The mid-tropospheric MSA level of 0.6-1 ppt in the summer Arctic is much lower than about 6 ppt in the boundary layer. At Alert, the ratio of MSA to non-sea-salt (nss) SO42− ranges from 0.02 to 1.13 and is about 10 times higher in summer than in spring. The summer ratios are higher than found over mid-latitude regions and, when combined with reported sulfur isotope compositions from the Arctic, suggest that on average a significant fraction (about 16–23%) of Arctic summer boundary layer sulfur is marine biogenic. The measurements show that the summer Arctic boundary layer has a significantly higher MSA/nss-SO42− ratio than aloft.

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.

Airborne observations related to ozone depletion at polar sunrise

Airborne observations were conducted in the high Arctic (69°N–83°N) during April 6–16, 1992, in support of the Polar Sunrise Experiment. Measurements of temperature, O3, NOx aerosol particles, inorganic aerosol species, inorganic bromide, alkyl nitrate species, and several organohalogens (including bromoform) were made from an altitude of 30 m above ground level (agl) up to 7000 m msl (mean sea level). The average temperature profile shows a strong surface-based inversion up to about 500 m, an isothermal region up to about 1.5 km, and steadily decreasing from 1.5 to 6.8 km. Ozone mixing ratios were frequently found to be depleted from the surface up to various altitudes within the boundary layer (maximum altitude for ozone <10 parts per billion by volume (ppbv) was 375 m). The average ozone profile increases from <10 ppbv near the surface up to about 40 ppbv at 1 km, remaining approximately constant up to 5 km, and increasing with altitude thereafter as the stratospheric source becomes evident. NOx, including a possible peroxyacetyl nitrate (PAN) interference, was typically <50±20 parts per trillion by volume (pptv), and frequently below detection limit (20 pptv). Accumulation-mode aerosol particle number concentrations in the boundary layer were 100–200 cm−3, and although CN increased low over a polynya, there were indications of an absence of nucleation-mode particles in ozone depleted air in the boundary layer compared with the free troposphere. Inorganic gaseous bromide, bromoform (CHBr3) and dibromochloromethane (CHClBr2) all exhibited strong anticorrelations with O3. Gaseous nitrate (HNO3 plus possibly some contribution from PAN interference) ranged up to 110 pptv but was <40 pptv in 11 of 14 samples. With the exception of 1-propyl nitrate the C3–C6 alkyl nitrates correlated positively with ozone, as did the isomer ratio C3/C6. Organohalogens were measured using charcoal cartridges {C} and Tenax cartridges {T}. CHBr3 was similar by both techniques (medians of 1.83{C} and 1.60{T} pptv), and negative correlations with O3 were indicated by both sets of samples (R2 = 0.75{C} and 0.71{T}). CHClBr2 was also very close in both sets of samples (median of 0.25{C} and 0.22{T} pptv), however, a negative correlation with O3 was present only in the Tenax samples (R2 = 0.63). Ln(CHClBr2/CHBr3) correlated negatively with In (CHBr3) with a coefficient of determination of 0.75, and with higher In(CHBr3) approached the value indicated by Li et al. (this issue) for air immediately above seawater at 0°C (i.e., 0.032). CHClBr2/CHBr3 was higher in the free troposphere than in the boundary layer and possibly less variant with In(CHBr3), indicating either different source regions for these free troposphere organohalogens and/or, as Li et al. suggest, faster chemical destruction of CHBr3 relative to CHClBr2 in the free troposphere. The airborne organohalogen data and that from ice camp SWAN (Hopper et al., this issue) and Alert (Yokouchi et al., this issue) were combined to produce a vertical profile of CHBr3. CHBr3 exhibited a well-defined logarithmic decrease with increasing altitude, indicating a strong surface source, opposite to the average O3 profile. In general, the low-level airborne observations related to O3 depletion are very consistent with the observations at Alert, indicating that the many features of this phenomenon are ubiquitous.

Measurement and modeling of particle nitrate formation

Nighttime measurements of particle number distribution and mass composition, and concentrations of NO2, O3, NH3, and HNO3, were made at night at a rural site in Ontario in August of 1992. A simple model of particle growth was constructed to simulate the observed rapid growth of aerosol mass in the 0.2 to 0.5 μm diameter range. Both measurements and model results indicate that the growth of accumulation mode aerosol mass was due to condensation of the nitrate radical, HNO3, and NH3 onto particles, with the formation of particle ammonium nitrate. The results show that the reaction of O3 with NO2 in the isolated nocturnal boundary layer can lead to the production of gas-phase nitric acid. When this occurs in the presence of local ammonia emissions and preexisting particles, rapid growth of particle ammonium nitrate takes place. The model results show that most of the observed variations can be accounted for by a coupled system of equations including dynamical and thermodynamic effects. The dynamical approach to equilibrium is sufficiently fast that the gas-phase nitric acid concentrations are more sensitive to the magnitude of the thermodynamic equilibrium concentration than the dynamical time constant. A simple parameterization for the effects of sulphate on particle nitrate formation was developed and shown to provide a good estimate of the equilibrium concentration of gas-phase nitric acid.

Contamination of arctic air at three sites during a haze event in late winter 1986

Abstract Interpretation of simultaneous measurements at three stations in different parts of the Arctic suggests that during wir masses forced into the Arctic from Eurasia in a surge Alaska and further return over the North Pole towards the European Arctic. On some occasions direct flow of the Eurasian air masses detected in the European Arctic. Simple statistical methods and dispersion modeling proved useful in studying source-receptor relationship in the Arctic.

High Resolution Model Simulations of the Canadian Oil Sands with Comparisons to Field Study Observations

The governments of Canada and Alberta are implementing a joint plan for oil sands monitoring that includes investigating emissions, transport and downwind chemistry associated with the Canadian oil sands region. As part of that effort, Environment Canada’s Global Environmental Multiscale—Modelling Air-quality And CHemistry (GEM-MACH) system was reconfigured for the first time to create nested forecasts of air quality at model grid resolutions down to 2.5 km, with the highest resolution domain including the Canadian provinces of Alberta and Saskatchewan. The forecasts were used to direct an airborne research platform during a summer 2013 monitoring intensive. Subsequent work with the modelling system has included an in-depth comparison of the model predictions to monitoring network observations, and to field intensive airborne and surface supersite observations. A year of model predictions and monitoring network observations were compared, as were model and aircraft flight track values. The relative impact of different model versions (including modified emissions and feedbacks between weather and air pollution) will be discussed. Model-based predictions of indicators of human-health (i.e., Air Quality Health Index) and ecosystem (i.e. deposition of pollutants) impacts for the region will also be described.