Variation in the sensitivity of predicted levels of atmospheric organic particulate matter (OPM)

This study examines the sensitivity in predicted levels of atmospheric organic particulate matter (M(0), mu g m(-3)) to changes in the governing gas/particle partitioning constants and the 97(levels of condensable organic compounds, mu g m(-3)). M(0) is given by the difference between Sigma J(j) and the corresponding sum for the gas-phase levels. It is demonstrated that the sensitivity in predicted M(0) levels increases rapidly as M(0) becomes very small relative to Sigma J(i) : as the J(i), decrease, the gas phase becomes increasingly capable of holding the majority of all J(i), and small changes in system parameters can cause large relative changes in M(0). These effects are illustrated using predictions for two values of the reacted hydrocarbon concentration (AHC) 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(0)/Sigma J(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 J(i) values for low volatility compounds) will be amplified into large errors in the predicted M(0) values.