Total sulphate vs. sulphuric acid monomer in nucleation studies: which represents the "true" concentration?

Abstract. Sulphuric acid is known to be a key component for atmospheric nucleation. Precise determination of sulphuric acid concentration is crucial factor for prediction of nucleation rates and subsequent growth. In our study, we have noticed a substantial discrepancy between sulphuric acid monomer and total sulphate concentrations measured from the same source of sulphuric acid vapour. The discrepancy of about one to two orders of magnitude was found with similar formation rates. The reason for this difference is not yet clear and it can have great impact on predicting atmospheric nucleation rates as well as growth rates. To investigate this discrepancy and its effect on nucleation, a method of thermally controlled saturator filled with pure sulphuric acid (~97%) for production of sulphuric acid vapour is introduced and tested. Sulphuric acid-water nucleation experiment was done using a laminar flow tube. Two independent methods of mass spectrometry and online ion chromatography were used for detecting sulphuric acid concentrations. The results are compared to our previous results, where a method of evaporating weak sulphuric acid-water solution droplets in a furnace was used to produce sulphuric acid vapour (Brus et al., 2010, 2011). Measured sulphuric acid concentrations are compared to theoretical prediction calculated using vapour pressure and simple mixing law. The calculated prediction of sulphuric acid concentrations agrees very well with the measured values when total sulphate is considered. Sulphuric acid monomer concentration was found to be about two orders of magnitude lower than the prediction, but with similar temperature dependency as the prediction and the results obtained with ion chromatograph method. Formation rates agree well when compared to our previous results with both sulphuric acid detection and sulphuric acid production methods separately.