Why organic aerosol acidity matters: Bridging molecular acidity and global aerosol–cloud chemistry

Atmospheric aerosol acidity governs a wide range of chemical processes, yet current global climate and chemical transport models calculate aerosol and cloud pH assuming that organic aerosol (OA) components are electrically neutral. This omission persists despite observations showing that organics comprise ~40% of global aerosol mass and frequently include weakly and strongly acidic species. As a result, a major contributor to particle-phase hydrogen ion budgets is systematically neglected in models.

Here we address this gap by introducing organic aerosol acidity into a global aerosol–chemistry–climate model. We first implement an idealized representation of OA acidity based on intrinsic bulk-phase acid dissociation, treating organic species as weak acids that contribute dynamically to aerosol hydrogen ion concentrations. This bulk-acidity case serves as an upper-limit, chemically ideal reference. To account for non-ideal behaviour under atmospheric conditions, we then introduce suppressed organic acid dissociation, representing deviations arising from surface effects in small droplets, mixed-acid systems, and other environmental constraints.

In parallel, we identify a second chemical inconsistency in the model: the oxidation of SO₂ by H₂O₂ is treated using a pH-insensitive kinetic formulation. We replace this with a pH-dependent general-acid catalysis mechanism, allowing organic acids to act as proton donors in aqueous sulfate formation. These developments are implemented first in a box-model framework and subsequently translated to the fully coupled global climate model ECHAM–HAMMOZ.

Including organic aerosol acidity substantially increases aqueous sulfate production, leading to enhanced cloud droplet number concentrations across large regions. The resulting changes strengthen shortwave cloud radiative cooling, yielding an additional cloud radiative forcing of approximately -0.6 to -1.0 W m^-2, depending on the degree of non-ideality assumed. This forcing is comparable to the current uncertainty range associated with aerosol–cloud interactions, demonstrating that organic aerosol acidity constitutes a previously missing and climatically significant chemical driver that should be represented in global models.