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<p>A large part of the uncertainty in climate projections comes from uncertain aerosol properties and aerosol–cloud interactions as well as the difficulty in remotely sensing them. The southeastern Atlantic functions as a natural laboratory to study biomass-burning smoke and to constrain this uncertainty. We address these gaps<span id="page13912"/> by comparing the Weather Research and Forecasting with Chemistry Community Atmosphere Model (WRF-CAM5) to the multi-campaign observations ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS), CLARIFY (CLoud–Aerosol–Radiation Interaction and Forcing), and LASIC (Layered Atlantic Smoke Interactions with Clouds) in the southeastern Atlantic in August 2017 to evaluate a large range of the model's aerosol chemical properties, size distributions, processes, and transport, as well as aerosol–cloud interactions. Overall, while WRF-CAM5 is able to represent smoke properties and transport, some key discrepancies highlight the need for further analysis. Observations of smoke composition show an overall decrease in aerosol mean diameter as smoke ages over 4–12 d, while the model lacks this trend. A decrease in the mass ratio of organic aerosol (OA) to black carbon (BC), <span class="inline-formula">OA:BC</span>, and the OA mass to carbon monoxide (CO) mixing ratio, <span class="inline-formula">OA:CO</span>, suggests that the model is missing processes that selectively remove OA from the particle phase, such as photolysis and heterogeneous aerosol chemistry. A large (factor of <span class="inline-formula">∼2.5</span>) enhancement in sulfate from the free troposphere (FT) to the boundary layer (BL) in observations is not present in the model, pointing to the importance of properly representing secondary sulfate aerosol formation from marine dimethyl sulfide and gaseous SO<span class="inline-formula">₂</span> smoke emissions. The model shows a persistent overprediction of aerosols in the marine boundary layer (MBL), especially for clean conditions, which multiple pieces of evidence link to weaker aerosol removal in the modeled MBL than reality. This evidence includes several model features, such as not representing observed shifts towards smaller aerosol diameters, inaccurate concentration ratios of carbon monoxide and black carbon, underprediction of heavy rain events, and little evidence of persistent biases in modeled entrainment. The average below-cloud aerosol activation fraction <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>(</mo><msub><mi>N</mi><mi mathvariant="normal">CLD</mi></msub><mo>/</mo><msub><mi>N</mi><mi mathvariant="normal">AER</mi></msub><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0e65ae8974414a05410b793df53d4703"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-23-13911-2023-ie00001.svg" width="65pt" height="14pt" src="acp-23-13911-2023-ie00001.png"/></svg:svg></span></span> remains relatively constant in WRF-CAM5 between field campaigns (<span class="inline-formula">∼0.65</span>), while it decreases substantially in observations from ORACLES (<span class="inline-formula">∼0.78</span>) to CLARIFY (<span class="inline-formula">∼0.5</span>), which could be due to the model misrepresentation of clean aerosol conditions. WRF-CAM5 also overshoots an observed upper limit on liquid cloud droplet concentration around <span class="inline-formula"><i>N</i>_CLD=</span> 400–500 cm<span class="inline-formula">⁻³</span> and overpredicts the spread in <span class="inline-formula"><i>N</i>_CLD</span>. This could be related to the model often drastically overestimating the strength of boundary layer vertical turbulence by up to a factor of 10. We expect these results to motivate similar evaluations of other modeling systems and promote model development to reduce critical uncertainties in climate simulations.</p>