HO_x budgets in a deciduous forest: Results from the PROPHET summer 1998 campaign

Results from a tightly constrained photochemical point model for OH and HO₂ are compared to OH and HO₂ data collected during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 intensive campaign held in northern Michigan. The PROPHET campaign was located in a deciduous forest marked by relatively low NO_x levels and high isoprene emissions. Detailed HO_x budgets are presented. The model is generally unable to match the measured OH, with the observations 2.7 times greater than the model on average. The model HO₂, however, is in good agreement with the measured HO₂. Even with an additional postulated OH source from the ozonolysis of unmeasured terpenes, the measured OH is 1.5 times greater than the model; the model HO₂ with this added source is 15% to 30% higher than the measured HO₂. Moreover, the HO₂/OH ratios as modeled are 2.5 to 4 times higher than the measured ratios, indicating that the cycling between OH and HO₂ is poorly described by the model. We discuss possible reasons for the discrepancies.