Technical note: Dispersion of cooking-generated aerosols from an urban street canyon

Shang Gao,Keith Ngan,Chak K Chan,Mona Kurppa

doi:10.5194/acp-22-2703-2022

Shang Gao, Keith Ngan + Show 2 more

Open Access

https://doi.org/10.5194/acp-22-2703-2022

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Abstract

Abstract. The dispersion of cooking-generated aerosols from an urban street canyon is examined with building-resolving computational fluid dynamics (CFD). Using a comprehensive urban CFD model (PALM) with a sectional aerosol module (SALSA), emissions from deep-frying and boiling are considered for near-ground and elevated sources. With representative choices of the source flux, the inclusion of aerosol dynamic processes decreases the mean canyon-averaged number concentration by 15 %–40 % for cooking emissions, whereas the effect is significantly weaker for traffic-generated aerosols. The effects of deposition and coagulation are comparable for boiling, but coagulation dominates for deep-frying. Deposition is maximised inside the leeward corner vortices, while coagulation increases away from the source. The characteristic timescales are invoked to explain the spatial structure of deposition and coagulation. In particular, the relative difference between number concentrations for simulations with and without coagulation is strongly correlated with the ageing of particles along fluid trajectories or the mean tracer age. It is argued that, for a specific emission spectrum, the qualitative nature of the aerosol dynamics within urban canopies is determined by the ratio of the aerosol timescales to the relevant dynamical timescale (e.g. the mean age of air).

Highlights

Computational fluid dynamics (CFD) is a well-established tool for studying urban pollutant dispersion (e.g. Rivas et al, 2019)
Deposition is usually the only aerosol process included in urban CFD models as it is often taken to be the most important loss process of ultra-fine particles emitted by traffic (Kumar et al, 2011; Kim et al, 2019)
To highlight the influence of the emission spectrum, we begin by comparing the aerosol number concentration fields generated by traffic and roadside restaurants, i.e. emission scenarios TR, NG-D and NG-B (Table 1)

Summary

Introduction

Computational fluid dynamics (CFD) is a well-established tool for studying urban pollutant dispersion (e.g. Rivas et al, 2019). Most urban CFD studies assume neutral (uniform density) flow and passive scalar dynamics. Deposition is usually the only aerosol process included in urban CFD models as it is often taken to be the most important loss process of ultra-fine particles emitted by traffic (Kumar et al, 2011; Kim et al, 2019). Using a single emission scenario, NGB, two cases are considered: (i) light pollution, Nb = 0.1N0; (ii) heavy pollution, Nb = 0.4N0, where N0 is the mean canyon-averaged number concentration for Nb = 0. These values are arbitrary; the increase in Nb is meant to capture the contrast between normal conditions and a severe pollution episode

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