Regional Transport Model Research Articles

European CO2 fluxes from atmospheric inversions using regional and global transport models

Approximately half of human-induced carbon dioxide (CO2) emissions are taken up by the land and ocean, and the rest stays in the atmosphere, increasing the global concentration and acting as a major greenhouse-gas (GHG) climate-forcing element. Although GHG mitigation is now in the political arena, the exact spatial distribution of the land sink is not well known. In this paper, an estimation of mean European net ecosystem exchange (NEE) carbon fluxes for the period 1998–2001 is performed with three mesoscale and two global transport models, based on the integration of atmospheric CO2 measurements into the same Bayesian synthesis inverse approach. A special focus is given to sub-continental regions of Europe making use of newly available CO2 concentration measurements in this region. Inverse flux estimates from the five transport models are compared with independent flux estimates from four ecosystem models. All inversions detect a strong annual carbon sink in the southwestern part of Europe and a source in the northeastern part. Such a dipole, although robust with respect to the network of stations used, remains uncertain and still to be confirmed with independent estimates. Comparison of the seasonal variations of the inversion-based net land biosphere fluxes (NEP) with the NEP predicted by the ecosystem models indicates a shift of the maximum uptake period, from June in the ecosystem models to July in the inversions. This study thus improves on the understanding of the carbon cycle at sub-continental scales over Europe, demonstrating that the methodology for understanding regional carbon cycle is advancing, which increases its relevance in terms of issues related to regional mitigation policies.

Impact of East Asian summer monsoon on the air quality over China: View from space

Tropospheric O3 columns retrieved from Ozone Monitoring Instrument and Microwave Limb Sounder measurements, CO columns retrieved from Measurements of Pollution in the Troposphere, and tropospheric O3 and CO concentrations retrieved from the Tropospheric Emission Spectrometer from May to August in 2006 are analyzed using the Regional Chemical and Transport Model to investigate the impact of the East Asian summer monsoon on the air quality over China. The observed and simulated migrations of O3 and CO are in good agreement, demonstrating that the summer monsoon significantly affects the air quality over southeastern China, and this influence extends to central East China from June to July. Enhancements of CO and O3 over southeastern China disappear after the onset of the summer monsoon and reemerge in August after the monsoon wanes. The premonsoon high O3 concentrations over southern China are due to photochemical production from pollutant emissions and the O3 transport from the stratosphere. In the summer monsoon season, the O3 concentrations are relatively low over monsoon‐affected regions because of the transport of marine air masses and weak photochemical activity. We find that the monsoon system strongly modulates the pollution problem over a large portion of East China in summer, depending on its strength and tempo‐spatial extension. Model results also suggest that transport from the stratosphere and long‐range transport from East China and South/central Asia all make significant contributions to O3 enhancements over West China. Satellite observations provide valuable information for investigating the monsoon impact on air quality, particularly for the regions with limited in situ measurements.

Ambiguous Hydraulic Heads and 14C Activities in Transient Regional Flow

A regional flow and transport model is used to explore the implications of significant variability in Pleistocene and Holocene climates on hydraulic heads and (14)C activity. Simulations involve a 39 km slice of the Death Valley Flow System through Yucca Mountain toward the Amargosa Desert. The long-time scale over which infiltration has changed (tens-of-thousands of years) is matched by the large physical extent of the flow system (many tens-of-kilometers). Estimated paleo-infiltration rates were estimated using a juniper pollen percentage that extends from the last interglacial (LIG) period (approximately 120 kyrbp) to present. Flow and (14)C transport simulations show that groundwater flow changes markedly as a function of paleoclimate. At the last glacial maximum (LGM, 21 kyrbp), the recharge to the flow system was about an order-of-magnitude higher than present, and water table was more than 100 m higher. With large basin time constants, flow is complicated because hydraulic heads at a given location reflect conditions of the past, but at another location the flow may reflect present conditions. This complexity is also manifested by processes that depend on flow, for example (14)C transport. Without a model that accounts for the historical transients in recharge for at least the last 20,000 years, there is no simple way to deconvolve the (14)C dates to explain patterns of flow.

