Transitions from functionalization to fragmentation reactions of laboratory secondary organic aerosol (SOA) generated from the OH oxidation of alkane precursors.

Functionalization (oxygen addition) and fragmentation (carbon loss) reactions governing secondary organic aerosol (SOA) formation from the OH oxidation of alkane precursors were studied in a flow reactor in the absence of NO(x). SOA precursors were n-decane (n-C10), n-pentadecane (n-C15), n-heptadecane (n-C17), tricyclo[5.2.1.0(2,6)]decane (JP-10), and vapors of diesel fuel and Southern Louisiana crude oil. Aerosol mass spectra were measured with a high-resolution time-of-flight aerosol mass spectrometer, from which normalized SOA yields, hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, and C(x)H(y)+, C(x)H(y)O+, and C(x)H(y)O(2)+ ion abundances were extracted as a function of OH exposure. Normalized SOA yield curves exhibited an increase followed by a decrease as a function of OH exposure, with maximum yields at O/C ratios ranging from 0.29 to 0.74. The decrease in SOA yield correlates with an increase in oxygen content and decrease in carbon content, consistent with transitions from functionalization to fragmentation. For a subset of alkane precursors (n-C10, n-C15, and JP-10), maximum SOA yields were estimated to be 0.39, 0.69, and 1.1. In addition, maximum SOA yields correspond with a maximum in the C(x)H(y)O+ relative abundance. Measured correlations between OH exposure, O/C ratio, and H/C ratio may enable identification of alkane precursor contributions to ambient SOA.

Intermediate-volatility organic compounds: a potential source of ambient oxidized organic aerosol.

Smog chamber experiments were conducted to investigate secondary organic aerosol (SOA) formation from intermediate volatility and semivolatile organic compounds (IVOCs and SVOCs). We present evidence for the formation of highly oxygenated SOA from the photooxidation of n-heptadecane, which is used as a proxy for IVOC emissions. The SOA is consistent with multiple generations of oxidation chemistry resulting from OH radical exposure equivalent to approximately 0.5 days of atmospheric processing under high-NO(x) and low-CoA conditions. The SOA has a calculated O/C ratio of 0.59, which is higher than typical for chamber-generated SOA. The mass spectrum of the SOA, as measured with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), is similar to the OOA-2 factor determined for Mexico City. SOA formed from the low-NO(x), low-C(OA), oxidation of n-heptadecane is less oxidized because of differences in the chemical mechanism and lower integrated OH exposure. SOA formed from both the oxidation of n-heptadecane under high-NO(x), high-C(OA) conditions and the oxidation of n-pentacosane, a proxy for semivolatile organic emissions, does not produce highly oxygenated SOA, largely because of the condensation of early generation oxidation products.