Christian Lohse

Anthropogenic contributions to atmospheric Hg, Pb and As accumulation recorded by peat cores from southern Greenland and Denmark dated using the 14C bomb pulse curve

Abstract Mercury concentrations are clearly elevated in the surface and sub-surface layers of peat cores collected from a minerotrophic (“groundwater-fed”) fen in southern Greenland (GL) and an ombrotrophic (“rainwater-fed”) bog in Denmark (DK). Using 14C to precisely date samples since ca. AD 1950 using the “atmospheric bomb pulse,” the chronology of Hg accumulation in GL is remarkably similar to the bog in DK where Hg was supplied only by atmospheric deposition: this suggests not only that Hg has been supplied to the surface layers of the minerotrophic core (GL) primarily by atmospheric inputs, but also that the peat cores have preserved a consistent record of the changing rates of atmospheric Hg accumulation. The lowest Hg fluxes in the GL core (0.3 to 0.5 μg/m2/yr) were found in peats dating from AD 550 to AD 975, compared to the maximum of 164 μg/m2/yr in AD 1953. Atmospheric Hg accumulation rates have since declined, with the value for 1995 (14 μg/m2/yr) comparable to the value for 1995 obtained by published studies of atmospheric transport modelling (12 μg/m2/yr). The greatest rates of atmospheric Hg accumulation in the DK core are also found in the sample dating from AD 1953 and are comparable in magnitude (184 μg/m2/yr) to the GL core; again, the fluxes have since gone into strong decline. The accumulation rates recorded by the peat core for AD 1994 (14 μg/m2/yr) are also comparable to the value for 1995 obtained by atmospheric transport modelling (18 μg/m2/yr). Comparing the Pb/Ti and As/Ti ratios of the DK samples with the corresponding crustal ratios (or “natural background values” for preanthropogenic peat) shows that the samples dating from 1953 also contain the maximum concentration of “excess” Pb and As. The synchroneity of the enrichments of all three elements (Hg, Pb, and As) suggests a common source, with coal-burning the most likely candidate. Independent support for this interpretation was obtained from the Pb isotope data (206Pb/207Pb = 1.1481 ± 0.0002 in the leached fraction and 1.1505 ± 0.0002 in the residual fraction) which is too radiogenic to be explained in terms of gasoline lead alone, but compares well with values for U.K. coals. In contrast, the lowest values for 206Pb/207Pb in the DK profile (1.1370 ± 0.0003 in the leached fraction and 1.1408 ± 0.0003 in the residual fraction) are found in the sample dating from AD 1979: this shows that the maximum contribution of leaded gasoline occurred approximately 25 yr after the zenith in total anthropogenic Pb deposition.

Urban and semi-rural observations of carboxylic acids and carbonyls

Abstract In a busy street in Central Copenhagen winter mean concentrations (ppbv) were for formic acid, (0.7 ± 0.3); acetic acid, (1.2 ± 0.5); formaldehyde, (2.6 ± 0.7); acetaldehyde, (1.0 ± 0.7); and acetone, (1.0 ± 0.5). Simultaneous measurements at a semi-rural site 30 km west of Copenhagen showed mean concentrations (ppbv) of: formic acid, (0.6 ± 0.3); acetic acid, (1.0 ± 0.5); formaldehyde, (0.9 ± 0.5); acetaldehyde, (0.7 ± 0.4); and acetone, (0.9 ± 0.4). The similar concentrations of formic acid and acetic acid at the two sites in Denmark when NOy concentrations were one order of magnitude lower at the semi-rural site indicate that primary emission from automobiles was not an important source of the carboxylic acids. In a busy street in Brussels, Belgium summer mean concentrations (ppbv) were for formic acid, (3.6 ± 1.6) and acetic acid, (4.0 ± 2.0). Weaker diurnal variation of formic acid and acetic acid than of the directly emitted NOy and CO in both Copenhagen and Brussels further support sources of the carboxylic acids other than car traffic. For formaldehyde the strong correlations with gas NOy and CO indicate that direct emission from automobile is a source. Acetone shows similar concentrations at the urban and semirural sites and weak correlations with CO and gas NOy which indicate other sources than traffic emission. For acetone and the carboxylic acids the similar concentrations at the urban and semi-rural site may be explained by a regional photochemical source, i.e. oxidation of reactive hydrocarbons in polluted air masses carried to the region by long-range transport.

