Jean Vankerkom

Improving local air quality in cities: To tree or not to tree?

Vegetation is often quoted as an effective measure to mitigate urban air quality problems. In this work we demonstrate by the use of computer models that the air quality effect of urban vegetation is more complex than implied by such general assumptions. By modelling a variety of real-life examples we show that roadside urban vegetation rather leads to increased pollutant concentrations than it improves the air quality, at least locally. This can be explained by the fact that trees and other types of vegetation reduce the ventilation that is responsible for diluting the traffic emitted pollutants. This aerodynamic effect is shown to be much stronger than the pollutant removal capacity of vegetation. Although the modelling results may be subject to a certain level of uncertainty, our results strongly indicate that the use of urban vegetation for alleviating a local air pollution hotspot is not expected to be a viable solution.

Modelling changes of aerosol compositions over Belgium and Europe

Seasonal changes in aerosol compositions over Belgium and Europe are simulated with an extended version of the EUROS model. EUROS is capable of modelling mass and chemical composition of aerosols in two size fractions (PM2.5 and PM10-2.5). The chemical composition is expressed in terms of seven components: ammonium, nitrate, sulphate, primary inorganic compounds, elementary carbon, primary organic compounds and Secondary Organic Compounds (SOA). A comparison of modelled and measured aerosol concentrations showed that modelled concentrations are generally consistent with observed concentrations. The chemical composition of the aerosol showed a strong dependence on the season. High aerosol concentrations during the summer were mainly due to high concentrations of the secondary components nitrate, ammonium, sulphate and SOA in the size fraction PM2.5. In contrast, during autumn and winter, increased PM-concentrations were mainly due to higher concentrations of primary components, especially in the size fraction PM10-2.5.

Making High Resolution Air Quality Maps for Flanders, Belgium

Using a combination of models, high resolution air quality maps for Flanders (Belgium) have been made. First of all, the Eulerian air quality model AURORA has simulated for a complete year the air pollutant concentrations over the region on a 3 × 3 km² resolution. These results are calibrated using the RIO-interpolation model on air quality station data. Thereafter, an extra simulation using the bi-Gaussian IFDM model is made on a resolution of 1 × 1 km², with a finer resolution (up to 25 m) close to the major roads. The nesting methodology of IFDM in AURORA is designed to avoid double counting of the roads. The results are highly detailed PM10, PM2.5, NO2 and EC maps for Flanders. Using station data and data from several measurement campaigns, the maps have been validated and it has been shown that the maps show indeed a highly detailed picture of the air quality in Flanders. These data will be used in assessing the air quality and human exposure to it in Flanders, and in assessing policy scenarios designed to improve the air quality.

The Role of Vegetation in Local and Urban Air Quality

We present the outcome of the international conference ‘Local Air Quality and its Interactions with Vegetation’, which took place in Antwerp, Belgium on January 21-22, 2010. Results of international CFD studies, measurement campaigns and experimental studies show that vegetation can have an important effect on dispersion patterns determining local air quality. However, there are many parameters involved (vegetation structure, local meteorology, urban canopy characteristics, mechanical turbulence properties) and the results show that the complexity of the mechanisms of vegetation affecting local air quality are often underestimated.

Combining Models for Assessment of Local Air Quality

Assessing local air quality can be a challenging task. Indeed, local air quality is strongly dependent on local factors but also regional and in some cases even global effects have to be taken into account when assessing local air pollution concentrations. Furthermore, large gradients in pollutant concentrations can be present in the urban environment. In order to assess the local air quality for the city of Antwerp, a combination of an Eulerian dispersion model, a measurement interpolation tool, a Gaussian plume model and a simplified version of the OSPM street canyon model have been coupled to each other, taking into account double counting effects of local emissions. The coupled model which combines the regional, urban and street canyon scale has been applied for the city centre of Antwerp and its harbour. This results in detailed maps with a resolution up to 30 m for four pollutants: PM10, PM2.5, EC (elementary carbon) and NO2. Furthermore, several abatement measures have been assessed in order to improve the urban air quality. It has been shown that local (traffic) measures only have a small effect on total mass PM10 and PM2.5 concentrations, but exhibit a larger effect on EC and NO2-concentrations.

