Secondary Organic Aerosol Formation Potential of Toluene and Products of Toluene Combustion

ANITA ANOSIKE, Omar El Hajj, Chase Glenn, Nicholas Dewey, Brandon Rotavera, Rawad Saleh, University of Georgia

     Abstract Number: 331
     Working Group: Combustion

Abstract
Gasoline engines are major sources of secondary organic aerosol (SOA) precursors. However, the relative contribution of these precursors from evaporative emissions (unburned fuel) versus tailpipe emissions (incomplete combustion) is still under debate. To address this gap, we investigated SOA formation from toluene and from emissions of toluene combustion at different combustion temperatures. Single-ring aromatics are responsible for the majority of SOA from evaporative emissions, therefore toluene is an appropriate surrogate for this investigation.

The experiments involved flowing toluene and air through an atmospheric-pressure reactor controlled at temperatures ranging between 293 K and 1123 K to capture evaporative emissions (lower temperatures) and partially burned fuel emissions (higher temperatures). The emissions were then diluted, passed through a PTFE filter to remove carbonaceous particles formed during combustion, and sent through an oxidation flow reactor (OFR) to initiate SOA formation from oxidation with OH radicals. The OH exposure in the OFR corresponded to an equivalent photochemical age of 1.5 days.

The SOA concentrations were stable for temperatures between 293 K and 850 K, and increased with increasing temperature reaching approximately a five-fold increase at 1123 K. This temperature-dependent SOA formation profile is consistent with the temperature-dependent toluene reactivity obtained using ChemKin simulations. These results indicate that products from incomplete combustion of toluene, including polycyclic aromatic hydrocarbons (PAHs), have higher SOA formation potential than toluene.

We performed chemical analysis on SOA samples collected at reactor temperature of 293 K and 1123 K using ultra-high resolution electrospray ionization mass spectrometry. Mass spectra of both samples exhibited repetitive peaks characteristic of oligomer formation. The average molecular formulae were C_16.0H_17.9O_9.5 and C_16.0H_16.7O_9.3 for the 293 K and 1123 K cases, respectively.

In summary, incomplete combustion products had higher SOA formation potential compared to unburned toluene, but the SOA from the two emissions retained similar chemical composition.