Bi-directional Ammonia Exchange Research Articles

Updates to the Noah Land Surface Model in WRF-CMAQ to Improve Simulated Meteorology, Air Quality, and Deposition.

Regional, state, and local environmental regulatory agencies often use Eulerian models to investigate the potential impacts on pollutant deposition and air quality from changes in land use, anthropogenic and natural emissions, and climate. The Noah land surface model (LSM) in the Weather Research and Forecasting (WRF) model is widely used with the Community Multiscale Air Quality (CMAQ) model for such investigations, but there are many inconsistencies that need to be changed so that they are consistent with dry deposition and emission processes. In this work, the Noah LSM in WRFv3.8.1 is improved in its linkage to CMAQv5.2 by adding important parameters to the WRF/Noah output, updating the WRF soil and vegetation reference tables that influence CMAQ wet and dry photochemical deposition processes, and decreasing WRF/Noah's top soil layer depth to be consistent with CMAQ processes (e.g., windblown dust and bidirectional ammonia exchange). The modified WRF/Noah‐CMAQ system (both off‐line and coupled) impacts meteorological predictions of 2‐m temperature (T2; increases and decreases), 2‐m mixing ratio (Q2; decreases), and 10‐m wind speed (WSPD10; decreases) in the United States. These changes are mostly driven by leaf area index values and aerodynamic roughness lengths updated in the vegetation tables based on satellite data, with additional impacts from soil tables updated based on recent soil data. Improvements in the consistency in the treatment of land surface processes between CMAQ and WRF resulted in improvements in both estimated meteorological (e.g., T2, WSPD10, and latent heat fluxes) and chemical (e.g., ozone, sulfur dioxide, and windblown dust) model estimates.

Nitrogen emission and deposition budget in West and Central Africa

Atmospheric nitrogen depends on land surface exchanges of nitrogen compounds. In Sub Saharan Africa, deposition and emission fluxes of nitrogen compounds are poorly quantified, and are likely to increase in the near future due to land use change and anthropogenic pressure. This work proposes an estimate of atmospheric N compounds budget in West and Central Africa, along an ecosystem transect, from dry savanna to wet savanna and forest, for years 2000−2007. The budget may be considered as a one point in time budget, to be included in long term studies as one of the first reference point for Sub Saharan Africa. Gaseous dry deposition fluxes are estimated by considering N compounds concentrations measured in the frame of the IDAF network (IGAC/DEBITS/AFrica) at the monthly scale and modeling of deposition velocities at the IDAF sites, taking into account the bi directional exchange of ammonia. Particulate dry deposition fluxes are calculated using the same inferential method. Wet deposition fluxes are calculated from measurements of ammonium and nitrate chemical content in precipitations at the IDAF sites combined with the annual rainfall amount. In terms of emission, biogenic NO emissions are simulated at each IDAF site with a surface model coupled to an emission module elaborated from an artificial neural network equation. Ammonia emissions from volatilization are calculated from literature data on livestock quantity in each country and N content in manure. NOx and NH3 emission from biomass burning and domestic fires are estimated from satellite data and emission factors. The total budget shows that emission sources of nitrogen compounds are in equilibrium with deposition fluxes in dry and wet savannas, with respectively 7.40 (±1.90) deposited and 9.01 (±3.44) kgN ha−1 yr−1 emitted in dry savanna, 8.38 (±2.04) kgN ha−1 yr−1 deposited and 9.60 (±0.69) kgN ha−1 yr−1 emitted in wet savanna. In forested ecosystems, the total budget is dominated by wet plus dry deposition processes (14.75 ± 2.36 kgN ha−1 yr−1), compared to emissions processes (8.54 ± 0.50 kgN ha−1 yr−1).

Review and parameterisation of bi-directional ammonia exchange between vegetation and the atmosphere

Abstract. Current deposition schemes used in atmospheric chemical transport models do not generally account for bi-directional exchange of ammonia (NH3). Bi-directional exchange schemes, which have so far been applied at the plot scale, can be included in transport models, but need to be parameterised with appropriate values of the ground layer compensation point (χg), stomatal compensation point (χs) and cuticular resistance (Rw). We review existing measurements of χg, χs as well as Rw and compile a comprehensive dataset from which we then propose generalised parameterisations. χs is related to Γs, the non-dimensional ratio of [NH4+]apo and [H+]apo in the apoplast, through the temperature dependence of the combined Henry and dissociation equilibrium. The meta-analysis suggests that the nitrogen (N) input is the main driver of the apoplastic and bulk leaf concentrations of ammonium (NH4 apo

A new model of bi-directional ammonia exchange between the atmosphere and biosphere: Ammonia stomatal compensation point

A new multi-layer canopy resistance model of bi-directional NH 3 exchange is presented. This new model, which is based on the Multi-Layer BioChemical deposition (MLBC) model [Wu, Y., Brashers, B., Finkelstein, P.L., Pleim, J.E., 2003a. A multiplayer biochemical dry deposition model. I. Model formulation. J. Geophys. Res. 108, D1; Wu, Y., Brashers, B., Finkelstein, P.L., Pleim, J.E., 2003b. A multiplayer biochemical dry deposition model. II. Model evaluation. J. Geophys. Res. 108, D1], incorporates a parameterization for the ammonia stomatal compensation point that is theoretically derived to consider the effects of leaf temperature and apoplastic concentrations of NH 4 + and H +. The new ammonia stomatal compensation point scheme accounts for the effects of deposition, emission and leaf temperature on the dynamics of apoplast [NH 4 +] and [H +]. The new model is evaluated against bidirectional NH 3 fluxes measured over fertilized soybean. The general patterns of observed deposition and emission are successfully reproduced when the ammonia stomatal compensation point is included. Driven by the effects of deposition, emission and leaf temperature, modeled apoplastic [NH 4 +] and [H +] display significant diurnal variation when the buffer effect of the underlying metabolic processes generating or consuming NH 4 + were ignored. Model predictive capability is improved slightly by incorporating the feedback into a dynamic stomatal compensation point. A simple implementation of the feedback mechanism in the current model provides opportunities for improvement. While the stomatal flux is shown to be an important process in the regulation of canopy-scale fluxes, it appears that exchange with leaf surface water and soil may also be important.

