Secondary Organic Aerosol Formation from Low-NO_x Photooxidation of Dodecane: Evolution of Multigeneration Gas-Phase Chemistry and Aerosol Composition

Abstract

The extended photooxidation of and secondary organic aerosol (SOA) formation from dodecane (C 12 H 26 ) under low-NO x conditions, such that RO 2 + HO 2 chemistry dominates the fate of the peroxy radicals, is studied in the Caltech Environmental Chamber based on simultaneous gas and particle-phase measurements. A mechanism simulation indicates that greater than 67% of the initial carbon ends up as fourth and higher generation products after 10 h of reaction, and simulated trends for seven species are supported by gas-phase measurements. A characteristic set of hydroperoxide gas-phase products are formed under these low-NO x conditions. Production of semivolatile hydroperoxide species within three generations of chemistry is consistent with observed initial aerosol growth. Continued gas-phase oxidation of these semivolatile species produces multifunctional low volatility compounds. This study elucidates the complex evolution of the gas-phase photooxidation chemistry and subsequent SOA formation through a novel approach comparing molecular level information from a chemical ionization mass spectrometer (CIMS) and high m/z ion fragments from an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). Combination of these techniques reveals that particle-phase chemistry leading to peroxyhemiacetal formation is the likely mechanism by which these species are incorporated in the particle phase. The current findings are relevant toward understanding atmospheric SOA formation and aging from the "unresolved complex mixture," comprising, in part, long-chain alkanes.

Size distribution dynamics reveal particle-phase chemistry in organic aerosol formation

“…The photooxidation of dodecane and subsequent SOA formation is studied using the Caltech Environmental Chamber (11,12). Dodecane at initial concentration of 34 parts per billion (ppb) in the presence of ammonium sulfate seed particles under dry conditions [relative humidity (RH) < 5%] was oxidized by OH radicals at low concentrations of NO x typical of nonurban conditions.…”

“…1A). Fig 1B shows the span of O:C ratio and gas-phase saturation concentrations for the surrogate SVOCs based on the dodecane oxidation mechanism of Yee et al (11). Some of the fourth generation products have been established to be multifunctional carbonyl compounds, aldehydes, and ketones.…”

“…Some of the fourth generation products have been established to be multifunctional carbonyl compounds, aldehydes, and ketones. The aldehydes can react with hydroperoxide, hydroxyl, and peroxycarboxylic acid groups, forming peroxyhemiacetal (PHA), hemiacetal, and acylperoxyhemiacetal, respectively, in the particle phase (2,11,18). Ketone functional groups can react with hydroxyl groups, catalyzed by acids, forming a variety of oligomers (4).…”

“…3D shows the measured evolution of gas-phase C 6 -carboxylic acid, which is a fourth-generation product and a proxy for the expected formation of aldehydes (11). Aerosol mass spectrometer (AMS) ions at mass-to-charge ratios (m/z) of 183 and 215 are also shown, which are representative fragments from the carbonyl hydroperoxide (CARBROOH; C 12 H 24 O 3 ) and its derived PHA (11). The onset of gas-phase aldehyde, particle-phase PHA, SOA mass growth, and production rate of low-volatility products at ∼4 h suggests that the formation of PHA triggers initial SOA growth.…”

Multi-generational oxidation model to simulate secondary organic aerosol in a 3-D air quality model

“…Finally, current schemes have not been tested against or constrained by measurements of multigenerational products (or classes of products) under realistic ambient conditions. Multi-generational VOC oxidation, in theory, can be explicitly modeled using detailed gas-phase chemical mechanisms such as the MCM (Master Chemical Mechanism; Jenkin et al, 2003;Saunders et al, 2003) or GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere; Aumont et al, 2005;Camredon et al, 2007) and have been put to use to develop a better understanding of the reaction chemistry leading to SOA formation (Yee et al, 2012;Aumont et al, 2012;Valorso et al, 2011). However, these mechanisms track thousands to millions of chemical species and are computationally impractical for modeling multi-generational oxidation in 3-D models.…”