Size-resolved aerosol fluxes above a broadleaved deciduous forest

In order to understand the aerosol exchange dynamics between the atmosphere and a peri-urban forest ecosystem located in the Po Valley, a region characterized by high PM concentrations, Eddy Covariance (EC) aerosol fluxes were measured between September and December 2017. The aerosol sampling, performed with an Electrical Low Pressure Impactor (ELPI +, DEKATI), involved a wide range of particle sizes including both ultrafine and fine aerosol. The monitoring campaign comprised a period with leaves (PL) and a period without leaves (PNL) to assess their influence on the emission and deposition fluxes. The diurnal profiles of PN (Particle Number) fluxes associated to the geometric mean diameters (GMD) of 0.02 µm and 0.48 µm were chosen as representative of the behaviour of ultrafine and fine particles, respectively. Fluxes of ultrafine particles showed a net emission pattern both in PL and PNL assuming values up to 5.6 ·106 m-2 s-1 and 4.5·106 m-2 s-1, respectively. Instead, fine particles fluxes showed a net deposition pattern in PL, assuming values up to -1.1 ·106 m-2 s-1, while in PNL a slight emission up to 4.1 ·105 m-2 s-1 occurred. The behaviour of the fluxes of the cumulative classes PM0.1 and PM1 was similar to the one of the PN fluxes of ultrafine and fine aerosol, respectively. Deposition velocities were calculated for PL and PNL depending on the atmospheric stability class. The values emerged from this study (from -0.25 cm s-1 up to 0.12 cm s-1) evidenced that under stable and very stable atmospheric conditions all size classes presented negative or slightly positive deposition velocities both in PL and PNL. Instead under unstable conditions fine particles showed deposition velocities whose direction changed in the two periods (PL and PNL).