Technical Note: A new coupled system for global-to-regional downscaling of CO<sub>2</sub> concentration estimation

Abstract. We introduce a global-to-regional nesting scheme for atmospheric transport models used in simulating concentrations of green house gases from globally distributed surface fluxes. The coupled system of the regional Stochastic Time-Inverted Lagrangian Transport (STILT) model and the global atmospheric transport model (TM3) is designed to resolve atmospheric trace gas concentrations at high temporal and spatial resolutions in a specified domain e.g. for regional inverse applications. The nesting technique used for the coupling is based on a decomposition of the atmospheric concentration signal into a far-field and a near-field contribution enabling the usage of different model types for global (Eulerian) and regional (Lagrangian) scales. For illustrating the performance of the coupled TM3-STILT system we compare simulated mixing ratios of carbon dioxide with available observations at 10 sites in Europe. For all chosen sites the TM3-STILT provided higher correlations between the modelled and the measured time series than the TM3 global model. Autocorrelation analysis demonstrated that the TM3-STILT model captured temporal variability of measured tracer concentrations better than TM3 at most sites.

Observation-based estimates of fossil fuel-derived CO2 emissions in the Netherlands using Δ14C, CO and 222Radon

Surface emissions of CO2 from fossil fuel combustion (FFCO2) are estimated for the Netherlands for the period of May 2006–June 2009 using ambient atmospheric observations taken at station Lutjewad in the Netherlands (6◦21E, 53◦24N, 1 m. a.s.l.). Measurements of 14C on 2-weekly integrations of CO2 and CO mixing ratios are combined to construct a quasi-continuous proxy record (ΦFFCO2 ∗ ) from which surface fluxes (ΦFFCO2 ∗ ) are determined using the 222Rn flux method. The trajectories of the air masses are analysed to determine emissions, which are representative for the Netherlands. We compared our observationally based estimates to the national inventories and we evaluated our methodology using the regional atmospheric transport model REMO. Based on 3 yr of observations we find annual mean ΦFFCO2∗ emissions of (4.7 ± 1.6) kt km−2 a−1 which is in very good agreement with the Dutch inventories of (4.5 ± 0.2) kt km−2 a−1 (average of 2006–2008).

An elevated large-scale dust veil from the Taklimakan Desert: Intercontinental transport and three-dimensional structure as captured by CALIPSO and regional and global models

Abstract. An intense dust storm occurred during 19–20 May 2007 over the Taklimakan Desert in northwestern China. Over the following days, the space-borne lidar CALIOP tracked an optically thin, highly elevated, horizontally extensive dust veil that was transported intercontinentally over eastern Asia, the Pacific Ocean, North America, and the Atlantic Ocean. A global aerosol transport model (SPRINTARS) simulated the dust veil quite well and provided a three-dimensional view of the intercontinental dust transport. The SPRINTARS simulation revealed that the dust veil traveled at 4–10 km altitudes with a thickness of 1–4 km along the isentropic surface between 310 and 340 K. The transport speed was about 1500 km/day. The estimated dust amount exported to the Pacific was 30.8 Gg, of which 65% was deposited in the Pacific and 18% was transported to the North Atlantic. These results imply that dust veils can fertilize open oceans, add to background dust, and affect the radiative budget at high altitudes through scattering and absorption. The injection mechanism that lifts dust particles into the free atmosphere is important for understanding the formation of the dust veil and subsequent long-range transport. We used a regional dust transport model (RC4) to analyze the dust emission and injection over the source region. The RC4 analysis revealed that strong northeasterly surface winds associated with low pressures invaded the Taklimakan Desert through the eastern corridor. These winds then formed strong upslope wind along the high, steep mountainsides of the Tibetan Plateau and blew large amounts of dust into the air. The updraft lifted the dust particles farther into the upper troposphere (about 9 km above mean sea level, MSL), where westerlies are generally present. The unusual terrain surrounding the Taklimakan Desert played a key role in the injection of dust to the upper troposphere to form the dust veil.

