Atmospheric Environment. Part A. General Topics最新文献

Possible intrusion of eruption material from the 1986 eruptions of Augustine volcano into Arctic Haze measurements is considered by comparing haze samples collected directly in the eruption plume. Based on aerosol particle size distribution, particle characteristics, and relative locations of the eruption plume and the haze sampling flight paths it is concluded that intrusion might have occurred during three of the haze flights cited here, but only to a very slight extent, affecting only the larger size particles which constitute a few percent of the total haze aerosol mass.

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.

Snowpack samples were collected from interior and arctic Alaska during March 1988 and analysed for pH, conductivity, NO₃⁻, SO₄²⁻ and other constituents. The mean snowpack NO₃⁻ and SO₄²⁻ concentrations in the interior Alaska snowpack were found to be 160 and 179 mg g⁻¹, respectively. The interior snowpack was observed to have concentrations and deposition fluxes of NO₃⁻ which are approximately 1.5 and 1–3 times, respectively, those observed in Greenland.

In the arctic samples, collected in the Sagavanirktok River Valley, wind-deposited loess substantially increases both pH and SO₄²⁻ concentrations in the snowpack. Snowpack nitrate in these samples is unaffected by the windborne loess and had a mean NO₃⁻ concentration of 688 ng g ⁻¹. The NO₃⁻ deposition flux in the Arctic is approximately two times that found in the interior snowpack.

The most plausible explanation for the elevated NO₃⁻ deposition flux is that the snowpack deposition is strongly influenced by the presence of the “arctic front”, a meteorological boundary which acts to contain the polluted, arctic air mass. Alternatively, local NO_x emissions on Alaska's arctic coast or substantial changes in the scavenging efficiencies may also influence the observe north-south gradient in NO₃⁻ concentrations in the snowpack.

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.

Backward air mass trajectories for Dye 3, Greenland (elevation 2.5 km) show source regions that vary with season: the direction of greatest transport distance is from the southwest in fall, west in winter, and northwest in spring; the trajectories in summer do not show a strong preferred direction. Based on 5 d transit times, the trajectories in fall suggest the importance of North America as a potential source region, with occasional trajectories from western Europe. The trajectories in spring, especially in April, suggest Eurasia (transport over the Pole), eastern North America, and Western Europe as potential source regions. Less transport of chemical constituents to Dye 3 is expected in summer when transport distances are shorter. Although some long-range transport to Greenland occurs in winter, the stability of the atmosphere over the ice sheet at this time of year is likely to limit the delivery of chemical constituents to the surface. Sources outside of these regions can also influence Dye 3 if transit times longer than 5 d are considered. These results are in contrast to trajectories reported by others for sea-level arctic locations such as Barrow, Alaska and Mould Bay, Canada, where transport over the Pole from Eurasia is responsible for high chemical species concentrations over much of the winter and early spring. Overall, the trajectories are consistent with aerosol chemical data for this time period at Dye 3 reported by several investigators, showing peak concentrations in spring and fall.

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.

Surface ozone, particulate bromine and inorganic and organic gaseous bromine species were measured at Barrow, AK, during March and April 1989 to examine the causes of surface ozone destruction during the arctic spring. Satellite images of the Alaskan Arctic taken during the same period were also studied in conjunction with calculated air mass trajectories to Barrow to investigate the possible origins of the ozone-depleted air. It was found that during major ozone depletion events (O₃<25 ppbv) concentrations of particulate bromine and the organic brominated gases bromoform and dibromochloromethane were elevated. Air mass trajectories indicated that the air had crossed areas of the Arctic Ocean where leads had been observed by satellite. The transport time from the leads was typically a day or less, suggesting a fast loss mechanism for ozone. A similarly fast production of particulate bromine was shown by irradiating ambient nighttime air in a chamber with actinic radiation that approximated daylight conditions. Such rapid reactions are not in keeping with gas-phase photolysis of bromoform, but further studies showed evidence for a substantial fraction of organic bromine in the particulate phase; thus heterogeneous reactions may be important in ozone destruction.

