Irina Nikolova

Dispersion modelling of traffic induced ultrafine particles in a street canyon in Antwerp, Belgium and comparison with observations

The aim of this study is to investigate the dispersion of ultrafine particles and its spatial distribution in a street canyon and its neighbourhood with the 3D CFD model ENVI-met®. The performance of the model at street scale is evaluated and the importance of the boundary conditions like wind field and traffic emissions on the UFP concentration is demonstrated. To support and validate the modelled results, a short-term measurement campaign was conducted in a street canyon in Antwerp, Belgium. The UFP concentration was measured simultaneously with P-TRACK (TSI Model 8525) at four different locations in the canyon. The modelled UFP concentrations compare well with the measured data (correlation coefficient R from 0.44 to 0.93) within the standard deviation of the measurements. Despite the moderate traffic flow in the street canyon, UFP concentrations in the canyon are in general double of the background concentrations, indicating the high local contribution for this particle number concentration. Some of the observed concentration profiles are not resembled by the model simulations. For these specific anomalies, further analysis is performed and plausible explanations are put forward. The role of wind direction and traffic emissions is investigated. The performance evaluation of ENVI-met® shows that in general the model qualitatively and quantitatively describes the dispersion of UFP in the street canyon study.

Size resolved ultrafine particles emission model--a continues size distribution approach.

A new parameterization for size resolved ultrafine particles (UFP) traffic emissions is proposed based on the results of PARTICULATES project (Samaras et al., 2005). It includes the emission factors from the Emission Inventory Guidebook (2006) (total number of particles, #/km/veh), the shape of the corresponding particle size distribution given in PARTICULATES and data for the traffic activity. The output of the model UFPEM (UltraFine Particle Emission Model) is a sum of continuous distributions of ultrafine particles emissions per vehicle type (passenger cars and heavy duty vehicles), fuel (petrol and diesel) and average speed representative for urban, rural and highway driving. The results from the parameterization are compared with measured total number of ultrafine particles and size distributions in a tunnel in Antwerp (Belgium). The measured UFP concentration over the entire campaign shows a close relation to the traffic activity. The modelled concentration is found to be lower than the measured in the campaign. The average emission factor from the measurement is 4.29E+14 #/km/veh whereas the calculated is around 30% lower. A comparison of emission factors with literature is done as well and in overall a good agreement is found. For the size distributions it is found that the measured distributions consist of three modes--Nucleation, Aitken and accumulation and most of the ultrafine particles belong to the Nucleation and the Aitken modes. The modelled Aitken mode (peak around 0.04-0.05 μm) is found in a good agreement both as amplitude of the peak and the number of particles whereas the modelled Nucleation mode is shifted to smaller diameters and the peak is much lower that the observed. Time scale analysis shows that at 300 m in the tunnel coagulation and deposition are slow and therefore neglected. The UFPEM emission model can be used as a source term in dispersion models.

Modelling traffic-induced multicomponent ultrafine particles in urban street canyon compartments: Factors that inhibit mixing

This study implements a two-box model coupled with ultrafine particle (UFP) multicomponent microphysics for a compartmentalised street canyon. Canyon compartmentalisation can be described parsimoniously by three parameters relating to the features of the canyon and the atmospheric state outside the canyon, i.e. the heterogeneity coefficient, the vortex-to-vortex exchange velocity, and the box height ratio. The quasi-steady solutions for the two compartments represent a balance among emissions, microphysical aerosol dynamics (i.e. evaporation/condensation of semi-volatiles, SVOCs), and exchange processes, none of which is negligible. This coupled two-box model can capture significant contrasts in UFP number concentrations and a measure of the volatility of the multi-SVOC-particles in the lower and upper canyon. Modelled ground-level UFP number concentrations vary across nucleation, Aitken, and accumulation particle modes as well-defined monotonic functions of canyon compartmentalisation parameters. Compared with the two-box model, a classic one-box model (without canyon compartmentalisation) leads to underestimation of UFP number concentrations by several tens of percent typically. By quantifying the effects of canyon compartmentalisation, this study provides a framework for understanding how canyon geometry and the presence of street trees, street furniture, and architectural features interact with the large-scale atmospheric flow to determine ground-level pollutant concentrations.

The influence of particle composition uponthe evolution of urban ultrafine dieselparticles on the neighbourhood scale

Abstract. A recent study demonstrated that diesel particles in urban air undergo evaporative shrinkage when advected to a cleaner atmosphere (Harrison et al., 2016). We explore, in a structured and systematic way, the sensitivity of nucleation-mode diesel particles to changes in particle composition and saturation vapour pressure. We use a multi-component aerosol microphysics model based on surrogate molecule (C 16 -C 32 n-alkane) volatilities. For standard atmospheric conditions (298 K, 1013.25 hPa), and over timescales (ca. 100 s) relevant for dispersion on the neighbourhood scale (up to 1 km), the choice of a particular vapour pressure dataset changes the range of compounds that are appreciably volatile by 2–6 carbon numbers. The nucleation-mode peak diameter, after 100 s of model runtime, is sensitive to the vapour pressure parameterisations for particles with compositions centred on surrogate molecules between C 22 H 46 and C 24 H 50 . The vapour pressures of components in this range are therefore critical for the modelling of nucleation-mode aerosol dynamics on the neighbourhood scale and need to be better constrained. Laboratory studies have shown this carbon number fraction to derive predominantly from engine lubricating oil. The accuracy of vapour pressure data for other (more and less volatile) components from laboratory experiments, is less critical. The influence of a core of involatile material is also considered. The new findings of this study may also be used to identify the Semi-Volatile Organic Compound (SVOC) compositions that play dominating roles in the evaporative shrinkage of the nucleation mode observed in field measurements (e.g. Dall’Osto et al., 2011). As well as reconciling model and observations, identifying the most significant vapour pressure regime for nucleation-mode dynamics offers a way to improve the computing efficiency of urban aerosol models by adopting simplified schemes for those less important components: e.g., an equilibrium scheme for low-carbon-number components and a linear scheme for high-carbon-number components.

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.