Local-scale Transport Research Articles

Silting in the Grand Canal in the Domain of Chantilly (Oise, France)—Catchment-Scale Hydrogeomorphological Reconnaissance and Local-Scale Hydro-Sedimentary Transport Modelling

The domain of Chantilly (Oise, France) includes a castle and a garden, both dating from the eighteenth century, which are seen as important legacies of France’s history. Nowadays, the 2.5 km canal that runs through the domain is subject to the phenomenon of silting, leading to the accumulation of sediments within the canal linked to the proliferation of algae, which has a dissuasive effect among visitors. HEC-RAS software (Hydrologic Engineering Centers River Analysis System, US army corps of Engineers, Washington, D.C., USA) was used to model and understand sediment accumulation within the canal. This model is widely used in the literature tackling sediment transport and accumulation, and allows the forecasting of which stretches of the canal are most susceptible to sediment accumulation. The simulation results highlight an accumulation of sediment near the entrance of the Nonette stream into the canal and a propagation through the canal. The total accumulated volume assessed by the model between 2001 and 2010 equals 3901 m3, when the reconnaissance of the catchment showed the sediment was not a limiting factor. However, the volumes determined are underestimated, as matter brought by vegetation or other systems different from the river (e.g., wind, rainfall) is not considered in the calculation. The quantity of sediment is also subject to uncertainties, as the bathymetry of the canal is not available.

Statistical atmospheric inversion of local gas emissions by coupling the tracer release technique and local-scale transport modelling: a test case with controlled methane emissions

Abstract. This study presents a new concept for estimating the pollutant emission rates of a site and its main facilities using a series of atmospheric measurements across the pollutant plumes. This concept combines the tracer release method, local-scale atmospheric transport modelling and a statistical atmospheric inversion approach. The conversion between the controlled emission and the measured atmospheric concentrations of the released tracer across the plume places valuable constraints on the atmospheric transport. This is used to optimise the configuration of the transport model parameters and the model uncertainty statistics in the inversion system. The emission rates of all sources are then inverted to optimise the match between the concentrations simulated with the transport model and the pollutants' measured atmospheric concentrations, accounting for the transport model uncertainty. In principle, by using atmospheric transport modelling, this concept does not strongly rely on the good colocation between the tracer and pollutant sources and can be used to monitor multiple sources within a single site, unlike the classical tracer release technique. The statistical inversion framework and the use of the tracer data for the configuration of the transport and inversion modelling systems should ensure that the transport modelling errors are correctly handled in the source estimation. The potential of this new concept is evaluated with a relatively simple practical implementation based on a Gaussian plume model and a series of inversions of controlled methane point sources using acetylene as a tracer gas. The experimental conditions are chosen so that they are suitable for the use of a Gaussian plume model to simulate the atmospheric transport. In these experiments, different configurations of methane and acetylene point source locations are tested to assess the efficiency of the method in comparison to the classic tracer release technique in coping with the distances between the different methane and acetylene sources. The results from these controlled experiments demonstrate that, when the targeted and tracer gases are not well collocated, this new approach provides a better estimate of the emission rates than the tracer release technique. As an example, the relative error between the estimated and actual emission rates is reduced from 32 % with the tracer release technique to 16 % with the combined approach in the case of a tracer located 60 m upwind of a single methane source. Further studies and more complex implementations with more advanced transport models and more advanced optimisations of their configuration will be required to generalise the applicability of the approach and strengthen its robustness.

Chronic exposure to odorous chemicals in residential areas and effects on human psychosocial health: Dose–response relationships

