Ozone Deposition to a Coniferous and Deciduous Forest in the Czech Republic

Estimates of ozone concentration and deposition flux to coniferous and deciduous forest in the Czech Republic on a 1 × 1 km grid during growing season (April–September) of the year 2001 are presented. Ozone deposition flux was derived from ozone concentrations in the atmosphere and from its deposition velocities. To quantify the spatial pattern in surface concentrations at 1 km resolution incorporating topography, empirical methods are used. The procedure maps ozone concentrations from the period of the day when measurements are representative for the forest areas of countryside. The effects of boundary layer stability are quantified using the observed relationship between the diurnal variability of surface ozone concentration and altitude. Ozone deposition velocities were calculated according to a multiple resistance model incorporating aerodynamic resistance (R _a ), laminar layer resistance (R _b ) and surface resistance (R _c ). Surface resistance (R _c ) comprises stomatal resistance (R _sto ). R _sto was calculated with respect to global radiation, surface air temperature and land cover. Modelled total and stomatal ozone fluxes are compared with the maps describing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb). For forests, the critical level (9,000 ppbh May–July daylight hours) is exceeded over 50% of forested territory. This indicates the potential for effects on large areas of forest. There is significiant correspondence between the exposure index AOT40 and the total ozone flux, but the relation between the total ozone flux and AOT40 exposure index is not clear in all parts of the forest territory.     

Ozone Dry Deposition Velocities for Coastal Waters

Air-sea exchange rates for ozone were measured by the eddy correlation technique at a site on the north Norfolk coast in the UK. The average surface resistance to ozone uptake was found to be, r_s(O₃) = 1,000 ± 100 s m^-1. Micrometeorological measurements of trace gas fluxes to ocean surfaces are rare but a review of available measurements suggests that we can constrain sea water surface resistance for ozone to between 1,000 (Regener (1974), and this work) and 1,890 s m^-1 (Lenschow et al., 1982), yielding surface deposition velocities between 0.53 and 1.0 mm s^-1. These values are more than an order of magnitude greater than can be explained by laboratory determined mass accommodation coefficients for ozone to water. The importance of dry deposition with respect to process air-sea exchange models is highlighted. A trend in surface deposition velocity with wind speed was also observed supporting a surface chemical enhancement mechanism of ozone uptake which in turn is enhanced by near surface mixing processes.     

Characteristics of an Ozone Deposition Module

An ozone deposition module is being developed to allow the estimation of stomatal fluxes of ozone into a number of vegetation types. This model is designed to be linked into a regional chemical-transport model for use within the European Monitoring and Evaluation Programme (EMEP), to provide information on possible risks to vegetation across Europe. This paper investigates some characteristics of this deposition module, for sites in very different climate zones. The model results suggest that both stomatal and non-stomatal fluxes are comparable in magnitude.     

Comparisons of Measured and Modelled Ozone Deposition to Forests in Northern Europe

The performance of a new dry deposition module, developedfor the European-scale mapping and modelling of ozone flux to vegetation, was tested against micrometeorological ozone and water vapour flux measurements. The measurement data are for twoconiferous (Scots pine in Finland, Norway spruce in Denmark) and one deciduous forest (mountain birch in Finland). On average, themodel performs well for the Scots pine forest, if local inputdata are used. The daytime deposition rates are somewhat over-predicted at the Danish site, especially in the afternoon. The mountain birch data indicate that the generic parameterisationof stomatal responses is not very representative of this northernspecies. The module was also tested by using modelled meteorological data that constitute the input for a photochemical transport model.     

Deposition Velocities of SO2 and O3 over Agricultural and Forest Ecosystems

The results of field studies that measured the flux anddeposition velocity of SO₂ and O₃ are reported. Three of the studies were over agricultural crops (pasture, corn, and soybean), and two were over forest (a deciduous forest and a mixed coniferous–deciduous forest). In all cases the deposition velocity for SO₂ was higher than that for O₃. Diurnal cycles of SO₂ deposition velocity were similar in shape, but not magnitude for all surfaces; however those for O₃ showed some difference between forest sites where the peak was in the morning, and the agricultural sites where the peak occurred at mid-day. Seasonal cycles of SO₂ were affected by deposition to surfaces when leaves were not active, yet surface conductance is significant, but not for O₃ where stomatal uptake is the primary pathway for deposition.(On assignment to the National Exposure Research Lab., US EPA) (e-mail:     