Controlled exposure of ice to a reactive gas, SO₂, demonstrated the importance of the chemical composition of the ice surface on the accumulation of acidity in snow. In a series of bench-scale continuous-flow column experiments run at four temperatures (−1, −8, −30 and −60°C), SO₂ was shown to dissolve and to react with other species in the ice-air interfacial region at temperatures approaching the melting point of ice. Experiments consisted of passing air containing SO₂ through glass columns packed with 100-μm ice spheres of varying bulk composition (0–5 μM H₂O₂, and 0–1 mM NaCl), and analysing SO₂ in the air and SO₄²⁻ in the ice. At all temperatures (−60 to −1°C), increased retention volumes were found for increasing ionic strength and oxidant concentration. At the coldest temperatures and with no NaCl, increased retention volumes for −60 vs −30°C are consistent with SO₂ uptake by physical adsorption. At warmer temperatures, −8 and −1°C, the observed tailing in the sorption curves indicated that other processes besides physical adsorption were occurring. The desorption curves showed a rapid decrease for the warmer temperatures, indicating the sorbed SO₂ is irreversibly oxidized to SO₄²⁻. Results indicate that aqueous-phase reactions can occur below −8°C (i.e. −30 and −60°C). Results for different salt concentrations show that increasing ionic strength facilitates SO₂ oxidation at colder temperatures, which is consistent with freezing point depression. One environmental implication is that snowpacks in areas with background SO₂, can accumulate acidity during the winter months. As acidity accumulates, the solubility of SO₂ will decrease causing a concomitant decrease in the air-to-surface flux of SO₂. Modeling dry deposition of gases to snow surfaces should incorporate the changing composition of the ice surface.

In six aircraft flights of AGASP-II, 2–15 April 1986, from ca. 1300–8100 m altitude, the most abundant elements measured in size separated aerosol samples were silicon, chlorine, and sulfur. Concentrations were higher than at ground level (G), particularly at highest altitudes (HT, 5600–8100 m, upper troposphere to lower stratosphere) compared to mid troposphere (MT, 1300–4700 m), especially for ultrafine particles <0.0625 μm aerodynamic diameter. HT and MT median and G average concentrations, μm⁻³ STP, respectively (1) Si = 3.64, 1.30, 0.092; (2) S = 1.44, 0.265, 0.087; (3) Cl = 1.62, 0.36, 0.213. The weight ratio Al/Si was less than half that expected for Earth crust material (0.3), evidence against fine silicon originating mainly by dispersion of volcanic debris or other eolian dust particles. Instead, pollution from high rank (mainly bituminous) coal combustion, which can form SiO vapors from quartz in the ash and fine alkaline aerosol with low Al/Si ratio, is a more likely source of apparently widespread aerosol silicon contamination of the Arctic atmosphere. Chlorine and sulfur gases may be scavenged by coarse alkaline dust particles and acidic chlorine and sulfur may be derived from coal combustion processes, thus also accounting for their high concentrations.

High volume filter samples were collected at Poker Flats in central Alaska, between 1984 and 1987, and analysed for a comprehensive suite of elements. In this report we focus on the results for the halogen elements Br, Cl and I, and their correlations with other selected elements (Al, As, Na, Se, and V). Seasonal cycles were observed for the halogens, including a pronounced spring peak in Br and a weak fall peak, pronounced spring and fall peaks in I, and increased winter/spring Cl. A significant correlation between Br and Se was shown to be partly due to common transport pathways, and possibly some common sources. Iodine showed enrichments of three orders of magnitude over sea water composition. Correlations to marine elements suggested a marine biogenic source. Chlorine evidently originated from sea salt aerosols, but showed evidence of substantial volatilization, correlated to the degree of pollution of the air mass.