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Ozone concentrations and fluxes have been measured continuously during 5 years (1996–2000) by the gradient method in a Norway spruce dominated forest stand in West Jutland, Denmark, planted in 1965. The method has been validated against other methodologies and a relatively good relationship was found. The data are analysed to quantify diurnal, seasonal and yearly fluxes, and non-stomatal and stomatal removal are estimated. Monthly means of climatic data are shown, and day and night values of the aerodynamic resistance, ra, viscous sub-layer resistance, rb, and the surface or canopy resistance, rc, are presented. The yearly ozone deposition is approximately 126 kg ha−1. The canopy ozone uptake is highest during the day and during the summer. This is interpreted as increased stomatal uptake and physical and chemical reactions. The daily means of ozone concentration and fluxes averaged over 5 years correlate, but the correlation is primarily based on two different uncoupled processes outside and inside the stomates: (1) The ozone destruction in the canopy occurring outside the stomates is much influenced by temperature, light and humidity, e.g. surface reactions, NO- and VOC-emissions. (2) The same factors have a strong influence on the stomatal opening, e.g. midday and night closure. Thus, looking at diurnal variations, the diurnal ozone concentration and ozone flux do not correlate at all during the growing season. The maximum diurnal difference for the ozone concentration is a factor 1.3 and the maximum diurnal difference for the ozone flux is a factor 3. From dawn to ca. 8:00 the ozone deposition increases and the ozone concentration decreases. The yearly stomatal uptake of ozone is estimated to minimum 21% of the total deposition, being highest in May–August (30–33%) and lowest in November–February (4–9%). The physiological ozone uptake per leaf area is estimated to 0.33 g ozone m−2 y−1.

Atmospheric day and night concentrations of particulate organic nitrates (PON) and several other air pollutants were measured in the summer 1995 over an open-land area in Denmark. The sampling of PON was evaluated comparing 24 h samples with two sets of 12 h samples. These results indicate that the observed low contribution of PON to NOy is real and not the result of an extensive loss during the sampling. Empirical relationships between the vapour pressure and chemical formula of organic compounds were established in order to evaluate the gas/particle distribution of organic nitrates. A positive correlation between the PON to NOz ratio and the inverse temperature may indicate that the organic nitrates associated with particles also may be present in the gas phase. The observed day and nighttime levels of PON were of the same magnitude, although the day to night ratio varied from 0.4 to 4.4.

Formic and acetic acid measured as daily averages in 1993–1994show equal and highly correlated concentrations up to 3 ppb in the summer(May–August). In the winter (October–March) the formicacid/acetic acid ratio was 0.6 and the formic acid concentrations wereusually below 1 ppb. In winter the carboxylic acids correlate withOx, NOy, SO2 and particulatesulphur. The main sources are suggested to be ozonolysis of anthropogenicalkenes and reactions between peroxyacetyl radicals and RO2radicals. In spring–summer the carboxylic acids correlate withO3, Ox, HNO3, PAN,NOy, SO2, particulate sulphur and temperature.In addition to the sources of the winter a contribution from ozonolysis ofbiogenic alkenes is likely. Quite similar formic acid/acetic acid ratios forall wind directions suggest that the source(s) are atmospheric oxidationprocesses distributed over large areas. The highest concentrations occurringfor winds from east to south and the correlation with e.g., particulatesulphur indicate chemical production in polluted air masses during longrange transport.