The size distribution of particles emitted from road traffic substantially influences urban air quality and human health. There is however a limited understanding on the importance of the emission number size distributions, especially for particles smaller than 50 nm, in neighborhood-scale air quality modelling. We used the large-eddy simulation PALM-SALSA model to investigate the influence of traffic emissions and their number size distributions on pedestrian-level particle concentrations in Beijing. The simulations were driven by particle size distributions from emission model GAINS, roadside field measurements, and engine laboratory experiments. With constant total traffic emissions and only modifying the shape of the emission number size distribution to better represent smaller particles, the modelled total particle number concentrations ( N t o t ) decreased less than 4% compared to the simulation using GAINS. The impact on health-related aerosol metrics, including PM 1 (Particulate Matter with a diameter smaller than 1 μ m) and LDSA (Lung-Deposited Surface Area) was larger. When experimental-based nucleation mode vehicle emissions were introduced, the total particle number emissions increased to 12-fold when compared to GAINS, leading to a pronounced rise in N t o t (up to 560%), particularly in the number concentration of smallest particles. Owing to aerosol dynamics, the increase in N t o t within the street canyon was lower than the relative increase in vehicle emissions. PM 1 remained largely unchanged. These results highlight the importance of nucleation-mode traffic emissions for modelling urban N t o t , whereas their influence on mass concentration and LDSA appears comparatively minor at the neighbourhood scale. • Using LES to study the importance of vehicle emission boundary conditions on modelled particle distributions. • Total particle number concentrations show limited sensitivity to the shape of vehicle emission particle size distribution. • A 12-fold increase in total particle number emissions due to adding nucleation-mode particles leads to only 229% increase in street-level particle number concentration. • The sensitivity of PALM-SALSA to vehicle emissions varies across aerosol metrics.
Li et al. (Wed,) studied this question.