Effect of radiation interaction and aerosol processes on ventilation and aerosol concentrations in a real urban neighbourhood in Helsinki

Abstract

Large-eddy simulation (LES) is an optimal tool to examine aerosol particle concentrations in detail within urban neighbourhoods. The concentrations are a complex result of local emissions, meteorology, aerosol processes and local mixing conditions due to thermal and mechanical effects. Despite this, most studies have focused on simplification of the affecting processes such as examining the impact of local mixing in idealised street canyons or treating aerosols as passive scalars. The aim of this study is to include all these processes into LES using the PALM model system and to examine the importance of radiative heating and aerosol processes in simulating local aerosol particle concentrations and different aerosol metrics within a realistic urban neighbourhood in Helsinki under morning rush hour with calm wind conditions. The model outputs are evaluated against mobile laboratory measurements of air temperature and total particle number concentration (Ntot) as well as drone measurements of lung-deposited surface area (LDSA). The inclusion of radiation interaction in LES has a significant impact on simulated near-surface temperatures in our study domain, increasing them on average from 8.6 to 12.4 ∘C. The resulting enhanced ventilation reduces the pedestrian-level (4 m) Ntot by 53 %. The reduction in Ntot due to aerosol processes is smaller, only 18 %. Aerosol processes particularly impact the smallest particle range, whereas radiation interaction is more important in the larger particle range. The inclusion of radiation interaction reduces the bias between the modelled and mobile-laboratory-measured air temperatures from −3.9 to +0.2 ∘C and Ntot from +98 % to −13 %. With both aerosol and radiation interaction on, the underestimation is 16 %, which might be due to overestimation of the ventilation. The results show how inclusion of radiative interaction is particularly important in simulating PM2.5, whereas aerosol processes are more important in simulating LDSA in this calm wind situation.