Christophe Flechard

Linking land-atmosphere-stream carbon fluxes in a lowland peatland system

Any change in the ability of northern peatlands to act as a sink for atmospheric CO2 will play a crucial part in the response of the Earth system to global warming. We argue that a true assessment of the sink-source relationships of peatland ecosystems requires that losses of C in drainage waters be included when determining annual net C uptake, thus connecting measurements of stream C fluxes with those made at the land surface-atmosphere interface. This was done by combining estimates of net ecosystem exchange (NEE) with stream water measurements of TOC, DIC, and gaseous C loss, in a 335-ha lowland temperate peatland catchment (55°48.80′N, 03°14.40′W) in central Scotland over a 2-year period (1996–1998). Mean annual downstream C flux was 304 (±62) kg C ha−1 yr−1, of which total organic carbon (TOC) contributed 93%, the remainder being dissolved inorganic carbon (DIC) and free CO2. At the catchment outlet evasion loss of CO2 from the stream surface was estimated to be an additional 46 kg C ha−1 yr−1. Over the study period, NEE of CO2-C resulted in a flux from the atmosphere to the land surface of 278 (±25) kg C ha−1 yr−1. Net C loss in drainage water, including both the downstream flux and CO2 evasion from the stream surface to the atmosphere, was therefore greater or equal to the net annual C uptake as a result of photosynthesis/respiration at the land surface. By combining these and other flux terms, the overall C mass balance suggests that this system was either acting as a terrestrial C source or was C neutral.

Regional estimation of pollutant gas dry deposition in the UK: model description, sensitivity analyses and outputs

A “big-leaf” resistance analogy model for dry deposition of sulphur dioxide, nitrogen dioxide, ammonia and nitric acid is described with a stomatal compensation point included to allow bi-directional exchange of ammonia. The model derivation is constrained by measurement data and it is parameterized for UK conditions. Monthly average dry deposition estimates are provided at the 5 km×5 km spatial scale. The model uses data available nationally at the appropriate spatial and temporal scales, such as gas concentration, land use, wind speed, temperature, rainfall and vapour pressure. A method is presented to overcome the lack of suitable solar radiation data. The effect of uncertainty in model inputs and in model parameterization is explored using sensitivity analyses. SO2 deposition is sensitive to variation in gas concentration, wind speed and wet surface uptake parameters. NO2 deposition is sensitive to parameters and inputs regulating stomatal behaviour, including solar radiation and temperature, as well as to gas concentration. The use of monthly or annual average NO2 concentrations may underestimate deposition substantially in some areas. HNO3 dry deposition is sensitive to wind speed and concentration. NH3 dry deposition to moorland and forest land uses, where the majority of deposition occurs, is sensitive to concentration, wind speed and choice of canopy resistance parameters. For arable and grassland areas, with both deposition and emission of NH3, the model is sensitive to all the model inputs and parameter choices. A full uncertainty analysis requires further work on the reliability of input variables and model parameter choices but these results quantitatively focus on the important areas of the model for each gas. Estimated dry deposition to the UK (excluding Northern Ireland) of SO2 is 135 Gg S yr−1 for 1996, for NH3 is 97 Gg N yr−1, for NO2 is 26 Gg N yr−1 and the preliminary estimate for HNO3 is 42 Gg N yr−1. For sulphur and reduced nitrogen, estimated dry deposition accounts for 40% of total deposition, including wet and cloud droplet deposition. NO2 dry deposition only accounts for 15% of total oxidised nitrogen deposition, but another 25% may come from the dry deposition of HNO3, giving a similar 40% overall by dry deposition. The sensitivity of the model to parameter values and the comparisons of modelled output with measurements show that parameter choices may be valid only at the scale of European countries rather than the whole continent.