Toward a general parameterization of N<sub>2</sub>O<sub>5</sub> reactivity on aqueous particles: the competing effects of particle liquid water, nitrate and chloride

Abstract. The heterogeneous reaction of N2O5 on mixed organic-inorganic aerosol particles was investigated using an entrained aerosol flow tube coupled to a custom-built chemical ionization mass spectrometer. Laboratory results on aqueous particles confirm a strong dependence of the reactive uptake coefficient (γ) on particle liquid water, for particle water concentrations below 15 M, and the molar ratio of particle water to nitrate. Measurements of γ (N2O5) on mixed chloride-nitrate particles indicate that the presence of trace chloride can negate the suppression of γ(N2O5) at high nitrate loadings with implications for polluted coastal regions. These results are used to construct a new parameterization for γ (N2O5), that when coupled to an aerosol thermodynamics model, can be used within regional and/or global chemical transport models.

Size‐resolved fluxes of sub‐100‐nm particles over forests

Dry deposition of atmospheric particles is critically dependent on particle size and plays a key role in dictating the mass and number distributions of atmospheric particles. However, modeling dry deposition is constrained by a lack of understanding of controlling dependencies and accurate size‐resolved observations. We present size‐resolved particle number fluxes for sub‐100‐nm particle diameters (Dp) over a deciduous forest derived using eddy covariance applied to data from a fast mobility particle sizer. The size‐resolved particle number fluxes in 18 diameters between 8 and 100 nm were collected during leaf‐on and are statistically robust. Particle deposition velocities normalized by friction velocity (vd+) are approximately four times smaller than comparable values for coniferous forests reported elsewhere. Comparison of the data with output from a new one‐dimensional mechanistic particle deposition model designed for broadleaf forest exhibits greater accord with the measurements than two previous analytical models, but modeled vd+ underestimate observed values by at least a factor of two for all Dp between 6 and 100 nm. When size‐resolved particle deposition velocities for Dp < 100 nm are normalized by friction velocity, the key controlling role of particle diffusivity is strongly manifest. On the basis of analyses of these new measurements and recently published size‐resolved particle number fluxes from a conifer forest, we present working parameterizations for size‐resolved particle deposition velocities over forests that could reasonably be applied in regional and global atmospheric chemistry transport models.

Satellite Remote Sensing and Mesoscale Modeling of the 2007 Georgia/Florida Fires

During April and May 2007, several hundred fires burned uncontrollably in Georgia and Florida. The smoke from these fire events were visible throughout the Southeastern United States and had a major impact on particulate matter (PM) air quality near the surface. In this study, we show the strength of polar orbiting and geostationary satellite data in capturing the spatial distribution and diurnal variability of columnar smoke aerosol optical depth from these fires. We quantitatively evaluate PM air quality from satellites and ground-based monitors, near and far away (> 300 km) from fire source regions. We also show the changes in organic carbon concentrations (a tracer for smoke aerosols) before, during and after these fire events. Finally, we use fire locations and emissions retrieved and estimated from satellite observations as input to a regional mesoscale transport model to forecast the spatial distribution of aerosols and their impact on PM air quality. During the fire events, near the source regions, total column 550 nm aerosol optical thickness (AOT) exceeded 1.0 on several days and ground-based PM2.5 mass (particles less than 2.5 mum in aerodynamic diameter) reached unhealthy levels ( > 65.5 mug m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ). Since the aerosols were reasonably well mixed in the first 1-2 km (as estimated from meteorology), the column AOT values derived from both geostationary and polar orbiting satellites and the surface PM2.5 were well correlated (linear correlation coefficient, r > 0.7). Several hundred miles away from the fire sources, in Birmingham, AL, the impact of the fires were also seen through the high AOT's and PM2.5 values. Correspondingly, PM2.5 mass due to organic carbon obtained from ground-based monitors showed a three fold increase during fire events when compared to background values. Satellite data were especially useful in capturing PM2.5 air quality in areas where there were no ground-based monitors. Although the mesoscale transport model captured the timing and location of aerosols, when compared to observations, the simulated mass concentrations are underestimated by nearly 70% due to various reasons including uncertainties in fire emission estimates, lack of chemistry in the model, and assumptions on vertical distribution of aerosols. Satellite products such as AOT, fire locations, and emissions from space-borne sensors are becoming a vital tool for assessing extreme events such as fires, smoke, and particulate matter air quality.

