Isolating a-Pinene Ozonolysis Pathways Reveals New Insights into Peroxy Radical Chemistry and Secondary Organic Aerosol Formation

alpha-Pinene ozonolysis is a key process that impacts the formation of new particles and secondary organic aerosol (SOA) in the atmosphere. The mechanistic understanding of this chemistry has been inconclusive despite extensive research, hindering accurate simulations of atmospheric processes. In this work, we examine the ozonolysis of two synthesized unsaturated carbonyl isomers (C11H18O) which separately produce the two Criegee intermediates (CIs) that would form simultaneously in alpha-pinene ozonolysis. Direct gas-phase measurements of peroxy radicals (RO2) from flowtube ozonolysis experiments by an iodide-adduct chemical ionization mass spectrometer suggest that the initial C10H15O4 center dot RO2 from the CI with a terminal methyl ketone undergo autoxidation 20-fold faster than the CI with a terminal aldehyde and always outcompete the bimolecular reactions under typical laboratory and atmospheric conditions. These results provide experimental constraints on the detailed RO2 autoxidation mechanisms for understanding new particle formation in the atmosphere. Further, isomer-resolved characterization of the SOA formed from a continuous-flow stirred tank reactor using ion mobility spectrometry mass spectrometry suggests that the two structurally different CIs predominantly and unexpectedly form constituents with identical structures. These results open up possibilities of diverse isomerization pathways that the two CIs may undergo that form mutual products to a large extent toward their way forming the SOA. This work highlights new insights into alpha-pinene ozonolysis pathways and call for future studies to uncover the detailed mechanisms.

Automated summary

The study investigates the ozonolysis of alpha-pinene and its impact on the formation of new particles and secondary organic aerosols in the atmosphere. Experimental measurements show that Criegee intermediates with a terminal methyl ketone undergo autoxidation 20 times faster than those with a terminal aldehyde, providing insights into the detailed mechanisms of particle formation. The research also suggests that structurally different Criegee intermediates can form constituents with identical structures, indicating diverse isomerization pathways that contribute to the formation of secondary organic aerosols.