1Division of Atmospheric Science, Department of Physics, P. O. Box 64, 00014 University of Helsinki, Finland
2University of Copenhagen, Department of Chemistry, Universitetsparken 5, 2100 København Ø, Denmark
3University of Eastern Finland, Yliopistonranta 1, P. O. Box 1627, 70211 Kuopio, Finland
4National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder CO 80305, USA
5Aerodyne Research, Inc. 45 Manning Rd, Billerica, MA 0182, USA
Abstract. The state-of-the art method for measuring atmospheric gas-phase sulfuric acid is chemical ionization mass spectrometry (CIMS) based on nitrate reagent ions. We have assessed the possible effect of the sulfuric acid molecules clustering with base molecules on CIMS measurements using computational chemistry. From the computational data, three conclusions can be drawn. First, a significant fraction of the gas-phase sulfuric acid molecules are very likely clustered with amines if the amine concentration is around or above a few ppt. Second, some fraction of these acid-amine clusters may not be charged by the CIMS instrument, though the most reliable computational methods employed predict this fraction to be small; on the order of ten percent or less. Third, the amine molecules will evaporate practically immediately after charging, thus evading detection. These effects may need to be taken into account in the interpretation of atmospheric measurement data obtained using chemical ionization methods. The purpose of this study is not to criticize the CIMS method, but to help understand the implications of the measured results.