Perceived air pollution, including environmental odor pollution, is known to be an environmental stressor that affects individuals' psychosocial health and well-being. However, very few studies have been able to quantify exposure–response associations based on individual-specific residential exposures to a proxy gas and to examine the mechanisms underlying these associations. In this study, individual-specific exposures in non-urban residential environments during 2005–2010 on a gas released from animal biodegradable wastes (ammonia, NH3) were calculated by the Danish Eulerian long-range transport model and the local-scale transport deposition model. We used binomial and multinomial logistic regression and mediation analyses to examine the associations between average exposures and questionnaire-based data on psychosocial responses, after controlling for person-specific covariates. About 45% of the respondents were annoyed by residential odor pollution. Exposures were associated with annoyance (adjusted odds ratio [ORadj]=3.54, 95% confidence interval [CI]=2.33–5.39), health risk perception (ORadj=4.94; 95% CI=1.95–12.5) and behavioral interference (ORadj=3.28; 95% CI=1.77–6.11), for each unit increase in loge(NH3 exposure). Annoyance was a strong mediator in exposure–behavior interference and exposure–health risk perception relationships (81% and 44% mediation, respectively). Health risk perception did not play a mediating role in exposure–annoyance or exposure–behavioral interference relationships. This is the first study to provide a quantitative estimation of the dose–response associations between ambient NH3 exposures and psychosocial effects caused by odor pollution in non-urban residential outdoor environments. It further shows that these effects are both direct and mediated by other psychosocial responses. The results support the use of NH3 as a proxy gas of air pollution from animal biodegradable wastes in epidemiologic studies.

Respiratory and sensory irritation symptoms among residents exposed to low-to-moderate air pollution from biodegradable wastes.

Previous studies have reported increased occurrence of respiratory and sensory irritation symptoms among residents living close to biodegradable waste sites. However, few studies have been able to quantify direct and annoyance-mediated effects based on individual-specific assessments of chemical exposures. We examined associations between residential exposures to a proxy gas (ammonia, NH3) from biodegradable wastes (mainly from farming, animal and agricultural activities) and odor annoyance and six respiratory and sensory irritation symptoms (self-reported), using adjusted logistic regression models and mediation analyses. Individual-specific NH3 exposures (n=454) in residential environments during 2005-2010 were calculated by the Danish Eulerian long-range transport model and the local-scale transport deposition model. Residential NH3 exposure was associated with increased frequency of four symptoms, including "eyes itching, dryness or irritation" and "cough" (ORadj=1.69; 95% CI: 1.09-2.61 and ORadj=1.75; 95% CI: 1.12-2.74, for each unit increase in loge(NH3 exposure)). Odor annoyance mediated the effect of exposure on cough and three sensory irritation symptoms. Mediation was either full (indirect-only effects) or partial (direct and indirect effects). This study provides support for the existence of indirect associations between residential exposures to low-to-moderate air pollution from wastes and symptoms, as well as direct dose-response associations for some of the symptoms.

Field-scale solute transport parameters derived from tracer tests in large undisturbed soil columns

Transport parameters obtained from laboratory tracer experiments were used to evaluate the stochastic form of the equilibrium convection–dispersion equation (CDE) in describing the transition of scale, i.e. from the column or local scale to a larger field scale. Local-scale solute breakthrough curves (BTCs) were measured in 1-m-long and 0.3-m-diameter undisturbed soil columns by means of time-domain reflectometry at six depths for a 79-h input pulse of chloride. The local-scale data were analysed in terms of the equilibrium CDE and the mobile–immobile non-equilibrium transport model (MIM). At the local scale, the MIM transport model better described the observed early breakthrough and the tailing of the BTC than did the CDE. A linear regression analysis indicated that the relationship between the hydrodynamic dispersion D and pore-water velocity v was of the form D = 31vl.92 (correlation ρv,D = 0.74). Averaging of the local-scale BTCs across the field produced a large-scale or field-scale mean BTC; at the greatest observation depth (0.8 m) the field-scale dispersivity <D>/<v> = λ equals 0.656 m. The results further showed that for large values of the mean dispersion coefficient, <D>, local-scale dispersion is an important mechanism for field-scale solute spreading, whereas the standard deviation, σD, and the correlation between v and D, ρvD, have negligible effects on field-scale transport. Stochastic stream tube models supplemented with statistical properties of local-scale transport parameters provide a practical and computationally efficient tool to describe heterogeneous solute transport at large spatial scales.

