Surface Exchange Fluxes Research Articles

Land‐atmosphere coupling in EURO‐CORDEX evaluation experiments

AbstractInteractions between the land surface and the atmosphere play a fundamental role in the weather and climate system. Here we present a comparison of summertime land‐atmosphere coupling strength found in a subset of the ERA‐Interim‐driven European domain Coordinated Regional Climate Downscaling Experiment (EURO‐CORDEX) model ensemble (1989–2008). Most of the regional climate models (RCMs) reproduce the overall soil moisture interannual variability, spatial patterns, and annual cycles of surface exchange fluxes for the different European climate zones suggested by the observational Global Land Evaporation Amsterdam Model (GLEAM) and FLUXNET data sets. However, some RCMs differ substantially from FLUXNET observations for some regions. The coupling strength is quantified by the correlation between the surface sensible and the latent heat flux, and by the correlation between the latent heat flux and 2 m temperature. The first correlation is compared to its estimate from the few available long‐term European high‐quality FLUXNET observations, and the latter to results from gridded GLEAM data. The RCM simulations agree with both observational datasets in the large‐scale pattern characterized by strong coupling in southern Europe and weak coupling in northern Europe. However, in the transition zone from strong to weak coupling covering large parts of central Europe many of the RCMs tend to overestimate the coupling strength in comparison to both FLUXNET and GLEAM. The RCM ensemble spread is caused primarily by the different land surface models applied, and by the model‐specific weather conditions resulting from different atmospheric parameterizations.

Eddy covariance measurements of the net turbulent methane flux in the city centre – results of 2-year campaign in Łódź, Poland

Abstract. To investigate temporal variability of methane (CH4) fluxes in an urban environment, air–surface exchange fluxes of CH4 were continuously measured using eddy covariance techniques at a city-centre site in Łódź, Poland, from July 2013 to August 2015. In the immediate vicinity of the measurement site, potential methane sources include vehicle traffic, dense sewerage infrastructure and natural gas networks. Sensible and latent heat fluxes have also been measured since 2000 and carbon dioxide fluxes since 2007 at this site. Upward CH4 fluxes dominated during the measurement period, indicating that the city centre is a net source of CH4 to the troposphere. The highest monthly fluxes were observed in winter (2.0 to 2.7 g m−2 month−1) and the lowest in summer (0.8 to 1.0 g m−2 month−1). Fluxes on working days were around 6 % higher than on weekends. The cumulative flux indicates that the city centre emitted a net quantity of nearly 18 g m−2 of CH4 in 2014. Stable values of the FCO2∕ FCH4 ratio in months (minimum 2.41 × 10−3, maximum 5.3 × 10−3) and the lack of a clear annual course suggest comparable magnitude of both fluxes.

A modified micrometeorological gradient method for estimating O<sub>3</sub> dry depositions over a forest canopy

Abstract. Small pollutant concentration gradients between levels above a plant canopy result in large uncertainties in estimated air–surface exchange fluxes when using existing micrometeorological gradient methods, including the aerodynamic gradient method (AGM) and the modified Bowen ratio method (MBR). A modified micrometeorological gradient method (MGM) is proposed in this study for estimating O3 dry deposition fluxes over a forest canopy using concentration gradients between a level above and a level below the canopy top, taking advantage of relatively large gradients between these levels due to significant pollutant uptake in the top layers of the canopy. The new method is compared with the AGM and MBR methods and is also evaluated using eddy-covariance (EC) flux measurements collected at the Harvard Forest Environmental Measurement Site, Massachusetts, during 1993–2000. All three gradient methods (AGM, MBR, and MGM) produced similar diurnal cycles of O3 dry deposition velocity (Vd(O3)) to the EC measurements, with the MGM method being the closest in magnitude to the EC measurements. The multi-year average Vd(O3) differed significantly between these methods, with the AGM, MBR, and MGM method being 2.28, 1.45, and 1.18 times that of the EC, respectively. Sensitivity experiments identified several input parameters for the MGM method as first-order parameters that affect the estimated Vd(O3). A 10% uncertainty in the wind speed attenuation coefficient or canopy displacement height can cause about 10% uncertainty in the estimated Vd(O3). An unrealistic leaf area density vertical profile can cause an uncertainty of a factor of 2.0 in the estimated Vd(O3). Other input parameters or formulas for stability functions only caused an uncertainly of a few percent. The new method provides an alternative approach to monitoring/estimating long-term deposition fluxes of similar pollutants over tall canopies.

