The overall goal of this project is to improve understanding of the influence of biogenic emissions on atmospheric chemistry through measurements of the concentrations and chemistry of OH (hydroxyl) and HO2 (hydroperoxyl) radicals in forested environments using a newly constructed instrument based on the technique of laser-induced fluorescence. The research will focus on measurements of OH and HO2 at various heights in and near the top of the forest canopy at both the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions and Transport) tower site in northern Michigan, as well as the AmeriFlux tower site in central Indiana. These measurements will help to determine whether unknown reactive biogenic emissions are a significant source of HOx (OH + HO2) radicals inside a forest canopy both during the day and at night. In addition to the proposed field measurements, laboratory experiments will be performed on various biogenic volatile organic compounds (VOCs) and their oxygenated products in order to better understand their oxidation mechanisms. The broader scientific results of this project will be an improved understanding of the contribution of biogenic emissions to ozone and aerosol formation in the atmosphere, leading to more effective control strategies for these pollutants. This project will also provide research and educational opportunities for postdoctoral associates, graduate students and undergraduates, including those from underrepresented groups. The HOx instrument may eventually be used in undergraduate and graduate laboratory-based atmospheric chemistry courses, allowing students to obtain and analyze ambient measurements of trace gases, and to experience the excitement of science through discovery.
Reactions involving 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. In addition to being the primary oxidant initiating the removal of many important greenhouse gases from the atmosphere, such as methane and the alternative chlorofluorocarbons, OH radical reactions initiate the oxidation of carbon monoxide and volatile organic compounds (VOCs) which in the presence of nitrogen oxides can lead to the production of ozone and secondary aerosols, the primary components of photochemical smog. However, recent measurements of OH and HO2 in the atmosphere have revealed serious disagreements with model predictions, especially in forested environments with high emissions of natural VOCs. 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. 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. The overall goal of this project was to make additional measurements of OH and HO2 radicals in forest environments in order to further test our understanding of the chemistry of biogenic emissions to the atmosphere. Measurements of these important radicals were made above the forest canopy in a northern Michigan forest using Indiana University’s newly developed laser-induced fluorescence instrument during the summers of 2008 and 2009 as part of the PROPHET (Program for Research on Oxidants: Photochemistry, Emission, and Transport) and CABINEX (Community Atmosphere Biosphere Interactions Experiment) measurement campaigns. These intensive campaigns included measurements of VOCs, nitrogen oxides, and other compounds that are sources and sinks of OH radicals in the atmosphere. The OH concentrations measured during these campaigns were found to be in good agreement with that predicted by a detailed model of atmospheric chemistry constrained by the measured OH sources and sinks. This is in contrast to previous measurements at this site and at similar sites around the world that found that the measured concentration of OH was 2-10 times larger than predicted. These previous studies have suggested that there may be a significant source of OH radicals missing from current models of atmospheric chemistry, which has important implications concerning our ability to predict how changes in emissions will affect the future composition of the atmosphere, including the ability of the atmosphere to remove important greenhouse gases such as methane. The agreement of the OH measurements made in this study are consistent with results from measurements made in the polluted urban environments of Mexico City and Los Angeles, where the measured OH and HO2 concentrations were found to be in good agreement with model predictions, giving further confidence in our ability to predict how changes in emissions will impact air quality in the future. The results of this study suggest that current models of atmospheric chemistry may be better at simulating the chemistry of biogenic emissions than previously believed. This gives confidence in the ability of these models to simulate airquality, including predictions of how the concentration of OH, ozone, and secondary aerosols in the atmosphere will change in the future. However, additional measurements are still needed in order to determine whether these measurements are characteristic of other forest environments around the world.
