Abstract
Abstract. New particle formation (NPF) provides a large source of atmospheric aerosols, which affect the climate and human health. In recent chamber studies, ion-induced nucleation (IIN) has been discovered as an important pathway of forming particles; however, atmospheric investigation remains incomplete. For this study, we investigated the air anion compositions in the boreal forest in southern Finland for three consecutive springs, with a special focus on H2SO4-NH3 anion clusters. We found that the ratio between the concentrations of highly oxygenated organic molecules (HOMs) and H2SO4 controlled the appearance of H2SO4-NH3 clusters (3<no.S<13): all such clusters were observed when [HOM] ∕ [H2SO4] was smaller than 30. The number of H2SO4 molecules in the largest observable cluster correlated with the probability of ion-induced nucleation (IIN) occurrence, which reached almost 100 % when the largest observable cluster contained six or more H2SO4 molecules. During selected cases when the time evolution of H2SO4-NH3 clusters could be tracked, the calculated ion growth rates exhibited good agreement across measurement methods and cluster (particle) sizes. In these cases, H2SO4-NH3 clusters alone could explain ion growth up to 3 nm (mobility diameter). IIN events also occurred in the absence of H2SO4-NH3, implying that other NPF mechanisms also prevail at this site, most likely involving HOMs. It seems that H2SO4 and HOMs both affect the occurrence of an IIN event, but their ratio ([HOMs] ∕ [H2SO4]) defines the primary mechanism of the event. Since that ratio is strongly influenced by solar radiation and temperature, the IIN mechanism ought to vary depending on conditions and seasons.
Highlights
Atmospheric aerosol particles are known to influence human health and the climate (Heal et al, 2012; Stocker et al, 2013)
We found that the ratio between the concentrations of highly oxygenated organic molecules (HOMs) and H2SO4 controlled the appearance of H2SO4NH3 clusters (3 < no.S < 13): all such clusters were observed when [HOM] / [H2SO4] was smaller than 30
Consistent with the findings by previous studies showing that H2SO4 clusters are the most abundant ions in the daytime (Ehn et al, 2010; Bianchi et al, 2017), we found that NH3-free H2SO4 clusters can contain up to three H2SO4 molecules when counting the HSO−4 as one H2SO4 molecule ((H2SO4)2HSO−4 ) and that NH3 is always present in clusters containing four or more H2SO4 molecules
Summary
Atmospheric aerosol particles are known to influence human health and the climate (Heal et al, 2012; Stocker et al, 2013). New particle formation (NPF) from gas-phase precursors contributes to a major fraction of the global cloud condensation nuclei population (Merikanto et al, 2009; Kerminen et al, 2012; Dunne et al, 2016; Gordon et al, 2017) and provides an important source of particulate air pollutants in many urban environments (Guo et al, 2014). NPF is an abundant phenomenon and has been observed in different places around the globe within the boundary layer (Kulmala et al, 2004), the detailed mechanisms at each location may differ and are still largely unknown. Experiments done in the CLOUD chamber (Cosmics Leaving Outside Droplets) at CERN explored different NPF mechanisms on a molecular level, including sulfuric acid (H2SO4) and ammonia (NH3) nucleation (Kirkby et al, 2011), H2SO4 and dimethylamine) nucleation (Almeida et al, 2013), and pure biogenic nucleation (Kirkby et al, 2016) from highly oxygenated organic molecules (HOMs) (Ehn et al, 2014). While chamber experiments can mimic some properties of ambient observations (Schobesberger et al, 2013), it is still unclear to what extent these chamber findings can be applied to understand NPF in the more complex atmosphere, mostly due to the challenges in atmospheric measurements and characterization of the nucleating species
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