Aqueous OH kinetics of aliphatic compounds in the context of formation and evolution of biogenic secondary organic aerosols

Hydroxyl radical (OH) is the major daytime oxidant, playing a key role in the atmospheric (photo)chemistry of numerous organics. More recently, there has been an increasing focus on multiphase reactions responsible for the formation and evolution of biogenic secondary organic aerosols (BSOAs). BSOAs are major components of fine particulate matter (PM), which strongly affects the climate and public health. The enhanced formation of SOAs in the aqueous phase (_aqSOAs) may, at least in part, explain the discrepancies between observed and modeled budgets of organic aerosols.

Models are essential for understanding and predicting how emissions and chemical transformations shape the atmospheric chemistry. Despite the now well-documented influence of the multiphase reactions on the formation and evolution of BSOAs, modeling such processes remains challenging. This is, in part, because encompassing the OH-mediated transformation of biogenic, water-soluble organic compounds (WSOCs) into the atmospheric models requires advanced predictive tools.

To resolve the extreme molecular complexity of chemical reactions leading to BSOAs, automated generators were introduced. These systems can provide near-explicit reaction schemes, often necessary to represent chemical transformations of the numerous, atmospherically widespread organics. Reaction rate coefficients - (k_OH M^-1s^-1) in case of OH-initiated oxidation in the aqueous phase, are a pivotal element of mechanism generators. However, kinetic databases exist for only a small subset of chemically diverse WSOCs present in the atmosphere. For this reason, generating (near)explicit mechanisms requires predicting the vast majority of rate coefficients. Hence, the reliability of these automated expert systems largely depends on kinetic models, primarily structure-activity relationships (SARs), which predict k_OH for structurally diverse reactants.

SARs are regression models that use the measured properties of the (model) molecules to predict the properties (here k_OH values) of a larger number of compounds, for which no experimental data exists. SARs are based on and evaluated against experimental data. At the same time, the kinetic data for many atmospherically widespread WSOCs remain limited.

In the work presented, the values of k_OH for aliphatic alcohols, carbonyls, carboxylic acids, and esters were measured with the relative rate technique. Measurements were conducted in a custom-designed aqueous photoreactor, and the WSOCs under investigation were quantified using gas and liquid chromatography. With this approach, ≈ 30 k_OH values can be measured in a single experiment, generating a significant amount of new data. To date, we have measured temperature-dependent values of k_OH for more than 100 aliphatic WSOCs. The values of activation parameters obtained from these measurements provided new insights into the mechanisms of aqueous oxidation of WSOCs by the OH. Furthermore, this new kinetic dataset was combined with the existing data to improve and expand the applicability domain of kinetic SARs.