Mhairi Coyle

The global nitrogen cycle in the twenty- first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr−1) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3−) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), contribute 100 Tg N yr−1 to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH4NO3) and ammonium sulfate (NH4)2SO4. Leaching and riverine transport of NO3 contribute 40–70 Tg N yr−1 to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr−1) to double the ocean processing of Nr. Some of the marine Nr is buried in sediments, the remainder being denitrified back to the atmosphere as N2 or N2O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of Nr in the atmosphere, with the exception of N2O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 102–103 years), the lifetime is a few decades. In the ocean, the lifetime of Nr is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N2O that will respond very slowly to control measures on the sources of Nr from which it is produced.

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.

Measurements of Ozone Deposition to Vegetation Quantifying the Flux, the Stomatal and Non-Stomatal Components

Ozone deposition to vegetation represents the major sink for boundary layer ozone and yet the underlying mechanism of reaction and uptake at the surface is poorly understood. While overall rates of O3 deposition are known, the fractions of the flux absorbed by stomata and deposited to non-stomatal surfaces in the field have been poorly quantified. This paper reports 4 years of continuous fluxes by micrometeorological methods to moorland vegetation in southern Scotland. The flux has been partitioned between stomatal and non-stomatal fluxes and shows over a seasonal scale that the non-stomatal deposition (50 kg O3 ha− y−1) dominates the overall flux (77 kg O3 ha−1 y−1) and represents 70% of the total deposition. The surface resistance for non-stomatal O3 deposition (rns) decreases with temperature from 400 s m−1 at 0°C to 200 s m−1 at 15°C in dry conditions and is consistent with thermal decomposition of ozone at the surface with an apparent activation energy of about 36 kJ mole−1. The 4 years of continuous flux measurements show that stomatal conductance, when O3 concentrations are 80 µg m−3, is substantially smaller than for smaller O3 concentrations, although whether this is a response to VPD or O3 remains unclear.

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.

Modelling photochemical oxidant formation, transport, deposition and exposure of terrestrial ecosystems.

The chemical processes responsible for production of photochemical oxidants within the troposphere have been the subject of laboratory and field study throughout the last three decades. During the same period, models to simulate the atmospheric chemistry, transport and deposition of ozone (O(3)) from individual urban sources and from regions have been developed. The models differ greatly in the complexity of chemical schemes, in the underlying meteorology and in spatial and temporal resolution. Input information from land use, spatial and temporally disaggregated emission inventories and meteorology have all improved considerably in recent years and are not fully implemented in current models. The development of control strategies in both North America and Europe to close the gaps between current exceedances of environmental limits, guide values, critical levels or loads and full compliance with these limits provides the focus for policy makers and the support agencies for the research. The models represent the only method of testing a range of control options in advance of implementation. This paper describes currently applied models of photochemical oxidant production and transport at global and regional scales and their ability to simulate individual episodes as well as photochemical oxidant climatology. The success of current models in quantifying the exposure of terrestrial surfaces and the population to potentially damaging O(3) concentrations (and dose) is examined. The analysis shows the degree to which the underlying processes and their application within the models limit the quality of the model products.

Advances in micrometeorological methods for the measurement and interpretation of gas and particle nitrogen fluxes

The application of micrometeorology for flux measurements of nitrogen species between terrestrial ecosystems and the atmosphere and some of their main limitations are reviewed. New methods which are gaining rapid acceptance such as relaxed eddy accumulation are also described. A new development to provide long term average fluxes by time averaged gradients is shown to yield long-term average NH3 fluxes over moorland within 10% of values obtained using continuous wet denuder methods and at less than 10% of the cost. The use of mass balance methods to quantify fluxes at the plot, landscape and regional scale are described, and show that in suitable conditions and for some countries, methods to check national inventories of radiatively active gases are now available.

