Abstract. High-resolution modelling is needed to understand urban air quality and pollutant dispersion in detail. Recently, the PALM model system 6.0, which is based on large-eddy simulation (LES), was extended with the detailed Sectional Aerosol module for Large Scale Applications (SALSA) v2.0 to enable studying the complex interactions between the turbulent flow field and aerosol dynamic processes. This study represents an extensive evaluation of the modelling system against the horizontal and vertical distributions of aerosol particles measured using a mobile laboratory and a drone in an urban neighbourhood in Helsinki, Finland. Specific emphasis is on the model sensitivity of aerosol particle concentrations, size distributions and chemical compositions to boundary conditions of meteorological variables and aerosol background concentrations. The meteorological boundary conditions are taken from both a numerical weather prediction model and observations, which occasionally differ strongly. Yet, the model shows good agreement with measurements (fractional bias <0.67, normalised mean squared error <6, fraction of the data within a factor of 2 >0.3, normalised mean bias factor <0.25 and normalised mean absolute error factor <0.35) with respect to both horizontal and vertical distribution of aerosol particles, their size distribution and chemical composition. The horizontal distribution is most sensitive to the wind speed and atmospheric stratification, and vertical distribution to the wind direction. The aerosol number size distribution is mainly governed by the flow field along the main street with high traffic rates and in its surroundings by the background concentrations. The results emphasise the importance of correct meteorological and aerosol background boundary conditions, in addition to accurate emission estimates and detailed model physics, in quantitative high-resolution air pollution modelling and future urban LES studies.

Abstract. High-resolution modelling is needed to understand urban air quality and pollutant dispersion in detail. Recently, the PALM model system 6.0, which is based on the large-eddy simulation (LES), was extended with a detailed aerosol module SALSA2.0 to enable studying the complex interactions between the turbulent flow field and aerosol dynamic processes. This study represents an extensive evaluation of the modelling system against the horizontal and vertical distributions of aerosol particles measured using a mobile laboratory and a drone in an urban neighbourhood in Helsinki, Finland. Specific emphasis is on the model sensitivity of aerosol particle concentrations, size distributions and chemical compositions to boundary conditions of meteorological variables and aerosol background concentrations. The meteorological boundary conditions are drawn from both a numerical weather prediction model and observations, which occasionally differ strongly. Yet, the model shows good agreement with measurements (fractional bias

Abstract. Large eddy simulation (LES) provides an optimal tool to examine air pollutant concentrations at high temporal and spatial resolutions within urban neighborhoods. The local mixing conditions are to a large extent a result of building morphology and thermal conditions impacting mechanically and thermally driven turbulence. However, the impact of thermal conditions on local air pollutant concentrations in real urban environments is not well understood nor the importance of including thermal processes in LES. Furthermore, LES of aerosol particle concentrations in urban areas rarely include aerosol processes, but rather aerosols are treated as passive scalars. The aim of this study is to examine the importance of radiative heating and aerosol processes in simulating local aerosol particle concentrations in a wide street canyon and its surroundings in Helsinki under morning rush hour with calm wind conditions using the LES model PALM. The model outputs are evaluated against mobile laboratory measurements of air temperature and total particle number concentration (Ntot), and 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 by 3.8 °C from 8.6 °C to 12.4 °C. The thermal processes further strengthen the flow field, and enhance the ventilation of air pollutants from the street canyon by altering the canyon vortex. The enhanced ventilation reduces the pedestrian level (4 m) Ntot by 53 %. The reduction of Ntot due to aerosol processes is smaller, only 18 %. Aerosol processes have a larger effect in the smallest particle range, decreasing particle concentrations below 10 nm by up to 2.5 orders of magnitude whereas radiation interaction is more important in the larger particle range. Aerosol processes have a stronger impact than ventilation on LDSA, whereas radiation interaction shows a larger decrease in PM2.5 than in other aerosol metrics. The inclusion of radiation interaction in PALM improves the modelled near-surface temperatures and Ntot when compared to mobile laboratory measurements reducing the bias between the modelled and measured temperatures from -3.9 °C to +0.2 °C, and concentrations from +98 % to -13 %. With both aerosol and radiation interaction on, the underestimation was 16 %, which might be due to overestimation of the ventilation. The results show how inclusion of radiative interaction, and to a lesser extent aerosol processes, on LES are important for realistic simulation of near surface aerosol particle concentrations. This particularly in a calm wind situation such as modelled in this study.

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