A multi-site climatology of aerosol mixing state from hygroscopicity measurements

Abstract. We present an expanded, size-resolved observational climatology of aerosol mixing state inferred from hygroscopicity measurements. Using κ-PDFs from long-term and campaign-based HTDMA observations at eight sites spanning urban, continental, coastal, marine environments, we infer the aerosol mixing state index χ across multiple particle sizes and four seasons. Building on a previously evaluated κ-to-χ framework, we synthesize these datasets to characterize spatial, seasonal, and size-dependent variability in aerosol mixing state, with attention to retrieval limitations and site-dependent. We find that continental and accumulation-mode aerosol populations generally exhibit consistently higher χ, whereas marine and urban-influenced sites tend to be more externally mixed and show greater variability. Across most sites, χ increases with particle size, consistent with the stronger atmospheric aging of accumulation-mode particles. The use of D_α-D_γ diagnostics provides insight into the processes controlling the observed variability in χ. These results provide observational constraints on the size dependence, seasonal variability, and environmental controls of aerosol mixing state, and establish benchmarks for evaluating mixing-state assumptions in aerosol models relevant to cloud activation and radiative effects.