The southeast Atlantic Ocean provides an excellent natural laboratory to study smoke-cloud interactions, a large driver of uncertainty in climate projections. The value of studying this in particular region is largely attributable to two factors---the expansive, bright, semi-permanent stratocumulus cloud deck and the fact that southern Africa is the largest source of biomass-burning aerosols in the world. We study this region using the WRF-Chem model with CAM5 aerosols and in situ observations from the ORACLES, LASIC, and CLARIFY field campaigns, all of which overlapped in August 2017. Across these campaigns, we compare aerosol, cloud, and thermodynamic variables to quantify model performance and expand upon observational findings of aerosol-cloud effects. Specifically, our approach is to analyze aerosol and cloud properties along flight tracks, picking out uniform legs within tropospheric smoke plumes and in the boundary layer. This unique approach allows us to sample the high spatiotemporal variability that can get lost to large-scale averaging. It also allows process-level comparison of local cloud responses to aerosol conditions, and measure model performance in those same processes. Along with better quantifying model predictive power, we find and justify updates to model parameters and processes to better emulate observations, notably aerosol size parameters. Preliminary results suggest that WRF-CAM5 is activating a smaller percentage of aerosols into cloud droplets than shown in observations, which could lead to biased modeling of aerosol indirect radiative effects on a larger scale. We explore this effect further with CCN activation tendency, updraft, particle sizing, and composition analysis, as well as broader dynamics like entrainment and removal rates. Comparing the model with similar instrument suites across multiple colocated campaigns also allows us to quantify instrument uncertainty in ways that a focus on a single campaign cannot and gives further context to the model performance.

Biomass-burning emissions impact almost all of the radiative forcing terms considered by the IPCC, yet little is known about smoke aerosol aging in nature beyond a few days. The marine southeast Atlantic free-troposphere is a natural testbed for examining aging through photolysis/oxidation of African continental fire emissions advected westward. In-situ measurements primarily from September, 2016 indicate highly-oxidized aerosol with minimal primary source signatures after 4-9 model-predicted days since emission. Aerosol loses approximately one-half of its organic aerosol over the ocean. The organic aerosol to black carbon mass ratios decrease from 14 to 10, significantly lower than many model predictions. This mass loss, combined with stability in black carbon, supports an observed 20% increase in solar absorptivity. The decreased single scattering albedos, reaching 0.83 at 9 days, arguably represent the lowest values measured globally. The relationship of the aerosol properties to model-derived time since emission suggests a useful new modeling constraint.