An air pollution model for use in epidemiological studies: evaluation with measured levels of nitrogen dioxide and benzene.

The aim of the study was to evaluate the predictions derived from the Danish Operational Street Pollution Model (OSPM) when the input data are obtained by simple methods that could be used in large-scale epidemiological studies. The model calculations were thus compared with passive sampler measurements of nitrogen dioxide and benzene at 103 street locations in Copenhagen, Denmark, and at 101 locations in rural areas. Data on traffic and street configuration were collected by means of a simple registration scheme in which forms were filled out by local municipal authorities. Meteorological data were derived from routine measurements at Copenhagen airport, and data on background air pollution were based on a simple empirical model. Differences in air pollution levels between rural areas and Copenhagen and differences in nitrogen dioxide concentrations at various locations in Copenhagen were well reproduced by the OSPM. The correlation coefficients (r) between the measured and the predicted half-year average concentrations of nitrogen dioxide in Copenhagen were between 0.75 and 0.80 for various degrees of precision of the input data for the model. The results indicate that the OSPM used with the presented methods for generation of input data might be useful in assessing long-term exposure to air pollutants in epidemiological studies.

Temporal variation of carbonyl compound concentrations at a semi-rural site in Denmark

Abstract The atmospheric concentrations of formaldehyde, acetaldehyde and acetone were measured by the DNPH-technique at the semi-rural site Lille Valby, Denmark (55°N) between May–July 1995. The average concentrations were observed to be 1.2 ppbv for formaldehyde, 0.8 ppbv for acetaldehyde and 1.9 ppbv for acetone. For the set of carbonyl compounds, concentrations were found to be highly correlated, though only during daytime. The weak correlations observed during nighttime are believed to be caused by the dry deposition of especially formaldehyde. During periods with low photochemical activity the carbonyl compounds also correlated with SO2 and the levels of carbonyl compounds were mainly controlled by meteorological parameters. The highest concentration levels were coincident with episodes of long-range transport from central Europe. A pronounced diurnal profile similar to those observed for PAN and ozone during high-pressure episodes also indicated that photochemical production was a major controlling factor. Here the highest concentrations of carbonyl compounds were observed in air masses with the highest photochemical age (PCA) and a likely source was determined to be the oxidation of hydrocarbons during long-range transport. Especially, the concentration levels of acetone showed a pronounced seasonal variation with the highest levels observed during summertime and lowest in winter and spring. The seasonal variation in the concentration levels of formaldehyde and acetaldehyde were small, thus indicating a low net photochemical production of these components. The measurements were validated by a laboratory intercomparison and good agreement was observed.

Relative contribution of biogenic and anthropogenic sources to formic and acetic acids in the atmospheric boundary layer

Formic and acetic acids are ubiquitous trace gases in ambient air, but their sources remain to be fully understood. They originate from photochemical oxidation of volatile organic compounds (VOC). Direct emissions of these carboxylic acids are supposedly negligible at North European conditions. Radiocarbon analysis of formic and acetic acids provides information on the origin of the precursor VOC, since biogenic VOC have the same 14C/12C ratio as atmospheric carbon dioxide, while VOC emitted from fossil fuels are devoid of 14C. The origin of atmospheric formic and acetic acids was studied by accelerator mass spectrometry of air and rainwater samples. Sampling sites were selected at different distances from centers of anthropogenic activities in Europe, ranging from urban via rural to remote sites. The procedure for preparation of samples was checked by several quality assurance samples, and no significant contamination from atmospheric carbon dioxide was observed. The results show very high (55–100%) biogenic contributions to both carboxylic acids in all samples. Only in semiurban and urban areas were the biogenic fractions less than 80%. The results indicate that biogenic VOC can have substantial influence on formation of photochemical oxidation products and thus photochemical oxidants.

