Atmospheric mercury concentrations: measurements and profiles near snow and ice surfaces in the Canadian Arctic during Alert 2000

Gaseous elemental mercury (GEM) concentration measurements were made during the Alert 2000 campaign in Alert, Nunavut, Canada, between February and May 2000. GEM exhibits dramatic mercury depletion events (MDE) concurrently with ozone in the troposphere during the Arctic springtime. Using a cold regions pyrolysis unit, it was confirmed that GEM is converted to more reactive mercury species during the MDEs. It was determined that on average 48% of this converted GEM was recovered through pyrolysis suggesting that the remaining converted GEM is deposited on the snow surfaces. Samples collected during this campaign showed an approximate 20 fold increase in mercury concentrations in the snow from the dark to light periods. Vertical gradient air profiling experiments were conducted. In the non-depletion periods GEM was found to be invariant in the air column between surface and 1–2 m heights. During a depletion period, GEM was found to be invariant in the air column except at the surface where a noticeable increase in the GEM concentration was observed. Concurrent ozone concentration profiles showed a small gradient in the air column but a sharp decrease in ozone concentration at the surface. Other profile studies showed a 41% average GEM concentration difference between the interstitial air in the snow pack and ∼2 m above the surface suggesting that GEM is emitted from the snow pack. Further profile studies showed that during MDEs surface level GEM exhibits spikes of mercury concentrations that were over double the ambient GEM concentrations. It is thought that the solar radiation may reduce reactive mercury that is deposited on the snow surface during a MDE back to its elemental form which is then increasingly released from the snow pack as the temperature increases during the day. This is observed when wind speeds are very low.     

Direct measurements and parameterisation of aerosol flux,concentration and emission velocity above a city

Articles have recently been published on aerosol size distributions and number concentrations in cities, however there have been no studies on transport of these particles. Eddy covariance measurements of vertical transport of aerosol in the size range 11 nm<D_p<3 μm are presented here. The analysis shows that typical average aerosol number fluxes in this size range vary between 9000 and 90,000 cm⁻² s⁻¹. With concentrations between 3000 and 20,000 cm⁻³ this leads to estimates of particle emission velocity between 20 and 75 mm s⁻¹. The relationships between number flux and traffic activity, along with emission velocity and boundary layer stability are demonstrated and parameterised. These are used to derive an empirical parameterisation for aerosol concentration in terms of traffic activity and stability. The main processes determining urban aerosol fluxes and concentrations are discussed and quantified where possible. The difficulties in parameterising urban activity are discussed.     

Trace gas and aerosol measurements at a remote site in the northeast U.S.

This paper presents the results of continuous measurements of trace atmospheric gases and aerosol composition made at the summit of Whiteface Mountain, New York, for 28 days in July 1982. The gas phase species NO, NO_x ( = NO + NO₂ + PAN), HNO₃, SO₂ and NH₃ were measured, as well as aerosol SO₄²⁻, NO₃⁻, H⁺ and NH₄⁺. Mean and median NO_x concentrations were 1.1 and 1.0 ppb, respectively, with maximum and minimum values of 3.2 and 0.3 ppb. HNO₃ concentrations were variable, occasionally exceeding the simultaneously measured NO_x levels. Mean and median SO₂ were 0.8 and 0.3 ppb, with concentrations up to 12 ppb in pollution episodes. Mean and median NH₃ were both 2.2 ppb. Monthly mean SO₄²⁻ was 5.3 μg m⁻³, with values in clean air of about 1.5 μg m⁻³, and in polluted air up to 80 μg m⁻³. Trajectory calculations indicate that episodes of high pollutant concentrations occur in air masses arriving at Whiteface from the southwest. These episodes contributed most of the SO₄²⁻, HNO₃ and aerosol acidity, and about half the SO₂ and NO_x to which the site was exposed during the measurement period. Limited comparisons of air chemistry data with the composition of cloudwater collected during the program are also presented.     

