Sensitivity of aerosol and cloud properties to coupling strength of marine boundary layer clouds over the northwest Atlantic

Quantifying the degree of coupling between marine boundary layer (MBL) clouds and the surface is critical for understanding the evolution of low clouds and explaining the vertical distribution of aerosols and microphysical cloud properties. Previous work has characterized the boundary layer as either coupled or decoupled, but this study rather considers four degrees of coupling, ranging from strongly to weakly coupled. We use aircraft data from the NASA Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) to assess aerosol and cloud characteristics for the following four regimes, quantified using differences in liquid water potential temperature (θ_{`</sub>) and total water mixing ratio (q<sub>t</sub>) between flight data near the surface level (∼ 150 m) and directly below cloud bases: strong coupling (1θ<sub>`} ≤ 1.0 K, 1q_t ≤ 0.8 g kg⁻¹), moderate coupling with high 1θ_{`</sub> (1θ<sub>`} > 1.0 K, 1q_t ≤ 0.8 g kg⁻¹), moderate coupling with high 1q_t (1θ_{`</sub> ≤ 1.0 K, 1q<sub>t</sub> > 0.8 g kg<sup>−1</sup>), and weak coupling (1θ<sub>`} > 1.0 K, 1q_t > 0.8 g kg⁻¹). Results show that (i) turbulence is greater in the strong coupling regime compared to the weak coupling regime, with the former corresponding to more vertical homogeneity in 550 nm aerosol scattering, integrated aerosol volume concentration, and giant aerosol number concentration (D_p > 3 µm) coincident with increased MBL mixing; (ii) cloud drop number concentration is greater during periods of strong coupling due to the greater upward vertical velocity and subsequent activation of particles; and (iii) sea salt tracer species (Na⁺, Cl⁻, Mg²⁺, K⁺) are present in greater concentrations in the strong coupling regime compared to weak coupling, while tracers of continental pollution (Ca²⁺, non-sea-salt (nss) SO₄²⁻, NO₃⁻, oxalate, and NH₄⁺) are higher in mass fraction for the weak coupling regime. Additionally, pH and Cl⁻ : Na⁺ (a marker for chloride depletion) are consistently lower in the weak coupling regime. There were also differences between the two moderate regimes: the moderate with high 1q_t regime had greater turbulent mixing and sea salt concentrations in cloud water, along with smaller differences in integrated volume and giant aerosol number concentration across the two vertical levels compared. This work shows value in defining multiple coupling regimes (rather than the traditional coupled versus decoupled) and demonstrates differences in aerosol and cloud behavior in the MBL for the various regimes.