C Davidson

A new cascade impactor for aerosol sampling with subsequent PIXE analysis

Abstract A small deposit area low pressure impactor (abbreviated to SDI) has been developed and tested. The device has been designed specifically to collect size-fractionated aerosol samples in remote locations for subsequent chemical analysis by PIXE. The SDI is a 12-stage, multinozzle device, but the deposit for each stage remains confined to an area with diameter less than 8 mm. It operates at a flow rate of 11 L/min and accepts the same, 25 mm diameter substrate rings as the PIXE International cascade impactor. The experimental cut-points for stages 12 through 1 are 8.50, 4.08, 2.68, 1.66, 1.06, 0.796, 0.591, 0.343, 0.231, 0.153, 0.086 and 0.045 μm equivalent aerodynamic diameter. The SDI has been tested in (and employed for) size-fractionated aerosol sampling in the Finnish Arctic and at Summit in Greenland. The data show that the SDI gives results very similar to those obtained with the PIXE International impactor, but with detection limits that are much lower. This suggests that the SDI can be used with shorter sampling times or in areas where concentrations are smaller to obtain reliable size distribution data. The results also suggest that data for a greater number of elements can be obtained with the SDI.

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42− serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.

Seasonal variations in sulfate, nitrate and chloride in the greenland ice sheet: Relation to atmospheric concentrations

Samples from three snowpits near Dye 3 in South Greenland have been used to study seasonal variations in contaminant transport from the atmosphere to the Ice Sheet. The snowpits cover the years 1982–1987. The samples have been dated by comparing δ18O values with meteorological data from Dye 3. Airborne concentrations of SO2−4 over the Ice Sheet have been estimated for the dates corresponding to each snowpit sample by statistically analyzing data from several air monitoring stations throughout the Arctic, and computing average values from the appropriate stations. Seasonal variations in concentrations in air, concentrations in snow, and mass-basis scavenging ratios (concentration in snow divided by concentration in air) have been identified. Results indicate that concentrations of SO2−4in the air show a strong peak in late February, resulting from long-range transport of mid-latitude anthropogenic emissions, while those in the snow show a broad peak in January, February and March with smaller seasonal variation overall. The smaller variation in the snow is attributed in part to the effect of riming, which results in more efficient scavenging during warm weather when airborne concentrations are low. The importance of riming is also supported by the annual cycle in scavenging ratio which peaks in mid-summer coincident with maximum temperatures. In agreement with previous estimates, dry deposition appears to account for 10–30% of the total SO2−4 in the snow. Concentrations of NO−3 in the snow show a strong peak in summer; natural material from the stratosphere as well as anthropogenic emissions transported from the mid-latitudes may be responsible. Concentrations of Cl− in the snow are maximum in January, with relatively high concentrations during October through March and a smaller peak in July. The winter peak is believed to reflect long-range transport (LRT) of marine aerosol from north Atlantic storms, while the summer peak is attributed to seaspray from nearby coastal Greenland. Riming also may influence the seasonal variations in NO−3 and Cl− in the snow.

Chemical constituents in the air and snow at Dye 3, Greenland—I. Seasonal variations

Chemical constituent concentrations in air and snow from the Dye 3 Gas and Aerosol Sampling Program show distinct seasonal patterns. These patterns are different from those observed at sea-level sites throughout the Arctic. Airborne SO42− and several trace metals ofcrustal and anthropogenic origin show strong peaks in the spring, mostly in April. Some species also have secondary maxima in the fall. The spring peaks are attributed to transport over the Pole from Eurasian sources, as well as transport from eastern North America and western Europe. The fall peaks are attributed primarily to transport from North America, and less frequent transport from Europe. Airborne 7Be and 210Pb show strong peaks in both spring and fall, suggesting that vertical atmospheric mixing is favored during these two seasons. Several other airborne constituents peak at other times. For example, Na peaks in winter due to transport of seaspray from storms in ice-free oceanic areas, while MSA peaks in summer due to biogenic production in the oceans nearby. Many trace gases such as freons and other chlorine-containing species show roughly uniform concentrations throughout the year. CO and CH4 show weak peaks in February–March. Concentrations of chemical constituents in fresh snow at Dye 3 also show distinct seasonal patterns. SO42− and several trace metals show springtime maxima, consistent with the aerosol data. Na shows a winter maximum and MSA shows a summer maximum in the snow, also consistent with the aerosols. 7Be and 210Pb in the snow do not show any strong variation with season. Similarly, soot and total carbon in snow do not show strong variation. When used with dry deposition models, these air and snow concentration data suggest that dry deposition of submicron aerosol species has relatively minor influence on constituent levels in the snowpack at Dye 3 compared to wet deposition inputs (including scavenging by fog); crustal aerosol, on the other hand, may have a more significant input by dry deposition. Overall, the results suggest that gross seasonal patterns of some aerosol species are constistent in the air and in fresh snow, although individual episodes in the air are not always reflected in the snow. The differences in data reported here compared with data sets for sea-level arctic sites demonstrate the need for sampling programs on the Ice Sheet in order to properly interpret Greenland glacial record data.

