Quantification of gas phase hydrogen peroxide and methyl peroxide in ambient air: Using atmospheric pressure chemical ionization mass spectrometry with O2−, and O2−(CO2) reagent ions

Abstract

Instrumentation and ion chemistry are described for the measurement of hydrogen peroxide (H2O2) and methyl peroxide (CH3OOH) in ambient air by chemical ionization mass spectrometry (CIMS). A CIMS, designed and certified for aircraft deployment, was used in this work. The reagent gas was ultrapure air containing 400ppm CO2. The resultant reagent ions, O2− and O2−(CO2), form cluster ions with CH3OOH and H2O2, respectively, and are monitored at 80m/z [O2−(CH3OOH)] and 110m/z [O2− (CO2)(H2O2)]. The CIMS instrument was periodically calibrated using gas-phase standards generated from aqueous solutions. A Carulite-200® catalyst was used to remove peroxides from ambient air to blank the system to account for variations in the ambient air matrix. H2O2 also forms a stable cluster ion with O2− though its calibration behavior was more complex than that for O2−(CO2) in ambient air. The instrument was deployed on the National Center for Atmospheric Research Gulfstream-V aircraft. Representative measurements from the May-June 2012 Deep Convective Clouds and Chemistry experiment spanning altitudes between 0 and 13km over the south-central midwest and southeastern United States are shown. Laboratory experiments, airborne experiments and theoretical molecular modeling approaches were utilized to identify and select reagent ions and to understand the ion-molecule reactions for the formation of peroxide-ion clusters in the ambient air matrix under tropospheric conditions. The analytical viability of a particular ion-molecule adduct was supported by ab-initio molecular orbital calculations.