Study of new particle formation events in southern Italy
Graphical abstract
Introduction
The interest on atmospheric new particle formation (NPF) has increased enormously in recent years because of the high importance that ultrafine particles (with <100 nm in diameter) play on the climate system, air quality and human health (Donaldson et al., 1998, 2002; Sioutas et al., 2005). Most ultrafine particles originate from anthropogenic activity such as industries, road traffic, aircraft, ships, and many other combustion processes; however, they are also produced in the atmosphere as secondary particles by gas-to-particle conversion. Anthropogenic precursor vapors, after oxidation in the atmosphere, can form small particles or clusters approximately 1 nm in diameter, which may then grow rapidly to larger sizes in the range of 10–50 nm (Berland et al., 2017; Kulmala et al., 2013). It is estimated that NPF is an important source of ultrafine particles (Kulmala et al., 2004), which provides more than half of the global cloud condensation nuclei (Gordon et al., 2017).
Atmospheric new particle formation is a global phenomenon, largely observed and documented in a wide variety of environments and climatic conditions (Birmili et al., 2003; Cusack et al., 2013; Hamburger et al., 2011; Manninen et al., 2010; Wiedensohler et al., 1996). Low concentrations of preexisting aerosol particles, high concentrations of gaseous precursors (e.g., SO2 and VOC) together with intense solar radiation, may all create the suitable conditions to trigger the formation process and the growth of new particles (Donahue et al., 2011; Hamburger et al., 2011; Kulmala et al., 2005; Pierce et al., 2011, 2012; Sihto et al., 2006). Along with these, also atmospheric ions could play a determining role (Kamra et al., 2015; Siingh et al., 2011; Yu and Turco, 2011), in particular during the initial step of process, by enhancing the stability of newly formed molecular clusters (Yu and Turco, 2001) and reducing their evaporation rates (Wagner et al., 2017), but their importance is still under investigation (Kirkby et al., 2016; Wang et al., 2017). A number of NPF events were observed even under different situations such as under cloudy conditions (Altstadter et al., 2018; Wiedensohler et al., 1997), polluted environments with high condensation sink values (Groβ et al., 2018; Wang et al., 2017) or during the nighttime (Kecorius et al., 2015; Lee et al., 2008; Ortega et al., 2012). The CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at CERN (European Council for Nuclear Research) is investigating the nucleation and growth of aerosol particles under controlled experimental parameters, simulating different atmospheric conditions. Although numerous results were obtained, many questions remain open and it is still not clear if the atmospheric nucleation process is dominated by one or multiple different factors (Dall’Osto et al., 2018). In this sense, the statistical analysis of time series that span long time periods (Heintzenberg et al., 2007) and in different environmental conditions could be a useful approach. Motivated by these reasons, the aim of this work is to identify and analyze favorable conditions that triggered new particle formation in an urban background area in Southern Italy where, a comprehensive long-term study of NPF was not done before. The study is focused on a flat area in Southeastern Italy recognized as hot-spot region both in terms of climate change (e.g. Giorgi and Lionello, 2008) and air-quality (Monks et al., 2009) being a crossroad where pollutants from different sources (natural and anthropogenic) converge (Dinoi et al., 2010, 2017; Perrone et al., 2011). Climatic features such as frequent conditions of clear skies during the whole year and intense solar radiation in spring-summer seasons make this site particularly interesting for the purpose of this study and, as a consequence, representative of many coastal areas of the central Mediterranean.
In this work, a comprehensive data set of one and half years of number particle size distributions was analyzed, including simultaneous measurements of the concentrations of gaseous pollutants (SO2, NO, NO2, and O3) in order to identify and characterize NPF events. Meteorological parameters together with back-trajectories analysis were additionally used to interpret the impact of meteorology and of the origin of air masses on the frequency and intensity of observed nucleation events.
Section snippets
Site description
All data evaluated in this work were collected at the Environmental-Climate Observatory of Lecce, ECO, regional station of GAW/ACTRIS Network (Global Atmosphere Watch Programme/Aerosol, Clouds and Trace Gases Research Infrastructure). The observatory (Fig. 1) is located on the roof of the ISAC-CNR Institute, inside the University Campus (40° 20′ 8″ N, 18° 07′ 28″ E) at about 13 m above the ground level and at about 5 km SW of the town of Lecce, a small-size town with a population of 95,000
Classification and frequency of events
Measurements of PNSD (mobility diameters range 10–800 nm) were considered from January 2015 to July 2016. Event classification was based on the methods described in Dal Maso et al. (2005) by visual inspection of the PNSD contour plots, with a day-by-day approach. A day was identified as “event” when there was a significant increase in number concentrations of ultrafine particles that gradually grew towards larger diameters for at least 4 h continuously, displaying the typical “banana” shape.
Conclusions
Frequent new particle formation (NPF) events were identified and studied in the urban background of Lecce at the ECO observatory, from January 2015 to July 2016. Throughout the observed period, NPF events occurred in about 21% of the days. The classification of events showed a seasonal trend with higher frequencies in spring and summer months and lower frequencies during the winter.
New particle formation events have always occurred under synoptic conditions associated with high surface pressure
Funding
This work was supported by the project I-AMICA (Infrastructure of High Technology for Environmental and Climate Monitoring - PONa3_00363), a project of Structural improvement financed under the National Operational Program (NOP) for “Research and Competitiveness 2007–2013″ co-funded with European Regional Development Fund (ERDF) and National resources.
CRediT authorship contribution statement
A. Dinoi: Conceptualization, Methodology, Data curation, Writing - original draft. K. Weinhold: Software, Validation, Data curation. A. Wiedensohler: Data curation, Writing - review & editing. D. Contini: Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Author Dinoi acknowledges the “short-term mobility” program of the CNR that funded her visit to the Leibniz Institute for Tropospheric Research (TROPOS). She would also like to express her sincere gratitude to all TROPOS staff and in particular to Dr. Birmili whose support made this work possible.
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