Sensitivity of a Q-ACSM to chamber generated SOA with different oxidation states

Abstract. The accuracy in quantification of secondary organic aerosols (SOA) using a Q-ACSM has been comprehensively investigated in this work. SOA samples were generated under simulated photochemical oxidation conditions in a 4.5 m³ Teflon chamber from three different volatile organic compounds (VOC) of atmospheric relevant concentrations (dozens of ppbv): α-pinene, isoprene, and toluene, representing both biogenic and anthropogenic VOC. Different SOA oxidation states were achieved by changing the relative ratio of the VOC precursor to the oxidants (O₃ or OH). A scanning mobility particle sizer (SMPS) and an aerosol particle mass analyzer (APM) were used to determine the number-size distribution and the exact mass of the chamber-generated SOA, which were then used to deduce the SOA effective density and mass concentration. Results showed that aerosol mass concentration measured by the Q-ACSM based on SMPS calibration alone may be associated with considerable errors due to the fact that the effective density of SOA at different oxidation state can change substantially. More importantly, the sensitivity of the Q-ACSM to a specific type of SOA was found to be anti-correlated with the aerosol oxidation state regardless of the VOC precursors. This may be due to the decreasing of relative ionization efficiency (RIE) or the collection efficiency (CE) of the Q-ACSM for more oxidized SOA. To pinpoint the actual cause, ammonium sulfate ((NH₄)₂SO₄) seed particles were injected into the chamber before SOAs were produced and the CE for a specific SOA sample was hence determined with reference to the changes in sulfate signals. Our experiment results along with previous literature reports strongly implied that as the SOA oxidation state increases, SOA will transform gradually from a liquid state (CE ≈ 1) into a solid (or glassy) state with a CE of 0.2~0.5. Meanwhile, the RIE of OA decreased substantially when SOA transformed from hydrocarbon-like OA (HOA) into more oxygenated OA (OOA) and may further decrease as O/C continued to increase. Our results indicated that the current Q-ACSM calibration procedure using a constant RIE may lead to somewhat underestimation of more oxidized OOA but overestimation of less oxidized HOA, i.e., a variable RIE shall be applied, most likely as a function of the SOA oxidation state.