Understanding measurements of OH, HO2, and OH reactivity in forest environments

The discrepancy between measured and modeled OH and HO2, collectively called HOx, remains a big challenge to the atmospheric chemistry community and has helped motivate the development of new chemical mechanisms for forest environments. For the past decade, studies have indicated that a significant OH source was needed to explain elevated amounts of OH that were observed in forests. Several laboratory and field studies have been designed to unravel the mystery of the missing OH source. Whether the discrepancy comes from real missing OH sources or from instrument artifacts in the presence of some BVOCs is still subject to debate. Recently, artifacts have been discovered in some measurements of both OH and HO2, including those from the Penn State HOx instrument (GTHOS on the ground and ATHOS in the air). Laboratory studies combined with field measurements have now verified these interferences and new methods for quantifying and removing them from field measurements have been devised. This presentation will demonstrate these methods for separating interference signals from OH and HO2 measurements and will then discuss the results for OH and HO2 measurements made during a ground-based field campaign in Sierra Nevada forest in 2009 (Biosphere Effects on Aerosols and Photochemistry Experiment II (BEARPEX09)) and during low-altitude flights over forests in the southern United States in 2012 (Deep Convective Clouds and Chemistry (DC3)).