AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
An Isomer-Resolved Picture of Evolving Organic Aerosol Composition during Heterogeneous OH-Oxidation under Different OH concentrations and Timescales
ZIXU ZHAO, Haofei Zhang, University of California, Riverside
Abstract Number: 494 Working Group: Aerosol Chemistry
Abstract Heterogeneous hydroxyl radical (OH) oxidation of organic aerosols (OA) and the subsequent reaction pathways are a critical but poorly understood question for predicting OA’s climate and air quality impacts. The complexity of the oxidation products from various pathways along multiple generations has largely hindered a coherent understanding. These products are usually multifunctional and contain structural isomers, adding analytical challenges for unambiguous separation and identification. Further, elucidating the difference of OA composition under different gas-phase OH concentrations, relative humidities (RH), and reaction timescales could be highly relevant to the real atmosphere. In order to address these questions, heterogeneous OH-oxidation of OA model compounds (glutaric acid and adipic acid) were studied in flow tube reactor (FTR) and continuous-flow stirred tank reactor (CFSTR) under identical conditions, except that the residence time differs by a factor of 60. Two RH conditions are studied (30% and 80%). In these experiments, the reaction kinetics was determined by an online thermal desorption time-of-flight chemical ionization mass spectrometer (TD-TOF-CIMS); the isomer-resolved composition of oxidation products was measured by an offline electrospray ionization ion mobility spectrometry mass spectrometer (ESI-IMS-TOFMS). From our measurements, highly dynamic product formation distributions were observed, varied with OH concentration, RH, and reaction timescales. The dramatic difference was evident in both the molecular dimension (m/Q) and the isomeric dimension. Mechanisms are proposed to explain the dynamic OA evolution. The outcome of this work will expand the currently limited mechanistic understanding of OH-initiated OA heterogenous oxidation into a broader context.