A. Ebel

Modeling the formation of secondary organic aerosol within a comprehensive air quality model system

The Secondary Organic Aerosol Model (SORGAM) has been developed for use in comprehensive air quality model systems. Coupled to a chemistry-transport model, SORGAM is capable of simulating secondary organic aerosol (SOA) formation including the production of low-volatility products and their subsequent gas/particle partitioning. The current model formulation assumes that all SOA compounds interact and form a quasi-ideal solution. This has significant impact on the gas/particle partitioning, since in this case the saturation concentrations of the SOA compounds depend on the composition of the SOA and the amount of absorbing material present. Box model simulations have been performed to investigate the sensitivity of the model against several parameters. Results clearly show the importance of the temperature dependence of saturation concentrations on the partitioning process. Furthermore, SORGAM has been coupled to the comprehensive European Air Pollution and Dispersion/Modal Aerosol Dynamics Model for Europe air quality model system, and results of a three-dimensional model application are presented. The model results indicate that assuming interacting SOA compounds, biogenic and anthropogenic contributions significantly influence each other and cannot be treated independently.

Modal aerosol dynamics model for Europe: development and first applications

Abstract The Modal Aerosol Dynamics model for Europe (MADE) has been developed as an extension to mesoscale chemistry transport models to allow a more detailed treatment of aerosol effects in these models. Due to the complexity of the atmospheric aerosol system an approach has been chosen which is, on the one hand, fast enough for this application and, on the other, provides sufficient information on the particle size distribution. In MADE, which is developed from the regional particulate model (RPM) the particle size distribution of the submicrometer aerosol is represented by two overlapping lognormal modes. The chemical composition is currently treated in the sulfate–nitrate–ammonium and water system. Sources for aerosol particles are modelled through nucleation and emission. Coagulation, condensation, transport and deposition are considered as processes modifying the aerosol population in the atmosphere. Aerosol dynamics calculations are performed on-line within the chemistry-transport model. Process studies with a one-dimensional version of the model system are used to investigate the relative importance of the individual aerosol dynamic processes and the important links between between the gas and aerosol phase as well as meteorological parameters. Results from a first three-dimensional application of the fully coupled system of MADE and the European Air Pollution Dispersion model system (EURAD) are presented, showing the suitability of MADE as an aerosol dynamics model even within complex air quality models. The application of the new aerosol model provides information on particle number, size and surface area in addition to the chemical concentration fields, which can be used to study a variety of aerosol-related air pollution issues in subsequent studies.

Variational data assimilation for tropospheric chemistry modeling

The method of variational adjoint data assimilation has been applied to assimilate chemistry observations into a comprehensive tropospheric gas phase model. The rationale of this method is to find the correct initial values for a subsequent atmospheric chemistry model run when observations scattered in time are available. The variational adjoint technique is esteemed to be a promising tool for future advanced meteorological forecasting. The stimulating experience gained with the application of four-dimensional variational data assimilation in this research area has motivated the attempt to apply the technique to air quality modeling and analysis of the chemical state of the atmosphere. The present study describes the development and application of the adjoint of the second-generation regional acid deposition model gas phase mechanism, which is used in the European air pollution dispersion model system. Performance results of the assimilation scheme using both model-generated data and real observations are presented for tropospheric conditions. In the former case it is demonstrated that time series of only few or even one measured key species convey sufficient information to improve considerably the analysis of unobserved species which are directly coupled with the observed species. In the latter case a Lagrangian approach is adopted where trajectory calculations between two comprehensively furnished measurement sites are carried out. The method allows us to analyze initial data for air pollution modeling even when only sparse observations are available. Besides remarkable improvements of the model performance by properly analyzed initial concentrations, it is shown that the adjoint algorithm offers the feasibility to estimate the sensitivity of ozone concentrations relative to its precursors.

