Differential characterization of air ions in boreal forest of Finland and a megacity of eastern China

Abstract. Air ions play a crucial role in new particle formation (NPF), which in turn has the potential to influence global climate and air quality. We conducted a comparative analysis of air ions in three size ranges (0.8–2 nm cluster ions, 2–7 nm intermediate ions, and 7–20 nm large ions) at two flagship sites: the Station for Measuring Ecosystem-Atmosphere Relations II (SMEAR II) site in a boreal forest of Finland and the Station for Observing Regional Processes of the Earth System (SORPES) site in a megacity of eastern China. Air ion number size distributions (0.8–42 nm) were measured using a Neutral cluster and Air Ion Spectrometer (NAIS) at these two sites from June 2019–August 2020. At both sites, rising temperatures reduced the difference between positive and negative cluster ion concentrations, likely due to the enhanced convection and turbulent mixing that diminish the Earth's electrode effect. The median cluster ion concentration at SMEAR II (1270 cm−3) was approximately six times higher than at SORPES (220 cm−3), which was caused by the high coagulation sink in the urban area. The median large-ion concentration at SORPES was nearly three times higher (197 cm−3) than that at SMEAR II (67 cm−3), which is due to the high number density of neutral aerosol particles facilitating ion attachment in the polluted megacity environment. The cluster ion concentration was negatively associated with the condensation sink (CS) at both sites, with a significantly stronger negative correlation at SORPES, suggesting that the CS was a decisive factor for reducing the cluster ion concentrations in this urban area. The median formation rates of 2 nm ions at SMEAR II (J2-: 0.033 cm-3s-1, J2+: 0.041 cm-3s-1) were similar to those at SORPES (J2-: 0.028 cm-3s-1, J2+: 0.025 cm-3s-1). The median ion-induced fractions were 19.9 % and 1.3 % at SMEAR II and SORPES, respectively, indicating a minor contribution of ions to NPF in polluted environments. Nevertheless, the charged particles were activated earlier than neutral particles at SORPES, indicating that the ion-induced nucleation could precede neutral nucleation in this polluted environment. In addition, the contribution of ion-induced nucleation at SORPES was higher at low NPF intensity, implying the non-negligible roles of air ions in urban aerosol production. This study underscores the need for long-term observations of air ions in both pristine boreal forests and polluted urban environments.