Phase partitioning and volatility of secondary organic aerosol components formed from α-pinene ozonolysis and OH oxidation: the importance of accretion products and other low volatility compounds

摘要

We measured a large suite of gas- and particle-phasemulti-functional organic compounds with a Filter Inlet for Gases andAEROsols (FIGAERO) coupled to a high-resolution time-of-flight chemicalionization mass spectrometer (HR-ToF-CIMS) developed at the University ofWashington. The instrument was deployed on environmental simulation chambersto study monoterpene oxidation as a secondary organic aerosol (SOA) source.We focus here on results from experiments utilizing an ionization methodmost selective towards acids (acetate negative ion proton transfer), but ourconclusions are based on more general physical and chemical properties ofthe SOA. Hundreds of compounds were observed in both gas and particlephases, the latter being detected by temperature-programmed thermaldesorption of collected particles. Particulate organic compounds detected bythe FIGAERO–HR-ToF-CIMS are highly correlated with, and explain at least 25–50 % of, the organic aerosol mass measured by an Aerodyne aerosol massspectrometer (AMS). Reproducible multi-modal structures in the thermogramsfor individual compounds of a given elemental composition reveal asignificant SOA mass contribution from high molecular weight organics and/oroligomers (i.e., multi-phase accretion reaction products). Approximately50 % of the HR-ToF-CIMS particle-phase mass is associated with compoundshaving effective vapor pressures 4 or more orders of magnitude lower thancommonly measured monoterpene oxidation products. The relative importance ofthese accretion-type and other extremely low volatility products appears tovary with photochemical conditions. We present a desorption-temperature-based framework for apportionment of thermogram signals into volatilitybins. The volatility-based apportionment greatly improves agreement betweenmeasured and modeled gas-particle partitioning for select major and minorcomponents of the SOA, consistent with thermal decomposition duringdesorption causing the conversion of lower volatility components into thedetected higher volatility compounds.