Measuring and Modeling the Tropospheric Hydroxyl Radical (OH)

Abstract

Efforts to measure the tropospheric hydroxyl radical concentration OH, the transient that plays a role in acid deposition formation and cleanses the atmosphere of many pollutants, began 20 years ago. In the past five years considerable progress has been made in local (in situ or point) measurement of OH (refer to other papers in this issue). New measurements put constraints on photochemical models that compute OH and other species. Reproducing observed OH with a model is more successful in some regimes than in others with calculated OH usually within 20% of measurements. This paper reviews OH photochemistry, comparisons of model-derived and measured OH in the continental boundary layer, and model calculation of OH in the marine boundary layer and free troposphere when high-frequency aircraft measurements of O₃, and related trace gases are assimilated into a photochemical model. Factors affecting model calculation of OH—nonlinearities, imprecisions in kinetics, differences among model formulation-are also discussed.