Making Activity Corrections Feasible: Adding Complexity to Modeling Organic Particulate Matter with Minimal Computational Expense

JOHN SCHNEIDER (1), William Asher (1,2), James Pankow (1)

(1) Portland State University, (2) University of Washington

     Abstract Number: 528
     Last modified: May 13, 2010

Abstract
SimAct.v1 was developed as a computationally inexpensive, simple algorithm for predicting activity coefficients of atmospherically relevant gas/particle partitioning species. For each compound, the method predicts activity coefficients as a function of temperature according to the carbon number and total molecular polarity. It represents a compromise between the computationally expensive UNIFAC method and the overly simplistic method of setting all activity coefficients to one, which can lead to large prediction errors in organic particulate matter (OPM) levels, especially when the total particulate matter (TPM) level is low. Currently, many large-scale models do not consider activity corrections when predicting atmospheric PM, but instead rely on the assumption that all activity coefficients are equal to one. SimAct.v1 makes activity corrections feasible in large-scale models by predicting activity coefficients using only simple, pure compound properties without the need for inefficient summation calculations required for methods such as UNIFAC.
More than 26,000 SimAct-predicted activity coefficients were fit to UNIFAC-predicted activity coefficient values over the temperature range of 273 K – 313 K using a basis set of water and 18 organic compounds closely representative of both primary and secondary organic aerosol. The basis set contained a wide variety of species and functional groups (hydroxyls, carboxylic acids, ketones, aldehydes, organic nitrates and sulfates, peroxy acyl nitrates, hydroperoxides, and peroxy acids). Box model testing has demonstrated that SimAct.v1 more reliably predicts total PM mass, the mass of a compound in the condensed phase, and water uptake relative to UNIFAC than for the case where all activity coefficients are set to one.