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Co-located long-term routine measurements of sulphur dioxide with the impregnated filter method and a pulsed fluorescence analyser at four Finnish background stations were compared. The data consist of 9 years of parallel measurements at the Virolahti station and 8, 7 and 2 years in Uto, Oulanka and Ahtari, respectively, between the years 1993–2001. During the whole study period, the daily SO2 concentrations measured with the analyser and the filter method showed excellent agreement in Virolahti (number of daily data pairs 2748, r 2 = 0.99 ) and Uto ( n = 2328 , r 2 = 0.97 ), and no systematic relative error between the two methods was found. Also in Oulanka ( n = 2131 , r 2 = 0.99 ) and Ahtari ( n = 643 , r 2 = 0.97 ) the linear model described the data well. However, in Oulanka the analyser produced 16% and in Ahtari 10% higher results than the filter method. The difference would decrease to 12% in Oulanka and to 8% in Ahtari if the concentrations were given at the same reference pressure. At all stations also a small constant error (analyser>filter) of the order of 0.03–0.06 μg S m−3 was discerned. The role of nitrogen oxide, volatile organic compounds and water vapour interferences as well as extreme temperatures were studied but no clear systematic influence by them on the daily differences between the two methods was detected.

Concentrations of different species of mercury in arctic air and precipitation have been measured at Ny-Alesund (Svalbard) and Pallas (Finland) during 1996–1997. Typical concentrations for vapour phase mercury measured at the two stations were in the range of 0.7–2 ng m−3 whereas particulate mercury concentrations were below 5 pg m−3. Total mercury in precipitation was in the range 3–30 ng l−1. In order to evaluate the transport and deposition of mercury to the arctic from European anthropogenic sources, the Eulerian transport model HMET has been modified and extended to also include mercury species. A scheme for chemical conversion of elemental mercury to other species of mercury and deposition characteristics of different mercury species have been included in the model. European emission inventories for three different forms of Hg (Hg0, HgCl2 and Hgp) have been implemented in the numerical grid system for the HMET model.

Observations of the concentration of several nitrogen containing compounds at five rural Scandinavian sites during March–June 1993 are reported. Total nitrate (NO3- + HNO3) and total ammonium (NH4+ + NH3) were measured by denuder and filter pack. In general the methods agree well. At all sites the particulate fraction dominated, with the largest fraction of NO3- and the lowest of NH4+ at the sites which were closest to the emission sources. The fraction of NO3- of total nitrate increased with increasing NO2 concentrations, indicating that the nighttime conversion of NO2 to NO3- is an important route of formation for NO3-. A positive correlation was found between HNO3 and O3 in June at all sites, while no correlation was found early in the spring. Model calculations were made with a lagrangian boundary layer photooxidant model for the whole period, and compared to the measured concentrations. The calculated ratio between mean observed and modelled daily maximum concentrations of ozone over the measurement period were within +/−10% at all sites. The models ability to describe the daily ozone maximum concentration was satisfactory with an average deviation of 19–22% from the observed concentrations. HNO3 was underestimated by over 50% at all sites except the one closest to the emission sources. The correlation between modelled and observed concentrations was generally best for the sites with shortest transport distance from the sources of emission.

Observations and model calculations of the concentration of hydrocarbonsat five Scandinavian rural sites during March–June 1993are reported.Decreasing concentrations from March to June are observedat all sites. The highest concentrations of hydrocarbons were found in air massescoming in from the southwest to southeast, indicating that long rangetransport fromcontinental Europe and the U.K. is important in pollution episodes. An episode of elevated concentrations of hydrocarbons observed at three of the sites in the middle of Marchis described and discussed in relation to the synoptic situation and thepresenceof other chemical compounds (NO2, PAN, total nitrate andozone).A Lagrangian numerical model is used to calculate the concentrations of theindividual hydrocarbons at the fivesites and comparison with observations is made.The calculated concentrations for nonmethane hydrocarbons with quite longchemicallifetimes agree well with the observations.For the sum of observed and calculated hydrocarbons the correlationcoefficientsare in the range of 0.65–0.88 for the five sitesand the ratio between calculated and measured concentrations was0.72–0.97, indicating thatthe European VOC emission inventory is quite well estimated.

The occurence of ozone concentrations and exposure indices related to the adverse effects of ozone upon vegetation are reported for four Finnish background stations. In Finllnd, ozone concentrations are often near the background tropospheric values of ca. 30 ppb. Very high concentrations are not observed. The maximum 1-h average in this data set was 79 ppb. The exposure parameter, which accumulates growing season 1-h aaverage concentrations above a 40 ppb base-line in daylight hours, gave clearly different exposure sums for the stations. These values varied between 4000 and 8500 ppb-h in the southern archipelago, 3000–6500 ppb-h in the southern coastal region. 2000–4000 ppb-h in central parts of the country, and 400–1000 ppb-h in the northern parts of the country. The date of the start of the vegetative season is important in high northern latitudes, because the spring maximum of ozone concentrations is relatively intense compared to the summer maximum. In northern Scandinavia, ozone exposures are particularly sensitive to the date of the start of the growing season. The long daylight period in northern Scandinavia is less important in this respect, since during the growing season ozone concentrations are usually below 40 ppb during the morning and evening hours. A good correlation was found between growing season average concentrations of the sum gaseous HNO 3 and particulate NO 3 , and on ozone exposure index which accumulates concentrations above a 40 ppb base-line, confirming the anthropogenic origin of the elevated ozone exposures.

