The Role Of H2SO4-NH3 Anion Clusters In Ion-induced Aerosol ... - ACP

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).

Although 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.

In the aforementioned chamber studies, ions have been shown to play a crucial role in enhancing new particle formation, which is known as ion-induced nucleation (IIN). The importance of IIN varies significantly depending on the temperature as well as the concentration and composition of the ion species. For instance, big H2SO4 ion clusters were not found in the sulfur-rich air mass from Atlanta, suggesting the minor role of IIN (Eisele et al., 2006). Similar conclusions were drawn based on the observations in Boulder (Iida et al., 2006) and Hyytiälä (e.g., Manninen et al., 2010), although the suggested importance of IIN in cold environments, such as upper troposphere, cannot be excluded (Lovejoy et al., 2004; Kürten et al., 2016). Recently, the CLOUD experiments have revealed that the importance of IIN can be negligible in the H2SO4-dimethylamine system (Almeida et al., 2013), moderate in the H2SO4-NH3 system (Kirkby et al., 2011), and dominating in the pure HOMs system (Kirkby et al., 2016). However, it is also important to note that the ion-pair concentration in Hyytiälä is lower than in the CLOUD chamber, which partly explains its smaller contribution of IIN (Wagner et al., 2017).

The recently developed atmospheric-pressure interface time-of-flight mass spectrometer (APi-TOF) (Junninen et al., 2010) has been used for measuring ion composition at the SMEAR II station in Hyytiälä since 2009. Ehn et al. (2010) first showed that the negative ion population varied significantly, with H2SO4 clusters dominating during the day and HOM-NO3- clusters doing so during the night. This variation was further studied by Bianchi et al. (2017), who grouped HOM-containing ions by separating the HOMs into non-nitrate- and nitrate-containing species as well as into ion adducts with HSO4- or NO3-. At nighttime, HOMs may form negatively charged clusters containing up to 40 carbons (Bianchi et al., 2017; Frege et al., 2018). In the daytime, H2SO4 and H2SO4-NH3 clusters appear to be the most prominent negative ions (Schobesberger et al., 2015, 2013). However, they have not yet been thoroughly studied regarding their appearance and their plausible links to atmospheric IIN.

Along with the changes in temperature and in ion concentration and composition, the importance of IIN is expected to vary considerably. In this study, we revisit the ion measurement in Hyytiälä, aiming to connect our current understanding of the formation of ion clusters to the significance of IIN, with a special focus on the fate of H2SO4-NH3 clusters. We also extend our analysis to ions other than H2SO4 clusters, i.e., HOMs, and identify their role in IIN, in addition to other measured parameters on site. Finally, this study confirms the consistency between chamber findings and atmospheric observations, even though it seems that at least two separate mechanisms alternately control the IIN in Hyytiälä.