CFD Simulations of Air Pollution in Urban Micro Environments

This paper presents the results of two recent case studies which were evaluated with the CFD model ENVI-met. In a first case, the impact of a vegetation barrier along a highway on the local air quality is studied. In a second case, the dispersion of total UFP concentrations within a street canyon is examined. In both cases, ENVI-met model output is compared with measurements resulting in an overall acceptable agreement of the model performance.

Comparing the impact of a road tunnel vs. a road viaduct by means of an integrated exposure assessment

Using the MOBILEE methodology, we performed a detailed air quality assessment for three scenarios for the ring road around Antwerp, a major city in Belgium, using PM10 and NOx emission inventories for 2003 (reference) and 2015 (projected future situation), followed by an assessment of the exposure of the population living in the vicinity of the planned constructions. PM10 turned out to be the dominant parameter in exposure assessment. Compared with the impact of the viaduct, a tunnel with an exhaust at a height of 5 or 30 m shows respectively a 40% increase or a 5% decrease in total exposure.

Modelling of Airborne Primary and Secondary Particulate Matter with the EUROS-Model

We used the Eulerian Chemistry-Transport Model EUROS to simulate the concentrations of airborne fine particulate matter above Europe. Special attention was paid to both primary as well as secondary particulate matter in the respirable size range up to 10 μm diameter. Especially the small particles with diameters up to 2.5 μm are often formed in the atmosphere from gaseous precursor compounds. Comprehensive computer codes for the calculation of gas phase chemical reactions and thermodynamic equilibria between the compounds in the gas phase and those ones in the solid phase had been implemented into the EUROS-model. Obtained concentrations of PM10for the year 2003 are compared to measurements. Additionally, calculations were carried out to assess the contribution of emissions derived from the sector agriculture in Flanders, the northern part of Belgium. The obtained results demonstrate the importance of ammonia emissions in the formation of secondary particulate matter. Hence, future abatement policy should consider more the role of ammonia in the formation of secondary particles.

Synergetic or Non-Linear Effects in PM10 and PM2.5 Scenario Calculations for 2015 in Belgium

We applied the extended version of the EUROS model to evaluate the impact of emission reductions on PM10 and PM2.5 concentrations in Flanders and Belgium for 2015. Individual sector contributions were assessed and the current and future changes in aerosol concentrations and compositions over Belgium and Europe were investigated. Contributions from anthropogenic sources in Flanders were found to be responsible for 34.3% of the annual averaged PM10 concentrations in Flanders in 2003. In 2015 this contribution is estimated to be 35.0%. For PM2.5 these contributions are 29.1% in 2003 and 27.8% in 2015 respectively. Results show that non-linear effects can not be neglected. Because of the non-linear processes that take place when secondary aerosols are formed, a small reduction in a gaseous compound (e.g. SO2) does not necessarily lead to the same amount of reduction of the secondary compound (e.g. sulphate). Another “non-linear” aspect is the formation of aerosols by contributions from two compounds that are delivered by two individual sectors. The synergetic effect of these “non-linear” contributions was found to be an additional 2.1% for PM10 and 3.7% for PM2.5, representing an increase of 6–13%, which is not negligible and might become relevant in abatement policies.

Chapter 5.4 Modelling seasonal changes of aerosol compositions over Belgium and Europe

Abstract The EUROpean Smog model (EUROS) was extended with two special modular algorithms for atmospheric particles. The first module is the Caltech Atmospheric Chemistry Mechanism (CACM) which describes in a mechanistic way the formation of precursors of secondary organic aerosols. The second module is the Model of Aerosol Dynamics, Reaction, Ionization and Dissolution (MADRID 2), which describes the formation of secondary aerosols by means of an equilibrium between the gas phase and the aerosol phase. It includes inorganic as well as hydrophilic and hydrophobic organic compounds. Through this extension, the EUROS model was able to model mass and chemical composition of aerosols in two size fractions (PM 2.5 and PM 10-2.5 ). The model was validated for 3 seasonal episodes in 2002 and 2003. A comparison between modelled and observed aerosol concentrations showed that the trends in PM 10 concentrations are well captured. A strong seasonal dependency was found in the chemical composition of the aerosols. Large contributions of secondary inorganic components were found for summer episodes with high aerosol concentrations, whereas during autumn/winter episodes the concentrations of secondary aerosol were less abundant.