Abstract
Abstract. Studying the vertical distribution of aerosol particle physical and chemical properties in the troposphere is essential to understand the relative importance of local emission processes vs. long-range transport for column-integrated aerosol properties (e.g. the aerosol optical depth, AOD, affecting regional climate) as well as for the aerosol burden and its impacts on air quality at the ground. The main objective of this paper is to investigate the transport of desert dust in the middle troposphere and its intrusion into the planetary boundary layer (PBL) over the Po Valley (Italy), a region considered one of the greatest European pollution hotspots for the frequency that particulate matter (PM) limit values are exceeded. Events of mineral aerosol uplift from local (soil) sources and phenomena of hygroscopic growth at the ground are also investigated, possibly affecting the PM concentration in the region as well. During the PEGASOS 2012 field campaign, an integrated observing–modelling system was set up based on near-surface measurements (particle concentration and chemistry), vertical profiling (backscatter coefficient profiles from lidar and radiosoundings) and Lagrangian air mass transport simulations by FLEXPART model. Measurements were taken at the San Pietro Capofiume supersite (44°39′ N, 11°37′ E; 11 m a.s.l.), located in a rural area relatively close to some major urban and industrial emissive areas in the Po Valley. Mt. Cimone (44°12′ N, 10°42′ E; 2165 m a.s.l.) WMO/GAW station observations are also included in the study to characterize regional-scale variability. Results show that, in the Po Valley, aerosol is detected mainly below 2000 m a.s.l. with a prevalent occurrence of non-depolarizing particles ( > 50 % throughout the campaign) and a vertical distribution modulated by the PBL daily evolution. Two intense events of mineral dust transport from northern Africa (19–21 and 29 June to 2 July) are observed, with layers advected mainly above 2000 m, but subsequently sinking and mixing in the PBL. As a consequence, a non-negligible occurrence of mineral dust is observed close to the ground ( ∼ 7 % of occurrence during a 1-month campaign). The observations unambiguously show Saharan dust layers intruding the Po Valley mixing layer and directly affecting the aerosol concentrations near the surface. Finally, lidar observations also indicate strong variability in aerosol on shorter timescales (hourly). Firstly, these highlight events of hygroscopic growth of anthropogenic aerosol, visible as shallow layers of low depolarization near the ground. Such events are identified during early morning hours at high relative humidity (RH) conditions (RH > 80 %). The process is observed concurrently with high PM1 nitrate concentration (up to 15 µg cm−3) and hence mainly explicable by deliquescence of fine anthropogenic particles, and during mineral dust intrusion episodes, when water condensation on dust particles could instead represent the dominant contribution. Secondly, lidar images show frequent events (mean daily occurrence of ∼ 22 % during the whole campaign) of rapid uplift of mineral depolarizing particles in afternoon–evening hours up to 2000 m a.s.l. height. The origin of such particles cannot be directly related to long-range transport events, being instead likely linked to processes of soil particle resuspension from agricultural lands.
Highlights
The Po River basin in northern Italy is one of the most important emissive areas in Europe, characterized by high concentration of both natural and anthropogenic aerosol and trace gases (Monks et al, 2009)
Nondepolarizing aerosol (Fig. 7c) is dominant throughout the campaign; this class of aerosol appears to be mainly confined below the planetary boundary layer (PBL) top
The presented paper provided a characterization of the effects of meteorological evolution and transport patterns on the aerosol variability, based on the observations collected during two major field campaigns (PEGASOS and SuperSito) in the eastern part of the Po Valley
Summary
The Po River basin in northern Italy is one of the most important emissive areas in Europe, characterized by high concentration of both natural and anthropogenic aerosol and trace gases (Monks et al, 2009). The relative proximity to the Sahara, which represents the major mineral dust source of the planet (Prospero et al, 2002; Washington et al, 2003), makes this region subject to long-range mineral dust transport, especially during the summer season (Kalivitis et al, 2007; Marinoni et al, 2008; Pederzoli et al, 2010; Carnevale et al, 2012) Such combination leads to unusually high concentrations of atmospheric pollutants and particulate matter, with frequent and prolonged periods of intense pollution. During summer 2012, under similar meteorological features (anticyclonic conditions and PBL air recirculation), Sandrini et al (2016) observed a significant enhancement of secondary organic and inorganic aerosol particle mass They pointed out differences in aerosol behaviour in rural and urban areas: rural areas, during night, were characterized by higher relative humidity and lower temperature compared to the urban areas, and showed higher fine nitrate nocturnal concentration and formation of ammonium nitrate in the large accumulation mode (0.42–1.2 μm). From here on, by “aerosol” we will refer to just the aerosol particle phase, excluding the carrier gas
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
