Effect of Water and Acidity on Formation of Secondary Organic Aerosol

California Institute Of Technology, Pasadena CA

Investigators

Abstract

This project supports a comprehensive study of the role of water in the formation of secondary organic aerosol (SOA). SOA forms as a result of atmospheric oxidation of volatile organic compounds, leading to products that partition between the gas and particle phases. Observed atmospheric levels of organic aerosol, of which the preponderance is SOA, exceed those predicted by current models. Water is ubiquitous in atmospheric particles, but the role of aerosol water in the gas-particle partitioning of organics and in particle-phase chemistry is unclear. The project will include: (1) application of a rigorous model of the phase behavior and gas-particle partitioning of inorganic-organic aerosol systems; (2) laboratory chamber studies with a full spectrum of volatile organic compounds (VOCs) designed to reveal the mechanism and importance of water in gas-particle partitioning and heterogeneous chemistry in SOA formation; and (3) analysis of ambient data with respect to the role of particle-phase chemistry and acidity in determining SOA formation. Thermodynamic modeling will be applied to: (1) artificial, well-defined organic-inorganic mixtures; (2) controlled aerosol chamber experiments; and (3) ambient data. The overall goal of the experimental chamber research program is to address comprehensively the effects of relative humidity (RH), aerosol acidity, and particle-phase chemistry on SOA formation. The experiments to be conducted will involve: (1) a full spectrum of VOCs; (2) high and low nitrogen oxide (NOx) levels; (3) variation of seed aerosol acidity; and (4) variation of RH. These include mechanisms of organic aerosol aging that lead to the highly oxidized state found in the atmosphere. In the laboratory chamber studies, unique marker compounds for both particulate-phase chemistry and acidity-enhanced SOA formation will be identified. Evaluation of the extent to which such compounds are present in ambient aerosol samples can shed light on the question of whether it is possible to discern the sources of organic carbon as the aerosol "ages" toward its homogenized, highly oxidized state. This work will contribute to understanding of climate sensitivity by improving our knowledge of the radiative forcing of atmospheric aerosols. Aerosol radiative forcing depends directly on the mass of aerosols in the atmosphere. Organic aerosols comprise roughly one-half of the global aerosol mass, and as much as 90% of the organic aerosol mass is formed as a result of the gas-phase oxidation of volatile organic compounds. The model of aerosol composition to be developed and the laboratory data to be obtained will be made available to the community. A postdoctoral scholar and two Ph.D. students will be trained in the project, all of whom will attend scientific conferences to present their research results, which will also be published in peer-reviewed journals.