H. W. Gäggeler

Heterogeneous production of nitrous acid on soot in polluted air masses

Polluted air masses are characterized by high concentrations of oxidized nitrogen compounds which are involved in photochemical smog and ozone formation. The OH radical is a key species in these oxidation processes. The photolysis of nitrous acid (HNO2), in the morning, leads to the direct formation of the OH radical and may therefore contribute significantly to the initiation of the daytime photochemistry in the polluted planetary boundary layer. But the formation of nitrous acid remains poorly understood: experimental studies imply that a suggested heterogeneous formation process involving NO2 is not efficient enough to explain the observed night-time build-up of HNO2 in polluted air masses. Here we describe kinetic investigations which indicate that the heterogeneous production of HNO2 from NO2 on suspended soot particles proceeds 105 to 107 times faster than on previously studied surfaces. We therefore propose that the interaction between NO2 and soot particles may account for the high concentrations of HNO2 in air masses where combustion sources contribute to air pollution by soot and NOx emissions. We believe that the observed HNO2 formation results from the reduction of NO2 in the presence of water by C–O and C–H groups in the soot. Although prolonged exposure to oxidizing agents in the atmosphere is likely to affect the chemical activity of these groups, our observations nevertheless suggest that fresh soot may have a considerable effect on the chemical reactions occurring in polluted air.

Pollen and charcoal in lake sediments compared with historically documented forest fires in southern Switzerland since AD 1920

Charcoal in unlaminated sediments dated by 210Pb was analysed by the pollen-slide and thin-section methods. The results were compared with the number and area of forest fires on different spatial scales in the area around Lago di Origlio as listed in the wildfire database of southern Switzerland since AD 1920. The influx of the number of charcoal particles > 75 µm2 in pollen slides correlates well with the number of annual forest fires recorded within a distance of 20-50 km from the coring site. Hence a size-class distinction or an area measurement by image analysis may not be absolutely necessary for the reconstruction of regional fire history. A regression equation was computed and tested against an independent data set. Its use makes it possible to estimate the charcoal area influx (or concentration) from the particle number influx (or concentration). Local fires within a radius of 2 km around the coring site correlate well with the area influx of charcoal particles estimated by the thin-section method measuring the area of charcoal particles larger than 20 000 µm2 or longer than 50 µm. Pollen percentages and influx values suggest that intensive agriculture and Castanea sativa cultivation were reduced 30-40 years ago, followed by an increase of forest area and a development to more natural woodlands. The traditional Castanea sativa cultivation was characterized by a complete use of the biomass produced, so abandonment of chestnut led to an increasing accumulation of dead biomass, thereby raising the fire risk. On the other hand, the pollen record of the regional vegetation does not show any clear response to the increase of fire frequency during the last three decades in this area.

Aerosol climatology at the high-alpine site Jungfraujoch, Switzerland

Continuous aerosol measurements have been performed at the high-alpine site Jungfraujoch (3450 m above sea level) since 1988 by means of an epiphaniometer. The instrument, which determines the Fuchs surface area of the aerosol particles, was operated with a time resolution of 30 min. High correlation coefficients (r>0.8) were found between the epiphaniometer signal and other aerosol parameters, which could be attributed to a rather constant size distribution of the Jungfraujoch aerosol in the accumulation range (0.1<d<1 μm). Well-defined diurnal variations with a peak in the late afternoon were observed on many days during summer, which was not the case during winter. Comparison with black carbon and radon daughter measurements revealed that these diurnal variations are due to vertical transport processes. A statistical analysis showed that the fraction of days with a well-defined diurnal pattern increased with decreasing stability of the atmosphere; however, late afternoon peaks also occurred during days when the potential temperature profile indicated a stable atmosphere. First simulations with ALPTHERM, a new convection model which takes topography into account, were able to explain the observed aerosol patterns. This indicates that slope winds over a certain catchment area are responsible for the transport to this high-elevation site. The distinct seasonal variation with summer values, which are about a factor of 10 higher than winter values, could therefore be attributed to seasonally varying transport processes, due to the seasonal variation of radiation. The data show that even sites at very high elevation cannot be assumed to be in the free troposphere all the time.

