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

Abstract

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.

Diesel exhaust nanoparticles and their behaviour in the atmosphere

“…[21], although earlier work has attached more importance to these processes, albeit on greater distance scales [86]. Vapour pressure parameterizations have a very significant impact on particle size and composition [60], as do factors affecting the dispersion of air through a street canyon [77]. The UFP aerosol module is incorporated into the three-dimensional large eddy simulation (LES) based on the Weather Research and Forecasting model (WRF) [87] to simulate the dispersion and evolution of nanoparticles at a neighbourhood-scale over central London.…”

Predicted ultrafine particulate matter source contribution across the continental United States during summertime air pollution events

“…Various methods, such as the source-resolved PMCAMx chemical transport model, the chemical mass balance (CMB) model, photochemical box models, and land use regression (LUR) models, have been used to track source contributions to primary organic matter, elemental carbon, and in some cases particle number concentration (N x ) over areas in the Eastern US and parts of Europe and Asia (Lane et al, 2007;Posner and Pandis, 2015;Wang et al, 2011;Cattani et al, 2017;Wolf et al, 2017;Simon et al, 2018;Gaydos et al, 2005;Zhong et al, 2018). However, these methods are limited in one or more aspects of their ability to predict population exposure to ultrafine particles over large analysis domains.…”

Urban form strongly mediates the allometric scaling of airshed pollution concentrations

“…Not all pollution is well-mixed throughout the urban airshed. Pockets of high concentration exist, particularly in poorly ventilated street canyons through which high volumes of traffic flow [10,38,41,42]. We expect the likelihood of the occurrence of pollution hotspots,…”

“…We have shown above that road length is a stronger and stronger linear predictor for NO x emissions as GB urban areas increase in size, but that a ∼10% pollution scale-related excess persists, presumably as a result of increasing congestion in the largest settlements. The scaling of F with population will depend on changes in urban form [42]. Some changes in urban form with scale-e.g.…”