Community-Based, Macrolevel Collision Prediction Model Use with a Regional Transportation Plan

This paper describes the application of previously developed community-based, macrolevel collision prediction models (CPMs) to evaluate the road safety of a regional transportation plan. The research objective was to present and test model-use guidelines in a regional road safety planning application. The data was extracted from over 400 Greater Vancouver neighborhoods in British Columbia, Canada, including output from the regional transportation model. The CPMs predicted a lower mean collision frequency region-wide due to a proposed three-year transportation plan, versus a do-nothing scenario. Recommendations have been made for future use of the CPMs in regional road safety planning applications. The application of macrolevel CPMs to this regional case study proved a solid step in the development of new and improved empirical tools for planners and engineers to include road safety in the planning process. It is hoped that these models and model-use guidelines will facilitate improved decisions by community planners and engineers, and ultimately, facilitate improved neighborhood traffic safety for residents and other road users.

Flow and transport modelling of a highly stressed aquifer to refine management strategies

Overexploitation of groundwater in the eastern part of San Jacinto Watershed Basin in conjunction with reservoir leakage in the western part has reversed the groundwater flow direction and has caused migration of groundwater with high total dissolved solids (TDS) into an area of better-quality groundwater. The regional groundwater flow and transport model presented here shows how decreased pumping and artificial recharge in the east and extraction of high-TDS groundwater in the west can impact the area.

Saharan dust transport and deposition towards the tropical northern Atlantic

Abstract. We present a study of Saharan dust export towards the tropical North Atlantic using the regional dust emission, transport and deposition model LM-MUSCAT. Horizontal and vertical distribution of dust optical thickness, concentration, and dry and wet deposition rates are used to describe seasonality of dust export and deposition towards the eastern Atlantic for three typical months in different seasons. Deposition rates strongly depend on the vertical dust distribution, which differs with seasons. Furthermore the contribution of dust originating from the Bodélé Depression to Saharan dust over the Atlantic is investigated. A maximum contribution of Bodélé dust transported towards the Cape Verde Islands is evident in winter when the Bodélé source area is most active and dominant with regard to activation frequency and dust emission. Limitations of using satellite retrievals to estimate dust deposition are highlighted.

Use of environmental isotopes to define the capture zone of a drinking water supply situated near a dredge lake

Summary Lake–ground water interactions in the vicinity of a dredge lake (Lake Leis; SW Germany) were studied by means of the stable isotopes ( 18 O, 2 H) mainly to determine the capture zone of a pumping well for the drinking water supply to the nearby town of Baden-Baden. The water supply is situated in a Quaternary gravel aquifer of the Rhine Valley. Numerical simulation performed using a two-dimensional water flow model was not able to characterise the capture zone uniquely. Several best fit options were possible by applying the values of hydraulic conductivities obtained from the pumping tests. As a result it was not possible to define whether Lake Leis is situated either completely inside or fully outside of the capture zone. To overcome this problem and to define the shape of the catchment area of the water supply precisely, a two-dimensional water flow and tracer transport model was established using both hydraulic heads and isotope data. The isotope data were applied to determine the proportion of lake water in down-gradient pumping and observation wells. Additionally, the isotope compositions of lake water and those in observation wells, measured as a function of time, were used to determine the mean water velocity ( v = 0.6 m/d) along some selected 200 m long flow-paths. Based on known porosity and hydraulic gradient values, a hydraulic conductivity of about k = 6 × 10 −4 m/s was determined for the central area down-gradient of the lake. This hydraulic conductivity and the proportion of lake water in the groundwater were used to calibrate a numerical two-dimensional regional scale transport model. The study demonstrates that it is possible to calibrate a water flow model in the vicinity of a dredge lake uniquely, only when stable isotope signals measured in a time series in both the lake and groundwater are taken into consideration. The calibrated model finally enables the capture zone of the production well to be precisely defined.