Chemical characterization of element 112

The heaviest elements to have been chemically characterized are seaborgium (element 106), bohrium (element 107) and hassium (element 108). All three behave according to their respective positions in groups 6, 7 and 8 of the periodic table, which arranges elements according to their outermost electrons and hence their chemical properties. However, the chemical characterization results are not trivial: relativistic effects on the electronic structure of the heaviest elements can strongly influence chemical properties. The next heavy element targeted for chemical characterization is element 112; its closed-shell electronic structure with a filled outer s orbital suggests that it may be particularly susceptible to strong deviations from the chemical property trends expected within group 12. Indeed, first experiments concluded that element 112 does not behave like its lighter homologue mercury. However, the production and identification methods used cast doubt on the validity of this result. Here we report a more reliable chemical characterization of element 112, involving the production of two atoms of 283112 through the alpha decay of the short-lived 287114 (which itself forms in the nuclear fusion reaction of 48Ca with 242Pu) and the adsorption of the two atoms on a gold surface. By directly comparing the adsorption characteristics of 283112 to that of mercury and the noble gas radon, we find that element 112 is very volatile and, unlike radon, reveals a metallic interaction with the gold surface. These adsorption characteristics establish element 112 as a typical element of group 12, and its successful production unambiguously establishes the approach to the island of stability of superheavy elements through 48Ca-induced nuclear fusion reactions with actinides.

A study of an outstanding Saharan dust event at the high-alpine site Jungfraujoch, Switzerland

Abstract From 20 until 23 March 1990 an outstanding Saharan dust episode was observed at the high-alpine site Jungfraujoch (3450 m a.s.l.), Switzerland. Detailed meteorological analyses including back trajectories showed a transport of the dust-loaded airmass from the Sahara desert over the Eastern Atlantic to Central Europe. During this episode, in situ studies of the aerosol revealed significantly different properties from the background conditions usually observed at the remote site Jungfraujoch. An increase in the coarse particle number concentration was accompanied by a depletion in the ultrafine particle concentration. This unusual absence of ultrafine particles during the first 1.5 days of the episode could be explained by coagulation processes in the Saharan air mass. The distinct coarse particle mode was reflected in the total mass concentration which was higher by a factor of 30 than the annual average at Jungfraujoch. The mineralogical and elemental composition of the dust point to a source region in North and West Africa which was corroborated by the trajectory calculations. The chemical composition of aerosol particles was characterized by high concentrations of soil and sea spray tracers. A snowfall event on 20 March showed “Saharan dust characteristics” and offered the unique possibility to study in situ the scavenging phenomena. Scavenging ratios were enhanced for most components-particularly for NH4+ and SO42− —during this event compared to typical values found for snowfall events without Saharan dust. This was most probably due to heavy riming. This single event contributed substantially to the annual Ca2+ and carbonate deposition at this site and could neutralize 50% of the 1991/1992 winter and spring deposition of NO3− and SO42−.

The epiphaniometer, a new device for continuous aerosol monitoring

A new device is described, which allows continuous monitoring of aerosols of any kind. Air is continuously pumped through a closed chamber containing short-lived 211Pb atoms delivered by a 227Ac source. These atoms attach onto the aerosol particles. After transportation through a thin capillary to a filter and counting station the particles are detected by means of an α-detector for measuring the decay of 211Pb (via 211Bi). Due to the relatively short half-life of 211Pb, the system allows continuous monitoring of aerosols without changing or transporting the filter. The measured signal is proportional to the exposed Fuchs surface of the aerosol particles. In the case of environmental applications, the device is therefore most sensitive to particles contained in the accumulation mode. Due to its high sensitivity it also works well at the lowest particle concentrations of less than 100 ng m−3 with gas flow rates as low as 1 l min−1.

