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

Abstract. Quantifying the degree of coupling between marine boundary layer clouds and the surface is critical for understanding the evolution of low clouds and explaining the vertical distribution of aerosols and microphysical cloud properties. In this study, we use aircraft data from the NASA Aerosol Cloud meTeorology Interactions oVer western ATlantic Experiment (ACTIVATE) to assess aerosol and cloud characteristics for the following four regimes of coupling strength as quantified using differences in liquid water potential temperature (θ_l) and total water mixing ratio (q_t) between a near-surface level (~ 150 m) and directly below cloud bases: strong coupling (Δθ_l  ≤ 1.0 K, Δq_t  ≤ 0.8 g kg^-1), moderate coupling with high Δθ_l (Δθ_l > 1.0 K, Δq_t ≤ 0.8 g kg^-1), moderate coupling with high Δq_t (Δθ_l  ≤ 1.0 K, Δq_t > 0.8 g kg^-1), weak coupling (Δθ_l > 1.0 K, Δq_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 smaller differences in 550 nm aerosol scattering, integrated aerosol volume concentration, and giant aerosol number concentration (D_p > 3 µm) between the near-surface level and just below marine boundary layer (MBL) cloud bases 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, (iii) sea-salt tracer species (Na⁺, Cl^-, Mg²⁺, K⁺) are present in greater concentrations in the strong coupling regime compared to weak coupling, while Ca²⁺, nss-SO₄^2-, NO₃^-, oxalate, and NH₄⁺ (tracers of continental pollution) 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 differences between the two moderate regimes: the moderate high Δq_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 between 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.