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

<p>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 (<span class="inline-formula"><i>θ</i>_ℓ</span>) and total water mixing ratio (<span class="inline-formula"><i>q</i>_t</span>) between flight data near the surface level (<span class="inline-formula">∼150</span> m) and directly below cloud bases: strong coupling (<span class="inline-formula">Δ<i>θ</i>_ℓ≤1.0</span> K, <span class="inline-formula">Δ<i>q</i>_t≤0.8</span> <span class="inline-formula">g kg⁻¹</span>), moderate coupling with high <span class="inline-formula">Δ<i>θ</i>_ℓ</span> (<span class="inline-formula">Δ<i>θ</i>_ℓ&gt;1.0</span> K, <span class="inline-formula">Δ<i>q</i>_t≤0.8</span> <span class="inline-formula">g kg⁻¹</span>), moderate coupling with high <span class="inline-formula">Δ<i>q</i>_t</span> (<span class="inline-formula">Δ<i>θ</i>_ℓ≤1.0</span> K, <span class="inline-formula">Δ<i>q</i>_t&gt;0.8</span> <span class="inline-formula">g kg⁻¹</span>), and weak coupling (<span class="inline-formula">Δ<i>θ</i>_ℓ&gt;1.0</span> K, <span class="inline-formula">Δ<i>q</i>_t&gt;0.8</span> <span class="inline-formula">g kg⁻¹</span>). 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 (<span class="inline-formula"><i>D</i>_p&gt;3</span> <span class="inline-formula">µm</span>) 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 (<span class="inline-formula">Na⁺</span>, <span class="inline-formula">Cl⁻</span>, <span class="inline-formula">Mg²⁺</span>, <span class="inline-formula">K⁺</span>) are present in greater concentrations in the strong coupling regime compared to weak coupling, while tracers of continental pollution (<span class="inline-formula">Ca²⁺</span>, non-sea-salt (nss) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn></msub><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="95945f53b3fbc040b883e7623294c88b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-2407-2025-ie00001.svg" width="34pt" height="16pt" src="acp-25-2407-2025-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="854abc5cffcc47c7a8d3c23f4d8e54ba"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-2407-2025-ie00002.svg" width="30pt" height="15pt" src="acp-25-2407-2025-ie00002.png"/></svg:svg></span></span>, oxalate, and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><msub><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn></msub><mo>+</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="23356e89e697acb5868d09068ed8ea2c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-2407-2025-ie00003.svg" width="29pt" height="14pt" src="acp-25-2407-2025-ie00003.png"/></svg:svg></span></span>) are higher in mass fraction for the weak coupling regime. Additionally, pH and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi mathvariant="normal">Cl</mi><mo>-</mo></msup><mo>:</mo><msup><mi mathvariant="normal">Na</mi><mo>+</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="de49dbf95b867cd2cc39cd77ac0f153e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-25-2407-2025-ie00004.svg" width="46pt" height="12pt" src="acp-25-2407-2025-ie00004.png"/></svg:svg></span></span> (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 <span class="inline-formula">Δ<i>q</i>_t</span> 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.</p>