Thermodynamics of the formation of sulfuric acid dimers in the binary (H₂SO₄–H₂O) and ternary (H₂SO₄–H₂O–NH₃) system

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.