Variation in the Sensitivity of Predicted Levels of Atmospheric Organic Particulate Matter (OPM)

Abstract

This study examines the sensitivity in predicted levels of atmospheric organic particulate matter (M(o), microg m(-3)) to changes in the governing gas/particle partitioning constants and the tau(I) (levels of condensable organic compounds, microg m(-3)). M(o) is given by the difference between sigma tau(i) and the corresponding sum for the gas-phase levels. It is demonstrated that the sensitivity in predicted M(o) levels increases rapidly as M(o) becomes very small relative to sigma tau(i): as the tau(i), decrease, the gas phase becomes increasingly capable of holding the majority of all tau(i) and small changes in system parameters can cause large relative changes in M(o). These effects are illustrated using predictions for two values of the reacted hydrocarbon concentration (deltaHC) for each of three secondary organic aerosol systems for relative humidity (RH) = 20-80%. Specific structures for the oxidation products allows consideration of the effects of varying activity coefficients and water uptake. At low M(o)/sigma tau(i) (as may be found in the atmosphere away from sources and at warm temperatures), relatively small errors in model input parameters (e.g., vapor pressures, vaporization enthalpies, activity coefficient parameters, and the tau(i) values for low volatility compounds) will be amplified into large errors in the predicted M(o) values.