Regional mass budgets of oxidized and reduced nitrogen and their relative contribution to the nitrogen inputs of sensitive ecosystems

Wet deposition of nitrogen is reasonably well monitored throughout Europe, whereas the dry deposition inputs are provided largely by models. Recent long-term measurements of NO2 and NH3 fluxes to semi-natural vegetation have shown that rates of NH3 deposition exceed those of NO2, typically by an order of magnitude. Incorporating the results of these dry deposition measurements in regional deposition budgets shows that the inputs of reduced nitrogen contribute the dominant fraction of the total nitrogen inputs in most regions of the UK. The results are illustrated by comparing the atmospheric mass-budget for oxidized nitrogen over the UK. Of the annual UK emissions of NOx, amounting to 780 kt N (Salway et al., 1997), only 5% is dry deposited to terrestrial surfaces within the country while 15% is wet deposited, whereas for the reduced nitrogen, 42% of emissions (of the 260 kt N, Salway et al., 1997) are dry deposited and 46% are wet deposited. Even more striking are the relative contributions of oxidized and reduced nitrogen to semi-natural vegetation, which is a particularly efficient sink for NH3. The species composition of semi-natural vegetation is also regarded as very sensitive to nitrogen inputs. The distribution of nitrogen deposition among different land uses shows that the average input to forest in the UK is 33 kg N ha−1 annually of which 78% is reduced nitrogen. The other land uses receive about 15 kg N ha−1 of nitrogen of which between 55% and 65% is NHx. Critical loads for nutrient nitrogen are exceeded primarily in forested and moorland areas as a consequence of NH3 dry deposition and wet NH4+ deposition. For forests the area in exceedance of 20 kg N ha−1 year−1 critical load represents 70% of the forest area (1.4 × 106 ha−1) while for moorland the area in exceedance is 13% of the moorland area and occupies 1.04 × 106 ha−1).

Effects of land use on surface–atmosphere exchanges of trace gases and energy in Borneo: comparing fluxes over oil palm plantations and a rainforest

This paper reports measurements of land–atmosphere fluxes of sensible and latent heat, momentum, CO2, volatile organic compounds (VOCs), NO, NO2, N2O and O3 over a 30 m high rainforest canopy and a 12 m high oil palm plantation in the same region of Sabah in Borneo between April and July 2008. The daytime maximum CO2 flux to the two canopies differs by approximately a factor of 2, 1200 mg C m−2 h−1 for the oil palm and 700 mg C m−2 h−1 for the rainforest, with the oil palm plantation showing a substantially greater quantum efficiency. Total VOC emissions are also larger over the oil palm than over the rainforest by a factor of 3. Emissions of isoprene from the oil palm canopy represented 80 per cent of the VOC emissions and exceeded those over the rainforest in similar light and temperature conditions by on average a factor of 5. Substantial emissions of estragole (1-allyl-4-methoxybenzene) from the oil palm plantation were detected and no trace of this VOC was detected in or above the rainforest. Deposition velocities for O3 to the rainforest were a factor of 2 larger than over oil palm. Emissions of nitrous oxide were larger from the soils of the oil palm plantation than from the soils of the rainforest by approximately 25 per cent. It is clear from the measurements that the large change in the species composition generated by replacing rainforest with oil palm leads to profound changes in the net exchange of most of the trace gases measured, and thus on the chemical composition of the boundary layer over these surfaces.

Sources of uncertainty in eddy covariance ozone flux measurements made by dry chemiluminescence fast response analysers

We present a systematic intercomparison study of eddy covariance ozone flux measurements made using two fast response dry chemiluminescence analysers. Ozone deposition was measured over a well characterised managed grassland near Edinburgh, Scotland, during August 2007. A data quality control procedure specific to these analysers is introduced. Absolute ozone fluxes were calculated based on the relative signals of the dry chemiluminescence analysers using three different methods and the results are compared for both analysers. It is shown that the error in the fitted analyser calibration parameters required for the flux calculations provides a substantial source of uncertainty in the fluxes. The choice of the calculation method itself can also constitute an uncertainty in the flux as the calculated fluxes by the three methods do not agree within error at all times. This finding highlights the need for a consistent and rigorous approach for comparable datasets, such as e.g. in flux networks. Ozone fluxes calculated by one of the methods were then used to compare the two analysers in more detail. This systematic analyser comparison reveals half-hourly flux values differing by up to a factor of two at times with the difference in mean hourly flux ranging from 0 to 23% with an error in the mean daily flux of± 12%. The comparison of analysers shows that the agreement in fluxes is excellent for some days but that there is an underlying uncertainty as a result of variable analyser performance and/or non-linear sensitivity. Correspondence to: J. B. A. Muller (jennifer.muller@postgrad.manchester.ac.uk)

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.