Effects of Precursor Structure on First-Generation Photo-Oxidation Organic Aerosol Formation

Abstract

The effect of precursor molecular structural features on secondary organic aerosol (SOA) growth was investigated for a number of precursor functional groups. SOA yields were determined for straight chain alkanes, some oxygenated, up to highly functionalized hydrocarbons, the largest being β-caryophyllene. Organic SOA yield was determined by comparing to standard particle size changes with SO2 in a photolytic flow reactor. SOA formation was initiated with OH radicals from HONO photolysis and continued with NO and NO2 present at single-digit nmol/mol levels. Seed particles of ∼10 nm diameter grew by condensation of SOA material and growth was monitored with a nanoparticle sizing system. Cyclic compounds dominate as the highest SOA yielding structural feature, followed by C-10 species with double bonds, with linear alkanes and isoprene most ineffective. Carbonyls led to significant increases in growth compared to the alkanes while alcohols, triple-bond compounds, aromatics, and epoxides were only slightly more effective than alkanes at producing SOA. When more than one double bond is present, or a double bond is present with another functional group as seen with 1, 2-epoxydec-9-ene, SOA yield is notably increased. Placement of the double bond is important as well with β-pinene having an SOA yield approximately 5 times that of α-pinene. In our photolytic flow reactor, first-generation oxidation products are presumed to be the primary species contributing to SOA thus the molecular structure of the precursor is determinant. We also conducted proton-transfer mass spectrometry measurements of α-pinene photooxidation and significant signals were observed at masses for multifunctional nitrates and possibly peroxy radicals. The mass spectrometer measurements were also used to estimate a HONO photolysis rate.

Secondary Organic Aerosol Formation and Chemistry from the OH-Initiated Oxidation of Monofunctional C10 Species

“…One potential way to ameliorate this difficulty would be to establish heuristics for which compounds warrant focus in future studies and modeling. Examining the effects of functional groups on SOA yields can help provide insight into which types of compounds should have higher yields and thus merit more attention in models. − This is especially useful for examining potential effects of Volatile Chemical Products (VCPs) such as household cleaners and industrial solvents which use less-studied compounds but have significant effects on indoor and outdoor air quality. VCPs are especially important as they constitute a growing fraction of U.S. VOC emissions in urban areas and lack significant regulations to limit their SOA formation .…”

“…VCPs are especially important as they constitute a growing fraction of U.S. VOC emissions in urban areas and lack significant regulations to limit their SOA formation . Furthermore, recent studies have shown that the oxygenated component of VCPs can serve as effective SOA precursors. − …”