Resolving Issues of Hydroxyl (OH) Measurements and Oxidation Chemistry in Forest Environments

Understanding atmospheric oxidation chemistry is critical for establishing the links between atmospheric composition, air quality, and climate change. Hydroxyl (OH) is the atmosphere's primary oxidant. Comparisons of measurements and models provide evidence that OH and hydroperoxyl (HO2) chemistry is generally understood in clean, remote environments, but less well understood in cities when nitrogen oxide (NOx) levels are high. Other evidence indicates that OH is most poorly understood in low-NOx, high biogenic VOC (BVOC) environments, particularly forests where measured OH has greatly exceeded modeled OH by a factor of 2 to 10, well beyond the typical ±30% 2ó uncertainties.

These forest environments are challenging the models, for which new oxidation mechanisms are being designed, and the measurements, which can have interferences and be near their detection limits. A previously suspected interference was recently confirmed for the combination of O3, OH, and alkenes in the Principal Investigator's (PI) laser-based Ground-based Hydrogen Oxides Sensor (GTHOS) by adding the ability to detect OH by chemical removal of OH (called OHchem) to the usual method wavelength modulation on and off an OH spectral line (called OHwave). Laboratory tests demonstrate that OHchem is the real OH for GTHOS.

This project will contribute measurements of OHchem, OHwave, HO2, alkene-based RO2, and OH reactivity to a multi-investigator study that will provide a consistent picture of the isoprene oxidation mechanism, demonstrate that OHchem is the real OH, and determine if the GTHOS OH interference is caused by an atmospherically relevant intermediate species. The PIs will also find the reason that GTHOS appears to be much more sensitive to this O3/OH/alkene interference than other OH-measuring instruments and to reduce that sensitivity if the interference is not atmospherically relevant. These goals of this two-year study will be accomplished primarily by participating with other groups who are proposing separately in a Focused Isoprene-chemistry eXperiment (FIX), which has a field component as part of the Secondary Organic Aerosol Study and a laboratory environmental chamber component. The results of this research will lead to improved oxidation mechanisms in the models that are used for regional and for global air quality, thus providing better guidance to policy makers.