Estimation of Cumulative Frequency Distribution for Carbon Monoxide Concentration from Wind-Speed Data, in Buenos Aires (Argentina)

In this article we apply and test a methodology to estimate cumulative frequency distribution for air pollutant concentration from wind-speed data. We use the inverse relationship after Simpson et al. (Atmospheric Environment, 19, 75–82, 1985) between the opposing percentile values in the statistical distributions for air pollutant concentrations and wind-speed data. This relationship is valid, irrespective of the statistical distributions of both variables, if an inverse relationship between them is also applicable. The available data are five years of 8-h average carbon monoxide concentration and 8-h mean wind-speed, observed in Buenos Aires (Argentina). The performance of the obtained empirical expressions in estimating cumulative frequency distributions for 8-h CO is statistically evaluated. The results show that it is possible to obtain an acceptable cumulative frequency distribution for 8-h CO concentration at the site if the cumulative frequency distribution for wind-speed is known. Q–Q plots show a good agreement between estimated and observed values. From our data, the mean relative error of the estimations was found to be as much as 8.0%.     

Application of Dispersion Modelling for Analysis of Particle Pollution Sources in a Street Canyon

The dominating source of particles in urban air is road traffic. In terms of number concentration, its main contribution is within the range of ultrafine particles (Dp < 100 nm). The dispersion conditions, i.e. transport and dilution, of the submicrometer particles are expected to be like for gases and therefore the particle concentrations in a street canyon can be calculated using gaseous pollutants dispersion models. Such processes, like coagulation or condensation, are less important due to the short residence time within the street canyon environment.Two extensive measuring campaigns were conducted in the street Jagtvej in Copenhagen, Denmark. The particle size distributions were measured by a Differential Mobility Analyser (DMA) coupled to a particle counter, providing high time resolution data (1/2 hourly) on a continuous basis. Measurements of NO_x, CO and meteorological parameters were also available. The measured particle number concentrations, especially below 100 nm, reveal very similar dependence on the meteorological conditions as the NO_x concentrations. This underpins the conclusion that dilution properties are similar for particles and NO_x. For particle sizes over 100 nm, somewhat different behaviour is observed. This points toward existence of additional particle sources, not related to traffic emissions within the street canyon. A significant contribution is believed here to be attributed to long-range transport. The total particle emission from traffic, including daily variation and size distribution, has been calculated using the OSPM dispersion model. Results are in accordance with a previous analysis based on statistical modelling.     

Ozone Episode in the Milan Metropolitan Area A Comparative Evaluation of UAM-V and CALGRID Models

Data from an ozone episode (2–5 June, 1998) in the Milan metropolitan area were used for an application of two photochemical grid models: UAM-V and CALGRID. To assure a fair comparison, the models were run on the same domain and grid size, with same source emission inputs, CALMET diagnostic meteorology, and initial and boundary conditions taken from air quality data and literature values. Hourly emissions were derived from the AutoOil-II programme inventory except for on-road mobile source emissions; a new traffic emission inventory, based on both COPERT II methodology and road classification has been developed. NO_x and O₃ concentration results were compared to local network monitoring data. Results indicate that both models predict the highest ozone values along the north-east direction and are able to reproduce the ozone daytime trend though differences can be found between the two models on ozone spatial distribution. Average normalised bias for both models is about 50%, peak daily ozone concentrations are underestimated, with simulated peak shapes broader than the observed ones and a temporal shift between the two models. Night-time concentration levels of pollutants were not successfully reproduced due to an incorrect parameterisation of vertical turbulence calling for further work.     

