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Sulfuric acid is known to be a key component for atmospheric nucleation. Precise determination of sulfuric-acid concentration is a crucial factor forprediction of nucleation rates and subsequent growth. In our study, we havenoticed a substantial discrepancy between sulfuric-acid monomer concentrations and total-sulfate concentrations measured from the same sourceof sulfuric-acid vapor. The discrepancy of about 1–2 orders of magnitude wasfound with similar particle-formation rates. To investigate this discrepancy,and its effect on nucleation, a method of thermally controlled saturatorfilled with pure sulfuric acid (97% wt.) for production of sulfuric-acid vapor is applied and rigorously tested. The saturator providedan independent vapor-production method, compared to our previous method ofthe furnace (Brus et al., 2010, 2011), to find out if the discrepancy iscaused by the production method itself. The saturator was used in aH2SO4–H2O nucleation experiment, using a laminar flow tube tocheck reproducibility of the nucleation results with the saturator method,compared to the furnace. Two independent methods of mass spectrometry andonline ion chromatography were used for detecting sulfuric-acid or sulfateconcentrations. Measured sulfuric-acid or total-sulfate concentrations arecompared to theoretical predictions calculated using vapor pressure and amixing law. The calculated prediction of sulfuric-acid concentrations agreesvery well with the measured values when total sulfate is considered. Sulfuric-acid monomer concentration was found to be about 2 orders ofmagnitude lower than theoretical predictions, but with a temperaturedependency similar to the predictions and the results obtained with theion-chromatograph method. Formation rates are reproducible when compared toour previous results with both sulfuric-acid or total-sulfate detection andsulfuric-acid production methods separately, removing any doubts that thevapor-production method would cause the discrepancy. Possible reasons for thediscrepancy are discussed and some suggestions include that the missingsulfuric acid is in clusters, formed with contaminants found in mostlaboratory experiments. One-to-two-order-of-magnitude higher sulfuric-acidconcentrations (measured as total sulfate in this study) would contribute toa higher fraction of particle growth rate than assumed from the measurementsby mass spectrometers (i.e. sulfuric-acid monomer). However, the observedgrowth rates by sulfate-containing vapor in this study does not directlyimply a similar situation in the field, where sources of sulfate are muchmore diverse

Total sulfate vs. sulfuric acid monomer concenterations in nucleation studies