Understanding the Lifecycle of Organic Carbon Through Multiple Generations of Aging

Abstract:

Emissions of organic carbon to the atmosphere undergo oxidation reactions to yield hundreds of products, forming a multiphase, chemically dynamic system of organic aerosol and gas-phase products that span a wide range of volatilities. A complete understanding of the fate and transformations of organic carbon in the atmosphere therefore requires a detailed quantitative description of both gas- and particle-phase carbon, but attempts to understand the evolution of carbon through atmospheric oxidation has in nearly all cases resulted in a large fraction of “missing” or unidentified carbon. In our work, a large suite of state-of-the-art mass spectrometric and spectroscopic instrumentation was brought to bear on the oxidation by both ozone and OH of common biogenic emissions (α-pinene, β-pinene, β-caryophyllene), with OH concentrations spanning hours to days of simulated atmospheric aging. Organic carbon across all volatilities and functionalities was measured, from aerosols to high-volatility gases, including CO and formic acid. New chemical ionization and electron-impact ionization mass spectrometric instruments allowed for the characterization and quantification of low-volatility gases, which are observed to form quickly then decrease with growth in aerosol mass. Reaction conditions were varied to better understand the role played by measurement artifacts, such as loss of vapor to the walls, in typical laboratory oxidation experiments. By observing all carbon through multiple generations of oxidation, we examine the transitions between particles and lower-volatility gases with unprecedented detail and place them in the context of current simplified (i.e. two-dimensional) model frameworks, providing a more complete understanding of the evolution of organics in the atmosphere.