Analysis of the St. Petersburg Traffic Data using the OSPM Model

Since October 1998 two DOAS instruments were installed at the level of the first floor and at the top of a building located in St. Petersburg at Pestelya Street. The collected datacovers the time period of December 1998–March 2001, and include concentrations of benzene, toluene, NO and NO₂, ozone and SO₂. There is also an additional information about the traffic intensity and meteorological conditions. The results of the analysis of this data set, using the OSPM model, are presented here with the goal to understand the features of the air pollution dispersion in this street canyon and to analyse the information about the emission factors of the vehicles. In particular, the model results are used for the solution of the inverse problem of reconstructing the emission factors from measured concentrations. The results obtained indicate that most of the concentrations are well inside the Russian standards with the only exception of NO₂ (mean and 98-th percentile are equal to 57.8 and 119.2 g m^-3 for the street level). The same values for benzene are 18.5 and 62.6, respectively. Emission estimates show that there is a possibility that the NO_x and benzene basic emission factors recommended by the Russian national guidelines could result in overestimating the traffic emissions. These considerations are supplemented with the model sensitivity tests carried out in connection with the problem of predictability of NO₂ concentrations in the street canyon. Tests indicate that NO₂ concentrations are not very sensitive to NO_x emissions because of the usually low urban background ozone levels.     

Improvements in Air Quality Modelling by Using Surface Boundary Layer Parameters derived from Satellite Land Cover Data

Many atmospheric dispersion models include only simpletreatment of surface features to estimate the wind profilesand stability parameters. Detailed characterisation of theland cover, particularly in large and complex urbanconurbations, is especially important, as the surfacefeatures can vary significantly over the area. This paperdiscusses the use of satellite land cover data to derivespatially resolved surface boundary layer (SBL) parameters.These parameters have been used in an air quality model,PEARL (Prediction Air Quality in Urban and RegionalLocations) for estimating monthly and annual COconcentrations. Land cover data, derived from LANDSATThematic Mapper Imagery, has been used to estimate SBLparameters (surface roughness length, albeedo, Bowen ratioand anthropogenic heat flux) for a study area of 10000km² encompassing Greater London and the surroundingcounties. The SBL parameters have been assigned according tomajor land cover types for the whole area at a spatialresolution of 1 × 1 km. Predictions from two versions of the PEARL model (one with land cover data and one without)have been compared with each other and with measured data forannual and monthly CO concentrations from seven London airquality monitoring sites. This comparison shows thatdifferences between predicted and observed values can bereduced by up to a factor of three. The use of SBLparameters derived from land cover data also yields moredetailed predicted annual CO spatial patterns especially inand around suburban areas. The performance of both versionsof the model for monthly CO concentrations has been comparedwith a range of statistical measures. This comparisonconfirms that improved agreement is observed betweenmodelled and measured monthly CO concentrations when use ismade of spatially resolved SBL parameters.     

Monitoring Studies of Precipitation and Cap Cloud Chemistry at Holme Moss in the Southern Pennines

Weekly collections of samples of precipitation and hill cloudwater have been made at Holme Moss (530 m.a.s.l.) in the southernPennines covering a six-year period (1994–1999). In addition continuous meteorological measurements have been conducted at thesite for a five year period (1995–1999). The concentrations of major ions in the samples have been determined by ion chromatography. Analysis of ion concentrations as a function ofwind direction reveals that the ions with anthropogenic sources (SO₄ ^–, NO₃ ^–, NH₄ ⁺, H⁺) exhibit higher concentrations during easterly wind directions whilst the ions with predominantly marine origins (Na⁺, Cl^–, Ca⁺⁺, Mg⁺⁺, K⁺) have concentrations thatare not significantly dependant on wind direction. Precipitation and cloud deposition are strongly correlated to south-westerly wind directions with a secondary peak occurring for north-easterly directions. Fifty nine per cent of ion deposition by rain was found to occur during wind from the SSW to W sector.The average concentrations of ions in cloud water were found tobe much higher than those in rain (by factors of between 2.5 and4.2). It is thought that the high precipitation and annual deposition of ions by precipitation at Holme Moss is due in partto the enrichment of ion concentrations in precipitation by thescavenging of more concentrated cap cloud droplets (the `seeder-feeder effect'). Comparison with data from a nearby lower level site shows encouraging agreement with the scheme currently used in the U.K. deposition mapping procedure to incorporate the influence of orography on deposition by precipitation.     