Identifying spatial and temporal dynamics of proglacial groundwater–surface-water exchange using combined temperature-tracing methods

The effect of proglacial groundwater systems on surface hydrology and ecology in cold regions often is neglected when assessing the ecohydrological implications of climate change. We present a novel approach in which we combined 2 temperature-tracing techniques to assess the spatial patterns and short-term temporal dynamics of groundwater–surface-water exchange in the proglacial zone of Skaftafellsjökull, a retreating glacier in southeastern Iceland. Our study focuses on localized groundwater discharge to a surface-water environment, where high temporal- and spatial-resolution mapping of sediment surface and subsurface temperatures (10–15 cm depth) were obtained by Fiber-Optic Distributed Temperature Sensing (FO-DTS). The FO-DTS survey identified temporally consistent locations of temperature anomalies at the sediment–water interface, indicating distinct zones of cooler groundwater upwelling. The high-resolution FO-DTS surveys were combined with calculations of 1-dimensional groundwater seepage fluxes based on 3 vertical sediment temperature profiles, covering depths of 10, 25, and 40 cm below the lake bed. The calculated groundwater seepage rates ranged between 1.02 to 6.10 m/d. We used the combined techniques successfully to identify substantial temporal and spatial heterogeneities in groundwater–surface exchange fluxes that have relevance for the ecohydrological functioning of the investigated system and its potential resilience to environmental change.

Disaggregation of screen-level variables in a numerical weather prediction model with an explicit simulation of subgrid-scale land-surface heterogeneity

The earth’s surface is characterized by small-scale heterogeneity attributable to variability in land cover, soil characteristics and orography. In atmospheric models, this small-scale variability can be partially accounted for by the so-called mosaic approach, i.e., by computing the land-surface processes on a grid with an explicit higher horizontal resolution than the atmosphere. The mosaic approach does, however, not account for the subgrid-scale variability in the screen-level atmospheric parameters, part of which might be related to land-surface heterogeneity itself. In this study, simulations with the numerical weather prediction model COSMO are shown, employing the mosaic approach together with a spatial disaggregation of the atmospheric forcing by the screen-level variables to the subgrid-scale. The atmospheric model is run with a 2.8 km horizontal grid resolution while the land surface processes are computed on a 400-m horizontal grid. The disaggregation of the driving atmospheric variables at screen-level is achieved by a three-step statistical downscaling with rules learnt from high-resolution fully coupled COSMO simulations, where both, atmosphere and surface, were simulated on a 400-m grid. The steps encompass spline interpolation of the grid scale variables, conditional regression based on the high-resolution runs, and an optional stochastic noise generator which restores the variability of the downscaled variables. Simulations for a number of case studies have been carried out, with or without mosaic surface representation and with or without atmospheric disaggregation, and evaluated with respect to the surface state variables and the turbulent surface exchange fluxes of sensible and latent heat. The results are compared with the high-resolution fully coupled COSMO simulations. The results clearly demonstrate the high importance of accounting for subgrid-scale surface heterogeneity. It is shown that the atmospheric disaggregation leads to clear additional improvements in the structures of the two-dimensional surface state variable fields, but to only marginally impacts on the simulation of the turbulent surface exchange fluxes. A detailed analysis of these results identifies strongly correlated errors in atmospheric and surface variables in the mosaic approach as the main reason for the latter. The effects of these errors largely cancel out in the flux parameterization, and thus explain the comparably good results for the fluxes in the mosaic approach without atmospheric disaggregation despite inferior performance for the surface state variables themselves. Inserting noise in the disaggregation scheme leads to a deterioration of the results.