Size distributions of atmospheric trace elements at dye 3, Greenland — I. Distribution characteristics and dry deposition velocities

Cascade impactor samples were collected at Dye 3 on the south-central Greenland Ice Sheet during March 1989. The impactor was calibrated in the laboratory, and the resulting collection efficiency curves were used to derive the impactor response for use in a data inversion procedure. The impactor samples were chemically analysed by proton-induced X-ray emission (PIXE), and the chemical concentration data were used with the inversion procedure to generate smooth size distributions for 15 elements. Results show three distinct size distribution categories. The first category includes elements that mainly originate from gas to particle conversion, with a substantial fraction from anthropogenic combustion (S, Pb, Zn, Br and Ni). These elements exhibit a unimodal size distribution with geometric mean aerodynamic diameter close to 0.6 μm, although S and Zn show a weak second mode centered at about 2 μm. Elements in the second category (Ti, Si, Fe, Mn, Ca, K) exhibit bimodal size distributions, with geometric mean diameters for the two modes of 0.6 and 2 μm, respectively. These elements results from a variety of sources, including crustal erosion as well as combustion from natural and anthropogenic sources. For elements in the third category (Al, Cl, Na, Mg), most of the mass occurs in particle sizes above 1 μm. Their size distribution is generally unimodal, with the geometric mean aerodynamic diameter around 2 μm. These elements are most likely to be of crustal and/or marine origin. The best-fit size distributions were used with curves of dry deposition velocity vs aerodynamic particle diameter to estimate the overall dry deposition velocity expected from the entire distribution. The deposition velocities for S, Pb, Zn, Br and Ni are all very low, with values less than about 0.02 cm s−1 if hygroscopic growth in the humid layer is neglected. For the other elements, deposition velocities are in the range 0.2-0.7 cm s−1. For those distributions that are bimodal, the upper mode generally dominates deposition even when most of he airborne mass is associated with the lower modes, as in the case of S and Zn.

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.

Biomass burning signatures in the atmosphere of central Greenland

Daily atmospheric concentrations of participate oxalate measured at the Summit of the Greenland Ice Sheet are presented for the summers 1992–1995. We believe that four episodes of elevated concentrations are due to biomass burning plumes passing over the site. In at least two cases the source regions of the fires are located in northern Canada. Further characteristics of the aerosol are examined during one of these events. A large increase of particle number concentrations in the accumulation mode can be observed, while the increase is much more limited for total particle number. The suite of chemical species enriched in the aerosol includes typical biomass burning tracers like fine K, large concentrations of ammonium, particulate formate and acetate, as well as other organic species like glycolate. The size distributions of K, oxalate, and glycolate are skewed toward the accumulation mode and exhibit the very same shape as sulfate, suggesting internal mixing of these species in the same particles. Molar ratios S/K indicate incorporation of S during transport, most probably by production of sulfate. Concentrations of these species were measured in fog samples for radiative events that occurred during the plume passage. There is a good agreement in the relative variation of concentrations between the aerosol and fog for oxalate and glycolate, while the gas phase probably dominates incorporation in the fog droplets for acetate, formate, chloride, nitrate, and sulfate (incorporated as SO2, which is further oxidized). The complexity of the transfer of the organic acids from the atmosphere to fog is underlined.

