Abstract. In the southeast Atlantic, well-defined smoke plumes from Africa advect over
marine boundary layer cloud decks; both are most extensive around September,
when most of the smoke resides in the free troposphere. A framework is put
forth for evaluating the performance of a range of global and regional
atmospheric composition models against observations made during the NASA
ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS)
airborne mission in September 2016. A strength of the comparison is a focus
on the spatial distribution of a wider range of aerosol composition and
optical properties than has been done previously. The sparse airborne
observations are aggregated into approximately 2∘ grid boxes and into
three vertical layers: 3–6 km, the layer from cloud top to 3 km, and the
cloud-topped marine boundary layer. Simulated aerosol extensive properties
suggest that the flight-day observations are reasonably representative of
the regional monthly average, with systematic deviations of 30 % or less.
Evaluation against observations indicates that all models have strengths and
weaknesses, and there is no single model that is superior to all the others
in all metrics evaluated. Whereas all six models typically place the top of
the smoke layer within 0–500 m of the airborne lidar observations, the
models tend to place the smoke layer bottom 300–1400 m lower than the
observations. A spatial pattern emerges, in which most models underestimate
the mean of most smoke quantities (black carbon, extinction, carbon
monoxide) on the diagonal corridor between 16∘ S, 6∘ E, and
10∘ S, 0∘ E, in the 3–6 km layer, and overestimate them
further south, closer to the coast, where less aerosol is present. Model
representations of the above-cloud aerosol optical depth differ more widely.
Most models overestimate the organic aerosol mass concentrations relative to
those of black carbon, and with less skill, indicating model uncertainties
in secondary organic aerosol processes. Regional-mean free-tropospheric
model ambient single scattering albedos vary widely, between 0.83 and 0.93
compared with in situ dry measurements centered at 0.86, despite minimal impact of
humidification on particulate scattering. The modeled ratios of the particulate
extinction to the sum of the black carbon and organic aerosol mass
concentrations (a mass extinction efficiency proxy) are typically too low
and vary too little spatially, with significant inter-model differences. Most
models overestimate the carbonaceous mass within the offshore boundary
layer. Overall, the diversity in the model biases suggests that different
model processes are responsible. The wide range of model optical properties
requires further scrutiny because of their importance for radiative effect
estimates.
Oxidation photochemistry in the Southern Atlantic boundary layer: unexpected deviations of photochemical steady state
