Particle Growth by Acid-Catalyzed Heterogeneous Reactions of Organic Carbonyls on Preexisting Aerosols

Abstract

Aerosol growth by the heterogeneous reactions of different aliphatic and alpha,beta-unsaturated carbonyls in the presence/absence of acidified seed aerosols was studied in a 2 m long flow reactor (2.5 cm i.d.) and a 0.5-m3 Teflon film bag under darkness. For the flow reactor experiments, 2,4-hexadienal, 5-methyl-3-hexen-2-one, 2-cyclohexenone, 3-methyl-2-cyclopentenone, 3-methyl-2-cyclohexenone, and octanal were studied. The carbonyls were selected based on their reactivity for acid-catalyzed reactions, their proton affinity, and their similarity to the ring-opening products from the atmospheric oxidation of aromatics. To facilitate acid-catalyzed heterogeneous hemiacetal/acetal formation, glycerol was injected along with inorganic seed aerosols into the flow reactor system. Carbonyl heterogeneous reactions were accelerated in the presence of acid catalysts (H2SO4), leading to higher aerosol yields than in their absence. Aldehydes were more reactive than ketones for acid-catalyzed reactions. The conjugated functionality also resulted in higher organic aerosol yieldsthan saturated aliphatic carbonyls because conjugation with the olefinic bond increases the basicity of the carbonyl leading to increased stability of the protonated carbonyl. Aerosol population was measured from a series of sampling ports along the length of the flow reactor using a scanning mobility particle sizer. Fourier transform infrared spectrometry of either an impacted liquid aerosol layer or direct reaction of carbonyls as a thin liquid layer on a zinc selenide FTIR disk was employed to demonstrate the direct transformation of chemical functional groups via the acid-catalyzed reactions. These results strongly indicate that atmospheric multifunctional organic carbonyls, which are created by atmospheric photooxidation reactions, can contribute significantly to secondary organic aerosol formation through acid-catalyzed heterogeneous reactions. Exploratory studies in 25- and 190-m3 outdoor chambers were also implemented to demonstrate the formation of high molecular weight organic structures. The reaction of ozone with alpha-pinene to generate secondary organic aerosols (SOAs) was performed in the presence of background aerosol consisting of a mixture of wood soot and diesel soot. Results strongly suggest that indigenous sulfuric acid associated with the combustion of fossil fuels (e.g., diesel soot) can initiate acid-catalyzed heterogeneous reactions of SOAs on the particle phase.

In situ measurements of gas/particle-phase transitions for atmospheric semivolatile organic compounds

“…It may also be possible that these small oxygenated compounds are present in the particle phase not only as parent compounds, but also as thermal decomposition products of unstable peroxides (24) and/or the decomposition product of larger oligomers (25)(26)(27). Ultimately, there are several explanations that would point toward additional SOA compared to theoretical SOA production.…”

Temperature dependence of yields of secondary organic aerosols from the ozonolysis of <i>α</i>-pinene and limonene

“…However, it has also been shown that other factors influence the SOA yield, i.e. the mass of organic aerosol that can serve as solvent for semivolatile organic compounds (Pankow, 1994;Odum et al, 1996), the acidity of and chemical reactions in the particulate phase (Jang et al, 2003;Gao et al, 2004;Iinuma et al, 2004;Kalberer et al, 2004), the humidity (Bonn et al, 2002;Jonsson et al, 2006), and the temperature that controls the phase equilibrium of the semivolatile organics and also influences the reaction pathways leading to condensable molecules (Sheehan and Bowman, 2001;Takekawa et al, 2003;Jenkin, 2004;Offenberg et al, 2006;Pathak et al, 2007b;Stanier et al, 2007;Johnson and Marston, 2008). There has been a considerable amount of work on aerosol yields from the ozonolysis of α-pinene or limonene near room temperature (290-303 K) e.g.…”

Effects of uncertainties in the thermodynamic properties of aerosol components in an air quality model – Part 1: Treatment of inorganic electrolytes and organic compounds in the condensed phase

“…A significant fraction of the organic aerosol material remains uncharacterised, and the thermodynamic properties of the secondary compounds that have been identified (e.g., Yu et al, 1999;Jaoui et al, 2005) have generally not been measured. Evidence suggests that oligomerisation and other aerosol phase reactions can be important, and enhance the formation of secondary organic aerosol (SOA) material (Jang et al, 2003;Kalberer et al, 2006). The most common atmospheric codes treat the organic components of aerosols, including SOA formation, using a lumped two component approach developed to represent the results of chamber experiments (Odum et al, 1996;Griffin et al, 1999).…”