Juha-Pekka Tuovinen

Modelling stomatal ozone flux across Europe.

A model has been developed to estimate stomatal ozone flux across Europe for a number of important species. An initial application of this model is illustrated for two species, wheat and beech. The model calculates ozone flux using European Monitoring and Evaluation Programme (EMEP) model ozone concentrations in combination with estimates of the atmospheric, boundary layer and stomatal resistances to ozone transfer. The model simulates the effect of phenology, irradiance, temperature, vapour pressure deficit and soil moisture deficit on stomatal conductance. These species-specific microclimatic parameters are derived from meteorological data provided by the Norwegian Meteorological Institute (DNMI), together with detailed land-use and soil type maps assembled at the Stockholm Environment Institute (SEI). Modelled fluxes are presented as mean monthly flux maps and compared with maps describing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb or nl l(-1)), highlighting the spatial differences between these two indices. In many cases high ozone fluxes were modelled in association with only moderate AOT40 values. The factors most important in limiting ozone uptake under the model assumptions were vapour pressure deficit (VPD), soil moisture deficit (for Mediterranean regions in particular) and phenology. The limiting effect of VPD on ozone uptake was especially apparent, since high VPDs resulting in stomatal closure tended to co-occur with high ozone concentrations. Although further work is needed to link the ozone uptake and deposition model components, and to validate the model with field measurements, the present results give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation.

Annual cycle of methane emission from a boreal fen measured by the eddy covariance technique

The northern wetlands are one of the major sources of methane into the atmosphere. We measured annual methane emission from a boreal minerotrophic fen, Siikaneva, by the eddy covariance method. The average wintertime emissions were below 1 mg m-2 h-1, and the summertime emissions about 3.5 mg m-2 h-1. The water table depth did have any clear effect on methane emissions. During most of the year the emission depended on the temperature of peat below the water table. However, during the high and late summer the emission was independent on peat temperature as well. No diurnal cycle of methane flux was found. The total annual emission from the Siikaneva site was 12.6 g m-2. The emissions of the snow free period contributed 91% to the annual emission. The emission pulse during the snow melting period was clearly detectable but of minor importance adding only less than 3% to the annual emission. Over 20% of the carbon assimilated during the year as carbon dioxide was emitted as methane. Thus methane emission is an important component of the carbon balance of the Siikaneva fen. This indicates need of taking methane into account when studying carbon balances of northern fen ecosystems.

Impact of the sulphur dioxide sources in the Kola Peninsula on air quality in northernmost Europe

Abstract 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%.

CO2 exchange of a sedge fen in southern Finland—the impact of a drought period

Eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) were conducted on a boreal sedge fen in southern Finland (61°50’N, 24.12’E) during a 1.5-yr period covering two summers in 2004.2005. The EC data were complemented by chamber measurements, which enabled the partition of the daytime NEE into respiration and photosynthesis. A special emphasis was put on the hydrometeorological responses of CO2 exchange during a drought period in July 2005. A mean CO2 efflux of 0.009 mg CO2 m-2 s-1 was observed during mid-winter (January.February), while the night-time respiration during the two Julys averaged 0.09 mgCO2 m-2 s-1. During both years the mean midday uptake in late July was about -0.16 mgCO2 m-2 s-1. An annual CO2 balance of -188 g CO2 m-2 was observed in 2005.Aslightly higher net sink of -219 gCO2 m-2 was estimated for 2004. The drought period experienced in July 2005 caused a clear depression in the daily NEE values. From the combined analysis of EC and chamber measurements it was concluded that this was mainly due to increased respiration, but evidence was also found of suppressed photosynthesis due to a high VPD.

Annual CO2 exchange of a peat field growing spring barley or perennial forage grass

[1] We report on net ecosystem CO 2 exchange (NEE) measurements conducted with the eddy covariance method over agricultural peat soil in the 2-year period between October 2000 and October 2002. In 2001, spring barley and undersown grass were sown on the site. After the barley harvest, the perennial forage grass was left to grow, so that in 2002 the field was growing grass. A higher maximum net CO 2 uptake was observed for barley than for grass during the height of the summer, peaking at about -1.0 and -0.75 mg CO 2 m s -1 , respectively. The maximum nighttime total ecosystem respiration was measured in July and was similar for both crops, about 0.35 mg CO 2 m -2 s -1 . During the growing season the field acted as a daily CO 2 sink for only 40 days in barley versus 84 days in grass. In the winter the average carbon dioxide efflux varied from 15.6 to 16.5 μg CO 2 m -2 s -1 . The annual NEE of the agricultural peat soil growing barley and grass was 771 ± 104 and 290 ± 91 g CO 2 m -2 , respectively. The longer net CO 2 uptake period was the main reason for the lower annual NEE for grass; however, owing to the higher amount of grass biomass produced the net ecosystem production (NEP), calculated as the sum of NEE and removed biomass, was slightly larger for grass (452 g C m -2 ) than for barley (336 g C m -2 ). These results show that the organic peat is still undergoing rapid decomposition after more than 100 years of cultivation activity. In addition, switching from an annual to a perennial crop did not turn the field into a CO 2 sink, at least during a 1-year period.