4D-variational data assimilation with an adjoint air quality model for emission analysis

Abstract The problem of analyzing the chemical state of the troposphere and the associated emission scenario on the basis of observations and model simulations is considered. The method applied is the four-dimensional variational data assimilation method (4D-var) which iteratively minimizes the misfit between modeled concentration levels and measurements. The overall model–observation discrepancy is measured in terms of a cost function, of which the gradient is calculated for subsequent minimization by adjoint modeling. The model applied is the University of Cologne EURopean Air pollution Dispersion model (EURAD) simulating the meso-alpha scale. The forward and adjoint components are Botts horizontal and vertical advection scheme (Bott, Mon. Wea. Rev. 117 (1989), 1006), implicit vertical diffusion, and the RADM2 gas phase chemistry. The basic feasibility of the adjoint modeling technique for emission rate assessment is demonstrated by identical twin experiments. The objective of the paper is to demonstrate the skill and limits of the 4D-var technique to analyze the emission rates of non-observed precursor constituents of ozone, when only ozone observations are available. It is shown that the space–time variational approach is able to analyze emission rates of NO directly. For volatile organic compounds (VOC), regularization techniques must be introduced, however.

Simulation of ozone intrusion caused by a tropopause fold and cut-off low

Abstract A tropopause fold and cut-off low developed over Europe at the end of April 1982 and enhanced the exchange of air between the stratosphere and troposphere. The episode has been simulated using the EURAD model which has been designed for long-range transport simulation for European conditions. Applying a linear relationship between potential vorticity and ozone, concentration fields of the tracer and their changes due to outflow of ozone-rich air from the stratosphere could be simulated. A considerable decrease of stratospheric ozone was also obtained. This indicates noticeable mixing of tropospheric air with reduced ozone content into the stratosphere during the episode. Strong downward fluxes more than an order of magnitude larger than normal when averaged over the model domain show up around levels close to the tropopause. Obviously, dynamical processes fovern teh ozone budget o the upper troposphere during the vigorous intrusion event. Drastic increases of ozone also occur in the lower troposphere but are probably underestimated since vertical mixing by clouds is not taken into account in the simulation experiments. It is intended to explore the interaction of ozone of stratospheric and tropospheric (anthropogenic) origin in further experiments.

Regional and Global Tropopause Fold Occurrence and Related Ozone Flux Across the Tropopause

This paper gives a synthesis of three algorithms to detect the presenceof tropopause folds from vertical ozone/radio-sounding profiles and frommeteorological analysis. Also an algorithm to identify injection ofstratospheric air into the lower troposphere fromozone/7beryllium time series is presented. Differences in theresults obtained from the algorithms are observed and discussed with respectto the criteria for fold detection and input data used. Spatial gradients inthe obtained folding frequencies are made evident on a global scale from thealgorithm based on meteorological analysis (Q-vector/potential vorticity)and probably also on a regional European scale from algorithms both basedmeteorological analyses and on ozone/PTU soundings. The observed seasonalvariation of folding occurrence is rather flat except during summer whenalso some differences appear between the algorithms. By combining thefolding frequencies with literature estimates of the cross-tropopause ozonetransfer in single folding events, an average stratospheric ozone influxinto the troposphere of 5.7 ± 1.3× 1010 mol.cm-2 s-1 is obtained for the Northern hemisphereand 12± 2.7× 1010 mol. cm-2s-1 for Western Europe. Potential additional contributions dueto other stratosphere-troposphere exchange processes than folds are not yetincluded in these estimates. Finally, the link between statistics fromozone/7beryllium data and folding statistics is brieflydiscussed.

Deep stratospheric intrusions: a statistical assessment with model guided analyses

Abstract A statistical assessment of deep intrusions of stratospheric air based on records of two adjacent mountain stations of the northern Alps at different altitudes is presented. Ten years recordings of beryllium activity, ozone concentrations, and relative humidity at the Zugspitze summit (2962 m a.s.l.), as well as ozone and relative humidity at the Wank summit (1776 m a.s.l., 15km distance) were analyzed. 195 stratospheric intrusion events could unambiguously be identified for the Zugspitze, whereas 85 intrusion events were found for the Wank. No event could be reliably identified at the valley floor station at Garmisch-Partenkirchen (740m a.s.l.). There is a pronounced seasonal cycle of the frequency of events showing highest activity during fall, winter, and spring, whereas low activity is found during summer. By assessing average events it was possible to infer the monthly mean enrichment rate of the lower tropospheric ozone concentration by deep stratospheric intrusions. It was found that at least 3% of the ozone burden is replaced every month on an annual average. Three events of moderate strength were taken to be further analyzed by mesoscale meteorological model simulations with subsequent trajectory studies. In two cases the intrusion of stratospheric air was induced by tropopause foldings. In the third case a cut-off low with an associated fold was responsible for the increased exchange. All three cases revealed that the ingress of stratospheric air observed at the mountain station is a non-local process induced more than 2000 km apart. Transport over these distances took about 2–4 days. Along the pathways through the tropopause the air parcels are shown to subside from the tip of the folds at 400–500 hPa down to about 700 hPa to reach the Zugspitze measurement station.