The mass budget of atmospheric ammonia in woodland within 1 km of livestock buildings

The emissions of ammonia from point sources close to the ground and the rapid deposition to vegetation lead to very large horizontal gradients in both concentration and deposition close to sources. The sources are primarily livestock related and some of the largest terrestrial N inputs occur in the proximity of intensive production facilities. This study quantifies the local fate of livestock NH 3 emissions from a poultry farm using measured NH 3 concentrations and the relationship between canopy resistance (r c ) and ambient NH 3 concentration from intensive flux measurements. The results of the measured concentrations and deposition are compared with those obtained using a dispersion model of the emission, transport and deposition close to point sources. The results of the measurements showed annual mean concentrations in the range 23 μg m -3 to 63 μg m -3 at a distance of 15 m from the source, declining to background concentrations for the region of 1 to 2 μg m -3 at a distance of 276 m and in reasonable agreement with the model. The deposition of NH 3 -N estimated from the measurements, declined from 42 kg N ha -1 at 15 m to 5 kg N ha -1 at 270 m and was smaller than the deposition estimated using the dispersion model by about a factor of two. Annual deposition within 270 m of the source to the woodland amounted to 155 kg N, and represented 3.2% of annual emissions from the poultry unit. The comparison between measurements and the model indicated substantial uncertainty in the deposition budget values, but supports the overall conclusion that local deposition of NH 3 to woodland within 300 m of the source represents a small fraction (3% to 10%) of the local emission source.

Deposition Monitoring in Europe

A monitoring station for atmospheric deposition was designed and constructed. Three such stations were applied in a pilot project for a year on three sites (Speulder forest in The Netherlands, Auchencorth in Scotland and Melpitz in Germany) in different regions in Europe to estimate local inputs and to validate deposition models which are currently used or developed to estimate ecosystem specific deposition in Europe. Fluxes at Auchencorth Moss are lowest for all components, except for those much influenced by the sea as a source. As Melpitz is located far away from seas, these components are lowest at this site. Wet deposition is the dominant source of input at Auchencorth, whereas at Speulder forest, through its roughness and pollution climate, dry deposition is dominant. At this site dry deposition velocities are highest. Melpitz is a polluted site. Particularly sulphur deposition is high. It is recommended to equip several locations in Europe with intensive deposition monitoring methods. Such a network will be an extension of existing monitoring programmes on air pollution, such as that run by Eurepean Monitoring and Evaluation Programme for the long-range transmission of air pollutants in Europe (EMEP). The intensive monitoring locations should be selected based on pollution climates and type of vegetation, common in Europe.

Dry Deposition Monitoring in Europe

Between 1993 and 1999 two EU funded projects wereexecuted aimed at (i) the development of drydeposition monitoring methods for core sites andlarge scale application, (ii) the installation andrunning of three core sites in Europe and (iii) the improvement and validation of models used forregional application. This article provides anoverview of the development of depositionmonitoring stations and the main results of thethree core sites, which were operated between1995 and 1998. Furthermore, the results of thedevelopment of a low cost monitoring system arepresented. Continuous measurements were made ofboth wet and dry deposition of sulphur andnitrogen components and base cations. The 4 yearsof data show a decrease in sulphur loads and notrend for the other components. It is shown thatthe surface affinities for sulphur depositionalso changed during the years, underpinning theneed for dry deposition monitoring. A conditionaltime average gradient system was successfullydeveloped and tested and provides a good meansfor low cost monitoring of dry deposition fluxes.The costs can be reduced by a factor of 3–4 without losing the accuracy of the annual average gas fluxes.

