Atmospheric Environment. Part A. General Topics最新文献

Physical and chemical parameters of the arctic aerosol were investigated at Ny Ålesund, Svalbard, in March and April 1989 in connection with the third Arctic Gas and Aerosol Project (AGASP III). The number size distribution of the particles was measured over the range of 0.02-1.0 μm. Filter samples were analysed for elemental composition and two integral chemical properties, hygroscopic growth and volatility, were measured. Along with the latter measurements, the distribution of these properties at specific particle sizes, i.e. the degree of internal mixing, was determined.

Both clean, marine conditions and “arctic haze” episodes were included in the series of measurements. The number size distribution indicated that the aerosol was well aged based on its narrowness and the relative low concentration of nuclei mode particles. It had a number mode at 0.22 μm diameter and geometric standard deviation of 1.4. Generally the particles exhibited uniform hygroscopic growth properties, i.e. they were largely internally mixed. The growth factor was 1.45 at 90% relative humidity. Approximately 40% of the overall particulate mass was volatile at a temperature of 50°C. The volatile fraction varied form particle to particle, i.e. the particles were externally mixed with respect to volatility.

In the spring of 1986 and 1989, particle nitrite was measured at Barrow, Alaska, by filter sampling and by ion chromatographic analysis. Particle nitrite concentrations averaged 2.9 ± 3.4 and 2.6 ± 2.0 ppt (molar ratio) in 1986 and 1989, respectively. Both seasons showed diurnal variations with higher concentrations during the day which might have been caused by daytime down mixing. In 1989, nitrite was determined in several snow samples with concentrations between 0 and 0.18 μmol ℓ⁻¹. Particle nitrite was probably in disequilibrium with gas phase, suggesting a heterogeneous source for gaseous HONO. A relationship between particle nitrite and sodium ions suggests that sea salt could be involved in nitrite formation, perhaps through hydrolysis of nitryl halides.

During six aircraft flights conducted as part of the third Arctic Gas and Aerosol Sampling Program (AGASP III, March 1989), 189 air samples were collected throughout the Arctic troposphere and lower stratosphere for analysis of CO₂, CH₄ and CO. The mixing ratios of the three gases varied significantly both horizontally and vertically. Elevated concentrations were found in layers with high anthropogenic aerosol concentrations (Arctic Haze). The mixing ratios of CO₂, CH₄ and CO were highly correlated on all flights. A linear regression of CH₄ vs CO₂ for pooled data from all flights yielded a correlation coefficient (r²) of 0.88 and a slope of 13.5 ppb CH₄/ppm CO₂ (n = 186). For CO vs CO₂ a pooled linear regression gave r² = 0.91 and a slope of 15.8 ppb CO/ppm CO₂ (n = 182). Carbon dioxide, CH₄ and CO also exhibited mean vertical gradients with slopes of 0.37, –4.4 and −4.2 ppb km⁻¹, respectively.

Since the carbon dioxide variations observed in the Arctic atmosphere during winter are due primarily to variations in the emissions and transport of anthropogenic CO₂ from Europe and Asia, the strong correlations that we have found suggest that a similar interpretation applies to CH₄ and CO. Using reliable estimates of CO₂ emissions for the source regions and the measured CH₄/CO₂ and CO/CO₂ ratios, we estimate a regional European CH₄ source of 47±6 Tg CH₄ yr⁻¹ that may be associated with fossil fuel combustion. A similar calculation for CO results in an estimated regional CO source of 82±2 Tg CO yr⁻¹.

Aerosol trace element concentrations spanning an eleven month period at Dye 3, Greenland are presented. Sea salt input into the lower atmosphere of the ice sheet occurs predominantly in the winter months of December-February. These aerosols are the product of vigorous Arctic winter storms. Long range transport of crustal material from lower latitude arid regions to the Greenland Ice Sheet takes place predominantly during the spring. The onset of Arctic sunrise and associated weakening of the surface and upper level inversion over the ice sheet appear to be important factors resulting in higher crustal aerosol concentrations in the lower levels of the Greenland atmosphere during the month of April. A strong pulse of crustal aerosol (260 ng Al scm⁻¹) was observed at Dye 3 on 14–15 April 1989. Meteorological evidence suggests that strong winds and deep convective activity injected dust high into the atmosphere over the Sahara desert region. This airmass then appears to have passed northwand over western Europe where it mixed with anthropogenic aerosols and arrived in the Dye 3 region some 4–6 d hence. Elevated concentrations of anthropogenic aerosol species were also observed at the surface during the months of April and May. Long range transport of these aerosols appears to be important during the Arctic winter and spring, while enhanced downward mixing due to a weakening inversion results in elevated concentrations at the surface during April and May. An increase in scavenging due to persistent Arctic stratus and the northward migration of the Polar Front in the spring results in very low anthropogenic aerosol concentrations during the summer months. Particulate aerosol iodine and bromine concentrations also peak during the month of April at Dye 3. It has been suggested that this spring particulate halogen peak, which is observed throughout the Arctic, may be the result of photochemical aerosol production from biogenic organo-halogen species. Regional meteorological phenomena as well as seasonal variations in source strength and long range transport appear to be important factors influencing aerosol concentrations in the surface atmosphere of the Greenland Ice Sheet.

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.

Concentrations of lead, cadmium, copper and zinc have been measured in a variety of samples of fresh or slightly aged snow collected at Dye 3, south Greenland, on a precipitation event basis from January to August 1989. Measured concentrations are found to be very variable from one snowfall to another, with high concentration peaks occurring in April and June. The four metals are shown to be mainly derived from anthropogenic sources, with the exception of Cu and Zn for some of the samples. The data obtained for several snow events are further discussed using 5 days backward air mass trajectories together with data for various other chemical species.

As part of the DGASP program on the Greenland Ice Sheet, an investigation was conducted into the nature of ice particle formation processes that result in the formation of snow. Ice particle habits were determined using Formvar replicas of falling snow crystals. From these measurements an assessment of the primary growth process and altitude of formation was made. Results indicate that the scavenging of cloud water by falling ice particles, growth by accretion or riming, rarely occurs. However, when riming does occur, it is usually associated with warmer air masses from the south. The occurrence of riming was also observed to be dependent on the season, with a greater frequency occurring during warmer months. It was estimated that ice particle riming contributes less than 5% of the average annual water mass, but up to 30% of the deposition of some chemical species, deposited to the Greenland Ice Sheet at Dye 3. Ice particle habits indicate that they originate at higher altitudes above the ice cap in summer than in winter. Variations in the magnitude of ice particle riming, the elevation of origin of ice particles, the meteorology and the season of the year are all essential when interpreting snow chemistry and comparing snow and aerosol chemistry at Dye 3.

Preliminary measurements of the gaseous and particulate fractions of atmospheric PAH have been performed in winter and spring during the DGASP program on the Greenland Ice Sheet. The results indicate the absence of gaseous fraction (<0.5 pg m⁻³) associated with very lw concentrations restricted to heavy compounds only (18–200 pg m⁻³ for total PAH) in the particulate phase; the repartition of the various PAH is very stable from one sample to another, indicative of near-complete reaction allowed by long transport time. Nevertheless, discrimination of the specific signal from the different sources involved may have been impaired by the long sampling time used.