Chemical ionization mass spectrometry (CIMS) may not measure all gas-phase sulfuric acid if base molecules are present
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. Using computed proton affinities and reaction thermodynamics for the relevant charging reactions, we show that in the presence of strong bases such as amines, which tend to cluster with the sulfuric acid molecules, a significant fraction of the total gas-phase sulfuric acid may not be measured by a CIMS instrument. If this is the case, this effect has to be taken into account in the interpretation of atmospheric sulfuric acid measurement data, as well as in intercomparison of different CIMS instruments, which likely have different susceptibilities to amine-sulfuric acid clustering.