Protecting the Groundwater Environment of Tulkarem City of Palestine from Industrial and Domestic Pollution

Domestic sewage, solid waste, industrial activities, and excessive use of fertilizers and pesticides are the main sources of pollution that threaten the main groundwater aquifers of Tulkarem City of Palestine which add to the problem of water scarcity already experienced by the Palestinians in the West Bank. These aquifers are a vital groundwater resource that provides high quality water. This paper presents the findings of the “Pro Aquifer” project funded by the EU, which aims to reduce groundwater polluting activities in the main municipalities such as Tulkarem. The scientific research of this paper was conducted on the potential and existing sources of groundwater pollution that threaten the Aquifer. GIS tools as well as vulnerability, hazard and risk mapping and pollution modeling were used to examine the impacts of pollution to the groundwater resources in the study area. The local-scale pollution transport model was developed to define the general characteristics of the groundwater flow system as well as to consequently assess the future transport trends of pollution in the groundwater water system of the Tulkarem area. The outputs of this scientific research were used to establish policies and guidelines for reducing water and environmental pollution in Palestinian municipalities. The recommendations produced aim to help Palestinian municipalities prevent pollution of the critical trans-boundary groundwater resources that underlie Tulkarem City. These guidelines provide a methodology to help municipalities begin solving the environmental issues they face. The goals of these guidelines are to protect and prevent further deterioration of the resources of the Mountain Aquifer and to enable and empower municipal officials and staff to achieve these goals.

Anomalous dispersion in chemically heterogeneous media induced by long-range disorder correlation

AbstractWe study transport in an idealized porous medium characterized by a spatially varying retardation factor, which models linear instantaneous chemical adsorption of a solute. Using a stochastic modelling approach, we study the impact of disorder correlation on the large-scale dispersion behaviour. We consider short, long-range and intermediate-range disorder correlations, and demonstrate that (truncated) power-law correlation causes anomalous dispersion, even in the presence of weak heterogeneity. We identify different preasymptotic and asymptotic regimes of anomalous dispersion that shed new light on the disorder and local-scale transport mechanisms leading to non-Fickian behaviour. The analytical results are complemented by numerical random walk particle tracking simulations, which are found to be in good agreement with the derived dispersion behaviour. We conclude the paper by deriving an effective transport equation for this system, which can be shown to be tied to the family of continuous-time random walk models.

Arid sedimentation in the oceans and atmospheric particulate matter

Recent studies on the continental sources of aerosol production, transport and deposition to the ocean (natural sink) allowed us to recognize the possibility of long-range high-altitude (transoceanic) transport of aerosol dust of specific composition and properties. The dust consists of fine (micro- and nanosized) particles (94%, less than 2 μm) originated in the arid (undrained) tropical and subtropical regions extending into the oceans and are similar in composition to deep-sea (pelagic) red clays. Satellite and aircraft observation data were used to track trajectories of long-range (and transoceanic) transport of dust clouds. These data were coupled with direct shipboard measurements on ice core records and data on nuclear explosions and volcanic eruptions. Several zones of arid sedimentation were identified based on climatic conditions (shortage of water), conditions of dust production, wind-blown transport at different altitudes and deposition onto the ocean surface. The main transport occurs at two altitude scales (from land to the cloud top and above) (5–7 km) where wind speed of 300 km/h will be critical for a long-range transport. Three types of transport are identified based on the particle dynamics, composition and properties: 1—local (0–10 km from the source); 2—regional (100–1000 km); 3—global (over 1000 km). The finer particles are a product of local-scale transport with a total flux of 1.6 billion t/yr, which is almost equal to the net influx of the riverine terrigenous material to the pelagic zones of the oceans (outside the marginal filters). There are four main sources of aerosol dust, which is transported and deposited over arid oceanic regions. The arid oceanic regions account for about 1/3 of the modern ocean surface. During glacial periods, the sea-level drop of 100–120 m caused a significant increase in the size of arid regions due to the exposure of the shelf areas, which is equal to the area of the African continent. This caused 3–5 times higher dust emission, a decrease in the heat flux and in the transparency of the atmosphere. Comparison of ice core records and deep drilling data provide basis for studies on the ancient arid sedimentation.

Numerical simulations of local circulations and pollution transport over Reunion Island

