ALBERT

Description: The processes of aerosol sulfate formation are vital components in the scientific understanding of perturbations of earth's radiative balance via aerosol direct and indirect effects. In this work, an analysis of the influence of changes in oxidant levels and sulfur dioxide oxidation pathways was performed to study the underlying pathways for sulfate formation. Sensitivities of this constituent were calculated from a series of photochemical model simulations with varying rates of NOx and VOC emissions to produce variations in oxidant abundances using a photochemical model (CMAQ) that covers the eastern US for part of the ICARTT 2004 campaign. Three different chemical mechanisms (CBIV, CB05, and SAPRC99) were used to test model responses to changes in NOx and VOC concentrations. Comparison of modeled results and measurements demonstrates that the simulations with all three chemical mechanisms capture the levels of sulfate reasonably well. However, the three mechanisms are shown to have significantly different responses in sulfate formation when the emissions of NOx and/or VOC are altered, reflecting different photochemical regimes under which the formation of sulfate occurs. Also, an analysis of the oxidation pathways that contribute to sulfur dioxide conversion to sulfate reveals substantial differences in the importance of the various pathways among the three chemical mechanisms. These findings suggest that estimations of the influence that future changes in primary emissions or other changes which perturb SO2 oxidants have on sulfate abundances, and on its direct and indirect radiative forcing effects, may be dependent on the chemical mechanism employed in the model analysis.

Description: The Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT), developed by NOAA’s Air Resources Laboratory, is one of the most widely used models for atmospheric trajectory and dispersion calculations. We present the model’s historical evolution over the last 30 years from simple hand-drawn back trajectories to very sophisticated computations of transport, mixing, chemical transformation, and deposition of pollutants and hazardous materials. We highlight recent applications of the HYSPLIT modeling system, including the simulation of atmospheric tracer release experiments, radionuclides, smoke originated from wild fires, volcanic ash, mercury, and wind-blown dust.

Description: The Hybrid Single-Particle Lagrangian Integrated Trajectories (HYSPLIT) model has been applied to calculate the spatial and temporal distributions of dust originating from North Africa. The model has been configured to forecast hourly particulate matter ≤10 μm (PM10) dust concentrations focusing on the impacts over the southern Iberian Peninsula. Two full years (2008 and 2009) have been simulated and compared against surface background measurement sites. A statistical analysis using discrete and categorical evaluations is presented. The model is capable of simulating the occurrence of Saharan dust episodes as observed at the measurement stations and captures the generally higher levels observed in eastern Andalusia, Spain, with respect to the western Andalusia station. But the simulation tends to underpredict the magnitude of the dust concentration peaks. The model has also been qualitatively compared with satellite data, showing generally good agreement in the spatial distribution of the dust column.

Description: Using ensembles to improve the simulation of plume dispersion is becoming a more common practice. One of the biggest challenges in creating ensembles is developing the appropriate member selection process to get the most accurate results, quantify ensemble uncertainty, and use computing resources more efficiently by avoiding the use of redundant model information. In this work, two reduction techniques are tested: one that is independent of observations and is based on the exclusion of redundant members by using uncorrelation as a measure of distance among the members and a second one based on measured data, which minimizes the mean square error (MSE) of the average of all possible model combinations. These techniques are applied to a 24-member ensemble, created by varying the boundary layer parameterizations in the meteorological WRF Model and dispersion Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) simulating six releases of the Cross-Appalachian Tracer Experiment (CAPTEX). Applying the first technique produced results statistically comparable to the full ensemble for four out of six releases, while the second technique shows a similar or superior performance for all cases. Furthermore, to mimic a forecast application, the first day of the tracer release is used to select the ensemble members and the subsequent days are utilized as a forecast proxy to evaluate their performance. The reduced ensembles chosen by applying the technique based on the minimization of the MSE statistically perform similarly to or better than the full ensemble for the forecasting time periods. This suggests that when observational data are available, the application of ensemble reduction techniques provides a potentially promising tool for improving the dispersion forecast.

Description: An investigation was conducted to identify the mechanistic differences between two versions of the carbon bond gas-phase chemical mechanism (CB05 and CBMIV) which consistently lead to larger ground-level ozone concentrations being produced in the CB05 version of the National Air Quality Forecasting Capability (NAQFC) modeling system even though the two parallel forecast systems utilize the same meteorology and base emissions and similar initial and boundary conditions. Box models of each of the mechanisms as they are implemented in the NAQFC were created and a set of 12 sensitivity simulations was designed. The sensitivity simulations independently probed the conceptual mechanistic differences between CB05 and CBMIV and were exercised over a 45-scenario simulation suite designed to emulate the wide range of chemical regimes encountered in a continental-scale atmospheric chemistry model. Results of the sensitivity simulations indicate that two sets of reactions that were included in the CB05 mechanism, but which were absent from the CBMIV mechanism, are the primary causes of the greater ozone production in the CB05 version of the NAQFC. One set of reactions recycles the higher organic peroxide species of CB05 (ROOH), resulting in additional photochemically reactive products that act to produce additional ozone in some chemical regimes. The other set of reactions recycles reactive nitrogen from less reactive forms back to NO2, increasing the effective NOx concentration of the system. In particular, the organic nitrate species (NTR), which was a terminal product for reactive nitrogen in the CBMIV mechanism, acts as a reservoir species in CB05 to redistribute NOx from major source areas to potentially NOx-sensitive areas where additional ozone may be produced in areas remote from direct NOx sources.