Inter-annual Variations of Ozone and Nitrogen Dioxide Over Europe During 1958–2003 Simulated with a Regional CTM

Inter-annual variability of surface ozone (O₃) and nitrogen dioxide (NO₂) over Europe has been studied over the period 1958–2003 using a three-dimensional Chemistry-Transport Model coupled to meteorological data from the ERA40 data set produced at the European Centre of Medium-range Weather Forecasts (ECMWF). Emissions and boundary conditions were kept at present levels throughout the simulation period. It was found that the annual mean NO₂ concentration varies between ±50% and the summer mean O₃ concentration varies between −10 and +20 percent (%) compared to the 46-year average over the model domain. There is also variation in ozone and NO₂ over longer time scales. The last 22 years display high concentrations of ozone in central and south-western Europe and low concentrations in north-eastern Europe. The first 22 years display very high concentrations of NO₂ over the North Sea. There is indication of trends in ozone and nitrogen dioxide but this has to be investigated further. Such information is one factor that should be taken into account when considering future control strategies.     

Characteristics of NO2 Pollution in the City of Gothenburg, South-West Sweden—Relation to NO x and O3 Levels, Photochemistry and Monitoring Location

Concentrations of NO₂, NO, and O₃ from a rooftop monitoring station in Gothenburg, Sweden (2002–2006) were analysed to characterise NO₂ pollution. [NO₂] was shown to correlate strongly and non-linearly with [NO _x ] (NO _x ?=?NO?+?NO₂), in line with observations in other cities. The [NO₂] to [NO _x ] fraction fell initially with increasing [NO _x ]. At [NO _x ] levels >200 ppb, the decline in [NO₂]/[NO _x ] with increasing [NO _x ] levelled out and [NO₂]/[NO _x ] converged towards approximately 0.15–0.16, independent of [NO _x ]. Data from a traffic route site showed the same pattern. This value of [NO₂]/[NO _x ] at high [NO _x ] can be interpreted as the NO₂ fraction of the NO _x emissions from vehicle exhaust. Situations with high NO _x pollution and minimum [NO₂]/[NO _x ] were always associated with [O₃] close to zero. Plotting [Ox] (Ox?=?NO₂?+?O₃) vs. [NO _x ] provided a strong linear correlation for situations dominated by local pollution ([NO]/[NO₂]>1). The slope of the regression, a measure of the primary NO₂ fraction in NO _x emissions, was 0.13 during the day and 0.14 during the night. With stronger winds, the rooftop monitoring station became more similar, in terms of NO₂ pollution, to a city street site and a traffic route site, although [NO₂] was almost always higher at the street/traffic route locations. The EU standard for the annual average of [NO₂] (40 μg m^?3) was exceeded, while the hourly standard (200 μg m^?3, not to be exceeded more than 18 times per year by 2010) was not exceeded at any of the sites.     

Simulation of Nitrogen Dynamics and Fluxes in Contrasting Catchments in Norway by Applying the Integrated Nitrogen Model for Catchments (INCA)

The process-based INCA model was applied to Dalelva Brook (3.2 km²) and the Bjerkreim River (685 km²) including several subcatchments, in order to test the model's ability to simulate streamwater nitrate (NO₃ ^-) dynamics and output fluxes under highly contrasting climatic conditions and nitrogen (N) loading. The simulated runoff volumes and mean NO₃ ^- concentrations at Dalelva and Bjerkreimwere within +2 to +10% of the measured average during 1993–1995 (–19 to +31% within individual years). INCA to a great extent also reproduced the observed streamwater flow dynamics at both study sites (coefficient of determination, r ² > 0.70). Temporal variation of streamwater NO₃ ^- during 1993–1995 was captured quite well by the model, especially at small catchments with a distinct seasonal NO₃ ^- pattern (r ² = 0.46–0.68). At the Bjerkreim River outlet, the relationship were somewhat weaker (r ² = 0.26, p < 0.01). Despite a few situations where the model failed to capturethe streamwater NO₃ ^- dynamics, INCA proved to be a quite robust tool for simulating NO₃ ^- dynamics and output fluxes in the two study catchments.     

