Vertically-resolved particle size distribution within and above the mixing layer over the Milan metropolitan area

摘要

Vertical aerosol profiles were directly measured over the city of Milanduring three years (2005–2008) of field campaigns. An optical particlecounter, a portable meteorological station and a miniaturized cascadeimpactor were deployed on a tethered balloon. More than 300 verticalprofiles were measured, both in winter and summer, mainly in conditions ofclear, dry skies.The mixing height was determined from the observed vertical aerosolconcentration gradient, and from potential temperature and relative humidityprofiles. Results show that inter-consistent mixing heights can be retrievedhighlighting good correlations between particle dispersion in the atmosphereand meteorological parameters. Mixing height growth speed was calculated forboth winter and summer showing the low potential atmospheric dispersion inwinter.Aerosol number size distribution and chemical composition profiles allowedus to investigate particle behaviour along height. Aerosol measurementsshowed changes in size distribution according to mixing height. Coarseparticle profiles (d_pp<1.6 μm) were, at different heights of themixing layer. The sedimentation process influenced the coarse particleprofiles, and led to a reduction in mean particle diameter for thoseparticles observed by comparing data above the mixing height with grounddata (−14.9±0.6% in winter and −10.7±1.0% in summer).Conversely, the mean particle diameter of fine particles increased above themixing height under stable atmospheric conditions; the average increase,observed by comparing data above the mixing height with ground data, was+2.1±0.1% in winter and +3.9±0.3% in summer. Ahierarchical statistical model was created to describe the changes in thesize distribution of fine particles along height. The proposed model can beused to estimate the typical vertical profile characterising launches withinpre-specified groups starting from: aerosol size and meteorologicalconditions measured at ground-level, and a mixing height estimation. Theaverage increase of fine particle diameter, estimated on the basis of themodel, was +1.9±0.5% in winter and +6.1±1.2% in summer,in keeping with experimental findings.