10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Mechanisms That Control the Contribution of Aromatic Highly Oxidized Multifunctional Compounds (Homs) to Initial Particle Growth in the Atmosphere
MINGYI WANG, Dexian Chen, Mao Xiao, Victoria Hofbauer, Penglin Ye, Alexander Lucas Vogel, Qing Ye, Roy Lee III Mauldin, Neil Donahue, Center for Atmospheric Particle Studies, Carnegie Mellon Uni
Abstract Number: 814 Working Group: Aerosol Chemistry
Abstract Organic condensation from the gas phase to the particle phase contributes to the initial growth of freshly nucleated particles and causes secondary organic aerosol (SOA) formation (Tröstl et al., 2016). SOA, in turn, may comprise well over half of the total fine-particle mass in the lower atmosphere, contributing to and possibly dominating health effects that include more than 4 million premature deaths per year (Apte et al., 2015). However, to what extent organic condensation affects the initial growth of newly formed particles smaller than 10 nm remains in doubt, and the mechanism forming "condensable" organic vapors is even less certain.
Ozonolysis of terpenes, followed by peroxy-radical auto-oxidation via a succession of internal H-atom transfer reactions, produces highly oxidized multifunctional compounds (HOMs) within a few seconds after the initial attack of ozone (Jokinen et al., 2014). This dominates new-particle formation in regions dominated by biogenic emissions. However, these processes can be suppressed substantially in the anthropogenic atmosphere, nitrogen oxides (NOx) can perturb peroxy-radical chemistry (Yan et al., in preparation). However, the urban atmosphere contains far more than NOx, laboratory studies have reported HOM formation from OH-triggered oxidation of aromatic hydrocarbons, highlighting the potential contribution of aromatic HOMs to nucleation and initial particle growth in the anthropogenic environment (Molteni et al., 2018).
In order to understand the role of aromatic hydrocarbons in new-particle formation, especially the initial growth of nucleated particles, experiments using toluene, 1,2,4-trimethylbenzene and naphthalene as oxidation precursors were conducted under typical urban conditions in the “Cosmics Leaving OUtdoor Droplets” (CLOUD) chamber. We measured the particle-phase volatility of aromatic HOMs via thermal desorption using an iodide-adduct Long Time-of-Flight Chemical-Ionization Mass Spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-LToF-CIMS). The particle-phase volatility measurements confirm that oxidation products of toluene, trimethylbenzene and naphthalene can and do contribute to the initial growth of newly formed particles. Naphthalene oxidation forms the least volatile products and thus is the strongest contributor to growth among the three aromatic precursors. Toluene-derived HOMs have slightly lower volatility than TMB-derived HOMs, and their volatility distribution is similar to that of monoterpene-derived HOMs. More detailed exploration of the oxidation mechanism suggests that rapid progression through multiple generations of oxidation is more pronounced in aromatic hydrocarbons, which resulting in more oxidation but crucially also favoring functional groups with much lower volatility per added oxygen atom. Ultimately, the relative contribution of the different aromatic precursors to nucleation and growth in highly polluted urban atmospheres will depend on the volatility distribution of the products as well as the relative concentrations of the precursors. It is likely that naphthalene will always dominate growth of the smallest particles, whereas the alkylbenzenes may overtake naphthalene once the particles have grown beyond the point where the Kelvin effect strongly influences the condensation driving force (roughly 10 nm diameter).