A Chronology of Nitrogen Deposition in the UK Between 1900 and 2000

Measurements of the concentrations of nitrogen compounds in air and precipitation in the UK have been made since the mid-19th century, but no networks operating to common protocols and having traceable analytical procedures were established until the 1950s. From 1986 onwards, a high-quality network of sampling stations for precipitation chemistry was established across the UK. In the following decade, monitoring networks provided measurement of NO₂, NH₃, HNO₃ and a satisfactory understanding of the dry deposition process for these gases allowed dry deposition to be quantified. Maps of N deposition for oxidized and reduced compounds at a spatial scale of 5 km × 5 km are available from 1986 to 2000. Between 1950 and 1985, the more limited measurements, beginning with those of the European Air Chemistry Network (EACN) provide a reasonable basis to estimate wet deposition of NO ₃ ^– –N and NH ₄ ⁺ –N. For the first half of the century, estimates of deposition were scaled with emissions assuming a constant relationship between emission and deposition for each of the components of the wet and dry deposition budget at the country scale. Emissions of oxidized N, which dominated total nitrogen emissions throughout the century, increased from 312 kt N annually in 1900 to a peak of 787 kt for the decade 1980–1990 and then declined to 460 kt in 2000. Emissions of reduced N, largely from coal combustion were about 168 kt N in 1900, increasing to a peak of 263 kt N in 2000 and by now dominated by agricultural sources. Reduced N dominated the deposition budget at the country scale, increasing from 163 kt N in 1900 to 211 kt N in 2000, while deposition of oxidized N was 66 kt N in 1900 and 191 kt N in 2000. Over the century, 68 Mt (Tg) of fixed N was emitted within the UK, 78% as NO _x , while 29 Mt of nitrogen was deposited (43% of emissions), equivalent to 1.2 t N ha^–1, on average, with 60% in the reduced form. Deposition to semi-natural ecosystems is approximately 15 Tg N, equivalent to between 1 and 5 t N ha^–1, over the century and appears to be accumulating in soil. The N deposition over the century is similar in magnitude to the total soil N inventory in surface horizons.     

Isotopic Assessment of Sources of Surface Water Nitrate within the Oldman River Basin, Southern Alberta, Canada

Concentrations and isotopic compositions of NO₃ ^- from the Oldman River (OMR) and some of its tributaries (Alberta, Canada) have been determined on a monthly basis since December 2000 to assess temporal and spatial variations of riverine NO₃ ^- sources within the OMR basin. For the OMR sites, NO₃ ^--N concentrations reached up to 0.34 mg L^-1, δ¹⁵N-NO₃ ^- values varied between –0.3 and +13.8‰, and δ¹⁸O-NO₃ ^- values ranged from –10.0 to +5.7‰. For the tributary sites, NO₃ ^--N concentrations were as high as 8.81 mg L^-1, δ¹⁵N-NO₃ ^- values varied between –2.5 and +23.4‰, and δ¹⁸O-NO₃ ^- values ranged from –15.2 to +3.4‰. Tributaries in the western, relatively pristine forested part of the watershed add predominantlyNO₃ ^- to the OMR with δ¹⁵N-NO₃ ^- values near +2‰ indicative of soil nitrification. In contrast, tributariesin the eastern agriculturally-urban-industrially-used part of the basin contribute NO₃ ^- with δ¹⁵N-NO₃ ^- valuesof about +16‰ indicative of manure and/or sewage derived NO₃ ^-. This difference in δ¹⁵N-NO₃ ^- values of tributaries was found to be independent of the season, but rather indicates a spatial change in the NO₃ ^- source, which correlates with land use changes within the OMR basin. As a consequence of tributary influx, δ¹⁵N-NO₃ ^- values in the Oldman River increased from <+3‰ to >+6‰ in the downstream direction (W to E), although [NO₃ ^--N] increased only moderately (generally <0.5 mg L^-1). This study demonstrates the usefulness of δ¹⁵N-NO₃ ^- and δ¹⁸O-NO₃ ^- values in identifying the addition of anthropogenic NO₃ ^- to riverine systems.     