Abstract. A series of high-resolution (1 km) numerical simulations with a limited-area numerical model has been performed over Reunion Island. In the dynamical context of a regular maritime flow perturbed by a major topographic obstacle such as Reunion Island, the objectives are to identify the main atmospheric circulations at local-scale over the island and to improve the understanding of local-scale transport and dispersion of pollutants emitted from local sources. To investigate the effects of topography and land surface heating on low-level flows over the island, simulations representative of austral winter were performed in idealized conditions keeping the radiative forcing plus a background east-south-easterly synoptic flux of varying strengths, typical of the prevailing trade-wind conditions. The numerical experiments show mainly that flow splitting of the trade-wind occurs around the island, with enhanced winds blowing along the coast lines parallel to the synoptic flux, due to the lateral constriction of the flow by the island and resulting Venturi effect. Blocking occurs on the island side facing the trade-wind. The north-western area on the leeside is screened from the trade-wind by high mountains, and this enables the development of diurnal thermally-induced circulations, combining downslope and land-breeze at night, and upslope and sea breeze at daytime. Flow splitting is modulated by radiative convergence toward the island at daytime, and divergence from the island at night. Stronger winds than the large-scale trade-wind occur along the coast at daytime (Venturi effect), whereas at night, the trade-wind flow is pushed few kilometres offshore by divergence of cooled air from the land. At night, the trade-wind flow is pushed few kilometres offshore by divergence of cooled air from the land. Consequently, a number of processes of pollution transport and dispersion have been identified. Vortices in the wake of the island were found to cause counterflow circulation and trapping of polluted air masses near the north-western coast. These air masses may in turn be sucked by anabatic wind systems during daytime (upslope and sea breezes) in the cirques and up to the summits of the island, and especially to Piton Maïdo (2200 m) where a new observatory of the Indian Ocean background atmosphere is being built. A "cap effect" above the mountains downstream from the volcano (to the south-east of the island), and especially above Piton Maïdo, might occur in case of development of inland and upslope breezes on the west coast. In this case, air pumped from lower layers may protect the observatory from the volcanic plume forced to pass over a "cap" of low-level air clean of volcanic emissions.

Effective transport dynamics in porous media with heterogeneous retardation properties

We study a transport model that is characterized by a spatially distributed retardation coefficient. The latter accounts for linear instantaneous mass transfer between a mobile and immobile phases as well as instantaneous equilibrium adsorption. Using a Lagrangian framework we upscale the local scale transport problem to an observation scale that is much larger than the variation scale of spatial heterogeneity. The derived effective Lagrangian transport equation describes the effective particle motion as a random walk in space‐time, or in other words, a continuous time random walk, which is characterized by a joint transition length and time distribution. The transition time distribution is obtained by an exact map from the spatial distribution of retardation coefficients. The effective model compares well with numerical simulations of the small scale transport problem. For broad disorder distributions transport is highly non‐Fickian and an (constant) effective retardation coefficient may not exist at practically relevant time and length scales.

The effects of roadside structures on the transport and dispersion of ultrafine particles from highways

Understanding local-scale transport and dispersion of pollutants emitted from traffic sources is important for urban planning and air quality assessments. Predicting pollutant concentration patterns in complex environments depends on accurate representations of local features (e.g., noise barriers, trees, buildings) affecting near-field air flows. This study examined the effects of roadside barriers on the flow patterns and dispersion of pollutants from a high-traffic highway in Raleigh, North Carolina, USA. The effects of the structures were analyzed using the Quick Urban & Industrial Complex (QUIC) model, an empirically based diagnostic tool which simulates fine-scale wind field and dispersion patterns around obstacles. Model simulations were compared with the spatial distributions of ultrafine particles (UFP) from vehicular emissions measured using a passenger van equipped with a Differential Mobility Analyzer/Condensation Particle Counter. The field site allowed for an evaluation of pollutant concentrations in open terrain, with a noise barrier present near the road, and with a noise barrier and vegetation present near the road. Results indicated that air pollutant concentrations near the road were generally higher in open terrain situations with no barriers present; however, concentrations for this case decreased faster with distance than when roadside barriers were present. The presence of a noise barrier and vegetation resulted in the lowest downwind pollutant concentrations, indicating that the plume under this condition was relatively uniform and vertically well-mixed. Comparison of the QUIC model with the mobile UFP measurements indicated that QUIC reasonably represented pollutant transport and dispersion for each of the study configurations.