Total and methyl mercury patterns in Arctic snow during springtime at Resolute,Nunavut, Canada

Patterns of gaseous elemental mercury (GEM) were monitored at 20 and 150 cm above the snowpack near Resolute Bay, Cornwallis Island, Nunavut, Canada near the Upper Air Station of Environment Canada (74°42′N, 94°58′W) from 7 May (day 127) to 12 June (day 163) 2003. At this time of year there was 24 h daylight but still a strong diel change in solar radiation. Daily patterns of GEM-tracked solar radiation with a lag of about 2 h and the GEM gradient between these two heights showed the direction of flux. In addition to the previously established autocatalytic reactions involving halogens where reactive gaseous mercury and fine particulate mercury result in direct deposition to the snow, both diffusion to and volatilization from the snow occurred on a regular basis. Total mercury (THg) in the snowpack increased to near 30 ng L⁻¹ following 8 d of atmospheric mercury depletion then decreased to values near 1 ng L⁻¹. Losses from the snow could not be accounted for in melt water as stream runoff values were also low. In other words, most of the mercury associated with increased levels in snow was volatilized back to the atmosphere either directly from the snow or from the water surfaces. However, using accepted mass transport coefficients, the flux appeared low and other mechanisms are suggested. In contrast to THg, methyl mercury (MeHg) in the snow reached values near 140 pg L⁻¹ but also declined to less than detection limit (10 pg L⁻¹) with the onset of warmer temperatures. MeHg in stream runoff water was similar to maximal values seen in the snow. This observation is consistent with the view that MeHg came in the snowfall or was deposited to the snow pack rather than produced in the snow. In contrast, much of the THg associated with mercury depletion events was volatilized back to the atmosphere.     

A preliminary climatology of trajectories related to atmospheric CO2 measurements at Alert and Mould bay

An attempt is made to establish a climatological relationship between anomalous CO₂ values observed at Alert and Mould Bay and trajectories of air parcels arriving at these stations. Measured atmospheric CO₂ values from 1981 to 1984 inclusive were used to obtain climatological winter and summer distributions of 5-day back trajectory origins associated with positive and negative CO₂ deviations relative to a ‘typical’ smoothed seasonal cycle obtained by a simple composite averaging process.Based on a climatological distribution of positive and negative trajectory origins, there are two basic ‘air masses’ over the Arctic Ocean with different CO₂ concentration levels which influence CO₂ deviation values at Alert and Mould Bay.     

The use of ambient measurements to identify which precursor species limit aerosol nitrate formation

A thermodynamic equilibrium model was used to investigate the response of aerosol NO3 to changes in concentrations of HNO3, NH3, and H2SO4. Over a range of temperatures and relative humidities (RHs), two parameters provided sufficient information for indicating the qualitative response of aerosol NO3. The first was the excess of aerosol NH4+ plus gas-phase NH3 over the sum of HNO3, particulate NO3, and particulate SO4(2-) concentrations. The second was the ratio of particulate to total NO3 concentrations. Computation of these quantities from ambient measurements provides a means to rapidly analyze large numbers of samples and identify cases in which inorganic aerosol NO3 formation is limited by the availability of NH3. Example calculations are presented using data from three field studies. The predictions of the indicator variables and the equilibrium model are compared.     

Locations and preferred pathways of possible sources of Arctic aerosol

The chemical composition of particles collected at Alert, Northwest Territories, Canada, show strong, persistent seasonal variations. In a previous study, a 2-way/3-way mixed factor model was performed on the weekly average concentrations of 24 aerosol components measured over the period from 1980 to 1991. The Multilinear Engine (ME), a new mathematical technique, was used to obtain the solution. The two modes of the 2-way model consist of the source composition profiles and mass contributions over the 11 yr, while for the three modes of the 3-way model, source profiles, mass contributions variations over the weeks within a year, and the year-to-year variation over the 11 yr within the measurement period. Five 2-way and two 3-way factors were found to provide a good fit to the data and were easily interpreted. In this investigation, potential source contribution function (PSCF) analysis was applied to the source contributions derived from the ME analysis by incorporating meteorological information in the form of 5-d air parcel back trajectories. The potential locations and/or the preferred pathways of these possible sources were then determined by the PSCF analysis.     

Satellite measurements of aerosol mass and transport

The aerosol optical thickness over land is derived from satellite measurements of the radiance of scattered sunlight. These data are used to estimate the columnar mass density of particulate sulfur on a day with a large amount of sulfur. The horizontal transport of the particulate sulfur is calculated using wind vectors measured with rawins.     