Aerodynamic gradient measurements of the NH<sub>3</sub>-HNO<sub>3</sub>-NH<sub>4</sub>NO<sub>3</sub> triad using a wet chemical instrument: an analysis of precision requirements and flux errors

Abstract. The aerodynamic gradient method is widely used for flux measurements of ammonia, nitric acid, particulate ammonium nitrate (the NH3-HNO3-NH4NO3 triad) and other water-soluble reactive trace compounds. The surface exchange flux is derived from a measured concentration difference and micrometeorological quantities (turbulent exchange coefficient). The significance of the measured concentration difference is crucial for the significant determination of surface exchange fluxes. Additionally, measurements of surface exchange fluxes of ammonia, nitric acid and ammonium nitrate are often strongly affected by phase changes between gaseous and particulate compounds of the triad, which make measurements of the four individual species (NH3, HNO3, NH4+, NO3− necessary for a correct interpretation of the measured concentration differences. We present here a rigorous analysis of results obtained with a multi-component, wet-chemical instrument, able to simultaneously measure gradients of both gaseous and particulate trace substances. Basis for our analysis are two field experiments, conducted above contrasting ecosystems (grassland, forest). Precision requirements of the instrument as well as errors of concentration differences and micrometeorological exchange parameters have been estimated, which, in turn, allows the establishment of thorough error estimates of the derived fluxes of NH3, HNO3, NH4+, and NO3−. Derived median flux errors for the grassland and forest field experiments were: 39% and 50% (NH3), 31% and 38% (HNO3), 62% and 57% (NH4+), and 47% and 68% (NO3−), respectively. Additionally, we provide the basis for using field data to characterize the instrument performance, as well as subsequent quantification of surface exchange fluxes and underlying mechanistic processes under realistic ambient measurement conditions.

Inter-comparison of ammonia fluxes obtained using the Relaxed Eddy Accumulation technique

Abstract. The exchange of Ammonia (NH3) between grassland and the atmosphere was determined using Relaxed Eddy Accumulation (REA) measurements. The use of REA is of special interest for NH3, since the determination of fluxes at one height permits multiple systems to be deployed to quantify vertical flux divergence (either due to effects of chemical production or advection). During the Braunschweig integrated experiment four different continuous-sampling REA systems were operated during a period of about 10 days and were compared against a reference provided by independent application of the Aerodynamic Gradient Method (AGM). The experiment covered episodes before and after both cutting and fertilizing and provided a wide range of fluxes −60–3600 ng NH3 m−2 s−1 for testing the REA systems. The REA systems showed moderate to good correlation with the AGM estimates, with r2 values for the linear regressions between 0.3 and 0.82. For the period immediately after fertilization, the REA systems showed average fluxes 20% to 70% lower than the reference. At periods with low fluxes REA and AGM can agree within a few %. Overall, the results show that the continuous REA technique can now be used to measure NH3 surface exchange fluxes. While REA requires greater analytical precision in NH3 measurement than the AGM, a key advantage of REA is that reference sampling periods can be introduced to remove bias between sampling inlets. However, while the data here indicate differences consistent with advection effects, significant improvements in sampling precision are essential to allow robust determination of flux divergence in future studies. Wet chemical techniques will be developed further since they use the adsorptive and reactive properties of NH3 that impedes development of cheaper optical systems.

Towards improving the simulation of meteorological fields in urban areas through updated/advanced surface fluxes description

Abstract. The increased resolution of numerical weather prediction models allows nowadays addressing more realistically urban meteorology and air pollution processes. This has triggered new interest in modelling and describing experimentally the specific features and processes of urban areas. Recent developments and results performed within the EU-funded project FUMAPEX on integrated systems for forecasting urban meteorology and air pollution are reported here. Sensitivity studies with respect to optimum resolution, parametrisation of urban roughness and surface exchange fluxes and the role of urban soil layers are carried out with advanced meso- or sub-meso meteorological models. They show that sensible improvements can be achieved by higher model resolution that is accompanied with better description of urban surface features. Recommendations, especially with respect to advanced urban air quality forecasting and information systems, are given together with an assessment of the needed further research and data.

Validation of Martilli's urban boundary layer scheme with measurements from two mid-latitude European cities

