No abstract

Abstract. Results from a tightly constrained photochemical point model for OH and HO2 are compared to OH and HO2 data collected during the Program for •e•earch 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 NOz levels and high isoprene emissions. Detailed HOz 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.

No abstract

Emissions of volatile chemicals control the hydroxyl radical (OH), the atmosphere's main cleansing agent, and thus the production of secondary pollutants. Accounting for all of these chemicals can be difficult, especially in environments with mixed urban and forest emissions. The first direct measurements of the atmospheric OH reactivity, the inverse of the OH lifetime, were made as part of the Southern Oxidant Study (SOS) at Cornelia Fort Airpark in Nashville, TN in summer 1999. Measured OH reactivity was typically 11 s(-1). Measured OH reactivity was 1.4 times larger than OH reactivity calculated from the sum of the products of measured chemical concentrations and their OH reaction rate coefficients. This difference is statistically significant at the 1sigma uncertainty level of both the measurements and the calculations but not the 2sigma uncertainty level. Measured OH reactivity was 1.3 times larger than the OH reactivity from a model that uses measured ambient concentrations of volatile organic compounds (VOCs), NO, NO2, SO2, and CO. However, it was within approximately 10% of the OH reactivity from a model that includes hydrocarbon measurements made in a Nashville tunnel and scaled to the ambient CO at Cornelia Fort Airpark. These comparisons indicate that 30% of the OH reactivity in Nashville may come from short-lived highly reactive VOCs that are not usually measured in field intensive studies or by US EPA's Photochemical Assessment Monitoring Stations.

No abstract