Gas to Particle Partitioning of Organic Acids in the Boreal Atmosphere

Gas to particle partitioning of carboxylic acids was investigated using a high-resolution chemical ionization time-of-flight mass spectrometer (HR?CI?ToF?MS) with the filter inlet for gases and aerosol (FIGAERO). Specifically, the partitioning coefficients of 640 components with unique molecular composition were calculated from an assumed linear relationship between [particle]/[gas] versus the mass of the organic fraction (Morg) according to Raoult?s law, i.e., equilibrium phase partitioning. We demonstrate that, using the full data set, most of the compounds do not follow a linear relationship. This is especially the case for low- and high-molecular-weight species. Using a subset of the data, with concurrent low sulfate ambient observations ([SO42? < 0.4 ?g m?3), the relationship improved significantly and Ki could be derived from the slope of a linear regression to the data. The 100 species with the highest R2 (?0.7) of this regression are presented. The restrictions during high sulfate conditions can be explained by changes in either the equilibrium conditions (e.g., the activity coeffient, ?i) or uptake kinetics (mass transfer limitation). This study demonstrates that partitioning of compounds in the complex ambient atmosphere follows ideal Raoult?s law for some limited conditions and stresses the need for studies also in more polluted environments.