Year-long variability of the mixing layer height at an urban Mediterranean location and association with air pollution levels

Abstract

This study examines the planetary boundary layer characteristics and the association with atmospheric pollutants in Athens, aiming to assess the effects of boundary-layer dynamics on pollution levels. Ceilometer (CL31) profiles of backscatter coefficient (BSC) were used to compute the mixing layer height (MLH) based on the gradient method under cloudless conditions, revealing higher values in summer (mean: 955 m) and lower in winter (mean: 556 m). The annual mean MLH displayed a pronounced diurnal pattern depending on season with a mean value of 902 ± 337 m at noon, decreased to 525 ± 336 m at midnight. The MLH maximized at 15:00 (UTC+2) (mean: 1293 ± 337 m) closely related to surface heating and turbulent mixing conditions. MLH variations are interrelated with the local wind patterns, with stronger winds mostly from northeast directions during May–September, facilitating higher MLH and dispersion of pollutants. PM_2.5, Black Carbon (BC) and NO_x concentrations were strongly linked to variations of MLH, exhibiting negative correlations with it, while O₃ exhibited a similar diurnal pattern with MLH (maximizing during early afternoon) due to its photochemical production and possible intrusion from upper atmospheric levels. Apart from changes in the emission sources due to domestic heating during wintertime, the shallow MLH along with stable atmospheric conditions, further exacerbate the accumulation of pollutants near the surface, with emphasis on BC. Two case studies regarding enhanced BC levels due to residential wood burning and transported smoke plumes were analyzed to assess the impacts of MLH variations on pollutant concentrations.