The Control of SO2 Dry Deposition on to Natural Surfaces by NH3 and its Effects on Regional Deposition

Two years of continuous measurements of SO2deposition fluxes to moorland vegetation are reported. The mean flux of 2.8 ng SO2 m-2 s-1 is regulated predominantly by surface resistance (rc) which, even for wet surfaces, was seldom smaller than 100 s m-1. The control of surface resistance is shown to be regulated by the ratio of NH3SO2 concentrations with an excess of NH3 generating the small surface resistances for SO2. A dynamic surface chemistry model is used to simulate the effects of NH3 on SO2 deposition flux and is able to capture responses to short-term changes in ambient concentrations of SO2, NH3 and meteorological conditions. The coupling between surface resistance and NH3/SO2 concentration ratios shows that the deposition velocity for SO2 is regulated by the regional pollution climate. Recent long-term SO2 flux measurements in a transect over Europe demonstrate the close link between NH3/SO2 concentrations and rc (SO2). The deposition velocity for SO2 is predicted to have increased with time since the 1970s and imply a 40% increase in vd at a site at which the annual mean ambient SO2 concentrations declined from 47 to 3 μg m-3 between 1973 and 1998.

Fluxes of total reactive atmospheric nitrogen (ΣN r ) using eddy covariance above arable land

ABSTRACT The amount and timing of reactive nitrogen exchange between agricultural land and the atmosphere play a key role in evaluating ecosystem productivity and in addressing atmospheric nitrogen budgets and transport. With the recent development of the Total Reactive Atmospheric Nitrogen Converter (TRANC) apparatus, a methodology has been provided for continuous measurement of the sum of all airborne nitrogen containing species (ΣNr) allowing for diurnal and seasonal investigations. We present ΣNr concentration and net flux data from an 11-month field campaign conducted at an arable field using the TRANC system within an eddy-covariance setup. Clear diurnal patterns of both ΣNr concentrations and fluxes with significant dependencies on atmospheric stability and stomatal regulation were observed in the growing season. TRANC data were compared with monthly-averaged concentrations and dry deposition rates of selected Nr compounds using DELTA denuders and ensemble-averages of four inferential models, respectively. Similar seasonal trends were found for Nr concentrations from DELTA and TRANC measurements with values from the latter being considerably higher than those of DELTA denuders. The variability of the difference between these two systems could be explained by seasonally changing source locations of NOx contributions to the TRANC signal. As soil and vegetation Nr emissions to the atmosphere are generally not treated by inferential (dry deposition) models, TRANC data showed lower monthly deposition rates than those obtained from inferential modelling. Net ΣNr exchange was almost neutral (~0.072 kg N ha−1) at the end of the observation period. However, during most parts of the year, slight but permanent net ΣNr deposition was found. Our measurements demonstrate that fertilizer addition followed by substantial ΣNr emissions plays a crucial role in a sites annual atmospheric nitrogen budget. As long-term Nr measurements with high temporal resolution are usually cost and labour-intensive, field application of the TRANC helps improve the understanding of ecosystem functioning, atmospheric transport and revising definitions of ecosystem-specific critical loads at a relatively moderate operational cost level.

Deposition of heavy metals and ionic aerosol species at a scottish moorland site

?While estimates of heavy metal deposition are often inferred from moss analyses (e.g. NORD, 1996) there are only few direct measurements of heavy metal loads. This is partly due to the size-dependent particle deposition velocity (v&) being still highly uncertain (Gallagher et al., 1997). Here micrometeorological measurements of v& are combined with particle size distributions to estimate the dry deposition of both ionic aerosols species and heavy metals at a remote, clean Scottish moorland site.

Modelling the air-surface exchange of ammonia from the field to global scale

The Working Group addressed the current understanding and uncertainties in the processes controlling ammonia (NH3) bi-directional exchange, and in the application of numerical models to describe these processes.

Impact of Leaf Surface and In-canopy Air Chemistry on the Ecosystem/Atmosphere Exchange of Atmospheric Pollutants

Chemical processes occurring on leaf surface and in air inside vegetation canopies play significant and sometimes dominant roles on pollutant dry deposition budgets. Yet, these processes are seldom explicitly treated in dry deposition parameterizations/models. This report briefly summarizes the current knowledge, outstanding issues, and recommendations for pollutant leaf surface exchange including ammonia (NH3), ozone (O3) and nitrogen oxides (NOx = NO + NO2), acidifying pollutants, volatile organic compounds (VOCs), and atmospheric aerosols.