Size-segregated measurements of particulate elemental carbon and aerosol light absorption at remote arctic locations

Size-segregated aerosol samples were taken during 2 winter pollution periods and in clean summer air at different remote locations in the European Arctic > 74°N. By means of a newly developed integrating sphere photometer these filter samples have been analysed for aerosol light absorption coefficients and particulate elemental carbon (PEC). The relatively high PEC concentrations in winter confirm other findings about the Arctic winter atmosphere having an aged continental aerosol burden. In summer very low light absorption coefficients of 4.5 × 10⁻⁸ m⁻¹ were measured, similar to upper tropospheric background values. For the climatically important months of March-May the key optical aerosol properties (extinction coefficient, single scattering albedo and absorption to backscatter ratio) were determined. Based on the approach of J.M. Mitchell (1971, in Man's Impact on Climate. MIT Press, Cambridge, MA) the Arctic haze aerosol is found to contribute to atmospheric heating, even in the summer. A first PEC size distribution was determined in a clean polar summer air. The results show systematic variations in the PEC size distribution from urban to remote locations and seasonal variations in the sink region which may be exploited to quantify aerosol removal process in long distance transport studies.     

Transport and chemical composition of the summer aerosol in the Norwegian Arctic

The chemical composition and transfer routes of the Arctic aerosol during summer have been studied at Ny-Alesund, Bjørnøya, Hopen and Jan Mayen in the period August/September 1983. Samples were also collected on mainland Norway to assess the origin of aerosols transported to the Norwegian Arctic. The concentrations of Si, Al, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Pb, Zn and Cu were measured in samples from a six-stage cascade impactor of Battelle design by particle-induced X-ray emissions (PIXE). The concentrations of Cd, Ni, Pb and Zn were also measured in samples from high-volume samplers by atomic absorption spectrophotometry (AAS).Three interesting periods were identified from the element concentrations. At the beginning of the measurement campaign, the air pollutants measured at Ny-Alesund and Hopen most likely originated in northern America and Greenland. A few days later, very high concentrations of Cd and Zn at Ny-Alesund seemed to be due to air mass transfer from the Soviet Union. During the last episode, observed at Ny-Alesund and Hopen in September, elevated concentrations of several anthropogenic pollutants appeared to be due to emissions in Europe.The results show that anthropogenic emissions from sources in western Europe, Eurasia and northern America may pollute the Arctic air not only in winter but in summer as well. Present levels of air pollutants in the Norwegian Arctic in summer are within the range of levels observed in other remote regions, but are one order of magnitude higher than in Antarctica.     

Columnar optical properties of tropospheric aerosol by combined lidar and sunphotometer measurements at Taipei,Taiwan

Vertical extinction profiles and columnar optical properties (optical depth, Angstrom exponent, lidar ratio, and particle depolarization) of aerosols were obtained by simultaneous measurements with a depolarization lidar and a sunphotometer at Taipei, Taiwan from February 2004 to January 2006. Columnar optical depths are high in Feb–Apr (0.61–0.75) by sunphotometer measurements. Lidar measurements show the contribution of aerosols in the free atmosphere on columnar optical depths are about 44–50% in Feb–Apr and about 26–37% in other months. Back-trajectory analyses and depolarization measurements show almost all of non-spherical aerosols originated from Northwest China which indicate Asian dusts frequently transported to Taipei from dust source regions in the free atmosphere. Aerosols with depolarization lower than 5% are found mostly originated from South China or Southeast Asia. Good correlations between columnar lidar ratio, particle depolarization, and Angstrom exponent are found for cases that columnar water vapor less than 1.5 cm. The effect of water vapor on particle depolarization is briefly discussed.     

The continuous field measurements of soluble aerosol compositions at the Taipei Aerosol Supersite,Taiwan

The characteristics of ambient aerosols, affected by solar radiation, relative humidity, wind speed, wind direction, and gas–aerosol interaction, changed rapidly at different spatial and temporal scales. In Taipei Basin, dense traffic emissions and sufficient solar radiation for typical summer days favored the formation of secondary aerosols. In winter, the air quality in Taipei Basin was usually affected by the Asian continental outflows due to the long-range transport of pollutants carried by the winter monsoon. The conventional filter-based method needs a long time for collecting aerosols and analyzing compositions, which cannot provide high time-resolution data to investigate aerosol sources, atmospheric transformation processes, and health effects. In this work, the in situ ion chromatograph (IC) system was developed to provide 15-min time-resolution data of nine soluble inorganic species (Cl⁻, NO₂⁻, NO₃⁻, SO₄²⁻, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺). Over 89% of all particles larger than approximately 0.056 μm were collected by the in situ IC system. The in situ IC system is estimated to have a limit of detection lower than 0.3 μg m⁻³ for the various ambient ionic components. Depending on the hourly measurements, the pollutant events with high aerosol concentrations in Taipei Basin were associated with the local traffic emission in rush hour, the accumulation of pollutants in the stagnant atmosphere, the emission of industrial pollutants from the nearby factories, the photochemical secondary aerosol formation, and the long-range transport of pollutants from Asian outflows.     

