Effect of relative humidity on the detection of sulfur dioxide and sulfuric acid using a chemical ionization mass spectrometer

Detection of sulfur dioxide and sulfuric acid at high relative humidity was studied using a chemical ionization mass spectrometer (CIMS). The reactant ions used in the experiments are CO_3^−·nH_2O (n=0–5), which react with SO_2 to form SO_5^−·nH_2O (n=0–2). H_2SO_4 reacts with the precursor ions to form HSO_4^− (m/z=97 amu) and H_2SO_4·CO_3^− (m/z=158 amu). We report the first use of the latter ionization scheme to detect sulfuric acid. High RH affects the detection of SO_2 and H_2SO_4 by forming clusters with the reactant and product ions, reducing sensitivity. Increasing the temperature breaks these clusters. For SO_2 at high RH, either SO_5^− (m/z=112 amu) or SO_5^−·H_2O (m/z=130 amu) can be used for SO_2 detection without a decrease in sensitivity. For H_2SO_4 at high RH, it is preferred to detect the ion H_2SO_4·CO_3^− because the background signal at 158 amu is small, and a better sensitivity can be achieved