03 Jan 2023
03 Jan 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

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

Jani Strömberg1, Xiaoyu Li1, Mona Kurppa2, Heino Kuuluvainen3, Liisa Pirjola1,4, and Leena Järvi1,5 Jani Strömberg et al.
  • 1Institute for Atmospheric and Earth System Research, Faculty of Science, University of Helsinki, Helsinki, Finland
  • 2Kjeller Vindteknikk, Espoo, Finland
  • 3Aerosol Physics Laboratory, Physics Unit, Faculty of Engineering and Natural Sciences, Tampere University, Tampere, Finland
  • 4Department of Automotive and Mechanical Engineering, Metropolia Applied University, Vantaa, Finland
  • 5Helsinki Institute of Sustainability Science, Faculty of Science, University of Helsinki, Helsinki, Finland

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.

Jani Strömberg et al.

Status: open (until 14 Feb 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-978', Anonymous Referee #1, 25 Jan 2023 reply

Jani Strömberg et al.

Data sets

Input files and scripts for creating PALM simulation input files on Mäkelänkatu in Helsinki, Finland | Datasets and scripts to create input files for running PALM simulations Mona Kurppa, Jani Strömberg

Mäkelänkatu Simulations 2022 | Datasets to run PALM around Mäkelänkatu, Helsinki to study aerosol processes and radiation interaction Mona Kurppa, Jani Strömberg

Mäkelänkatu Scripts 2022 | Scripts used to analyze the output data from Jani Strömberg, Mona Kurppa

Jani Strömberg et al.


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Short summary
In this study we concluded that with low wind speeds, solar radiation has a larger decreasing effect (53 %) on pollutant concentrations than aerosol processes (18 %). Additionally, our results showed that with solar radiation included, pollutant concentrations were closer to observations (-13 %) than with only aerosol processes (+97 %). This has implications when planning simulations under calm conditions such as our case and deciding whether or not simulations need to include these processes.