The overall goal of this project is to measure the concentration and chemistry of hydroxyl and hydroperoxyl radicals in a forest environment in order to improve understanding of the influence of biogenic emissions on the complex photochemistry of the atmosphere. Although the number of hydroxyl radical measurements in forested environments has grown over the past few years, there are few measurements at a single site designed to address whether these individual measurements are representative of the overall chemistry of these regions and whether there are still unknown interferences with the measurement technique. The research will focus on minimizing potential interferences with the detection of hydroxyl using laser-induced fluorescence techniques, as well as extending the record of hydroxyl, hydroperoxyl, and hydroxyl reactivity measurements both above and below the forest canopy at the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions and Transport) tower site in Northern Michigan. In addition, laboratory experiments will be conducted to measure the rate constants of reactions of some of the oxygenated products of various biogenic volatile organic compounds in order to improve understanding of their oxidation mechanisms.
The inability of current numerical models of atmospheric chemistry to explain measured concentrations of hydroxyl and hydroperoxyl radicals in forested environments has important implications to current issues of air quality and climate change, as it calls into question understanding of the contribution of biogenic emissions to the production of ozone and aerosols in the atmosphere. In addition to being a greenhouse gas, ozone has known impacts on humans and the biosphere, while atmospheric aerosols have also been shown to have serious health impacts in addition to impacting solar radiation. The results of this project will help to improve understanding of the chemistry of biogenic emissions, their impact on ozone and aerosol formation, and how this chemistry will change with future climate change. This project will also provide research and educational opportunities for postdoctoral associates, graduate students and undergraduates, including several students from underrepresented groups. At the new Teaching and Research Preserve laboratory facility near the Indiana University Bloomington campus, instruments will be operated for periods during the academic year as part of a laboratory-based atmospheric chemistry curriculum for courses at both the undergraduate and graduate levels, allowing students to participate in obtaining and analyzing ambient measurements of hydroxyl, hydroperoxyl, ozone, and nitrogen oxides, allowing them to experience the excitement of science through discovery.
The hydroxyl radical (OH) and peroxy radicals, both the hydroperoxy (HO2) and organic peroxy radicals (RO2) play a central role in the chemistry of the atmosphere. The hydroxyl radical controls the atmospheric lifetime of methane, alternative chlorofluorocarbons, carbon monoxide and many other trace gases important to air quality and climate change. In addition the OH radical initiates the oxidation of volatile organic compounds (VOCs) that lead to the production of ozone and secondary organic aerosols in the atmosphere. In addition to being a greenhouse gas, ozone has known impacts on humans and the biosphere, while atmospheric aerosols have also been shown to have serious health impacts in addition to affecting the amount of solar radiation reaching the surface. However, recent measurements of OH radicals in the atmosphere have revealed serious disagreements with model predictions, especially in forested environments with high emissions of natural VOCs. Intellectual Merit: The inability of current models of atmospheric chemistry to explain measured concentrations of OH radicals in forested environments has important implications to current issues of air quality and climate change, as it brings into question our understanding of the contribution of biogenic emissions to the production of ozone and aerosols in the atmosphere as well as our ability to predict how well the atmosphere will be able to remove methane and other greenhouse gases in the future. However, recent studies have suggested that some instruments that measure OH in the atmosphere may suffer from unknown interferences, which could explain the observed discrepancies with models. The original goal of this project was to make measurements of the concentration and chemistry of OH and HO2 radicals in different forest environments in order to improve our understanding of the influence of biogenic emissions on the complex chemistry of the atmosphere. Based on reviewer comments, the overall goal was modified and focused on experiments designed to determine whether measurements of OH and HO2 radicals using the Indiana University Fluorescence Assay by Gas Expansion (IU-FAGE) instrument is free from interferences, and to make additional measurements of OH and HO2 radicals at the PROPHET (Program for Research on Oxidants: Photochemistry, Emission, and Transport) forested site in northern Michigan in order to determine whether previous measurements at this site were free from interferences. During July-August of 2012, we did extensive measurements at the PROPHET site to quantify potential interferences with the IU-FAGE instrument using an ambient chemical scrubbing technique. These measurements did not reveal any unknown interferences associated with the technique. These results suggest that previous measurements of OH radical by the IU-FAGE instrument at this site were not influenced by an unknown interference, in contrast to recent results from other groups. In addition, we performed extensive laboratory tests to quantify potential interferences, and discovered that the IU-FAGE instrument is susceptible to an interference from the reaction of ozone with several biogenic VOCs. However, under typical atmospheric concentrations the interference appears to be below the detection limit of the instrument, suggesting that this interference is negligible under ambient conditions, consistent with the measurements at PROPHET. Based on these experiments, future measurements of OH will employ a chemical scrubbing technique to quantify any unknown interference. We also performed extensive laboratory tests and discovered that measurements of HO2 by the IU-FAGE instrument are sensitive to interferences from some organic peroxy radicals, in particular peroxy radicals produced from the oxidation of biogenic VOCs such as isoprene. As a result, measurements of peroxy radicals by the IU-FAGE instrument in forest environments reflect the sum of HO2 and isoprene-based RO2 radicals (together HO2*). Comparing the measured HO2* to the model reveals that the model overpredicts the concentration of these radicals, suggesting that the model is missing a radical significant sink. Broader Impacts: These results suggest that our previous measurements of OH were likely free from unknown interferences. These measurements of OH were in good agreement with that predicted by a detailed model of atmospheric chemistry, suggesting that current models of atmospheric chemistry may be better at simulating the chemistry of biogenic emissions than previously believed. However, the inability of atmospheric chemistry models to reproduce the observed HO2* concentrations bring into question the ability of these models to accurately predict rates of ozone production in forested environments. Additional measurements are still needed to improve our understanding of the impact of biogenic emissions on atmospheric chemistry and improve our ability to predict how this chemistry may change with future climate change.