Identification of source nature and seasonal variations of Arctic aerosol by the multilinear engine

Samples of airborne particulate matter were collected over a continuous sequence of 1 week intervals at Alert, Canada beginning in 1980 and analyzed for a number of chemical species. It was found that the measured weekly average concentrations display strong, persistent seasonal variations. In another recent study, the measured concentration of 24 constituents were arranged into both 2-way and 3-way data arrays and bilinear and trilinear models were used to fit the data using a new mathematical technique, positive matrix factorization. Five factors were found to explain the data for both 2-way and 3-way modeling with each factor representing a likely particle source. In the 2-way modeling, the yearly cyclical seasonal variations were not directly retrieved since the whole 11 yr of data was regarded as a single mode in the fitting. In the 3-way analysis, assuming the week-to-week patterns of the source contributions recur from year to year imposed fixed seasonality on the solutions. The resulting fit becomes worse if the year-to-year pattern of variation is not identical for any given source. These results suggested that a mixed model containing both 2-way and 3-way components might provide the best representation of the data. The methodology to calculate such a mixed model has just been developed. The multilinear engine is introduced in this study to estimate a mixed 2-way/3-way model for the Alert aerosol data. Five 2-way and two 3-way factors have been found to provide the best fit and interpretation of the data. Each factor represented probable source with a distinctive compositional profile and seasonal variations. The five 2-way factors are (i) winter Arctic haze dominated by SO^2-₄ including metallic species with highest concentrations from December to April, (ii) soil represented by Si, Al, Ca, (iii) sea salt, (iv) sulfate with high acidity peaking in late March and April and (v) iodine representing most of the observed I with two maxima, one around September and October and another around March and April. The two 3-way factors are (i) bromine characterized by a maximum in the spring around March and April; and (ii) biogenic sulfur which includes sulfate and methanesulfonate with maxima in May and August. The acidic sulfate, bromine, and iodine factors have a common maximum around March/April, just after polar sunrise, suggesting the influence of increased photochemistry at that time of year. The strength of the year-to-year biogenic sulfur factor showed a moderate correlation (r²=0.5) with the yearly average Northern Hemisphere Temperature Anomaly suggesting a relationship of temperature with biogenic sulfur production. The results obtained are consistent with those obtained in the previous study and agree with the Arctic aerosol.     

Study of organochlorine pollutants in snow at North Pole and comparison to the snow at north, central and south Finland

Organic pollutants, especially polychlorinated hydrocarbons, phenols, guaiacols and catechols have been studied by analyses of snow samples from North Pole, May 1984. All of these pollutants were below the limit of determination which was estimated to be as fallout 0.1–0.05 μg/m² for individual compounds. For comparison, snow samples from Central Finland and South Finland 1983–1985 also showed non-detectable levels of chlorinated hydrocarbons but well measurable levels of chlorophenol compounds which were significantly higher at urban (heavy traffic) than rural and higher at South than Central Finnish places, respectively. One sample from Lapland, North Finland 1985, however, had no measurable amounts of chlorophenols like the North Pole sample.     

Measurements of SOx,NOx and aerosol species on Bermuda

During August 1982 and January and February 1983, General Motors Research Laboratories operated an air monitoring site on the southwest coast of Bermuda. The data show that the levels of the NO_x and SO_x species reaching Bermuda are determined by the direction of the air flow. The highest levels of sulfate (mean = 4.0 μg m⁻³), nitric acid (126 ppt) and other species are observed when air masses arrive from the northeastern United States while the lowest levels (sulfate = 1.1 μg m⁻³; nitric acid = 41 ppt) occur during air flow from the SE direction. With westerly air flow, increases in many anthropogenic constituents such as particulate sulfate, lead, elemental carbon, sulfur dioxide, nitrogen dioxide, nitric acid and ozone are observed. These species are generally the lowest during SE winds which bring high concentrations of soil- and crustal-related aerosol species. The source of this crustal material appears to be the Sahara Desert. On the average, the levels of anthropogenic constituents are higher in winter because of frequent intrusions of N American air masses. Conversely, the levels of crustal materials are higher in summer when the SE flow is more prevalent.     

