The partitioning theory for organic aerosol, as presented by Pankow, is modified to use an effective saturation concentration, \( C^* \), instead of the partitioning coefficient. This modification simplifies the treatment by eliminating the need for an average molar weight and using specific molar weights. The effective saturation concentration is derived from Raoult's law, adjusted for molality-based activity coefficients. This approach is particularly useful in scenarios involving oligomerization, where the modified activity coefficient ensures more realistic behavior. The kinetic transformation section outlines a method for modeling the chemical evolution of semi-volatile materials using a transformation matrix. This matrix describes the movement of material between different volatility bins, representing both the condensed phase and the vapor phase. The transformation matrices account for processes such as gas-phase oxidation, condensed-phase oxidation, and oligomerization. Reversible processes, like oligomerization, require additional bins to differentiate between reversible and irreversible low-volatility mass. The simplified example uses a single transformation matrix for both phases, with a basis set that conserves mass, except for the volatile bin which decays completely upon reaction. The framework can accommodate various forms of chemistry affecting organic aerosol mass, including gas-phase oxidation of intermediate volatility compounds, condensed-phase oxidation, and oligomer formation. These processes can significantly influence the aerosol volume and composition, highlighting the importance of considering these reactions in aerosol models.The partitioning theory for organic aerosol, as presented by Pankow, is modified to use an effective saturation concentration, \( C^* \), instead of the partitioning coefficient. This modification simplifies the treatment by eliminating the need for an average molar weight and using specific molar weights. The effective saturation concentration is derived from Raoult's law, adjusted for molality-based activity coefficients. This approach is particularly useful in scenarios involving oligomerization, where the modified activity coefficient ensures more realistic behavior. The kinetic transformation section outlines a method for modeling the chemical evolution of semi-volatile materials using a transformation matrix. This matrix describes the movement of material between different volatility bins, representing both the condensed phase and the vapor phase. The transformation matrices account for processes such as gas-phase oxidation, condensed-phase oxidation, and oligomerization. Reversible processes, like oligomerization, require additional bins to differentiate between reversible and irreversible low-volatility mass. The simplified example uses a single transformation matrix for both phases, with a basis set that conserves mass, except for the volatile bin which decays completely upon reaction. The framework can accommodate various forms of chemistry affecting organic aerosol mass, including gas-phase oxidation of intermediate volatility compounds, condensed-phase oxidation, and oligomer formation. These processes can significantly influence the aerosol volume and composition, highlighting the importance of considering these reactions in aerosol models.