Hydroxyl and Hydroperoxy Chemistry at the CalNex-LA 2010 Site: Model comparisons and Radical Budgets

The Hydroxyl (OH) and hydroperoxy (HO₂) radicals drive the oxidation of organic trace gases that can lead to the production of ozone and secondary organic aerosols in the atmosphere. Understanding the behavior of OH and HO₂ radicals is critical for the development of effective control strategies for the reduction of these pollutants. Previous measurements of these radicals in urban areas have shown discrepancies with modeled concentrations, bringing into question our understanding of this important chemistry in polluted environments. Measurements of OH and HO₂ radicals made during the CalNex LA campaign in May-June, 2010 were compared with predictions from several models, including the Regional Atmospheric Chemistry Mechanism (RACM), and the Master Chemical Mechanism (MCM). An extensive suite of supporting measurements was used to constrain the models, including photolysis rates and concentrations of NO_x, biogenic and other anthropogenic VOCs. On average, weekend OH concentrations are overpredicted and weekday HO₂ concentrations are underpredicted by the models with some models resulting in better accuracy than others. Average campaign and weekend/weekday radical budgets are examined to investigate discrepancies between the modeled and measured values.