Abstract
Abstract. This study evaluates the influence of aerosol processes on the particle number (PN) concentrations in three major European cities on the temporal scale of 1 h, i.e., on the neighborhood and city scales. We have used selected measured data of particle size distributions from previous campaigns in the cities of Helsinki, Oslo and Rotterdam. The aerosol transformation processes were evaluated using the aerosol dynamics model MAFOR, combined with a simplified treatment of roadside and urban atmospheric dispersion. We have compared the model predictions of particle number size distributions with the measured data, and conducted sensitivity analyses regarding the influence of various model input variables. We also present a simplified parameterization for aerosol processes, which is based on the more complex aerosol process computations; this simple model can easily be implemented to both Gaussian and Eulerian urban dispersion models. Aerosol processes considered in this study were (i) the coagulation of particles, (ii) the condensation and evaporation of two organic vapors, and (iii) dry deposition. The chemical transformation of gas-phase compounds was not taken into account. By choosing concentrations and particle size distributions at roadside as starting point of the computations, nucleation of gas-phase vapors from the exhaust has been regarded as post tail-pipe emission, avoiding the need to include nucleation in the process analysis. Dry deposition and coagulation of particles were identified to be the most important aerosol dynamic processes that control the evolution and removal of particles. The error of the contribution from dry deposition to PN losses due to the uncertainty of measured deposition velocities ranges from −76 to +64 %. The removal of nanoparticles by coagulation enhanced considerably when considering the fractal nature of soot aggregates and the combined effect of van der Waals and viscous interactions. The effect of condensation and evaporation of organic vapors emitted by vehicles on particle numbers and on particle size distributions was examined. Under inefficient dispersion conditions, the model predicts that condensational growth contributes to the evolution of PN from roadside to the neighborhood scale. The simplified parameterization of aerosol processes predicts the change in particle number concentrations between roadside and urban background within 10 % of that predicted by the fully size-resolved MAFOR model.
Highlights
Motor vehicle exhaust emissions constitute the major source of ultrafine particle (UFP, < 100 nm in aerodynamic diameter) pollution in urban environments (Harrison et al, 2011; Morawska et al, 2008; Pey et al, 2009; Johansson et al, 2007)
Measured particle number (PN) concentrations based on hourly averages or 10 min averages showed a wide range of PN concentrations (20 000– 100 000 particles cm−3) for the traffic sites considered, depending on the season of the year, traffic density, and distance from the road
The “mean of traffic sites” distribution is characterized by a fraction of ultrafine particles (Dp: 10–100 nm) and accumulation mode (ACC) particles (Dp: 100–500 nm) of 80 and 4 %, respectively, while 16 % of the particles were below 10 nm
Summary
Motor vehicle exhaust emissions constitute the major source of ultrafine particle (UFP, < 100 nm in aerodynamic diameter) pollution in urban environments (Harrison et al, 2011; Morawska et al, 2008; Pey et al, 2009; Johansson et al, 2007). Reliable information on the number concentrations, together with the size distributions, is needed to better assess the health effects of urban particulate pollution. The exposure of the population in urban areas to particles may be assessed by modeling the spatial distribution of particles emitted from road transport and other sources in various micro-environments (e.g., Soares et al, 2014). Aerosol dynamic models (i.e., process models of aerosol microphysics, often employing Lagrangian approaches to the fluid flow) have been used to model the spatial and temporal evolution of ultrafine particles in the initial vehicle exhaust plume during the first seconds after emission Aerosol dynamic processes continuously change the number and size distribution, after the particles have been released into air
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