Size resolved aerosol fluxes above a deciduous forest: seasonal variability and underlying processes

Atmospheric aerosol is a global concern for its detrimental effects on human health and climate. Vegetation has been identified as a possible sink for atmospheric particles. However, the interactions between vegetation and aerosols are not completely understood, and they are not limited to deposition only since also bidirectional aerosol exchanges can appear. Moreover, the knowledge of aerosol-vegetation interactions is quite limited for deciduous forests, as well as its seasonality which follows the appearance and disappearance of leaves on the crowns. Much of research, in fact, has been focused on conifers which have almost constant leaf area during the year.

For this sake, eddy covariance measurements of size-resolved aerosol fluxes were conducted in a broadleaf deciduous forest in the Po Valley, by means of a fast electric low-pressure impactor able to resolve 14 dimensional classes with cut-off diameters ranging from 6 nm to 10 µm. The measurements spanned approximately eight months and covered both a leaf-off period and a leaf on period, from leaf sprout to leaf senescence.

Overall, aerosol fluxes exhibited a distinct seasonality and variations in exchange direction depending on particle diameter. During the winter, when leaves were absent, most aerosol classes exhibited positive upward fluxes. In contrast, during the leaf-on season, ultrafine (dp<100 nm) and fine aerosols (100 nm<dp<1 µm) displayed contrasting exchange patterns, with fine aerosol being predominantly deposited and ultrafine aerosol primarily emitted.

The vertical exchange of aerosols was found to be dependent on leaf area index, friction velocity and surface wetness. Vertical fluxes increased as LAI increased, both for fine aerosols -which were deposited- and for the ultrafine ones, which were emitted.

In dry conditions and in presence of leaves, at moderate friction velocities (u*<0.75 m/s) aerosol deposition increased as u* increased. However, above a u* value of 0.75 m/s upward fluxes appeared, and they increased as friction velocity increased becoming rapidly dominant. In wet conditions, leaf wetness did not enhance aerosol deposition and the latter started only when friction velocity increased above a certain threshold (0.4 m/s). No upward aerosol fluxes were observed in wet conditions.

An attempt to explain the observed deposition and resuspension processes will be provided.