Trends in Water Chemistry in a Maple Forest on a Steep Slope

Year-to-year variation in SO₄ ^2-,NO₃ ^-, Ca²⁺, K⁺, and Mg²⁺concentrations in forest floor and mineral soil percolatefrom a forested, podzolic soil at the Turkey Lakes Watershedon the Precambrian Shield was assessed for monotonic trendsbetween 1986 and 1995. Our objective was to examine howrapidly ion concentrations in soil percolate equilibratedafter stabilization of SO₄ ^2- concentrations inprecipitation. Significant negative trends were detected inmonthly Ca²⁺, and Mg²⁺ concentrations in forestfloor and SO₄ ^2-, Ca²⁺, and Mg²⁺ inmineral soil percolate during the 10-year-period. Thedecline in Ca²⁺ and Mg²⁺ was greater than annualdecreases in SO₄ ^2- and NO₃ ^- in forestfloor percolate and proportional to the reduction inSO₄ ^2- in mineral soil percolate. Response ofmineral soil percolate to a 15 mol_c L^-1SO₄ ^2- decrease in wet-only precipitation between1985 and 1986 was a gradual decline in SO₄ ^2-concentration through 1995. The five-year meanSO₄ ^2- concentration in bulk precipitation, forestfloor percolate, and mineral soil percolate decreased 8, 9and 18 mol_c L^-1 from 1986–90 to 1991–95.Microbial (mineralization of organic S) and sorption(release from and/or retention in the pool of insolubleSO₄ ^2-) processes in the soil were logicalexplanations for the observed changes in SO₄ ^2- inmineral soil percolate.     

Simulations of Atmospheric Dispersion Following a Spillage of Petroleum at Sea

In January 1997 7000 t of refined petrol were spilled into the English Channel following a collision in dense fog off Ostend. The following day reports of petrol odours were recorded throughout central England at distances up to 300 km from the spill site. One possibility is that mass hysteria was induced by media reports of the accident coupled with the publicised wind direction. Indeed, the simulated gas cloud trajectory does not pass over central England if the petrol is assumed to have evaporated over a period of about 3 h. However, a feasible advection path can be reproduced by assuming that the water/atmosphere exchange of vapours occurred on a time scale of 12 h, the extended evaporation being due to the formation of subsurface droplets by wave action. Evidence to support the simulated trajectory has been obtained from the UK National Air Quality Information Archive which contains hourly data for benzene and NO_x concentrations. By regressing the benzene concentration against NO_x which is assumed to be an indicator of traffic density, it is shown that an increase in benzene concentrations on the day following the spillage was consistent with the simulated movement of the vapour cloud.     

Winter and Spring Thaw Measurements of N2O,NO and NOx Fluxes using a Micrometeorological Method

The impact of nitrogen fertilizers on gaseous emissions duringwinter and spring-thaw is not well understood and was the objective of this research. Using a micrometeorological method,N₂O, NO and NO_x fluxes from ryegrass were measured from November 1997 to March 1998. Three different mineralfertilizers were applied in November: urea (U), slow-release urea(SRU) and ammonium nitrate (AN). N₂O emissions during the winter were small, increasing significantly in March. Total losses of N₂O-N were significantly higher from SRU and U plots, with winter N₂O emissions accounting for 50% of annual losses. Nitric oxide fluxes from all plots weresmall during the measurement period (<0.9 ng N m^-2 s^-1). The NO fluxes from U and AN fertilized plots were significantly higher than from SRU and control plots. NO₂ fluxes were always negative (–6 ng N m^-2 s^-1)indicating deposition, but decreased to –2 ng N m^-2s^-1 when snow was present on the soil surface. Our resultsindicate that the form of inorganic N applied has an effect on NO+ N₂O emissions but not on NO₂ fluxes.Sponsored by CAPES – Brasília, Brazil     

Protozooplankton in the Deep Oligotrophic Traunsee (Austria) Influenced by Discharges of Soda and Salt Industries

