The Near-Surface Vertical Variability of Aerosol Single-Scattering Properties over Warsaw: Case Study

Abstract Introduction This study investigates the vertical variability of aerosol optical properties within the surface layer over Warsaw, Poland, using low-cost aerosol sensors SPS30 mounted on the Palace of Culture and Science skyscraper at five different heights (3, 19, 57, 109, and 166 m a.g.l.). Data was collected between March and September 2023. Methods SPS30 sensors were and calibrated against an Aurora 4000 nephelometer to provide aerosol scattering coefficient (ASC). Results ASC vertical gradient was estimated to − 3.0 ± 8.2 Mm −1 /100 m, indicating a slight decrease in ASC with altitude. During night time, the mean ASC gradient was − 6.1 Mm −1 /100 m, while during solar noon it is only − 0.1 Mm −1 /100 m. The diurnal ASC cycle exhibited a later morning maximum at higher altitudes, corresponding to the disappearance of surface inversion and the initiation of vertical mixing in the near-ground layer. This gradient was found to be significantly influenced by temperature inversion, with a Pearson correlation coefficient of − 0.55 between temperature lapse rate and ASC gradient. The mean scattering aerosol optical depth (sAOD) at 525 nm was 0.007 ± 0.004, representing a 3–4% contribution to the total sAOD. The research also highlights the impact of weather conditions on the ASC profile, with diurnal variations in ASC being more closely linked to vertical mixing processes (sensible heat flux, turbulence kinetic energy) than to changes in traffic emissions. Notably, under strong surface inversion conditions (lapse rate > 8 °C/100 m), the ASC between edge stations (163 m of vertical difference) variates by 30%. The study also demonstrated the sensitivity of the low-cost SPS30 sensor to cloud droplets, emphasizing the need to filter data based on typical particle size below 0.65 μm to exclude low-cloud and fog conditions. Conclusion Overall, this research demonstrates the effectiveness of using low-cost sensors to measure vertical profiles of aerosol optical properties in urban environments, offering valuable insights into aerosol behaviour and atmospheric conditions that can complement Lidar observations (in overlap region) and enhance aerosol transport models. The findings emphasize the significant role of temperature inversion in shaping aerosol vertical variability and highlight the potential of this methodology for improving our understanding of aerosol processes in the surface layer. Graphical Abstract