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.
Airborne particles (aerosols) are ubiquitous in the Earth's atmosphere. They arise from natural sources such as wind-blown dust, sea salt, biogenic vegetation emissions, and fires, and from anthropogenic sources such as combustion of fuels. Atmospheric aerosols are a mixture of inorganic and organic substances. The inorganic fraction contains predominantly sulfate, from burning of sulfur-containing fuels, nitrates, from conversion of gaseous nitrogen oxides in the air, ammonium, from emissions of ammonia, and a variety of other trace compounds. The organic fraction contains a myriad of species, largely a result of the gas-phase oxidation of volatile organic compounds to yield products of sufficiently low volatility to condense into the particle phase, so-called Secondary Organic Aerosol (SOA). Globally, airborne particles are roughly 50% or more organic species, and in some locations can be as much as 90% organics. Water vapor is ubiquitous in the atmosphere, and, as a result, a certain amount of condensed water is present in each airborne particle, as many of the particulate species are water-friendly. An important branch of atmospheric science aims to understand the contributions of different emission sources and of gas-phase atmospheric chemistry to the amount and make-up of airborne aerosols. An important motivation for this study is the dominant role atmospheric aerosols play in human health effects; another important role is in the Earth's climate. In their climatic role, particles reflect sunlight back to space and thereby serve to act as a cooling influence on the Earth; they also are the nuclei on which clouds form. If the Earth were totally devoid of airborne aerosols, there would be no clouds on the Earth, and Earth would be a totally different planet. We seek to quantify the contribution of particles to the Earth's energy balance. We have learned that airborne aerosols are microscopic floating chemistry laboratories, and understanding that chemistry is paramount to tracing particles to their sources and predicting their role in climate. This project aimed to quantify, through measurements and fundamental chemical theory, how water vapor in the air interacts with aerosols; in general, the higher the humidity, the more water a particle will take up, the larger the particle will be, and the greater its effect on scattering of light and in the human lung. One of the important outcomes of the project is a detailed model of the thermodynamic interactions among the inorganic and organic constituents and water in a particle. This model has been made available to the entire scientific community through a website. As mentioned above, the principal route by which organic species enter the particle phase is via gas-phase chemistry that produces low volatility compounds. But, once in the particles, these compounds may continue to chemically react to produce even more complex products. Studying such chemistry in particles that are many times smaller than the width of a human hair presents significant analytical challenges. In this project, we also studied, through mass spectrometry, the chemical reactions that occur in aerosols. These studies were carried out using two large transparent Teflon reactors, each about the size of a small room, in which we can tailor-make our own atmosphere and its particles, to simulate the dynamics that occur in the actual atmosphere. One reactor is set up to simulate the conditions characteristic of the pristine atmosphere that is present over remote continental areas, and the other reactor has conditions like those of an urban atmosphere. As a result of the research carried out in this project, we have a better understanding of the types of chemical reaction that occur in airborne particles and the types of products that result from them. The project resulted in 21 peer-reviewed publications.