Assessing ozone abatement strategies in terms of their effectiveness on the regional and urban scales

Integrated ozone abatement strategies should take into account that an emission intervention which is effective on the regional scale may not necessarily be effective for a city and its surroundings. In the context of a study performed for the European Commission a methodology was developed for assessing ozone abatement strategies in terms of their effectiveness on both the regional and the urban scale. Towards this aim, the assumptions made for three regional emission reduction scenarios were assumed to be valid also for the emission situation in the urban areas of Athens and Stuttgart and the corresponding emission inventories were compiled. The EMEP MSC-W ozone model was used to calculate the regional scale ozone distribution over a 6 month summer period applying the 1990 meteorology. Local scale transport and chemical transformation processes were analysed with the ozone fine structure (OFIS) model. Both the regional and the local scale simulations were performed for the base case (1990 situation) and the three emission reduction scenarios. The significance of regional scale emission reductions was demonstrated by the aid of a second series of simulations assuming that the emission interventions were implemented only at local scale. The results of the simulations reveal that ozone exposure in conglomerations as the ones considered in this study depends on both urban and regional scale influences. Urban VOC control is found to be effective in reducing ozone primarily on the local or urban scale, whereas urban NO x control may cause an increase of urban peak ozone while contributing to an effective reduction of regional ozone.

Statistical evaluation of centreline concentration estimates by atmospheric dispersion models

Within the American Society for Testing and Materials (ASTM), a standard practice (Z6849Z1) is being developed to provide an objective statistical procedure for comparing air quality simulation modelling results with tracer field data. The practice is limited to steady state, local-scale transport from isolated point sources in simple terrain. Evaluation data having similar external conditions are grouped together, and comparisons are made of the model's ability to replicate without bias the average of the centreline maximum concentrations for each group. Centreline concentrations measured during three field studies are compared with estimates from three steady-state plume models: Industrial Source Complex (ISC), Hybrid Plume Dispersion Model (HPDM) and AMS/EPA Regulatory Model (AERMOD). These results, combined with those presented in Irwin and Rosu (1998), provide a complete examination of the draft ASTM standard practice under development. It is concluded that the evaluation methodology is capable of objectively discerning differences in skill between models in their ability to estimate the centreline maximum concentration.

Upscaling local-scale transport processes in large-scale relief dynamics

The objective of this paper is to discuss the form and parameters of the macroscopic continent-scale erosion law. The law comes from a spatial integration of local transport processes on the resulting topography. It thus depends on the relief/drainage organization that results from the development of geomorphic instabilities such as differential incisions.Assuming local transport processes to depend on local slope and water discharge, we have calculated topographic evolution to derive the characteristic time scales of erosion dynamics. We especially focused on the case of a declining plateau which presents two main phases: a first phase when the drainage pattern establishes, and a declining phase when topography decreases almost exponentially in the absence of tectonic input. The latter phase is characterized by a time scale which depends on the system size, on the organization of the drainage network, and on the parameters of the transport process. We show that the macroscopic erosion law has the characteristics of an abnormal diffusion whose basic time-length exponent a is determined by the parameters of the fluvial process. This result sheds new light on the observed negative correlation between current denudation rates and drainage areas in world-wide fluvial watersheds.

An integrated study of controls on solute transport in the Lincolnshire limestone

Abstract A study site located in a quarry on the Lincolnshire Limestone has been selected to examine the validity of the ‘dual porosity’ hypothesis and its application in models for the prediction of solute migration. An integrated approach has been adopted for the experimental programme, combining data obtained from both field and laboratory-scale investigations with mathematical modelling. The results of the first to stages of the experimental developments at the site are reported and the results are discussed in relation to he development of a full tracer test programme. Fracture mapping has been conducted on the face and the floor of the quarry where the vertical fractures are seen to be discontinuous over depth. However, extensive horizontal bedding plane fissures are readily identifiable in the boreholes and represent the main paths for lateral flows. An extensive data collection programme has been conducted to characterize the hydraulic properties of these fissures, as well as the adjacent altered and unaltered matrix. Important results for future tracer testing were obtained from the hydraulic tests which suggest the strong possibility of non-Darcian flow in the fissures under pumping and tracer test conditions and the probability of strong spreading of solutes at injection wells under forced inflows. Preliminary tracer tests confirm the difficulty of employing laboratory defined diffusion coefficients as input data to ‘dual porosity’ models of local-scale transport.

