Modeling Tropospheric Aqueous Interfacial Chemistry and Bulk Interaction with CAPRAM-HET2.0

Reactions at the air–water interfaces of any aqueous tropospheric particles, such as water-containing aerosol particles, haze, fog, cloud and rain droplets, can be important for atmospheric chemistry through their specific properties, which can increase the rates for certain reactions. Such accelerations can occur through (i) increased concentrations, (ii) increased rate constants, or (iii) a combination of both. A proper process description in models remains challenging due to the lack of data for both the above issues (i), (ii), and (iii). The first challenge was overcome by deriving a relationship between bulk–interface partition coefficients and octanol–water partition coefficients. This allowed us to calculate the interfacial concentration for numerous species. Results show that less soluble species prefer the interface, while the more soluble species prefer the bulk. A developed interfacial reaction mechanism was coupled to the CAPRAM bulk mechanism and applied for model simulations with an urban scenario. The simulation results show that interfacial chemistry can influence both the gas and aqueous composition, and systems with important effects are identified. Among the gas-phase species, HONO and the halogen compounds (Cl₂, Br₂, and I₂) were most affected. A HONO concentration increase by up to 348% was modeled during cloud periods. Despite a decrease of Cl₂ modeled on average, a daytime in-cloud concentration increase by 62% was modeled, mainly due to the interfacial reaction of HOCl with Cl^– and H⁺. Moreover, the modeling demonstrated that less soluble organic species can get more efficiently oxidized at the interface due to their stronger enrichment. This enables higher concentrations of some oxidized organic compounds, such as lactic acid (+18%), indicating that interfacial chemistry can support aqSOA formation.