The carbon number-polarity grid: A means to manage the complexity of the mix of organic compounds when modeling atmospheric organic particulate matter

Current regional models for predicting atmospheric organic particulate matter (OPM) levels rely heavily on the computationally simple, lumped "two-product" (2p) approach for parameterizing the formation of secondary organic aerosol (SOA). At the other extreme, molecular kinetic models of SOA formation are available that use numerous complex reaction pathways to produce large numbers of potentially condensable organic oxidation products, but the complexities of those models currently limit implementation in large-scale air quality models. This study proposes use of a carbon number (n_C) vs. polarity grid (with concentration bins) for tracking the various OPM-relevant compounds and their time-dependent concentrations T_i (μg m^-3). The grid can be used when adding complexity to the 2p approach, or for managing and limiting the complexity of molecular kinetic models. For the former, an expanded n p + m P approach is proposed with n products and m possible types of very low volatility polymer (P) materials. For the latter, the grid limits system complexity by allowing compounds with similar structures to be lumped together in the same grid bin even when they are formed by different routes. With either an n p + m P approach or a molecular kinetic approach, the n_C vs. polarity grid will allow consideration of the effects of variation in the activity coefficients ζ_i of the partitioning compounds, variation in the mean molecular weight of the absorbing OPM phase, water uptake, and the possibility of phase separation in the OPM phase.