Fluxes of chemical species to the Greenland ice sheet at Summit by fog and dry deposition

Abstract Experiments were performed during June–July 1992 to determine the impact of dry deposition and fog deposition on surface snow chemical inventories. The fluxes of SO2−4, NO−3, Cl−, MSA, Na, Ca, and Al were measured by collecting deposited fog on flat polyethylene plates. Dry deposition fluxes of SO2−4 were measured using aerodynamic surfaces. Real-time concentrations of atmospheric particles greater than 0.5 um and greater than 0.01 um were measured using continuous monitors. Filter samplers were used to determine daily average atmospheric SO2−4 and MSA concentrations. Also, daily surface snow samples were taken and analyzed for SO2−4, NO−3, Cl−, Na+, Ca2+, and NHJ+4. The real-time concentration data indicate that aerosol particles greater than 0.5 urn are efficiently incorporated into fog droplets. Results also show that condensation nuclei (CN) are not as greatly affected by fog as the larger particles. Fog fluxes of SO2−4 and NO−3 have similar values and are approximately 4 times greater than those of Cl−, an order of magnitude greater than those of MSA, Na, and Ca, and two orders of magnitude greater than those of Al. The fog deposition flux of SO2−4 appears to be much greater than the dry deposition flux, based on experimental data. This indicates that dry deposition has a negligible effect on surface snow SO2−4 concentrations on days when there is fog. Such a finding is consistent with significant increases in surface snow SO2−4, NO−3, and NH+4 inventories seen after fog events. Cl− surface snow inventories are affected by fog but not as greatly. Variation in surface snow chemical inventories makes it difficult to obtain quantitative estimates of daily chemical fluxes. Surface snow Ca2+ and Na+ are relatively unaffected by post snowfall processes due to low atmospheric concentrations relative to the amount of material in fresh snow. Model results suggest that the fog fluxes have been underestimated by the current sampling technique.

Chemical constituents in the air and snow at Dye 3, Greenland—II. Analysis of episodes in April 1989

Detailed examination of a two-week period in April 1989 during the Dye 3 Gas and Aerosol Sampling Program shows that episodes of relatively high concentration of certain chemical constituents occur at this time of year. Airborne concentrations of crustal metals such as Al and Ca can exceed 100 ng m−3, while concentrations of SO42− can exceed 1000 ng m−3. Elevated concentrations of MSA, 7Be and 210Pb are also noted. Consideration of synoptic maps and backward air mass trajectories suggests that the episodes are due to transport from a variety of source regions, including Eurasia (transport over the Pole), North America and western Europe. In addition to elevated airborne concentrations, levels of these constituents in surface snow are high during April. However, it is difficult to develop quantitative relationships between concentrations in air and in snow due to the difficulty in measuring airborne concentrations at cloud-level; variations in scavenging by clouds may also be significant. It is concluded that the springtime maxima in airborne concentrations resulting from long-range transport from a variety of source regions are responsible for strong identifiable signals in ice cores and snowpits from this region.

Temporal and spatial variability of snow accumulation in central Greenland

Snow accumulation records from central Greenland are explored to improve the understanding of the accumulation signal in Greenland ice core records. Results from a “forest” of 100 bamboo poles and automated accumulation monitors in the vicinity of Summit as well as shallow cores collected in the Summit and Crete areas are presented. Based on these accumulation data, a regression has been calculated to quantify the signal-to-noise variance ratio of ice core accumulation signals on a variety of temporal (1 week to 2 years) and spatial (20 m to 200 km) scales. Results are consistent with data obtained from year-round automated accumulation measurements deployed at Summit which suggest that it is impossible to obtain regional snow accumulation data with seasonal resolution using four accumulation monitors positioned over a length scale of ∼30 km. Given this understanding of the temporal and spatial dependence of noise in the ice core accumulation signal, the accumulation records from 17 shallow cores are revisited. Each core spans the time period from 1964 to 1983. By combining the accumulation records, the regional snow accumulation record has been obtained for this period. The results show that 9 of the 20 years can be identified as having an accumulation different from the 20 year mean with 99% confidence. The signal-to-noise variance ratio for the average accumulation signal sampled at annual intervals is 5.8±0.5. The averaged accumulation time series may be useful to climate modelers attempting to validate their models with accurate regional hydrologic data sets.