A regional scale chemical transport modeling of Asian aerosols with data assimilation of AOD observations using optimal interpolation technique

A regional chemical transport model assimilated with monthly mean satellite and ground based aerosol optical depth (AOD) observations was used to produce three dimensional distributions of aerosols throughout Asia for a period of four years. The model was evaluated with daily assimilation of AOD for the month of April 2005. Sulfur Transport dEposition Model (STEM), a regional chemical transport model, was used to simulate aerosol distributions at a resolution of 50 × 50 km with a time interval of 3 h. Monthly mean Moderate Resolution Imaging Spectroradiometer (MODIS) AOD along with AErosol RObotic NETwork (AERONET) AOD was used in an optimal interpolation assimilation scheme to yield regional distributions of aerosols. The MODIS AOD and aerosol fine mode fraction information (where available) were used in the assimilation technique. The daily assimilation of AOD results shows that the optimal interpolation algorithm is able to significantly improve model aerosol mass prediction skills at the two sites in Asia. Sensitivity studies were also conducted with different assimilation parameters on a daily assimilation scale and these results are discussed. The assimilation results of four-year aerosol fields were used to study the spatial and temporal distribution of aerosols in Asia. Two remote sites, Hanimaadhoo and Gosan were chosen as the case studies to study the outflow from the Indian subcontinent and East Asia. Seasonal and vertical structures of the aerosols are presented at these two sites. Positive Matrix Factorization (PMF), a factor analytic method was also used to characterize the source profile and source contribution at these two locations. A three-factor solution was able to explain more than 80% of the variation in the individual species at Hanimaadhoo and 90% variation of aerosol loadings at Gosan. The four-year averaged PMF model results were able to capture the seasonality of anthropogenic and dust loadings at both these locations. In addition, the PMF model identified the differences in the composition of anthropogenic aerosols over Hanimaadhoo and Gosan reflecting the differences in regional emissions. The PMF derived factors could be used as additional constraints for future assimilation studies.

An Ensemble Kalman Filter for severe dust storm data assimilation over China

Abstract. An Ensemble Kalman Filter (EnKF) data assimilation system was developed for a regional dust transport model. This paper applied the EnKF method to investigate modeling of severe dust storm episodes occurring in March 2002 over China based on surface observations of dust concentrations to explore the impact of the EnKF data assimilation systems on forecast improvement. A series of sensitivity experiments using our system demonstrates the ability of the advanced EnKF assimilation method using surface observed PM10 in North China to correct initial conditions, which leads to improved forecasts of dust storms. However, large errors in the forecast may arise from model errors (uncertainties in meteorological fields, dust emissions, dry deposition velocity, etc.). This result illustrates that the EnKF requires identification and correction model errors during the assimilation procedure in order to significantly improve forecasts. Results also show that the EnKF should use a large inflation parameter to obtain better model performance and forecast potential. Furthermore, the ensemble perturbations generated at the initial time should include enough ensemble spreads to represent the background error after several assimilation cycles.

Coupled hydrogeological and reactive transport modelling of the Simpevarp area (Sweden)

The Simpevarp area is one of the alternative sites being considered for the deep geological disposal of high level radioactive waste in Sweden. In this paper, a coupled regional groundwater flow and reactive solute transport model of the Simpevarp area is presented that integrates current hydrogeological and hydrochemical data of the area. The model simulates the current hydrochemical pattern of the groundwater system in the area. To that aim, a conceptual hydrochemical model was developed in order to represent the dominant chemical processes. Groundwater flow conditions were reproduced by taking into account fluid-density-dependent groundwater flow and regional hydrogeologic boundary conditions. Reactive solute transport calculations were performed on the basis of the velocity field so obtained. The model was calibrated and sensitivity analyses were carried out in order to investigate the effects of heterogeneities of hydraulic conductivity in the subsurface medium. Results provided by the reactive transport model are in good agreement with much of the measured hydrochemical data. This paper emphasizes the appropriateness of the use of reactive solute transport models when water-rock interaction reactions are involved, and demonstrates what powerful tools they are for the interpretation of hydrogeological and hydrochemical data from site geological repository characterization programs, by providing a qualitative framework for data analysis and testing of conceptual assumptions in a process-oriented approach.

