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

Abstract

Abstract. Vertical aerosol profiles were directly measured over the city of Milan during three years (2005)(2006)(2007)(2008) of field campaigns. An optical particle counter, a portable meteorological station and a miniaturized cascade impactor were deployed on a tethered balloon. More than 300 vertical profiles were measured, both in winter and summer, mainly in conditions of clear, dry skies.The mixing height was determined from the observed vertical aerosol concentration gradient, and from potential temperature and relative humidity profiles. Results show that inter-consistent mixing heights can be retrieved highlighting good correlations between particle dispersion in the atmosphere and meteorological parameters. Mixing height growth speed was calculated for both winter and summer showing the low potential atmospheric dispersion in winter.Aerosol number size distribution and chemical composition profiles allowed us to investigate particle behaviour along height. Aerosol measurements showed changes in size distribution according to mixing height. Coarse particle profiles (d p >1.6 µm) were distributed differently than the fine ones (d p <1.6 µm) were, at different heights of the mixing layer. The sedimentation process influenced the coarse particle profiles, and led to a reduction in mean particle diameter for those particles observed by comparing data above the mixing height with ground data (−14.9±0.6% in winter and −10.7±1.0% in summer). Conversely, the mean particle diameter of fine particles increased above the mixing height under stable atmospheric conditions; the average inCorrespondence to: L. Ferrero (luca.ferrero@unimib.it) crease, 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. A hierarchical statistical model was created to describe the changes in the size distribution of fine particles along height. The proposed model can be used to estimate the typical vertical profile characterising launches within prespecified groups starting from: aerosol size and meteorological conditions measured at ground-level, and a mixing height estimation. The average increase of fine particle diameter, estimated on the basis of the model, was +1.9±0.5% in winter and +6.1±1.2% in summer, in keeping with experimental findings.

Estimation of atmospheric mixing layer height from radiosonde data

“…Although some studies have suggested that coherence exists among the various methods, most of the research was conducted in clear sky conditions (Kim et al, 2007;Ferrero et al, 2011Ferrero et al, , 2010McGrath-Spangler and Denning, 2013). We suggest that the presence of clouds is an important factor that reduces the agreement of different methods.…”

“…Water vapour acts as a tracer of the atmospheric dispersion state which contains and integrates the effects of physical atmospheric forces (both thermal and mechanical) on substance dispersion (Ferrero et al, 2010). The h can be estimated as the level of the minimum vertical gradient of relative humidity (h RH ) or specific humidity (h q ) with a significant reduction in atmospheric moisture (Ao et al, 2008;Seidel et al, 2010).…”

“…However, broad application of the remote sensing data has been hindered by the sharp extinction of the remote sensing signal in high humidity conditions (e.g. cloud or fog) (Ferrero et al, 2010;McGrath-Spangler and Denning, 2012). Many methods have been proposed to estimate h from the temperature and atmospheric composition profiles (Seibert et al, 2000;Basha and Ratnam, 2009;Seidel et al, 2010;Ferrero et al, 2011;Seidel et al, 2012); however, inconsistencies exist among the methods (Seidel et al, 2010;von Engeln and Teixeira, 2013).…”

Fast particulate nitrate formation via N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; uptake aloft in winter in Beijing

“…The pNO − 3 formation via N 2 O 5 uptake contributes to the gradients in the compound percentage and size distribution of the particle (Ferrero et al, 2010(Ferrero et al, , 2012. On nights when NO 3 cannot accumulate in the surface layer owing to high NO emissions, N 2 O 5 uptake can still be active aloft without NO titration.…”

Vertical distributions of aerosol optical properties during the spring 2016 ARIAs airborne campaign in the North China Plain

“…Given the potential impact of aerosols on climate, additional accurate measurements of the vertical profiles of aerosol optical properties are needed. These data can be obtained directly or indirectly from platforms such as meteorological towers (Zhao et al, 2017), tethered balloons (Stratmann et al, 2003;Ferrero et al, 2010), and unmanned aerial vehicles (Corrigan et al, 2008). Although limited in terms of temporal and geographic coverage, airborne sensing provides direct, highresolution, in situ aerosol vertical profiles, used to evaluate numerical models and satellite retrievals (Chazette and Liousse, 2001).…”