Traunsee is a deep oligotrophic lake in Austria characterised by an artificial enrichment of chloride in the hypolimnion (up to 170 mg L^-1) caused by waste disposal of soda and salt industries. Protists were collected monthly over one year, observed alive and after Quantitative Protargol Staining (ciliates) or via epifluorescence microscopy (heterotrophic flagellates). Three sites within the lake (0–40 m depths) were compared to deeper water layers from 60–160 m depths where chloride concentrations and conductivity were increased. In addition, we observed the protozooplankton of two neighbouring lakes, i.e. reference systems, during one sampling occasion. In Traunsee the abundance of ciliates was low (200–36 600 cells L^-1) in contrast to high species diversity (at least 60 different species; H_S = 2.6) throughout the year. The main pelagic species in terms of abundance were small oligotrichs and prostomatids like Rimostrombidium brachykinetum/hyalinum, Balanion planctonicum and Urotricha spp. throughout the investigation period. Among free-living heterotrophic flagellates, which occurred at densities of 40–2800 cells mL^-1, small morphotypes dominated in the pelagial. No differences at the community level between the three lakes could be observed and pelagic ciliates and flagellates seemed not to be affected by increased chloride concentrations or by enhanced conductivity.     

Concentrations and Pools of Heavy Metals in Urban Soils in Stockholm,Sweden

The concentrations of heavy metals (Cd, Cr, Cu, Hg, Ni, Pb andZn) and arsenic (As) were surveyed and the metal pools estimatedin soils in Stockholm Municipality. The sampling sites were distributed all over the entire municipality with a higher sampling density in the city centre. Soils were sampled to a maximum depth of 25 to 60 cm. Soil texture, total-C content, electrical conductivity and pH were analysed. Heavy metal concentrations were determined after wet digestion with boiling7 M HNO₃.The results showed a wide range in heavy metal concentrations, as well as in other soil properties. The city centre soils constituted a rather homogeneous group whereas outside this areano geographical zones could be distinguished. These soils were grouped based on present land use, i.e. undisturbed soils, public parks, wasteland (mainly former industrial areas), and roadside soils. The city centre and wasteland soils generally hadenhanced heavy metal concentrations to at least 30 cm depth compared to park soils outside the city centre and rural (arable)soils in the region, which were used to estimate background levels. For example, the mean Hg concentration was 0.9 (max 3.3)mg kg^-1 soil at 0–5 cm and 1.0 (max 2.9) at 30 cm depth in the city centre soils, while the background level was 0,04 mg kg^-1. Corresponding values for Pb were 104 (max 444) and135 (max 339) mg kg^-1, at 0–5 and 30 cm, respectively, while the background level was 17 mg kg^-1.The average soil pools (0–30 cm depth) of Cu, Pb and Zn were 21,38 and 58 g m^-2 respectively, which for Pb was 3–4 timeshigher and for Cu and Zn 1.5–2 times higher than the backgroundlevel. The total amount of accumulated metals (down to 30 cm)in the city centre soils (4.5^*10 ⁶ m² public gardens and green areas) was estimated at 80, 1.1, 120 and 40 t for Cu, Hg, Pb and Zn, respectively. The study showed (1) thatfrom a metal contamination point of view, more homogeneous soilgroups were obtained based on present land use than on geographicdistance to the city centre, (2) the importance of establishing a background level in order to quantify the degree of contamination, and (3) soil samples has to be taken below the surface layer (and deeper than 30 cm) in order to quantify theaccumulated metal pools in urban soils.     