Abstract. Martilli's urban parameterization scheme is improved and implemented in a mesoscale model in order to take into account the typical effects of a real city on the air temperature near the ground and on the surface exchange fluxes. The mesoscale model is run on a single column using atmospheric data and radiation recorded above roof level as forcing. Here, the authors validate Martilli's urban boundary layer scheme using measurements from two mid-latitude European cities: Basel, Switzerland and Marseilles, France. For Basel, the model performance is evaluated with observations of canyon temperature, surface radiation, and energy balance fluxes obtained during the Basel urban boundary layer experiment (BUBBLE). The results show that the urban parameterization scheme represents correctly most of the behavior of the fluxes typical of the city center of Basel, including the large heat uptake by the urban fabric and the positive sensible heat flux at night. For Marseilles, the model performance is evaluated with observations of surface temperature, canyon temperature, surface radiation, and energy balance fluxes collected during the field experiments to constrain models of atmospheric pollution and transport of emissions (ESCOMPTE) and its urban boundary layer (UBL) campaign. At both urban sites, vegetation cover is less than 20%, therefore, particular attention was directed to the ability of Martilli's urban boundary layer scheme to reproduce the observations for the Marseilles city center, where the urban parameters and the synoptic forcing are totally different from Basel. Evaluation of the model with wall, road, and roof surface temperatures gave good results. The model correctly simulates the net radiation, canyon temperature, and the partitioning between the turbulent and storage heat fluxes.

An Analysis and Implications of Alternative Methods of Deriving the Density (WPL) Terms for Eddy Covariance Flux Measurements

We explore some of the underlying assumptions used to derive the density or WPL terms (Webb et al. (1980) Quart J Roy Meteorol Soc 106:85–100) required for estimating the surface exchange fluxes by eddy covariance. As part of this effort we recast the origin of the density terms as an assumption regarding the density fluctuations rather than as a (dry air) flux assumption. This new approach, which is similar to the expansion/compression approach of Liu (Boundary-Layer Meteorol 115:151–168, 2005), eliminates the dry-air mean advective vertical velocity from the development of the WPL terms and allows us to directly compare Liu’s assumptions for deriving the WPL terms with the analogous assumptions appropriate to the original expression of the WPL terms. We suggest, (i) that the main difference between these two approaches lies in the interpretation of the turbulent exchange flux, and (ii) that the original WPL formulation is the more appropriate approach. Given the importance of the WPL terms to accurate and reliable measurements of surface exchange fluxes, a careful analysis of their origins and their proper mathematical expression and interpretation is warranted.

Oxygen diffusion through perovskite membranes

The oxygen permeation flux through the perovskite membrane made of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) has been measured as a function of both the temperature and the oxygen pressure gradient across the membrane. A simple model for the surface exchange fluxes was used to understand the mechanism of the oxygen transport. The limiting step of the oxygen transport was found to be the bulk diffusion for the BSCF perovskite at temperatures above 700 oC. Furthermore, the oxygen vacancy diffusion coefficient (Dv) can be deduced from the dependence of the oxygen permeation flux on the oxygen pressure gradient. From permeation measurement on the BSCF membrane tube, Dv is between 2.82×10-9 m2/s at 900 oC and 0.82×10-9 m2/s at 700 oC.

Gas-particle interactions above a Dutch heathland: III. Modelling the influence of the NH<sub>3</sub>-HNO<sub>3</sub>-NH<sub>4</sub>NO<sub>3</sub> equilibrium on size-segregated particle fluxes

Abstract. Micrometeorological measurements of size-segregated particle number fluxes above Dutch heathlands and forests have repeatedly shown simultaneous apparent emission of particles with a diameter (Dp)<0.18 µm and deposition of larger particles when measured with optical particle counters. In order to assess whether this observation may be explained by the equilibrium reaction of ammonia (NH3), nitric acid (HNO3) and ammonium (NH4+), a new numerical model is developed to predict the vertical concentration and flux profiles of the different species as modified by the interaction of equilibration and surface/atmosphere exchange processes. In addition to former studies, the new approach explicitly models the height-dependence of the NH4+ and total aerosol size-distribution. Using this model, it is demonstrated that both gas-to-particle conversion (gtpc) and aerosol evaporation can significantly alter the apparent surface exchange fluxes, and evoke the observed bi-directional particle fluxes under certain conditions. Thus, in general, the NH3-HNO3-NH4NO3 equilibrium needs to be considered when interpreting eddy-covariance particle fluxes. Applied to an extensive dataset of simultaneous flux measurements of particles and gases at Elspeet, NL, the model reproduces the diurnal pattern of the bi-directional exchange well. In agreement with the observation of fast NH4+ deposition, slow nitric acid deposition (both as measured by the aerodynamic gradient method) and small concentration products of NH3×HNO3 at this site, this study suggests that NH4+ evaporation at this site significantly alters surface exchange fluxes.

