Abstract
Abstract. Secondary organic aerosol (SOA) yields were measured for cyclododecane, hexylcyclohexane, n-dodecane, and 2-methylundecane under high-NOx conditions, in which alkyl proxy radicals (RO2) react primarily with NO, and under low-NOx conditions, in which RO2 reacts primarily with HO2. Experiments were run until 95–100% of the initial alkane had reacted. Particle wall loss was evaluated as two limiting cases using a new approach that requires only suspended particle number-size distribution data and accounts for size-dependent particle wall losses and condensation. SOA yield differed by a factor of 2 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. SOA yields ranged from 3.3% (dodecane, low-NOx conditions) to 160% (cyclododecane, high-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 mixing ratios 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.
Highlights
Alkanes are emitted from combustion sources and can comprise up to 90 % of anthropogenic emissions in urban areas (Rogge et al, 1993; Fraser et al, 1997; Schauer et al, 1999, 2002) and 67.5 %, 56.8 %, and 82.8 % of the mass of diesel fuel, liquid gasoline, and non-tailpipe gasoline sources, respectively (Gentner et al, 2012)
Upon atmospheric oxidation by OH and NO3 radicals, alkanes form lower-volatility products that can condense as secondary organic aerosol (SOA)
For most SOA precursors, a larger initial hydrocarbon mixing ratio results in a larger source of semivolatile oxidation products, assuming that reactions occur at the same temperature and oxidizing conditions and that the vapor-phase product distributions do not vary over the range of initial hydrocarbon mixing ratios considered
Summary
Alkanes are emitted from combustion sources and can comprise up to 90 % of anthropogenic emissions in urban areas (Rogge et al, 1993; Fraser et al, 1997; Schauer et al, 1999, 2002) and 67.5 %, 56.8 %, and 82.8 % of the mass of diesel fuel, liquid gasoline, and non-tailpipe gasoline sources, respectively (Gentner et al, 2012). SOA yields, defined as mass of SOA formed divided by mass of alkane reacted, have been measured in the laboratory for C7–C25 alkanes with linear, branched, and cyclic structures (Lim and Ziemann, 2005, 2009b; Presto et al, 2010; Tkacik et al, 2012). In these studies, SOA yields are reported after 50–85 % of the alkane had reacted and may not represent the maximum possible yield. SOA yield was found to increase with increasing carbon number or the presence of a cyclic structure and decrease with branching of the carbon chain
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