Bidirectional ammonia exchange above a mixed coniferous forest

Two canopy compensation point models were used to study the bidirectional exchange of ammonia over a mixed coniferous forest subjected to high nitrogen deposition. The models were tested for 16 time series, average fluxes of which ranged between −270 and +1 ng m −2 s −1. The static model consisted of a bidirectional stomatal flux and a unidirectional cuticular flux component. The dynamic model also allowed for desorption of ammonia from the leaf surface and took into account ammonia fluxes from precedent periods. The apoplastic ammonium/hydrogen ion ratio ( Γ), which was derived to estimate the stomatal compensation point ( χ s), amounted to 3300 in spring and 1375 during the summer/autumn. Empirical descriptions for cuticular resistances ( R w) in the static model, developed as a function of micrometeorological conditions and codeposition effects, failed to reproduce the measured fluxes. A better match with measurements was obtained using the dynamic model, which succeeded in simulating net-emission during the daytime.

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. SO 2 deposition is sensitive to variation in gas concentration, wind speed and wet surface uptake parameters. NO 2 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 NO 2 concentrations may underestimate deposition substantially in some areas. HNO 3 dry deposition is sensitive to wind speed and concentration. NH 3 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 NH 3, 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 SO 2 is 135 Gg S yr −1 for 1996, for NH 3 is 97 Gg N yr −1, for NO 2 is 26 Gg N yr −1 and the preliminary estimate for HNO 3 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. NO 2 dry deposition only accounts for 15% of total oxidised nitrogen deposition, but another 25% may come from the dry deposition of HNO 3, 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.

A dynamic chemical model of bi-directional ammonia exchange between semi-natural vegetation and the atmosphere

A dynamic chemical model of bi-directional ammonia exchange between semi-natural vegetation and the atmosphere

The exchange of atmospheric ammonia with vegetated surfaces. II: Fertilized vegetation

Measurements of the exchange of gaseous ammonia (NH3) have been made over fertilized plant canopies using micrometeorological techniques. The NH3 exchange over ungrazed grassland and arable crops was examined in a range of environmental conditions: a bidirectional exchange of ammonia was observed, with warm dry conditions favouring emission (NH3 fluxes up to 24 ng m−2s−1) and cool wet conditions favouring deposition (fluxes in the range 1–35 ng m−2s−1). Limited measurements made when the ground was frozen suggest an increased surface resistance may occur in these conditions (Measured Rc up to 80 s m−1), resulting in reduced rates of deposition. The fluxes are contrasted with measurements over unfertilized vegetation, and it is suggested that the net canopy exchange is the combined result of leaf surface, stomatal and soil exchange processes; the increased tendency of fertilized vegetation to emit ammonia is attributed to the larger fixed nitrogen concentrations in these ecosystems. The bidirectional fluxes may be related to the existence of ammonia compensation points. Estimates of the stomatal compensation point for fertilized croplands were in the range 1-7 μg NH3m−3, though because of deposition to leaf surfaces, this frequently does not control net fluxes. The Mean surface concentration of ammonia X(z'o), or canopy compensation point, was therefore also estimated and found to vary, with environmental conditions, in the range 0-3 μg m−3. A simplified procedure is used to estimate the net annual exchange of ammonia over an example fertilized crop in the British Isles. This predicts an emission of fixed nitrogen of less than 1 kg per hectare, which is the net result of emission and deposition throughout the year.

The exchange of atmospheric ammonia with vegetated surfaces. II: Fertilized vegetation

AbstractMeasurements of the exchange of gaseous ammonia (NH3) have been made over fertilized plant canopies using micrometeorological techniques. The NH3 exchange over ungrazed grassland and arable crops was examined in a range of environmental conditions: a bidirectional exchange of ammonia was observed, with warm dry conditions favouring emission (NH3 fluxes up to 24 ng m−2s−1) and cool wet conditions favouring deposition (fluxes in the range 1–35 ng m−2s−1). Limited measurements made when the ground was frozen suggest an increased surface resistance may occur in these conditions (Measured Rc up to 80 s m−1), resulting in reduced rates of deposition.The fluxes are contrasted with measurements over unfertilized vegetation, and it is suggested that the net canopy exchange is the combined result of leaf surface, stomatal and soil exchange processes; the increased tendency of fertilized vegetation to emit ammonia is attributed to the larger fixed nitrogen concentrations in these ecosystems. The bidirectional fluxes may be related to the existence of ammonia compensation points. Estimates of the stomatal compensation point for fertilized croplands were in the range 1‐7 μg NH3m−3, though because of deposition to leaf surfaces, this frequently does not control net fluxes. The Mean surface concentration of ammonia X(z'o), or canopy compensation point, was therefore also estimated and found to vary, with environmental conditions, in the range 0‐3 μg m−3. A simplified procedure is used to estimate the net annual exchange of ammonia over an example fertilized crop in the British Isles. This predicts an emission of fixed nitrogen of less than 1 kg per hectare, which is the net result of emission and deposition throughout the year.