Gas-particle interactions above a Dutch heathland: II. Concentrations and surface exchange fluxes of atmospheric particles

Abstract. Size-dependent particle number fluxes measured by eddy-covariance (EC) and continuous fluxes of ammonium (NH4+) measured with the aerodynamic gradient method (AGM) are reported for a Dutch heathland. Daytime deposition velocities (Vd) by EC with peak values of 5 to 10 mm s-1 increased with particle diameter (dp) over the range 0.1–0.5 µm, and are faster than predicted by current models. With a mean Vd of 2.0 mm s-1 (daytime: 2.7; night-time 0.8 mm s-1) NH4+ fluxes by AGM are overall in agreement with former measurements and NH4+-N dry deposition amounts to 20% of the dry input of NH3-N over the measurement period. These surface exchange fluxes are analyzed together with simultaneous gas-phase flux measurements for indications of gas-particle interactions. On warm afternoons the apparent fluxes of acids and aerosol above the heathland showed several coinciding anomalies, all of which are consistent with NH4+ evaporation during deposition: (i) canopy resistances for HNO3 and HCl of up to 100 s m-1, (ii) simultaneous particle emission of small particles (Dp<0.18 µm) and deposition of larger particles (Dp>0.18 µm), (iii) NH4+ deposition faster than derived from size-distributions and size-segregated EC particle fluxes. These observations coincide with the observations of (i) surface concentration products of NH3 and HNO3 well below the thermodynamic equilibrium value and (ii) Damköhler numbers that indicate chemical conversion to be sufficiently fast to modify exchange fluxes. The measurements imply a removal rate of volatile NH4+ of 3−30×10-6 s-1 averaged over the 1 km boundary-layer, while NH3 deposition is underestimated by typically 20 ng m-2 s-1 (28%) and flux reversal may occur.

Estimating net air‐sea fluxes from ocean bulk data: Methodology and application to the heat cycle

A novel method to estimate annual mean heat, water, and gas exchange fluxes between the ocean and the atmosphere is proposed that is complementary to the traditional approach based on air‐sea gradients and bulk exchange parameterization. The new approach exploits the information on surface exchange fluxes contained in the distribution of temperature, salinity, and dissolved gases in the ocean interior. We use an Ocean General Circulation Model to determine how the distribution in the ocean interior is linked to surface fluxes. We then determine with least squares the surface fluxes that are most compatible with the observations. To establish and test the method, we applyit to ocean temperature data to estimate heat fluxes across the air‐sea interface for which a number of climatological estimates exists. We also test the sensitivity of the inversion results to data coverage, differences in ocean transport, variations in the surface flux pattern and a range of spatial resolutions. We find, on the basis of the World Ocean Circulation Experiment (WOCE) data network augmented with selected high‐quality pre‐WOCE data, that we are able to constrain heat exchange fluxes for 10–15 regions of the ocean, whereby these fluxes nearly balance globally without enforcing a conservation constraint. Our results agree well with heat flux estimates on the basis of bulk exchange parameterization, which generally require constraints to ensure a global net heat flux of zero. We also find that the heat transports implied by our inversely estimated fluxes are in good agreement with a large range of heat transport estimates based on hydrographic data. Increasing the number of regions beyond the 10–15 regions considered here is severely limited because of modeling errors. The inverse method is fairly robust to the modeling of the spatial patterns of the surface fluxes; however, it is quite sensitive to the modeling of ocean transport. The most striking difference between our estimates and the heat flux climatologies is a large heat loss of 0.64 PW to the atmosphere from the Southern Ocean and a large heat gain by the subpolar South Atlantic of 0.56 PW. These results are consistent with the large gain of carbon dioxide called for by Takahashi et al. [1999] in his recent analysis of the air‐sea flux of carbon dioxide but inconsistent with the large loss of oxygen and carbon dioxide such as those of Stephens et al. [1998].

