On the interplay between upper and ground levels dynamics and chemistry in determining the surface aerosol budget

Abstract

We use the WRF/Chem model to interpret observations of the aerosol concentration and its chemical composition both at surface level and along vertical profiles performed during an intensive campaign in July 2007 in Milan urban area. The model is added with a new diagnostic for aerosol budget analysis, building on that available for gas species, in order to study the contribution of upper levels processes on the aerosol formation at ground level. The analysis illustrates a quite variegated evolution of budget terms, which we found to depend strongly on the hour of the day, the vertical level, the aerosol compound, and the aerosol size. Primary components are generally emitted near the ground and rapidly transported by turbulent motions to the upper levels, where they gradually disperse and age. For some secondary components, such as nitrate, we calculate a net chemical destruction in the bottom layers, as opposed to a net chemical production higher in the boundary layer, which supply new material to ground level aerosol through turbulent mixing.