Modelling of Air Quality with CAMx: A Case Study in Switzerland

The three-dimensional Eulerian model CAMx (Comprehensive Air QualityModel with Extensions) was applied for the first time to simulate bothgaseous and particulate photochemical air pollution in Switzerland during July 28–30, 1993. The meteorological input data were prepared using the Systems Applications International Mesoscale Model (SAIMM). The CAMx model results were compared with the measurements carried out at ground level andfrom airborne measuring platforms within the frame of the Swiss POLLUMET research programme. In general, the CAMx performance for gaseous species wasfound to be better than that of the previously used Urban Airshed Model (UAM)and the Variable Grid Urban Airshed Model (UAM-V). The most significant improvement for the gaseous species is in the prediction of HNO₃ concentrations, due to the inclusion of aerosol chemistry. Aerosol species such as NO₃ ^-, NH₄ ⁺, SO₄ ^2-, and secondary organic aerosols were calculated in one particle size range (0.04–2.5 m) and compared with a few measurements available. Although July 29 was reasonably well simulated, overestimated wind speeds by SAIMM for July 30 caused a too fast transport of pollutants. Similarly to measurements, significant spatial correlation of the secondarily formed aerosols with ozone and formaldehyde is found in the afternoon.     

Atmospheric Levels of Nitrogen Oxides at a Greek Oil Refinery Compared with the Urban Measurements in Athens

Oil refining is among the industrial activities that emit considerable amounts of air pollutants into the atmosphere. Nitrogen oxides are important air pollutants that are emitted by oil refineries as products of combustion processes. The ambient air concentrations of nitrogen oxide (NO) and nitrogen dioxide (NO₂) were monitored continuously at a site close to an oil refinery, near the city of Corinth in Greece, during autumn 1997 together with the main meteorological parameters. The contribution of the oil refinery to the measured atmospheric levels of nitrogen oxides was estimated. The ambient air concentration of nitrogen oxides in the area surrounding the oil refinery were generally lower than the ambient air concentrations in the urban area of Athens in Greece, and the NO₂ levels were always below the existing air quality standards. The influence of the refinery emitted NO_x in the photochemical production of ozone seems to be more important in terms of human and vegetation exposure given the high ozone backgrounds measured in the area.     

Air–Soil Exchange of NO,NO2 and O3 in Forests

The exchange of NO, NO₂ and O₃ at the soil surface wasmeasured with automatic dynamic chambers in a spruce forest and in abeech forest during periods of several months.NO was emitted from the soil at a rate of0–8 ng N m^-2 s^-1(spruce) and 0–15 ng Nm^-2 s^-1(beech), but there was no simple relationship between the flux andeither soil temperature or soil moisture. NO₂ and O₃ weredeposited at the soil surface. Deposition velocities forNO₂were on average 0.3 mm s^-1 (spruce) and 0.1 mms^-1(beech), and the deposition velocities of O₃ were on average 1.6 mm s^-1 (spruce) and 1.4 mm s^-1 (beech). The depositionvelocity of O₃ is fairly constant whereas the deposition velocityof NO₂ varies greatly, but the reasons remain to be investigated.     

Long Term Trends in Concentration of Major Pollutants (SO<Subscript>2</Subscript>, CO,NO, NO<Subscript>2</Subscript>, O<Subscript>3</Subscript> and PM<Subscript>10</Subscript>) in Prague – Czech Republic (Analysis of Data Between 1992 and 2005)

To assess the effect of changes in traffic density and fuels used for heating at the beginning of the 1990s, 1992–2005 monthly averages of PM₁₀, SO₂, NO₂, NO, CO and O₃ from Prague, the Czech capital, were analyzed together with long term trends in emissions of major pollutants, fuel consumption and number of vehicles registered in Prague. The data from all monitoring stations were retrieved from the database of the state automated monitoring system. Correlation coefficients between ambient monthly averaged temperature and all pollutants of concern showed distinct seasonal trends. The results showed that while SO₂ and to some extent also CO concentrations dropped namely in the first half of the analyzed period (1992–1997) as a result decreased fossil fuel consumption for local heating, the behaviour of other pollutant concentrations followed a different pattern. PM₁₀ concentrations decreased during the beginning of the 1990s but showed a sign of increase after 2000. Concentrations of ozone and NO₂ did not reveal any significant change throughout the whole studied period. It can be concluded that during the studied period traditional urban sources of pollution, such as coal and oil combustion, lost their importance but were simultaneously substituted by pollutants from automotive transport (namely PM and NO₂) making the problem of air quality even worse.