Insights into Particle Growth from the Incorporation of Organic Nitrates into SOA from the Ozonolysis of α-Pinene

Abstract:

Secondary organic aerosol (SOA) particles are formed in the atmosphere by oxidation of volatile organic compounds (VOC). However, the mechanism by which these particles form and grow, and how other compounds are taken up into the particle, is currently not well characterized. It appears that in many cases, the SOA is a highly viscous material such that uptake of semi-volatile oxidation products is irreversible. The goal of the present experiments was to study the incorporation of organic nitrates into SOA formed in the ozolonylsis of α-pinene using a large volume, slow-flow, stainless steel aerosol flow reactor. Gas phase 2-ethylhexyl nitrate (2EHN) was monitored by GC/MS using a simultaneous scan/single ion monitoring (SIM) mode following the signal at m/z 46 (NO₂⁺) specific to organic nitrate. In addition, particle size distributions were monitored using a scanning mobility particle sizer (SMPS) and polydisperse particles were collected onto a Ge-ATR crystal using a custom impactor located at the end of the flow reactor (30 min reaction time). Particulate 2EHN was quantified by GC/MS after extraction of the SOA impacted on the Ge crystal. In parallel, evaporation of the SOA and its components was monitored by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). Insights into phase of the SOA and the uptake coefficient of 2EHN in the O₃/α-pinene SOA are presented.