Abstract
Abstract. Sulfuric acid is an important gas influencing atmospheric new particle formation (NPF). Both the binary (H2SO4–H2O) system and the ternary system involving ammonia (H2SO4–H2O–NH3) may be important in the free troposphere. An essential step in the nucleation of aerosol particles from gas-phase precursors is the formation of a dimer, so an understanding of the thermodynamics of dimer formation over a wide range of atmospheric conditions is essential to describe NPF. We have used the CLOUD chamber to conduct nucleation experiments for these systems at temperatures from 208 to 248 K. Neutral monomer and dimer concentrations of sulfuric acid were measured using a chemical ionization mass spectrometer (CIMS). From these measurements, dimer evaporation rates in the binary system were derived for temperatures of 208 and 223 K. We compare these results to literature data from a previous study that was conducted at higher temperatures but is in good agreement with the present study. For the ternary system the formation of H2SO4·NH3 is very likely an essential step in the formation of sulfuric acid dimers, which were measured at 210, 223, and 248 K. We estimate the thermodynamic properties (dH and dS) of the H2SO4·NH3 cluster using a simple heuristic model and the measured data. Furthermore, we report the first measurements of large neutral sulfuric acid clusters containing as many as 10 sulfuric acid molecules for the binary system using chemical ionization–atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometry.
Highlights
The formation of new particles from the gas phase is a frequent and important process in the atmosphere
We report the first measurements of large neutral sulfuric acid clusters containing as many as 10 sulfuric acid molecules for the binary system using chemical ionization–atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometry
During the galactic cosmic rays (GCRs) stage of the experiment, the dimer signal gradually increases. This could be due to either neutral dimers being charged in the chemical ionization mass spectrometer (CIMS) or charged dimer ions forming within the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber
Summary
The formation of new particles from the gas phase is a frequent and important process in the atmosphere. Observed atmospheric boundary-layer nucleation rates typically correlate with the concentration of gaseous sulfuric acid (Kulmala et al, 2004; Kuang et al, 2008). Nucleation under typical ground-level conditions cannot be explained by the binary nucleation of sulfuric acid and water vapor (Kulmala et al, 2004; Kerminen et al, 2010), even if the enhancing effect due to ions is taken into account (Kirkby et al, 2011). Recent chamber experiments showed that the observed atmospheric boundary-layer nucleation rates can, in principle, be explained by sulfuric acid acting in combination with either amines or the oxidation products from α-pinene (Almeida et al, 2013; Schobesberger et al, 2013; Riccobono et al, 2014)
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