Sources to formic acid studied by carbon isotopic analysis and air mass characterization

Abstract The carbon isotopic composition of formic acid samples from a rural and a semi-remote area in Denmark has been investigated by collection of 5–6 day integrated samples during winter and spring. The results show that 80–100% of formic acid stems from biogenic VOC emitted from terrestrial sources. Even during winter there is a dominating biogenic contribution, which is surprising since biogenic emissions are very low at these latitudes in that period. Quality assurance samples have been used to check the reliability of the collection and pre-treatment procedures for atmospheric formic acid samples. The carbon isotopic investigation was supplemented by measurements of benzene, toluene, formaldehyde, acetone, acetaldehyde and nitrogen dioxide in order to characterise the air masses of one formic acid sample. The general air mass analysis showed that the sampling site, a semi-remote area, was without significant local air pollution sources. During the formic acid sampling period, the air masses were influenced by both direct anthropogenic emissions (benzene, toluene, nitrogen dioxide and acetone) and compounds formed during long-range transport of anthropogenic hydrocarbons (formaldehyde and acetaldehyde). Nevertheless, formic acid still had a predominantly (89±5%) biogenic origin.

Performance of a new diffusive sampler for Hg0 determination in the troposphere

Environmental context. Mercury is of concern to both the public and to the scientific community because it is found at high levels in some marine predators, prompting the US EPA and others to make guidelines restricting the consumption of some species. Most mercury in the environment is emitted to the atmosphere, but it is not known how it is transferred from the atmosphere to the marine environment. Therefore, it is important to study the connection between emission of mercury, its transport and removal from the atmosphere. We have developed a new sampler that is inexpensive, easy to use and with a sufficiently high detection limit that it can be used to measure the low mercury concentrations in the atmosphere at a reasonable time resolution.

Measurements of nitrogen dioxide in Greenland using Palmes diffusion tubes.

Measurements of nitrogen dioxide using the Palmes diffusion tubes in Uummannaq, Aasiaat, and Nuuk. all located along the west-coast of Greenland, have demonstrated that the levels of pollution at the most heavily impacted sites are comparable to levels in much larger towns in Denmark. The highest concentrations were, in general, observed near sites influenced by car traffic (peak concentrations of up to 16 ppbv), medium concentrations were observed in the residential areas (2 6 ppbv), and very low levels were found at the background locations in the town outskirts (1-2 ppbv). Observations of nitrogen dioxide concentrations less than 0.1 ppbv at a remote site, Akia, 25 km from Nuuk, indicate that, compared to local sources, long-range transport of nitrogen dioxide is not important in western Greenland.

Exposure of Danish children to traffic exhaust fumes

This exposure study addresses the validity of the exposure assessment method of an epidemiological study of traffic-related air pollution and childhood cancer. In particular, this paper concerns the question of whether the concentration of nitrogen dioxide (NO2) outside the front door is a valid marker of the exposure of the child living at the address. The study includes 100 children living on streets with dense traffic in central parts of Copenhagen and 100 children living in rural areas. Preliminary results, based on 25% of the study subjects, suggest that both the outdoor NO2-concentration and the exposure of the children are two to three times higher in Copenhagen than in the rural districts. Moreover, the results suggest that the NO2-concentration outside the front door is a poor marker of the exposure of the children in Copenhagen, but a marker of some relevance for the exposure of the children in rural districts. The preliminary results must be treated with caution, as among other things, the analysis did not consider seasonal changes and indoor NO2-sources such as passive smoking, candles, and gas appliances.

Photochemistry of sulphur compounds related to the 1,2-dithiole system. Part 6. Photoisomerization of 1,6,6aλ4-trithiapentalenes

A series of 1,6,6aλ4-trithiapentalenes are transformed upon irradiation into photoproducts which revert to starting materials by a dark process obeying first-order kinetics. The photoproducts have been assigned a trans-1,2-dithiol-3-ylidene thione structure. The stability of the trans-forms has been shown to be controlled mainly by steric factors; it is strongly dependent on the ability of the thiocarbonyl group to be coplanar with the dithiole ring. The rate constants and energy and entropy of activation for the thermal back reactions are reported.