Towards more accurate particulate organic nitrate quantification through aerosol mass spectrometry
Résumé
A large fraction of particulate organic nitrates (pON) arise from reactions involving volatile organic compounds (VOC) and nitrate radicals (NO3) contributing significantly to tropospheric particulate matter. However, their quantification remains challenging. Previous studies with Aerosol Mass Spectrometers (AMS) show that the fragmentation patterns of inorganic ammonium nitrate and pON significantly differ from each other resulting in a clear contrast in the ratio between the major nitrate fragments, i.e., the NO+/NO2+ ratio. Detection of pONs leads to larger NO+/NO2+ values, but with high variability depending on the pON precursors. Additional uncertainty is also introduced while performing unit mass resolution (UMR) analysis as an organic ion fragment (CH2O+) is typically present at the same unit mass as NO+. Underestimation of this organic fragment can lead to significant overestimation of pON. As part of the Aerosol Chemical Monitor Calibration Centre (ACMCC) pON experiment, we generated pON via NO3 oxidation of four different VOCs (limonene, β-pinene, guaiacol, and acenaphthylene) in a Potential Aerosol Mass (PAM) oxidation flow reactor. The pON were detected with a High Resolution (HR) AMS equipped with a Long Time-of-Flight chamber facilitating a mass resolution approaching 8000 M/ΔM, further enabling separation of nitrogen containing peaks in the mass spectrum. We observed precursor-dependent differences in the nitrate loading when comparing UMR and HR results mainly linked to CH2O+ underestimation with the default AMS fragmentation table. Here we present a modified fragmentation table with the goal of providing more accurate pON quantification in ambient air via UMR aerosol mass spectrometry such as Aerosol Chemical Speciation Monitor (ACSM). We also compare the abundance of other nitrogen-containing HR-AMS ion signals (e.g. combinations of carbon/hydrogen/nitrogen and carbon/hydrogen/nitrogen/oxygen atoms) obtained for the different pON types. We acknowledge the European COST Action CA16109 COLOSSAL, the H2020 ACTRIS-2 project (grant agreements no. 654109 and the ERC Starting Grant 638703-COALA.