Studies of Isoprene SOA Formation Under Atmospherically-Relevant RO2 Conditions

NADIA TAHSINI, Hannah Kenagy, Matthew Goss, Yaowei Li, Frank Keutsch, Jesse Kroll, MIT

     Abstract Number: 273
     Working Group: Aerosol Chemistry

Abstract
Isoprene is an important SOA precursor in the atmosphere, and thus the yields and composition of isoprene SOA have been major targets of past laboratory chamber studies. However, most such studies were carried out under very high-NO or very low-NO conditions, with short peroxy radical (RO₂) lifetimes. As a result, the RO₂ fate, including both bimolecular processes and isomerization reactions, is not representative of the fate in the real atmosphere, likely affecting product distributions and the formation of SOA.

Here, we access a range of atmospherically-relevant RO₂ conditions—including long RO₂ lifetimes and moderate NO levels—in a laboratory chamber to investigate gas-phase composition and SOA formation from OH-initiated isoprene oxidation. Gas-phase species are characterized with an ammonium chemical ionization mass spectrometer (NH₄⁺-CIMS) and a proton transfer reaction mass spectrometer (PTR-MS). For the particle-phase, an aerosol mass spectrometer (AMS) is used to characterize the amount and composition of aerosol formed. RO₂ reaction pathways and lifetimes are assessed using both measurement of oxidation products and mechanistic modeling. Results from this study include gas-phase product composition, SOA yields, and SOA oxidation state, all as a function of RO₂ conditions.