Insights into the biosphere-atmosphere exchange of organic gases from seasonal observations over a ponderosa pine forest
Fulgham, S. Ryan, author
Farmer, Delphine, advisor
Ham, Jay M., committee member
Ravishankara, Akkihebbal R., committee member
Van Orden, Alan, committee member
The biosphere-atmosphere exchange of organic gases over forests contributes to the formation of air pollution and the availability of forest nutrients. Forests can be both sources and sinks of volatile and semi-volatile organic compounds to the atmosphere. The role that forests play in controlling organic acid concentrations remains poorly understood, with multiple model-measurement comparisons reporting missing sources of formic acid. Large, missing sources of organic acids have been identified over different forested environments. Despite substantial seasonal variability in forest productivity and environmental conditions, a paucity of observations, during seasons other than summertime, is available. Although forest fires are a major source of hazardous organic gases and particulate matter, few measurements of semi-volatile organic compounds emitted by forest fires are available from within 1 km of the fire. Detection further-afield cannot disambiguate between chemistry at the source of the fire and chemical aging as a smoke plume traverses the atmosphere. Near-field observations are needed to characterize emissions attributable to combustion and pyrolysis processes. To improve understanding of processes that control the atmospheric budgets of organic acids, water-soluble pollutants with physicochemical properties similar to organic acids, and fire-emitted phenolic compounds, this dissertation reports measurements of the biosphere-atmosphere exchange of a suite of organic gases over a Rocky Mountain ponderosa pine forest in Colorado over four, seasonally-representative measurement campaigns. First, we report seasonally persistent, upward fluxes of organic acids, which are neither explained by direct emissions nor secondary production. Second, we present evidence for equilibrium partitioning into and out of water films on forest surfaces as both a missing source and sink of isocyanic acid and small alkanoic acids. Finally, we report significant enhancement of organic acids, phenolic compounds, and other nitrogen containing compounds during initiation of a controlled forest fire compared with the remainder of the burn. Nitrated phenols are rapidly produced and enhanced more than phenolic precursors during initial, higher temperature conditions. We attribute greater enhancement of nitrated phenols to high NOx emissions under higher temperature conditions.
Includes bibliographical references.