The effect of aging conditions at equal OH exposure in an oxidation flow reactor on the composition of toluene-derived secondary organic aerosols

Abstract

Oxidation flow reactors (OFRs) have been increasingly used to conduct research on secondary aerosol formation potential and composition in laboratory and field studies by exposing aerosols to high levels of oxidants in short time periods. In order to assess the atmospheric relevance of the triggered chemical reactions, kinetic models have been developed to reveal the production of atmospheric oxidants and the fate of volatile organic compounds (VOCs). However, it is unknown how different OFR conditions generating the same OH exposure affect the chemical and physical properties of the secondary aerosol particle phase because the model is based on gas phase chemistry. Toluene as a well-investigated precursor of secondary organic aerosols (SOAs) was aged in the high-volume OFR “PEAR” at three different external OH reactivities (OHR_ext) and in an environmental chamber at the same OH exposure of (1.09 ± 0.09) × 10¹¹ s cm⁻³. These specific OFR conditions altered the majority of the investigated chemical and physical properties of the toluene-derived SOA (tol-SOA). However, OFR-aging at low OHR_ext associated with atmospherically relevant conditions did not lead to physical and chemical SOA properties most similar to chamber-generated SOAs at the same low OHR_ext. Particularly, for the most detailed chemical analysis by electrospray (ESI) high-resolution Orbitrap mass spectrometry (HRMS), tol-SOA from the PEAR at high OHR_ext, deviating from atmospherically relevant conditions, and the chamber were most different to tol-SOA from the PEAR with low and medium OHR_ext. Our study challenges the concept of atmospherically relevant OFR aerosol aging and motivates further exploration of OFR conditions on secondary aerosol composition.