Study of new particle formation events in southern Italy

Highlights

• New particle formation shows a trend with higher frequencies in warm seasons.

• All events occurred with no cloud cover, high pressure, and dry air (RH<55%).

• NO, NO₂, and O₃ did not play an evident role in the formation process.

• SO₂ concentrations were on average 40% higher during days with nucleation events.

• Event days were associated to air masses circulating from inland (N-NW).

Abstract

New particle formation (NPF) events were studied in the urban background of Lecce (Italy) at the ECO Environmental-Climate Observatory, GAW/ACTRIS regional station. The objective of this study was to obtain long-term information on nucleation phenomenon in the Salentum peninsula, a coastal area of the central Mediterranean basin for which no extensive data exists. Particle number size distributions of the atmospheric aerosol were continuously collected by a Mobility Particle Size Spectrometer (MPSS) in the size range of 10–800 nm. For this investigation, data was analyzed for the period from January 2015 to July 2016 together with simultaneous measurements of gaseous pollutants (SO₂, NO, NO₂, and O₃) and meteorological parameters. Throughout the observation period, nucleation events have been identified in approximately 21% of days. The classification of the events showed a seasonal trend with higher frequencies in warm seasons with a maximum in September (38%) while the highest non-event days fraction was observed in cold seasons, ~90% in November and December. The fraction of undefined days peaked in August and September, 15 and 19% respectively. The analysis of reactive gas showed that NO, NO₂, and O₃ did not play an evident role in the formation process, as observed by the comparison between event and non-event monthly median concentrations. Nucleation phenomenon seemed instead to be very sensitive to SO₂ that was present with higher (about 40%) monthly median concentrations during event days compared to non-event days. The condensation sink (CS), calculated for the complete observational period, showed similar values comparing event and non-event days. It is noteworthy that all the events occurred during the central hours of the day, high pressure, dry air (RH<55%) and without significant cloud cover. The event days were characterized by winds originating predominately from the north and northwest directions, as shown by the analysis of the back-trajectories.

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.