Seasonal CO2 balances of a subarctic mire

Micrometeorological measurements of CO2 and energy fluxes were carried out in a peatland ecosystem in northern Finland (69°08′N, 27°17′E) during a measurement period from April to the end of October 1997. The summer of 1997 was exceptionally warm and dry as compared to the climatological normal period of 1961–1990, and the effects of the high temperature and lowered water table were clearly seen in the CO2 fluxes. The highest individual downward flux densities of about −0.25 mg(CO2) m−2 s−1 took place at the end of July, while the highest respiration rates of 0.15 mg(CO2) m−2 s−1 were observed later in August. During the first days of measurements in April the median of respiration flux densities through the snow cover was about 0.006 mg(CO2) m−2 s−1. In correspondence to the CO2 fluxes the strongest sink terms in the daily net ecosystem exchange (NEE) balances of about −6 g(CO2) m−2 d−1 were observed in July. The highest positive balances of about 4 g(CO2) m−2 d−1 were observed in early June and in August. The daily balances in April were about 0.6 g(CO2) m−2 d−1. The net balances for the sink period (June 15 to August 26, 1997) and for the 6-month measurement period were −188 g m−2 and −30 g m−2, respectively. The wintertime CO2 balance was estimated by modeling the NEE using the NEE values from the first measurement week in April. The wintertime balances obtained yielded estimates for annual balances in the range of 62 to 72 g m−2 yr−1.

The uncertain climate footprint of wetlands under human pressure

Significance Wetlands are unique ecosystems because they are in general sinks for carbon dioxide and sources of methane. Their climate footprint therefore depends on the relative sign and magnitude of the land–atmosphere exchange of these two major greenhouse gases. This work presents a synthesis of simultaneous measurements of carbon dioxide and methane fluxes to assess the radiative forcing of natural wetlands converted to agricultural or forested land. The net climate impact of wetlands is strongly dependent on whether they are natural or managed. Here we show that the conversion of natural wetlands produces a significant increase of the atmospheric radiative forcing. The findings suggest that management plans for these complex ecosystems should carefully account for the potential biogeochemical effects on climate. Significant climate risks are associated with a positive carbon–temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence from natural to managed conditions to quantify the “cost” of CH4 emissions for the benefit of net carbon sequestration. With a sustained pulse–response radiative forcing model, we found a significant increase in atmospheric forcing due to land management, in particular for wetland converted to cropland. Our results quantify the role of human activities on the climate footprint of northern wetlands and call for development of active mitigation strategies for managed wetlands and new guidelines of the Intergovernmental Panel on Climate Change (IPCC) accounting for both sustained CH4 emissions and cumulative CO2 exchange.

Modelling and Mapping Ozone Deposition in Europe

A new dry deposition module has been developed for European-scale mapping and modelling of ozone deposition fluxes (Emberson et al., 2000a,b). The module is being implemented in the photochemical long-range transport model of EMEP that is currently used to estimate exceedance of the existing critical levels for ozone within the UN ECE LRTAP programme. The deposition model evaluates the atmospheric, boundary layer and surface resistances to ozone transfer with the calculation of the dry deposition velocity performed according to a standard resistance formulation. The approach differs from other existing methods through the use of a detailed stomatal uptake model that describes stomatal conductance as a function of plant species, phenology and four environmental variables (air temperature, solar radiation, water vapour pressure deficit and soil moisture deficit). Comparison of preliminary model outputs for selected land-cover types indicate that the model is capable of predicting the seasonal and diurnal range in deposition velocities that have been reported previously in the literature. The application of this deposition scheme enables calculations of ambient ozone concentrations to be made using a biologically based method that can distinguish stomatal and non-stomatal components of total ozone deposition. The ability to estimate stomatal ozone fluxes (according to vegetation type, phenology and spatial location) that are consistent with evaluations of atmospheric ozone concentrations will be helpful in future assessments of ozone impacts to vegetation.

Environmental controls on the CO2 exchange in north Europea mires

Net CO2 exchange measured under well-mixed atmospheric conditions in four different mires in Sweden and Finland were used to analyse which factors were controlling photosynthesis and respiration. The parameters of a light response function showed strong seasonal variations with similar behaviour for all mires. The half-monthly nighttime respiration rates in the central part of the growing season were about two times higher in the southernmost, warmest site, Fåje, as compared to the northernmost, coldest site, Kaamanen. However, Kaamanen had high photosynthesis rates, and this in combination with the long daylight periods in the middle of the summer caused Kaamanen to have the largest net ecosystem exchange (NEE) during the summer period. Fåje that showed the highest productivity had also the highest respiration and therefore, the lowest NEE during summer. Correlation between half-monthly components and different environmental variables showed the highest correlation between the components themselves. Thereafter came temperature except for Fåje where water table depth (WTD) explained most of the variance both for detrended and temperature-normalized components. All sites showed dependencies between WTD and the respective components during drying up periods. Temperature sensitivity was higher for productivity than for respiration indicating that CO2 uptake would increase during global warming.

Assessing vegetation exposure to ozone : properties of the AOT40 index and modifications by deposition modelling

A discussion is presented on the application of micrometeorological deposition modelling principles to improve the characterisation of vegetation exposure to ozone and thus the use of critical levels as the basis of targeted emission control. The AOT40 (accumulated exposure over a threshold of 40 ppb or nl l(-1)) ozone exposure index is shown to impose a differential weighting that results in a high sensitivity, by a factor of two to 10 depending on the pollution climate, with respect to concentration. This makes it necessary to correct for systematic effects, such as the concentration profile below the measurement height, in order to justify a comparison with the biological data obtained from well-mixed exposure chambers. Available studies indicate a 50-70% lower AOT40 at the vegetation height. The resistance method for estimating the profile is extended to allow for stomatal effects that potentially bias the plant response predicted with an exposure index. This integrated profile-uptake correction refines the current approach and serves as a transitional step towards a real flux-based approach. For the latter, a new deposition parameterisation is tested against field observations.