Simulation of the chernobyl radioactive cloud over Europe using the eurad model

Abstract The Eur opean A cid D eposition Model (EURAD) is used to investigate the long-range transport (LRT) and deposition of radioactive material in Europe during the first week after the Chernobyl accident. Emphasis is laid on using the model system in a forecast mode as possibly would be done shortly after such an event. Thus, meteorological fields are predicted with the PSU/NCAR mososcale model MM4. The multilayer Eulerian model CTM ( C hemistry T ransport Model ) is applied to compute transport and deposition of Cs-137 and I-131 using the predicted meteorological fields. However, the accident scenario was estimated using published data. The model results and performance are discussed by comparison with observations. It is demonstrated that the model can reproduce certain observed characteristics of the radioactive cloud, i.e. trends in surface air concentrations, arrival times and wet deposition patterns. This leads to the suggestion that the predictive capability of the EURAD-system has a relatively high level considering the fact that several simple approaches were used.

Evaluation studies with a regional chemical transport model (EURAD) using air quality data from the EMEP monitoring network

Abstract The EURAD model has been applied to simulate the transport and chemical transformations of atmospheric constituents over central Europe for a 3 week late winter episode in February/March 1982. The TADAP model (the European version of ADOM) carried out simulations for the same episode with the same emission data set but with different meteorological input. The episode is characterized by two typical European meteorological conditions: a relatively quiet blocking situation at the beginning of the episode is followed by rather disturbed conditions with frontal systems passing through central European. The evolution of the pollutant fields is markedly different during these two conditions. Evaluation studies with the focus on the sulfur species were performed using air quality data from the EMEP monitoring network for the comparison of model predictions with observations. The first simulation with predicted meteorology and the PHOXA emission data set revealed an underprediction of airborne sulfate and wet deposition, while SO 2 was overpredicted. A simulation with meteorological data created by a so-called nudging technique corrected the transport of SO 2 to remote regions and drastically improved the predictions of rainfall and subsequently the amount of wet deposition. However, the ambient sulfate aerosol concentrations were still underpredicted. As the same bias in sulfate was also observed for TADAP simulations of the same episode a hypothesis for the underprediction is a very low primary sulfate emission inherent in the emission database. By artificially increasing the primary sulfate emission to an obviously too high value, the underprediction of sulfate was changed into a strong overprediction for the major source regions and into a reasonable range as observed for the remote regions. An analysis of the precipitation amounts, the aqueous concentrations and the resulting wet deposition showed that during this episode the aqueous were precipitation limited.

Long-term simulations of particulate matter in Europe on different scales using sequential nesting of a regional model

A regional model system for the simulation of air quality in Europe (EURAD) has been employed for long-term calculations of atmospheric pollutants on different scales. The model allows sequential nesting. Starting with a domain covering most of Europe, the nesting procedure was used to scale down the simulations to a highly populated and industrialized area, namely NorthRhine-Westphalia (NRW). The analysis of results is focusing on the simulation of particulate matter (PM10) for the year 1997. This parameter is of special interest in the framework of air pollution control as demanded by the European Union. The model takes account of various components of the atmospheric aerosol, including secondary organic aerosols (SOA). Their role in the budget of particulate matter is briefly shown. The model data are compared with measurements of the monitoring network of NRW. Hit rates for a given range of accuracy are discussed for PM10 and NO2. It is found that a large part of deviating results may be ascribed to shortcomings in the applied aerosol emission inventories, in particular, and that it is worthwhile to improve them for the application to future air quality assessments based on numerical simulations.