Secondary organic aerosol yields of 12-carbon alkanes

Secondary organic aerosol (SOA) yields were measured for cyclododecane, hexylcyclohexane, n-dodecane, and 2-methylundecane under high- and low-NOx conditions, in which alkyl peroxy radicals (RO2) react primarily with NO and HO2, respectively, for multiple initial alkane concentrations. Experiments were run until 95–100% of the initial alkane had reacted. Particle wall loss was evaluated as two limiting cases. SOA yield differed by 2 orders of magnitude between the two limiting cases, but the same trends among alkane precursors were observed for both limiting cases. Vapor-phase wall losses were addressed through a modeling study and increased SOA yield uncertainty by approximately 30%. SOA yields were highest from cyclododecane under both NOx conditions. Under high-NOx conditions, SOA yields increased from 2-methylundecane < dodecane ~ hexylcyclohexane < cyclododecane, consistent with previous studies. Under low-NOx conditions, SOA yields increased from 2-methylundecane ~ dodecane < hexylcyclohexane < cyclododecane. The presence of cyclization in the parent alkane structure increased SOA yields, whereas the presence of branch points decreased SOA yields due to increased vapor-phase fragmentation. Vapor-phase fragmentation was found to be more prevalent under high-NOx conditions than under low-NOx conditions. For different initial concentrations of the same alkane and same NOx conditions, SOA yield did not correlate with SOA mass throughout SOA growth, suggesting kinetically limited SOA growth for these systems.

α-pinene photooxidation under controlled chemical conditions – Part 1: Gas-phase composition in low- and high-NO_x environments

The OH oxidation of α-pinene under both low- and high-NOx environments was studied in the Caltech atmospheric chambers. Ozone was kept low to ensure OH was the oxidant. The initial α-pinene concentration was 20–50 ppb to ensure that the dominant peroxy radical pathway under low-NOx conditions is reaction with HO2, produced from reaction of OH with H2O2, and under high-NOx conditions, reactions with NO. Here we present the gas-phase results observed. Under low-NOx conditions the main first generation oxidation products are a number of α-pinene hydroxy hydroperoxides and pinonaldehyde, accounting for over 40% of the yield. In all, 65–75% of the carbon can be accounted for in the gas phase; this excludes first-generation products that enter the particle phase. We suggest that pinonaldehyde forms from RO2 + HO2 through an alkoxy radical channel that regenerates OH, a mechanism typically associated with acyl peroxy radicals, not alkyl peroxy radicals. The OH oxidation and photolysis of α-pinene hydroxy hydroperoxides leads to further production of pinonaldehyde, resulting in total pinonaldehyde yield from low-NOx OH oxidation of ~33%. The low-NOx OH oxidation of pinonaldehyde produces a number of carboxylic acids and peroxyacids known to be important secondary organic aerosol components. Under high-NOx conditions, pinonaldehyde was also found to be the major first-generation OH oxidation product. The high-NOx OH oxidation of pinonaldehyde did not produce carboxylic acids and peroxyacids. A number of organonitrates and peroxyacyl nitrates are observed and identified from α-pinene and pinonaldehyde.

α-pinene photooxidation under controlled chemical conditions – Part 1: Gas-phase composition in low- and high-NO_x environments

The OH oxidation of α-pinene under both low- and high-NOx environments was studied in the Caltech atmospheric chambers. Ozone was kept low to ensure OH was the oxidant. The initial α-pinene concentration was 20–50 ppb to ensure that the dominant peroxy radical pathway under low-NOx conditions is reaction with HO2 and under high-NOx conditions, reactions with NO. Here we present the gas-phase results observed. Under low-NOx conditions the main first generation oxidation products are α-pinene hydroxy hydroperoxide and pinonaldehyde, accounting for over 40% of the yield. In all, 65–75% of the carbon can be accounted for in the gas phase; this excludes first-generation products that enter the particle phase. We suggest that pinonaldehyde forms from RO2 + HO2 through an alkoxy radical channel that regenerates OH, a mechanism typically associated with acyl peroxy radicals, not alkyl peroxy radicals. The OH oxidation and photolysis of α-pinene hydroxy hydroperoxides leads to further production of pinonaldehyde, resulting in total pinonaldehyde yield from low-NOx OH oxidation of ~33%. The low-NOx OH oxidation of pinonaldehyde produces a number of carboxylic acids and peroxyacids known to be important secondary organic aerosol components. Under high-NOx conditions, pinonaldehyde was also found to be the major first-generation OH oxidation product. The high-NOx OH oxidation of pinonaldehyde did not produce carboxylic acids and peroxyacids. A number of organonitrates and peroxyacyl nitrates are observed and identified from α-pinene and pinonaldehyde.

Rate Coefficients and Mechanisms for the Atmospheric Oxidation of the N-Atom-Containing Oxygenates

The many different nitrogen-containing oxygenated volatile organic compounds that are present in the troposphere play important roles in the chemistry of our atmosphere. They can be emitted directly into the atmosphere, such as in the case of amides that are widely used as organic solvents, starting materials, or intermediates in different industries (e.g., synthetic polymers, manufacture of dyes, and synthesis of pesticides). Amides are formed in situ as intermediate products in the degradation of amines (e.g., see Tuazon et al., 1994; Finlayson-Pitts and Pitts, 2000). Nitrogen-containing oxygenated organic compounds are formed in the atmosphere also via reactions of alkoxy (RO) and alkyl peroxy radicals (RO2) with NO or NO2 leading to alkyl nitrates, alkyl nitrites, and peroxy acetyl nitrates. However, primary sources of these organic species have also been suggested such as diesel and other engines and biomass burning (e.g., see Simpson et al., 2002). Alkyl nitrates (RONO2) have been detected in both the urban and the remote troposphere (e.g., see Roberts, 1990; Walega et al., 1992; Atlas et al., 1992; Ridley et al., 1997; and Stroud et al., 2001; see also section I-D). Nitrates are formed as minor products in the reaction of peroxy radicals with NO. The nitrate yield increases with the size of peroxy radicals and can be as high as 20–30% for large (>C6) radicals (Calvert et al., 2008). Peroxyacyl nitrates (RC(O)O2NO2) are important constituents of urban air pollution. They have been identified in ambient air (e.g., see Bertman and Roberts, 1991; Williams et al., 1997, 2000; Nouaime et al., 1998; Hansel and Wisthaler, 2000; also see section I-D). They are formed from photochemical reactions via RC(O)O2 + NO2. A major role of these compounds is their capacity to act as a reservoir for NOx that can be transported from polluted urban to remote regions that are poor NOx regions and where their presence can increase NOx levels (Roberts, 1990). As with other volatile organic compounds (VOCs), once released to the atmosphere, nitrogen-containing organic compounds are expected to undergo degradation primarily via reaction with hydroxyl and nitrate radicals, reaction with ozone, and photolysis. Thermal decomposition is an important loss process for the peroxyacyl nitrates.