Understanding how gas-phase reaction conditions affect organic aerosol formation
Fast, Jerome D — Pacific Northwest National Laboratory
Area of research
Aerosol particles affect visibility, human health, and climate change. They are required for cloud droplet formation, making understanding their formation important. Most aerosol forms in the atmosphere when carbon-based, or organic, gases released by plants undergo chemical reactions. Representing these processes in models is difficult due to their complexity. Researchers conducted laboratory experiments investigating the influence of reaction conditions on the mass and chemical composition of aerosol formed from the oxidation of organic gases emitted by trees, known as monoterpenes. The reaction conditions were chosen to mimic common real-world conditions and will improve scientific understanding of aerosol formation.
Organic aerosol (OA) represents a substantial and often dominant fraction of the total fine aerosol mass over continents. Researchers characterized the gas and particle compositions of OA formed from monoterpene oxidation under different oxidation conditions. They connected the measurements to OA formation and physical properties to infer mechanistic insights controlling growth by OA formation. The results suggest that gas-phase nitric oxide (NO) and hydroperoxy radical (HO2) levels, ozone concentration, and the molecular structure of organic precursors have considerable influence on the chemical and physical properties of monoterpene OA. Lumping all monoterpenes into a single category may produce errors in modeled OA concentrations. At the same time, the results suggest that simplification of NOx impacts may be reasonable in models primarily interested in reproducing OA mass.
Monoterpene photooxidation plays an important role in secondary organic aerosol (SOA) formation in the atmosphere. Products of monoterpene oxidation can enhance new-particle formation and particle growth, influencing climate feedbacks. Researchers performed α-pinene and Δ-3-carene photooxidation experiments in continuous-flow mode in an environmental chamber under multiple reaction conditions. The results illustrate the roles of oxidants, addition of NO, and volatile organic compound molecular structure in influencing SOA yield. SOA yield from α-pinene photooxidation shows a weak dependence on hydrogen peroxide, a proxy for HO2, concentration. The high O:C ratios observed in the α-pinene photooxidation products suggest the production of highly oxygenated organic molecules (HOM). The addition of ozone to the chamber during low-nitrogen oxides photooxidation experiments leads to a higher SOA yield. Adding NO enhances the production of N-containing HOMs. The SOA yield shows a modest, non-linear dependence on the input NO concentration. Carene photooxidation leads to higher SOA yield than α-pinene under similar reaction conditions. These results improve the understanding of SOA formation from monoterpene photooxidation and could be applied to refine representations of biogenic SOA formation in models.