Surface/atmosphere exchange and chemical interaction of gases and aerosols over oilseed rape

Measurements of NH 3, HCl, HNO 3 and HNO 2 gas as well as NH 4 +, NO 3 −, Cl − and SO 4 2− aerosol are used to investigate their surface exchange fluxes and the potential for gas–particle interactions at a clean coastal Scottish site. Mean concentrations of HNO 3 and HCl were small at 0.68 and 0.32 μg m −3, respectively. At relative humidities ( h)<85% measured gas concentration products ( K m) were smaller than the predicted dissociation constants ( K e), suggesting potential for aerosol evaporation, but at high h, K e of NH 4Cl was exceeded at the mean canopy height. Above the canopy, small aerosol concentrations resulted in estimated chemical time-scales of >3 min. Thus, chemical reactions should not have affected NH 3 flux measurements by aerodynamic gradient methods (AGMs), except for very low turbulence when AGM is not applicable. Within the canopy, however, the diffusive transport provided enough time for NH 4Cl to be generated. This was substantiated by measurements of NH 4 + emission and high Cl − aerosol concentrations within the canopy. Micrometeorological measurements above the canopy indicated that gaseous Cl compounds were emitted for most of the time, and this was supported by the source/sink distributions of gaseous and aerosol Cl compounds calculated from in-canopy profiles as well as high apoplastic Cl − concentrations. Although emission of CH 3Cl has been reported for other Brassica species, an unrealistically large emission would be necessary to cause the observed above-canopy gradients. Emission of HCl liberated from unidentified water pools of high Cl − or leaf surface reactions is a more likely source of gaseous Cl compounds.

A coarse grid three‐dimensional global inverse model of the atmospheric transport: 2. Inversion of the transport of CO2 in the 1980s

Models of atmospheric transport can be used to interpret spatiotemporal differences in the observed concentrations of CO2 in terms of its surface exchange fluxes. Inversion of the atmospheric transport is the systematic search for both a flux field that yields an optimal match between modeled and observed concentrations and, equally importantly, the uncertainties in this inferred flux field. The present inversion study combines observations of the CO2 concentration at the global station network of the NOAA/CMDL in the 1980s with a representation of the atmospheric transport model TM2 by its Jacobian matrix, which has been previously computed by the adjoint model of TM2. This Jacobian matrix maps monthly fluxes on the approximately 8° latitude by 10° longitude horizontal model grid onto the resulting changes in the monthly CO2 concentration at every station. Since the number of observational sites is much smaller than the number of grid cells, the inverse problem is highly underdetermined. A unique solution is determined by including a priori information on the surface exchange fluxes derived from output of high‐resolution models of both the terrestrial biosphere and the ocean, combined with statistics of fossil fuel burning and land use change. Performing a Bayesian synthesis inversion, for a target period in the 1980s, the average seasonal cycle and the mean annual magnitude of CO2 surface fluxes on the TM2 grid are inferred. The resulting simulated concentration compares well with independent observations. On a global scale, an oceanic sink of 1.5±0.4 gigatons of carbon (GtC) is estimated. This sink is stronger in the northern than in the southern hemisphere. On a regional scale, however, the inferred exchange fluxes exhibit high uncertainty, indicating a low capacity of the global observational network to monitor regional trace gas emissions. These findings are relatively insensitive to the year of meteorological driving data, suggesting interannual changes in concentration should primarily result from source not transport changes.

A coarse grid three‐dimensional global inverse model of the atmospheric transport: 1. Adjoint model and Jacobian matrix

TM2 is a global three‐dimensional model of the atmospheric transport of passive tracers. The adjoint of TM2 is a model that allows the efficient evaluation of derivatives of the simulated tracer concentration at observational locations with respect to the tracer's sources and sinks. We describe the generation of the adjoint model by applying the Tangent linear and Adjoint Model Compiler in the reverse mode of automatic differentiation to the code of TM2. Using CO2 as an example of a chemically inert tracer, the simulated concentration at observational locations is linear in the surface exchange fluxes, and thus the transport can be represented by the model's Jacobian matrix. In many current inverse modeling studies, such a matrix has been computed by multiple runs of a transport model for a set of prescribed surface flux patterns. The computational cost has been proportional to the number of patterns. In contrast, for differentiation in reverse mode, the cost is independent of the number of flux components. Hence, by a single run of the adjoint model, the Jacobian for the approximately 8° latitude by 10° longitude horizontal resolution of TM2 could be computed efficiently. We quantify this efficiency by comparison with the conventional forward modeling approach. For some prominent observational sites, we present visualizations of the Jacobian matrix by series of illustrative global maps quantifying the impact of potential emissions on the concentration in particular months. Furthermore, we demonstrate how the Jacobian matrix is employed to completely analyze a transport model run: A simulated monthly mean value at a particular station is decomposed into the contributions to this value by all flux components, i.e., the fluxes into every surface model grid cell and month. This technique also results in a series of global maps.

