Two-stage Secondary Organic Aerosol Formation in Low-temperature Combustion
ANITA ANOSIKE, Omar El Hajj, Chase Glenn, Samuel Hartness, Annabelle W. Hill, Nicholas Dewey, Daelyn Moore, Heeseung Choi, Jonathan Amster, Brandon Rotavera, Rawad Saleh, University of Georgia
Abstract Number: 644
Working Group: Urban Aerosols
Abstract
We have previously demonstrated that n-alkanes that exhibit two-stage ignition also exhibit two-stage primary organic aerosol (POA) formation. The second-stage POA forms at temperatures larger than 1000 K via the soot-formation route, while the first-stage POA forms at 500 K – 700 K via alkylperoxy radical chemistry that involves sequential oxidation reactions of cyclic ethers. Here, we investigate secondary organic aerosol (SOA) formation during first-stage and second-stage ignition of n-pentane.
We conducted combustion experiments in a steady-flow reactor at fuel-rich (equivalence ratio = 2) and dilute (O2 / N2 = 0.1) conditions, at temperatures between 500 K – 1300 K to capture first-stage and second-stage ignition. To generate SOA through oxidation with OH radicals in an oxidation flow reactor (OFR), the emissions were first passed through a filter to remove POA particles and isolate the gaseous emissions.
SOA exhibited temperature-dependent two-stage formation, consistent with n-pentane two-stage ignition behavior. The peak second-stage SOA mass concentration (1250 K) was a factor of two larger than POA mass concentration at the same temperature, while the peak first-stage SOA concentration (700 K) was two orders of magnitude larger than the peak first-stage POA concentration (600 K). These results are consistent with previous reports of significantly higher levels of SOA compared to POA in emissions of gasoline engines.
Gas-phase measurements revealed that the second-stage SOA precursors were predominantly aromatic species (benzene and toluene), which are typically associated with anthropogenic emissions. While, the first-stage SOA precursors featured n-pentane first-stage ignition intermediates, including cyclic ethers, which have not been previously associated with anthropogenic emissions.
This study provides evidence that SOA precursors are emitted at high levels during low-temperature combustion, which is encountered during cold-start conditions. With most urban driving occurring at cold engine conditions, our results highlight the importance of low-temperature combustion emissions as SOA precursors in urban atmospheres.