Post 17th century changes of European lead emissions recorded in high-altitude Alpine snow and ice

The occurrence of organic pollutants in European Alpine snow/ice has been reconstructed over the past three centuries using a new online extraction method for polycyclic aromatic hydrocarbons (PAH) followed by liquid chromatographic determination. The meltwater flow from a continuous ice core melting system was split into two aliquots, with one aliquot directed to an inductively coupled plasma quadrupole mass spectrometer for continuous trace elements determinations and the second introduced into a solid phase C18 (SPE) cartridge for semicontinuous PAH extraction. The depth resolution for PAH extractions ranged from 40 to 70 cm, and corresponds to 0.7-5 years per sample. The concentrations of 11 PAH were determined in dated snow/ice samples to reconstruct the atmospheric concentration of these compounds in Europe for the last 300 years. The PAH pattern is dominated by phenanthrene (Phe), fluoranthene (Fla), and pyrene (Pyr), which represent 60-80% of the total PAH mass. Before 1875 the sum of PAH concentration (SigmaPAH) was very low with total mean concentrations less than 2 ng/kg and 0.08 ng/kg for the heavier compounds (SigmaPAH*, more than four aromatic rings). During the first phase of the industrial revolution (1770-1830) the PAH deposition showed a weak increase which became much greater from the start of the second phase of the industrial revolution at the end of 19th Century. In the 1920s, economic recession in Europe decreased PAH emissions until the 1930s when they increased again and reached a maximum concentration of 32 ng/kg from 1945 to 1955. From 1955 to 1975 the PAH concentrations decreased significantly, reflecting improvements in emission controls especially from major point sources, while from 1975 to 2003 they rose to levels equivalent to those in 1910. The Fla/(Fla+Pyr) ratio is often used for source assignment and here indicates an increase in the relative contribution of gasoline and diesel combustion with respect to coal and wood burning from 1860 to the 1980s. This trend was reversed during the last two decades.

Influence of sublimation on stable isotope records recovered from high-altitude glaciers in the tropical Andes

Sublimation dominates the ablation process on cold, high-altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30°08′S, 69°55′W) revealed a strong enrichment in the 2H and 18O content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of δ2H and δ18O help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2–4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.

Historical record of carbonaceous particle concentrations from a European high‐alpine glacier (Colle Gnifetti, Switzerland)

Historical records of the concentrations of black carbon (BC) and elemental carbon (EC), as well as of water insoluble organic carbon (OC) and total carbon (TC) covering the time period ∼1755–1975 are presented. Concentrations were obtained from an ice core of a European high-alpine glacier, using an optical and a thermal method. Concentrations were found to vary between 7 and 128 μg L−1 for BC, between 5 and 130 μg L−1 for EC, between 53 and 484 μg L−1 for OC, and between 66 and 614 μg L−1 for TC. From preindustrial (1755–1890) to modern times (1950–1975) BC, EC, OC, and TC concentrations increased by a factor of 3.7, 3.0, 2.5, and 2.6, respectively. The sum of BC emissions of Germany, France, Switzerland, and Italy, calculated from fossil fuel consumption, and the EC concentration record correlate well (R2 = 0.56) for the time period from 1890 to 1975; this indicates that the ice core record reflects the emissions of western Europe. High pre-1860 concentrations indicate that by that time BC emissions to the atmosphere were already significant.

Chemical properties of element 106 (seaborgium)

The synthesis, via nuclear fusion reactions, of elements heavier than the actinides, allows one to probe the limits of the periodic table as a means of classifying the elements. In particular, deviations in the periodicity of chemical properties for the heaviest elements are predicted as a consequence of increasingly strong relativistic effects on the electronic shell structure. The transactinide elements have now been extended up to element 112 (ref. 8), but the chemical properties have been investigated only for the first two of the transactinide elements, 104 and 105 (refs 9,10,11,12,13,14,15,16,17,18,19). Those studies showed that relativistic effect render these two elements chemically different from their lighter homologues in the same columns of the periodic table (Fig. 1). Here we report the chemical separation of element 106 (seaborgium, Sg) and investigations of its chemical behaviour in the gas phase and in aqueous solution. The methods that we use are able to probe the reactivity of individual atoms, and based on the detection of just seven atoms of seaborgium we find that it exhibits properties characteristic of the group 6 homologues molybdenum and tungsten. Thus seaborgium appears to restore the trends of the periodic table disrupted by relativistic effects in elements 104 and 105.