Description: The processes of aerosol sulfate formation are vital components in the scientific understanding of perturbations of earth's radiative balance via aerosol direct and indirect effects. In this work, an analysis of the influence of changes in oxidant levels and sulfur dioxide oxidation pathways was performed to study the underlying pathways for sulfate formation. Sensitivities of this constituent were calculated from a series of photochemical model simulations with varying rates of NOx and VOC emissions to produce variations in oxidant abundances using a photochemical model (CMAQ) that covers the Eastern US for the ICARTT 2004 campaign. Three different chemical mechanisms (CBIV, CB05, and SAPRC99) were used to test model responses to changes in NOx and VOC levels. Comparison of modeled results and measurements demonstrates that the simulations with all three chemical mechanisms capture the levels of sulfate reasonably well. However, the three mechanisms are shown to have significantly different responses in sulfate formation when the emissions of NOx and/or VOC are altered, reflecting different photochemical regimes under which the formation of sulfate occurs. Also, an analysis of the oxidation pathways that contribute to sulfur dioxide conversion to sulfate reveals substantial differences in the importance of the various pathways among the three chemical mechanisms. These findings suggest that estimations of the influence that future changes in primary emissions or other changes which perturb SO2 oxidants have on sulfate abundances, and on its direct and indirect radiative forcing effects, may be dependent on the chemical mechanism employed in the model analysis.

Description: An investigation was conducted to identify the mechanistic differences between two versions of the carbon bond gas-phase chemical mechanism (CB05 and CBMIV) which consistently lead to larger ground-level ozone concentrations being produced in the CB05 version of the National Air Quality Forecasting Capability (NAQFC) modeling system even though the two parallel forecast systems utilize the same meteorology and base emissions and similar initial and boundary conditions. Box models of each of the mechanisms as they are implemented in the NAQFC were created and a set of 12 sensitivity simulations was designed. The sensitivity simulations independently probed the conceptual mechanistic differences between CB05 and CBMIV and were exercised over a 45-scenario simulation suite designed to emulate the wide range of chemical regimes encountered in a continental-scale atmospheric chemistry model. Results of the sensitivity simulations indicate that two sets of reactions that were included in the CB05 mechanism, but which were absent from the CBMIV mechanism, are the primary causes of the greater ozone production in the CB05 version of the NAQFC. One set of reactions recycles the higher organic peroxide species of CB05 (ROOH), resulting in additional photochemically reactive products that act to produce additional ozone in some chemical regimes. The other set of reactions recycles reactive nitrogen from less reactive forms back to NO2, increasing the effective NOx concentration of the system. In particular, the organic nitrate species (NTR), which was a terminal product for reactive nitrogen in the CBMIV mechanism, acts as a reservoir species in CB05 to redistribute NOx from major source areas to potentially NOx-sensitive areas where additional ozone may be produced in areas remote from direct NOx sources.

Description: A three-dimensional air quality model based on a set of chemical species mass conservation equations describes the time evolution of chemical species in the atmosphere. In order to solve this set of equations, proper choices of initial and boundary conditions are needed. Ideally, initial and boundary conditions should be determined on the basis of observations. However, since such high-resolution observations are generally not available, it becomes necessary to use other information sources to specify the initial and boundary values. The fact that both the initial and the boundary conditions are specified with some degree of presumption makes it important to evaluate their influence in the model results. In this paper we present a study of the impact of initial and boundary concentrations on the modelled surface ozone concentration over two environments: Huelva and Badajoz, an industrial and a rural zone, respectively. The impacts are analysed for the same meteorological period (10–15 August 2003).

Description: Throughout Europe the summer of 2003 was exceptionally warm, especially July and August. The European Environment Agency (EEA) reported several ozone episodes, mainly in the first half of August. These episodes were exceptionally long-lasting, spatially extensive, and associated to high temperatures. In this paper, the 10$ndash;15 August 2003 ozone pollution event has been analyzed using meteorological and regional air quality modelling. During this period the threshold values of the European Directive 2002/3/EC were exceeded in various areas of the Iberian Peninsula. The aim of this paper is to computationally understand and quantify the influence of biogenic volatile organic compound (BVOC) emissions in the formation of tropospheric ozone during this high ozone episode. Being able to differentiate how much ozone comes from biogenic emissions alone and how much comes from the interaction between anthropogenic and biogenic emissions would be helpful to develop a feasible and effective ozone control strategy. The impact on ozone formation was also studied in combination with various anthropogenic emission reduction strategies, i.e., when anthropogenic VOC emissions and/or NOx emissions are reduced. The results show a great dependency of the BVOC contribution to ozone formation on the antropoghenic reduction scenario. In rural areas, the impact due to a NOx and/or VOC reduction does not change the BVOC impact. Nevertheless, within big cities or industrial zones, a NOx reduction results in a decrease of the biogenic impact in ozone levels that can reach 85 μg/m3, whereas an Anthropogenic Volatile Organic Compound (AVOC) reduction results in a decrease of the BVOC contribution on ozone formation that varies from 0 to 30 μg/m3 with respect to the contribution at the same points in the 2003 base scenario. On the other hand, downwind of the big cities, a decrease in NOx produces a minor contribution of biogenic emissions and a decrease in AVOCs results in greater contributions of BVOCs to the formation of ozone.