Intercalibration of a passive wind-vane flux sampler against a continuous-flow denuder for the measurements of atmospheric ammonia concentrations and surface exchange fluxes

A passive wind-vane flux sampler is a simple low-cost device used to estimate long-term vertical fluxes of ammonia in the atmospheric surface boundary layer. The passive flux sampler measures the horizontal flux of ammonia. A vertical gradient of the horizontal flux, combined with micro-meteorological measurements of wind speed and temperature, is used to estimated the vertical flux of ammonia using a modified aerodynamic gradient technique. The passive wind-vane flux sampler gradient was calibrated against a gradient measured with fast response (6 min) continuous-flow denuders. The measurements were carried out at a heathland located in an intensive farming area in the centre of the Netherlands. A field campaign took place over 70 day period in the summer of 1996, during which the sampling periods of the passive wind-vane flux sampler varied between 3 and 9 days. The comparison clearly showed that the long-term measurements with the passive wind-vane flux samplers gave accurate average ammonia deposition values for the field campaign as a whole which deviated by only 18% from the reference flux. However, there was no significant correlation between the fluxes from the passive samplers and the reference method for the individual 10 periods which were compared. Possible explanations found for the lacking correlation were (I) a high percentage number of half-hour emission events within each period resulted in a significant large relative deviation between the fluxes, and (II) uncertainties in the reference method might also explain the lacking correlation. The passive wind-vane flux samplers proved to be a stable method for long-term measurements (months to years) due to a close to 100% optimal functioning during the field campaign.

A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory

As part of the Reactive Chlorine Emissions Inventory, a global model of chemical processes in the marine boundary layer (MBL), Marine Aerosol and Gas Phase Interactions (MAGPI), was developed to calculate direct monthly production of HCl and ClNO2 from sea salt dechlorination on a 2.8 × 2.8 latitude‐longitude grid. Sea salt mass and size distributions and associated surface exchange fluxes were calculated using the Canadian General Circulation Model; integrated annual production of sea salt Cl− was 1785 Tg Cl yr−1. Corresponding distributions of gas‐phase HNO3, SO2, N2O5, H2O2, O3, H2SO4 and NH3 were calculated using different global chemical transport models in which sea salt reactions were not considered. A chemical scheme was developed to estimate the monthly mean steady‐state phase partitioning of product and reactant species at each grid point. Average annual gridded fluxes of HCl and ClNO2 varied spatially from 1 to 300 mg Cl m−2 yr−1 and from 1 to 8 mg Cl m−2 yr−1, respectively. Maxima occurred in polluted coastal regions of the North Atlantic, the western North Pacific and the Mediterranean where up to 20% of the total Cl and 80% of the sub‐micron Cl volatilized. In remote oceanic regions, available acidity was insufficient to titrate all sea salt alkalinity, thus, significant HCl was not produced via acid displacement. However, in these regions virtually all HNO3 was scavenged by sea salt. The integrated annual global fluxes for HCl and ClNO2 were 7.6 Tg Cl yr−1 and 0.06 Tg Cl yr−1, respectively; virtually all in the Northern Hemisphere. Largest HCl and ClNO2 fluxes occur in northern hemisphere winter due to high sea salt loading and elevated HNO3, SO2 and N2O5 concentrations. 70% of the HCl dechlorination occurs on particles between 0.75 μm and 4 μm radius; ClNO2 volatilized from slightly larger particles. The aerosol pH of each particle size bin equilibrates towards the same value once the alkalinity has been titrated.

Sensitivity of the seasonal cycle of CO 2 at remote monitoring stations with respect to seasonal surface exchange fluxes determined with the adjoint of an atmospheric transport model

The adjoint model to a global three-dimensional atmospheric transport model can be used to efficiently perform a sensitivity analysis, i.e. the computation of the partial derivatives of a particular model output feature with respect to many control variables. We demonstrate this approach by investigating the dependence of the magnitude of the modeled seasonal cycle of CO 2 at remote monitoring stations with respect to the magnitudes of the seasonal cycle of the net CO 2 surface fluxes prescribed from a simple diagnostic terrestrial biosphere model. The technique results in global maps of those source regions that predominately influence the magnitude of the seasonal cycle at the different monitoring stations.