An account is presented of sulphur pollution in northernmost continental Europe, based on analysis of recent observations and on dispersion model calculations. To complement the routine daily observations made at background stations, SO 2 has also been measured using an hourly registering monitor at Janiskoski in the Russian Kola Peninsula. Sulphur dioxide emissions from the Kola Peninsula, totalling 600 Gg (10 3 tonnes) yr −1 , have a dominant impact on SO 2 concentrations and S deposition over large areas, producing an environmental load exceeding the thresholds for potential detrimental effects. The major part of these emissions is produced by two non-ferrous smelters. Due to these sources, SO 2 concentrations vary considerably within the study area, from the low “remote background” values to pollution episodes with −100 μ g(SO 2 )m −3 as an hourly average, depending on wind direction. High concentrations also occur in summer, providing a great potential for dry deposition and suggesting that exposure is a highly episodic process. On an annual basis, dry deposition predominates over wet deposition. Precipitation acidity is related mainly to sulphate, and neutralization by alkaline cations is exceptionally low in the north. According to model calculations, a potentially critical deposition of 0.3 g(S) m −2 yr −1 is exceeded over an area of 150,000 km 2 , 32,000 km 2 of which are in Finland and 19,000 km 2 in Norway. Within this area the contribution of smelter emissions to sulphur deposition ranges from 40% to almost 100%.

Measurements of ozone, nitrogen dioxide and meteorological parameters at the two Finnish EMEP background stations of Ahtari (forested site) and Uto (an offshore island) show clear indications of the influence of the precursor source areas of Western and Eastern Europe on surface ozone behaviour at these higher latitudes. The mean ozone levels are relatively high, with maximum monthly values of 41 and 42 ppb, respectively. These values occur in April at both sites nearly irrespectively of wind direction, pointing to a global feature. Other spring, as well as summer and autumn, months have lower ozone values for wind direction sectors corresponding to clean air masses (north-westerlies and north-easterlies). Surface uptake is an important sink in the local ozone budget, especially during late spring, summer and early autumn. Chemical losses are more efficient in winter, when the ground is covered by snow.

CO, CH4, and non-methane hydrocarbons (NMHCs) are important precursors of photo-oxidants. Therefore, these species have been measured at a number of TOR stations in western Europe. An interlaboratory comparison of the analytical methods employed by the various participating laboratories (PLs) and subsequent data harmonisation were performed to ensure data compatibility within TOR. The relative deviations of the PLs from the CO reference values by the Fraunhofer Institute for Atmospheric Environmental Research (IFU), acting as the central laboratory, covered the range from —2.21 to +2.38 percent, while the relative deviations of the PLs from the respective CH4 reference values were found to range from +0.66 to +2.31 percent. For most of the nineteen species considered in the intercomparison of atmospheric C2—C7 NMHC measurements, the relative deviations of IFU and the various PLs from the respective reference values U* for each round were found to typically fall into the range of -35 to +35 percent.

The performance characteristics of five commercial sulphur dioxide monitors were studied. The experiments included both laboratory tests and field monitoring in an urban environment. According to the laboratory tests, the most sensitive concentration region was between 100 and 500 ppb [1 ppb(SO2) = 2.86 μgm−3 at NTP]. The lower detection limits were 2 to 5 ppb, and the precision varied from 1 to 26 ppb in the concentration range up to 500 ppb. The field data were analyzed with orthogonal regression and principal component analysis, and the results were less favourable than in the laboratory tests.

Water-soluble nitrogen compounds play an important role in the acidification and eutrophication of our environment. The most abundant species are NH3 and HNO3 which both are present in gaseous and particulate forms. They can both be transported over long distances, especially in particulate form. In order to study this transport a network of stations in different countries has to be used. A prerequisite is of course that all laboratories involved use comparable measurement techniques. Another prerequisite is that the methods are so simple and inexpensive to use that a dense network can be established even at remote places. Several field experiments have been performed in which techniques to measure the gaseous and particulate forms separately are compared. However, they have not been very successful. In this study three techniques to measure the total (gaseous and particulate) concentrations of NH4+ and NO3− were compared in field experiments. Comparison of measurements made by different laboratories in different countries using the same sampling technique at a given place yielded a relative standard deviation of 20% for total NH4+ and 15% for total NO3−. The same relative standard deviations were obtained when different sampling techniques (denuder, total filter, filter pack) were compared.

Total (particulate + gaseous) ammonium and total nitrate were measured on a 24-h basis continuously at six Nordic EMEP stations from August 15 to October 31, 1984. They were both measured using impregnated filters mounted in open face filter holders. High concentrations in air occurred episodically at several sites almost simultaneously. Calculated backtrajectories indicated that the air had passed over the western parts of Europe during the episodes. The concentrations of H+, NH 4 + , NO 3 − and SO 4 2− in precipitation was also measured. From the data it is clear that NO 3 − and SO 4 2− are enriched in precipitation compared to NH 4 + . One possible explanation to this is the in-cloud oxidation of SO2 and NO2. Another contributing factor is a higher apparent scavenging rate for SO 4 2− and NO 3 − that may be caused by a lower concentration decrease with altitude.

We present preliminary results of 14 months of ozone observations carried out on a small island in the Baltic Sea. Gross statistics from the data indicate a typical maritime environment with small diurnal amplitude and high average concentrations. Comparison with a few months’ observations at a site in Central Finland confirms the importance of different loss mechanisms at the surface. Enhanced ozone concentrations produced by either long-range transport, photochemical processes or stratospheric intrusion were identified.