Abstract
Binary nucleation of sulphuric acid‐water particles is expected to be an important process in the free troposphere at low temperatures. SAWNUC (Sulphuric Acid Water Nucleation) is a model of binary nucleation that is based on laboratory measurements of the binding energies of sulphuric acid and water in charged and neutral clusters. Predictions of SAWNUC are compared for the first time comprehensively with experimental binary nucleation data from the CLOUD chamber at European Organization for Nuclear Research. The experimental measurements span a temperature range of 208–292 K, sulphuric acid concentrations from 1·106 to 1·109 cm−3, and distinguish between ion‐induced and neutral nucleation. Good agreement, within a factor of 5, is found between the experimental and modeled formation rates for ion‐induced nucleation at 278 K and below and for neutral nucleation at 208 and 223 K. Differences at warm temperatures are attributed to ammonia contamination which was indicated by the presence of ammonia‐sulphuric acid clusters, detected by an Atmospheric Pressure Interface Time of Flight (APi‐TOF) mass spectrometer. APi‐TOF measurements of the sulphuric acid ion cluster distributions ( (H2SO4)i·HSO4− with i = 0, 1, ..., 10) show qualitative agreement with the SAWNUC ion cluster distributions. Remaining differences between the measured and modeled distributions are most likely due to fragmentation in the APi‐TOF. The CLOUD results are in good agreement with previously measured cluster binding energies and show the SAWNUC model to be a good representation of ion‐induced and neutral binary nucleation of sulphuric acid‐water clusters in the middle and upper troposphere.
Highlights
New particle formation is an important source of cloud condensation nuclei [Kerminen et al, 2005; Laaksonen et al, 2005; Kuang et al, 2009]
Comparison With CLOUD Particle Formation Rates For the binary experiments during CLOUD 3, a data set of particle formation rates at 1.7 nm exists [Kirkby et al, 2011]
These data sets contain steady state particle formation rates that were corrected for losses [Kürten et al, 2015, 2016]
Summary
New particle formation is an important source of cloud condensation nuclei [Kerminen et al, 2005; Laaksonen et al, 2005; Kuang et al, 2009]. The main compound responsible for new particle formation in the upper troposphere is considered to be sulphuric acid [Weber et al, 1999]. A large number of experimental studies of binary homogenous nucleation of sulphuric acid and water have been made, mostly limited to room temperature [see, for example, Raes and Janssens, 1986; Wyslouzil et al, 1991; Ball et al, 1999; Young et al, 2008; Zollner et al, 2012]. Ball et al [1999] showed that small amounts of ternary vapor, in this case ammonia, can strongly enhance the observed nucleation rates.
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