Anion Transport in a Piedmont Ultisol: II. Local-Scale Parameters

Field-scale transport parameters obtained from tile drain data in two large conventionally tilled plots were reported in the first paper in this series. Our objectives in this paper were to determine the relationship between local-scale transport parameters and macropore distributions measured with columns in one of these plots and to compare local-scale transport parameters with the field-scale parameters. Forty undisturbed soil columns (15 cm in diameter, 33-64 cm in length) were taken from Plot 1 described in the first paper. Transport parameters were determined from a Br breakthrough curve (BTC) with CXTFIT. Macropore distributions were determined from computed tomography (CT) scanning and by sectioning the columns after adding methylene blue dye. Local-scale saturated hydraulic conductivity (K s ), pore water velocity (V), and dispersivity (λ) were highly variable (coefficient of variation of 77, 74, and 41%, respectively) and lognormally distributed. The K s was positively correlated with the number of 5- to 10-mm-diameter CT scan macropores at a depth of 35 to 40 cm near the top of the Bt horizon. Dye patterns indicated that preferential flow began at about this depth. This may be the zone where flow becomes restricted and highly dependent on macropores. Mean local-scale λ (6.6 cm) was similar to the estimated field-scale values (3.4 and 5.3 cm) for the unsaturated zone to a depth of 20 cm. This implied that variation among local-scale V in the field experiment was much less than that observed in the columns, probably because the surface was not ponded in the field. A deterministic approach with mean values of local-scale parameters adequately described the estimated field-scale BTC.

Convective-Dispersive Stream Tube Model for Field-Scale Solute Transport: I. Moment Analysis

Field-scale solute transport is typically difficult to model due to the complexity and heterogeneity of flow and transport in natural soils. The stream tube model attempts to stochastically describe transport across the field for relatively short travel distances by viewing the field as a series of independent vertical soil columns. This study investigates the stream tube model with the chemical equilibrium and nonequilibrium convection-dispersion equation (CDE) for local-scale transport. A bivariate (joint) lognormal probability density function was used for three pairs of random transport parameters: (i) the dispersion coefficient, D, and the pore-water velocity, v; (ii) the distribution coefficient for linear adsorption, &, and v; and (iii) the firstorder rate coefficent for nonequilibrium adsorption, a, and v. Expressions for travel time moments as a rsult of a Dirac input were derived to characterize field-scale transport according to the stream tube model. The mean breakthrough time for the field-scale flux-averaged concentration, 21, was found to be identical to that for the deterministic CDE. Variability in D has generally a minor effect on solute spreading compared with variability in v. Spreading of reactive solutes increased for negatively correlated v and Kd, even if the variability in K,, was relatively small, while nonequilibrium adsorption further increased spreading. If a was variable, a negative correlation between v and a enhanced the skewness of the breakthrough curve for&while spreading was independent of the correlation between (I and v.

Field Scale Vertical Transport from Horizontally Variable Local Scale Transport Parameters

In field soils, transport properties vary vertically and horizontally. With experimental measurements it is distinctly easier to consider the vertical than the horizontal variability. We present a simple approach for considering the horizontal variability of the transport behavior of the soil in a one‐dimensional transport model. The model approach assumes independence on depth of the vertical dispersion coefficient, which is justified by the relatively small effects of the vertical dispersion in comparison to those of the horizontal dispersion only. The approach is based on numerical experiments using different parameter statistics. An analytical expression of the dependence of effective parameters, describing the transport at the field scale, on the statistical lognormal distributions of transport parameters at the local scale is derived heuristically by careful inspection of the numerical experimental results. The model was tested in (1) a reevaluation of a one‐dimensional field experiment with chloride tracer in a heavy loam soil of Roth (1989), and (2) a two‐dimensional leaching field experiment with the same tracer in a sandy soil plot of 5600 m2. For both experiments the model using lognormal local parameter distributions provides a satisfactory representation of field averages of the vertical transport derived from local measurements.

Association of Non-Marine Sulfate Aerosol with Sea Breeze Circulation in Tampa Bay

Peak concentrations of aerosol sulfur in Tampa, Florida may be the result of either regional-scale transformation and transport processes or local-scale transport from nearby air pollution sources. The existence of the latter has been demonstrated in Tampa through correspondence of sulfur with sea breeze circulation patterns and the resulting chloride concentration maxima (which serve as indicators of the marine aerosol), vanadium concentration maxima (which indicate times of high concentrations of certain plume constituents), and the locations of sources favorable for high concentrations of air pollution-derived sulfate during occurrences of the sea breeze. The analysis indicates that locally derived sulfate in the Tampa atmosphere, which may be less abundant than sulfate due to regional-scale processes, can be identified by the use of combined meteorological and chemical tracer interpretation.