A51A-03 Laboratory studies of the oxidative lifecycle of reactive organic carbon (Invited)

The oxidation of reactive organic carbon (ROC) forms a number of secondary products central to air quality and climate, including ozone (O₃), secondary organic aerosol (SOA), and carbon dioxide (CO₂). The initial oxidation steps (the first 1-2 generations, occurring over timescales of hours to days) are the focus of most laboratory studies of ROC oxidation, due to their importance in the formation of SOA and ground-level ozone. Later oxidation steps – the formation and loss of oxidized products, occurring over timescales of several days to weeks – have received considerably less study; however,such reactions can have an important influence on ozone production and radical cycling in the remote atmosphere. Here we describe laboratory studies aimed at mapping out the entire ROC oxidative lifecycle, from the initial oxidation steps to the complete mineralization (conversion to CO or CO₂) of the carbon. In these experiments, a model organic compound (isoprene, α-pinene, or 1-hexene) is introduced into a small (150 L) Teflon chamber and exposed to high levels of OH radicals (formed from the 254 nm photolysis of O₃ in the presence of water vapor). The formation and evolution of reaction products are measured in real time by a suite of analytical instruments, over the equivalent of several weeks of atmospheric oxidation. In all cases, early-generation reaction products (both gas-phase species and SOA) are produced but then react away, forming a range of successively smaller and long-lived oxygenates. After the equivalent of ~1 day of aging, total ROC levels begin decreasing, likely due to mineralization; such decays enable estimates of the overall oxidative lifetimes of atmospheric ROC. Laboratory results (product distributions, OH reactivity, and ROC lifetimes) are compared against predictions from several chemical mechanisms used in air quality and atmospheric chemistry. These comparisons provide insights into the limitations both of current mechanisms and of laboratory studies of ROC aging, and highlight major uncertainties in ROC oxidation, particularly over long timescales.