Evaluation and Improvement of a Dry Deposition Model using SO2 and O3 Measurements over a Mixed Forest

A dry deposition model (RDM) for operational application has beenevaluated and modified in the present study. Field measurements of friction velocity and dry deposition velocity of SO₂ andO₃ over a mixed forest have been used to evaluate RDM. It was found that RDM predicts friction velocities very close to measurements and thus it can predict reasonable aerodynamic resistance. RDM overestimated O₃ deposition during dry nighttime conditions and underestimated both O₃ andSO₂deposition for early morning hours. It could not predict the mean diurnal variation in deposition velocity for either O₃ or SO₂ deposition under wet surface conditions. Modifications have been made for O₃ and SO₂ dry deposition based on the comparison of results and based upon additional published data. Compared to an earlier version of RDM, the modified versionpredicts better results for O₃ and SO₂ dry deposition,especially under rain and dew conditions.     

A Coupled Land-Surface and Dry Deposition Model and Comparison to Field Measurements of Surface Heat,Moisture, and Ozone Fluxes

We have developed a coupled land-surface and drydeposition model for realistic treatment of surface fluxes ofheat, moisture, and chemical dry deposition within acomprehensive air quality modelling system. A new land-surfacemodel (LSM) with explicit treatment of soil moisture andevapotranspiration and an indirect soil moisture nudging schemehas been added to a mesoscale meteorology model. The new drydeposition model uses the same aerodynamic and bulk stomatalresistances computed for evapotranspiration in the LSM. Thisprovides consistent land-surface and boundary layer propertiesacross the meteorological and chemical components of the system. The coupled dry deposition model also has the advantage of beingable to respond to changing soil moisture and vegetationconditions. Modelled surface fluxes of sensible and latent heatas well as ozone dry deposition velocities were compared to twofield experiments: a soybean field in Kentucky during summer 1995and a mixed forest in the Adirondacks of New York in July 1998.(on assignment to the National Exposure Research Laboratory, U.S. Environmental Protection Agency) (author for correspondence, e-mail(on assignment to the National Exposure Research Laboratory, U.S. Environmental Protection Agency)     

Model Evaluation of Dry Deposition to Vegetation for Volatile and Semi-Volatile Organic Compounds in a Multimedia Environment

Gas-phase atmospheric deposition wasevaluated in a screening level model of themultimedia environmental distribution of toxics(MEND-TOX). Algorithmic additions to MEND-TOXfor the estimation of gas-phase depositionvelocity over vegetated surfaces were analyzedusing recently published dry deposition fluxmeasurements for nitric acid. Model outputs arecompared to similar estimates from the NOAAmultilayer dry deposition model. Results of theevaluation indicate that MEND-TOX performs wellas a screening level model for the estimation ofgas-phase dry deposition velocity of nitric acidover soybeans. The present study expandsprevious laboratory results for organic speciesto include an inorganic species and open fieldand dry leaf, conditions.(On assignment to the National Exposure Research Laboratory, U.S. Environmental Protection Agency); (author for correspondence, e-mail     

Modelling Change in Ground Vegetation Response to Acid and Nitrogen Pollution, Climate Change and Forest Management at in Sweden 1500–2100 a.d.

The ForSAFE model, designed for modelling biogeochemical cycles (water, acidity, base cation, nitrogen and carbon) in terrestrial ecosystems, was modified with a vegetation response module (VEG), incorporating the effects of: nitrogen pollution, acidification, soil moisture, temperature, wind chill exposure, light and shading by trees, grazing by animals, competition between plants, above ground for light and below ground for water and nutrients. The model calculates the response of number ground vegetation plant groups. The integrated model was tested and validated at integrated level II forest monitoring sites across Sweden, four have been shown here, and used to assess the effect of acidification and nitrogen pollution in relation to factors such as climate change, forest management and changing grazing pressure. The response functions have been derived from single-factor experiments and integrated through the model structure for use on whole systems. The tests with the model suggest that the ground vegetation composition is reasonably well predicted, that much research remains before the model is fully tested and operational, and that the model may serve as a tool for assessing impacts of climate change, acid rain and forest management on plant biodiversity in forested areas.