Spring to summer northward migration of high O3 over the western North Atlantic

Tropospheric O3 columns retrieved from OMI and MLS measurements, NO2 columns from OMI, and upper tropospheric O3 concentrations from TES over North America and the western North Atlantic from April to August 2005 are analyzed using the Regional chEmical and trAnsport Model (REAM). Large enhancements of column and upper tropospheric O3 over the western North Atlantic comparable to those over the eastern United States are found in the satellite measurements and REAM simulations. The O3 enhancement region migrates northward from spring to summer. Model analysis indicates that the northward migration is driven by seasonal shifts of O3 transported from the stratosphere and that produced through photochemistry from surface emissions and lightning NOx. As their uncertainties improve, satellite measurements of O3 and its precursors will be able to provide more quantitative constraints on pollutant outflow from the continents.

Regional model for formation, dynamics, and long-range transport of atmospheric aerosol

This paper is concerned with various aspects of regional transport modeling of atmospheric aerosols, or particulate matter (PM), including their physical and chemical transformations and presents a recently developed aerosol model. The model accounts for all of main aerosol processes, including aerosol emissions, formation, dynamics, atmospheric transport, and removal. The model includes primary and secondary aerosols of both anthropogenic and natural origin and allows calculating mass and number concentrations and size distribution of multicomponent aerosols. A flexible model structure makes it possible to change a description of aerosol size distribution, to include new aerosol components, to modify numerical schemes used in the model, and to include new schemes. The combination of model computational efficiency with satisfactory accuracy of results facilitates the use of the model for operational calculations of the current transboundary air pollution and pollution trends in Europe within EMEP (Co-operative Programme for Monitoring and Evaluation of the long-range transmission of air pollutants in Europe), as well as for estimation of future air pollution scenarios to support the strategy development for optimal emission regulations in Europe.

Modelling of near-surface ozone over South Asia

Hourly, three-dimensional, fields of tropospheric ozone have been produced for 12 consecutive months on a domain covering South Asia, using the regional Eulerian off-line chemistry transport model MATCH. The results were compared with background observations to investigate diurnal and seasonal variations of near-surface ozone in the region. MATCH reproduced the seasonality of near-surface ozone at most locations in the area. However, the current, and previous, studies indicate that the model consequently overestimate night-time concentrations, while it occasionally underestimates the day-time, near-surface, ozone concentrations. The lowest monthly-mean concentrations of near-surface ozone are typically experienced in June–September, coincident with the rainy season in most areas. The seasonality is not identical across the domain; some locations have a completely different trend. Large areas in Northern India and Nepal show a secondary minimum during the cold winter season (December–January). High concentrations of near-surface ozone are found over the oceans, close to the Indian subcontinent, due to the less efficient dry deposition to water surfaces; over parts of Tibet due to influence of free tropospheric air and little deposition to snow covered surfaces; and along the Gangetic valley due to the large emissions of precursors in this region. Monthly-mean ozone concentrations in the densely populated northern India range from 30–45 ppb(v). The model results were also used to produce maps of AOT40. The results point towards similar levels of AOT40 in India as in Europe: large areas of India show 3-month AOT40 values above 3 ppm(v) hours.

Network Equilibrium with Activity-Based Microsimulation Models

Travel demand models and network simulation models are distinct sets of procedures that are combined and interact within the framework of regional transportation modeling systems. Conventional four-step models have numerous limitations compared with more advanced activity-based microsimulation models, primarily with respect to internal consistency and detailed behavioral realism. However, two of the remaining advantages of four-step models are an established theory and an effective set of practical rules for achieving global network equilibrium so that travel time and cost simulated in the networks exactly correspond to the demand (trip tables) generated by the model. Nonetheless, this issue remains less explored and somewhat obscure for activity-based models. These models have a more complicated analytical structure compared with four-step models, which makes it difficult to derive equilibrium conditions in a rigorous theoretical manner. In addition, implementation of an activity-based model requires microsimulation of individual outcomes in the form of “crisp” discrete choices that is very different from the summation of fractional probabilities implemented in conventional models. This paper documents the results of testing various equilibrium strategies implemented with the New York City activity-based microsimulation regional travel demand model used by the New York Metropolitan Transportation Council. The purpose of the paper is twofold. First, it is intended to outline some fundamental research directions and extensions of the network equilibrium theory to cover activity-based microsimulation models in a more rigorous way. Second, it describes realistic levels of convergence that can be achieved with activity-based microsimulation models in practice and establishes practical rules and protocols for using these types of models for different projects and policies.