Abstract
Abstract. New particle formation (NPF) events occur almost everywhere in the world and can play an important role as a particle source. The frequency and characteristics of NPF events vary spatially, and this variability is yet to be fully understood. In the present study, long-term particle size distribution datasets (minimum of 3 years) from 13 sites of various land uses and climates from across Europe were studied, and NPF events, deriving from secondary formation and not traffic-related nucleation, were extracted and analysed. The frequency of NPF events was consistently found to be higher at rural background sites, while the growth and formation rates of newly formed particles were higher at roadsides (though in many cases differences between the sites were small), underlining the importance of the abundance of condensable compounds of anthropogenic origin found there. The growth rate was higher in summer at all rural background sites studied. The urban background sites presented the highest uncertainty due to greater variability compared to the other two types of site. The origin of incoming air masses and the specific conditions associated with them greatly affect the characteristics of NPF events. In general, cleaner air masses present higher probability for NPF events, while the more polluted ones show higher growth rates. However, different patterns of NPF events were found, even at sites in close proximity (< 200 km), due to the different local conditions at each site. Region-wide events were also studied and were found to be associated with the same conditions as local events, although some variability was found which was associated with the different seasonality of the events at two neighbouring sites. NPF events were responsible for an increase in the number concentration of ultrafine particles of more than 400 % at rural background sites on the day of their occurrence. The degree of enhancement was less at urban sites due to the increased contribution of other sources within the urban environment. It is evident that, while some variables (such as solar radiation intensity, relative humidity, or the concentrations of specific pollutants) appear to have a similar influence on NPF events across all sites, it is impossible to predict the characteristics of NPF events at a site using just these variables, due to the crucial role of local conditions.
Highlights
Ultrafine particles, while not yet regulated, are believed to have adverse effects upon air quality and public health (Atkinson et al, 2010; Politis et al, 2008; Tobías et al, 2018), as well as having a direct or indirect effect on atmospheric properties (Makkonen et al, 2012; Seinfeld and Pandis, 2012)
The frequency of new particle formation (NPF) events was consistently found to be higher at rural background sites, while the growth and formation rates of newly formed particles were higher at roadsides, underlining the importance of the abundance of condensable compounds of anthropogenic origin found there
The annual number of NPF events, growth rate, and formation rate for all the sites is found in Table S6, for which no clear interannual trend is found for any of the sites in this study
Summary
Ultrafine particles (particles with diameter smaller than 100 nm), while not yet regulated, are believed to have adverse effects upon air quality and public health (Atkinson et al, 2010; Politis et al, 2008; Tobías et al, 2018), as well as having a direct or indirect effect on atmospheric properties (Makkonen et al, 2012; Seinfeld and Pandis, 2012). The source of ultrafine particles can either be from primary emissions (Harrison et al, 2000; Masiol et al, 2017), including delayed primary emissions (Hietikko et al, 2018; Olin et al, 2020; Rönkkö et al, 2017), or from secondary formation from gaseous precursors (Brean et al, 2019; Chu et al, 2019; Kerminen et al, 2018; Kulmala et al, 2004a; Yao et al, 2018), which is considered an important source of cloud condensation nuclei (CCN) in the atmosphere (Dameto de España et al, 2017; Kalivitis et al, 2015; Spracklen et al, 2008) For the latter, while the process of formation of initial clusters that subsequently lead to particle formation has been extensively studied (Dal Maso et al, 2002; Kulmala et al, 2014; Riipinen et al, 2007; Weber et al, 1998), there is no consistent explanation of the factors which determine the occurrence and development of new particle formation (NPF) events in the atmosphere. NPF events in different locations do not appear to follow consistent trends with the concentrations of these compounds and meteorological parameters (McFiggans et al, 2019; Minguillón et al, 2015; Riipinen et al, 2007), though links between NPF events and sulfuric acid vapour concentrations (Petäjä et al, 2009; Weber et al, 1995) and organics (Bianchi et al, 2019; Ehn et al, 2014) have been reported
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