Measurement report: Dependence of Aerosol–Sea Fog Interactions on the Cloud-Aerosol Mass Mixing Ratio (MCr) and Liquid Water Content (LWC) in Advection Sea Fog over the East China Sea

Abstract. Aerosol effects on sea fog microphysics are nonmonotonic with aerosol concentration, and complex aerosol-sea fog interactions leave substantial uncertainty in microphysical responses. Five sea fog events observed over the East China Sea during March–May 2025 are analyzed with the aerosol-cloud interaction index (ACI) and cloud-aerosol mass mixing ratio (MCr). Positive ACI time fraction generally decreases with fog duration. MCr ranges from 2.17 to 2.93. The MCr-ACI transition range is 2.50–2.63, and 2.63 is selected as an empirical grouping threshold. Under liquid water content (LWC) constraints, ACI_re at high-MCr range increases from 0.02 to 0.14 with LWC, consistent with a progressively stronger Twomey effect in droplet-size reduction. At low-MCr, ACI_re changes from -0.09 and -0.03 at low and intermediate LWC to 0.21 at high-LWC, indicating that the droplet-size response can change direction with LWC. Fog droplet size spectra show that, in the high-MCr range, the response of the small-droplet mode to increased aerosol loading strengthens with LWC. Under high-LWC, the small-droplet modal peak for high aerosol loading at low-MCr is about four times the corresponding high-MCr value. This pattern exhibits pronounced small-droplet accumulation and relatively constrained subsequent growth. This difference indicates MCr = 2.63 can distinguish relatively complete and constrained activation-growth microphysical states. The autoconversion threshold function (T) response further indicates that aerosol effects on droplet coalescence also depend on the MCr-LWC state. These results show that jointly considering MCr-LWC better distinguishes the direction and strength of aerosol effects on sea fog microphysics.