Effect of dimethylamine on the gas phase sulfuric acid concentration measured by Chemical Ionization Mass Spectrometry

Sulfuric acid is widely recognized as a very important substance driving atmospheric aerosolnucleation. Based on quantum chemical calculations it has been suggested that the quantitative detectionof gas phase sulfuric acid (H2SO4) by use of Chemical Ionization Mass Spectrometry (CIMS) could be biased inthe presence of gas phase amines such as dimethylamine (DMA). An experiment (CLOUD7 campaign) was setup at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber to investigate the quantitative detection ofH2SO4in the presence of dimethylamine by CIMS at atmospherically relevant concentrations. For the first time inthe CLOUD experiment, the monomer sulfuric acid concentration was measured by a CIMS and by two CI-APi-TOF(Chemical Ionization-Atmospheric Pressure interface-Time Of Flight) mass spectrometers. In addition, neutralsulfuric acid clusters were measured with the CI-APi-TOFs. The CLOUD7 measurements show that in the presenceof dimethylamine (<5 to 70 pptv) the sulfuric acid monomer measured by the CIMS represents only a fraction ofthe total H2SO4, contained in the monomer and the clusters that is available for particle growth. Although it wasfound that the addition of dimethylamine dramatically changes the H2SO4cluster distribution compared to binary(H2SO4-H2O) conditions, the CIMS detection efficiency does not seem to depend substantially on whether anindividual H2SO4monomer is clustered with a DMA molecule. The experimental observations are supported bynumerical simulations based on A Self-contained Atmospheric chemistry coDe coupled with a molecular processmodel (Sulfuric Acid Water NUCleation) operated in the kinetic limit.