Atmospheric gas and particle measurements at a rural northeastern U.S. site

Ammonia, nitric acid, sulfur dioxide and particles in two size ranges were collected at a rural site in northeastern U.S. in January–March 1984. Ammonia was collected with an oxalic acid coated denuder, all other components were collected on filters. The concentrations of ammonia ranged between 0.0 and 0.5 ppbv, nitric acid: 0.1 and 2.3 ppbv and sulfur dioxide: 1 and 52 ppbv. Ammonium and sulfate in the fine particles were highly correlated, the regression line indicated that the most abundant compound was ammonium sulphate. The content of free hydronium ions in the fine particles was well below the ammonium content. No correlation between NH₃ and NH⁺₄, HNO₃ and NO₃ SO₃ and SO⁻²₄ could be observed.     

International field intercomparison measurements of atmospheric mercury species at Mace Head,Ireland

Eleven laboratories from North America and Europe met at Mace Head, Ireland for the period 11–15 September 1995 for the first international field intercomparison of measurement techniques for atmospheric mercury species in ambient air and precipitation at a marine background location. Different manual methods for the sampling and analysis of total gaseous mercury (TGM) on gold and silver traps were compared with each other and with new automated analyzers. Additionally, particulate-phase mercury (Hg_part) in ambient air, total mercury, reactive mercury and methylmercury in precipitation were analyzed by some of the participating laboratories. Whereas measured concentrations of TGM and of total mercury in precipitation show good agreement between the participating laboratories, results for airborne particulate-phase mercury show much higher differences. Two laboratories measured inorganic oxidized gaseous mercury species (IOGM), and obtained levels in the low picogram m^-3 range.     

Organic aerosol characterization by complementary measurements of chemical bonds and molecular fragments

Organic aerosol chemical markers from normalized concentrations of independent measurements of mass fragments (using Aerosol Mass Spectrometry, AMS) are compared to bond-based functional groups (from Fourier Transform Infrared spectroscopy, FTIR) during eight field projects in the western hemisphere. Several field projects show weak correlations between alcohol group fractions and m/z 60 fractions, consistent with the organic hydroxyl groups and the fragmentation of saccharides, but the weakness of the correlations indicate chemical differences among the relationships for ambient aerosols in different regions. Carboxylic acid group fractions and m/z 44 fractions are correlated weakly for three projects, with correlations expected for aerosols dominated by di-acid compounds since their fragmentation is typically dominated by m/z 44. Despite differences for three projects with ratios of m/z 44 to m/z 57 fragments less than 10, five projects showed a linear trend between the project-average m/z 44 to m/z 57 ratio and the ratio of acid and alkane functional groups. While this correlation explains only a fraction of the fragment and bond variability measured, the consistency of this relationship at multiple sites indicates a general agreement with the interpretation of the relative amount of m/z 44 as a carboxylic acid group marker and m/z 57 as an alkane group marker.     

Measurement of aerosol size distributions for nitrate and major ionic species

Nitrate particle size distributions were determined at the Nitrogen Species Methods Comparison Study at Claremont, CA during September 1985 with the Berner nine stage cascade impactor. Particle bounce was minimized by an inert fluorocarbon grease substrate. Interference from nitric acid was eliminated with a vertical acid gas denuder designed to prevent large particle loss. Samples were taken with and without the grease or denuder to test the techniques. Volatile nitrate loss was less than 10%, based on comparisons with fine nitrate from parallel filter samplers.Aerosol size distributions for the major ionic species were determined under varying atmospheric conditions and good ion balances achieved. Nitrate was bimodal during both day and night-time periods. Fine mode nitrate was associated with ammonium while coarse mode nitrate was associated with both ammonium and sodium. Extensive evidence including size distributions, temporal associations and species balances is given for the reaction of nitric acid and sea salt to produce hydrochloric acid and coarse mode nitrate. Sulfate was primarily in two submicrometer modes. Strong acid was associated with the smaller sulfate mode.