Relationship between the planetary boundary layer height and the particle scattering coefficient at the surface

The relationship between the Planetary Boundary Layer (PBL) height and the particle scattering coefficient (σp) at the surface has been investigated with the main goal of estimating the PBL height from the ground-level particle optical properties that are simpler to measure and are provided by instruments as nephelometers, which can run continuously. A lidar system and an integrating nephelometer operating within the European infrastructure ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure) have been used to simultaneously monitor the daily evolution of both the PBL height and σp. Measurements have been performed at a coastal site of south-eastern Italy, characterized by a shallow PBL (<1000 m), during a two-year period. The standard deviation technique has been applied to lidar signals to determine the daily evolution of the PBL height, being this technique independent on the lidar overlap function. The maximum value of the PBL height hourly mean was reached around midday and was equal to 470 ± 160 m in spring-summer (SS) and 580 ± 170 m in autumn-winter (AW). A statistically significant inverse correlation between the PBL height and σp was found both in AW and in SS, since σp decreased with the increase of the PBL height, because of the increase of the ground-level particles' vertical dispersion. The retrieved relationships between the PBL height and σp have been used to estimate the daily evolution of the PBL height from σp values both in SS and in AW. We found a satisfactory accordance, within experimental uncertainties, between estimated and experimentally determined PBL heights. Therefore, a new experimental methodology to estimate the PBL height from ground-based nephelometer measurements has been suggested in the paper. The analysis of the scattering Ångström exponent has revealed that in AW the mean size of the particles at the surface on average increased during the central hours of the day, since the PBL height increase likely favoured the vertical dispersion of fine particles more than the coarse ones. The comparison between the lidar-derived PBL heights and the corresponding ones calculated by the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model has revealed that the HYSPLIT PBL height seasonal and daily trends were similar to the corresponding ones retrieved from lidar measurements. Nevertheless, the HYSPLIT model on average overestimated by 40% and underestimated by 20% the experimentally determined PBL height in autumn-winter and in spring-summer, respectively.