. Southern Africa produces almost a third of the Earth’s biomass burning (BB) aerosol particles, yet the fate of these particles and their influence on regional and global climate is poorly understood. ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) is a five-year NASA EVS-2 (Earth Venture Suborbital-2) investigation with three Intensive Observation Periods designed to study key atmospheric processes that determine the climate impacts of these aerosols. During the Southern Hemisphere winter and spring (June-October), aerosol particles reaching 3–5 km in altitude are transported westward over the South-East Atlantic, where they interact with one of the largest subtropical stratocumulus subtropical stratocumulus (Sc) cloud decks in the world. The representation of these interactions in climate models remains highly uncertain in part due to a scarcity of observational constraints on aerosol and cloud properties, and due to the parameterized treatment of physical processes. Three ORACLES deployments by the NASA P-3 aircraft in September 2016, August 2017 and October 2018 (totaling ~350 science flight hours), augmented by the deployment of the NASA ER-2 aircraft for remote sensing in September 2016 (totaling ~100 science flight hours), were intended to help fill this observational gap. ORACLES focuses on three fundamental science questions centered on the climate effects of African BB aerosols: (a) direct aerosol radiative effects; (b) effects of aerosol absorption on atmospheric circulation and clouds; (c) aerosol-cloud microphysical interactions. This paper summarizes the ORACLES science objectives, describes the project implementation, provides an overview of the flights and measurements in each deployment, and highlights the integrative modeling efforts from cloud to global scales to address science objectives. Significant new findings on the vertical structure of BB aerosol physical and chemical properties, chemical aging, cloud condensation nuclei, rain and precipitation statistics, and aerosol indirect effects are emphasized, but their detailed descriptions are the subject of separate publications. The main purpose of this paper is to familiarize the broader scientific community with the ORACLES project and the data set it produced.

/pdf/an-overview-of-the-oracles-observations-of-aerosols-above-54wcpzyuso.pdf

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) project: aerosol–cloud–radiation interactions in the southeast Atlantic basin

Observations from June to October 2016, from a surface-based ARM Mobile Facility deployment on Ascension Island (8∘S, 14.5∘W) indicate that refractory black carbon (rBC) is almost always present within the boundary layer. The rBC mass concentrations, light absorption coefficients, and cloud condensation nuclei concentrations vary in concert and synoptically, peaking in August. Light absorption coefficients at three visible wavelengths as a function of rBC mass are approximately double that calculated from black carbon in lab studies. A spectrally-flat absorption angstrom exponent suggests most of the light absorption is from lens-coated black carbon. The single-scattering-albedo increases systematically from August to October in both 2016 and 2017, with monthly means of 0.78 ± 0.02 (August), 0.81 ± 0.03 (September), and 0.83 ± 0.03 (October) at the green wavelength. Boundary layer aerosol loadings are only loosely correlated with total aerosol optical depth, with smoke more likely to be present in the boundary layer earlier in the biomass burning season, evolving to smoke predominantly present above the cloud layers in September–October, typically resting upon the cloud top inversion. The time period with the campaign-maximum near-surface light absorption and column aerosol optical depth, on 13–16 August 2016, is investigated further. Backtrajectories that indicate more direct boundary layer transport westward from the African continent is central to explaining the elevated surface aerosol loadings. Plain Language Summary First findings from the remote Ascension Island midway between Africa and South America in the Atlantic Ocean indicate that smoke is present much more often near the surface than has been previously thought. The new measurements from a 17-month-long campaign suggest that August is the smokiest month near the surface. The smoke includes other aerosols besides black carbon, and is most absorptive of sunlight in June and least in October. The smoke is more present near the surface earlier in the biomass burning season, or June, while later on toward September and October, more of the smoke resides above the cloud layer. This has implications for which aerosol-cloud microphysical and radiative interactions are dominant when. The campaign-maximum aerosol loading event is investigated further and attributed to an unusual direct westward flow from the continental African fire sources at low altitudes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017GL076926

The Ascension Island Boundary Layer in the Remote Southeast Atlantic is Often Smoky

. We study 41 d with daily median surface
accumulation mode aerosol particle concentrations below 50 cm −3 
(ultra-clean conditions) observed at Ascension Island (ASI; 7.9 ∘  S, 14.4 ∘  W) between June 2016 and October 2017 as part of the
Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign.
Interestingly, these days occur during a period of great relevance for
aerosol–cloud–radiation interactions, the southeast Atlantic (SEATL)
biomass-burning season (approximately June–October). That means that these
critical months can feature both the highest surface aerosol numbers, from
smoke intrusion into the marine boundary layer, as well as the lowest. While
carbon monoxide and refractory black carbon concentrations on ultra-clean
days do not approach those on days with heavy smoke, they also frequently
exceed background concentrations calculated in the non-burning season from
December 2016 to April 2017. This is evidence that even what become
ultra-clean boundary layers can make contact with and entrain from an
overlying SEATL smoke layer before undergoing a process of rapid aerosol
removal. Because many ultra-clean and polluted boundary layers observed at
Ascension Island during the biomass burning season follow similar isobaric
back trajectories, the variability in this entrainment is likely more
closely tied to the variability in the overlying smoke rather than
large-scale horizontal circulation through the boundary layer. Since
exceptionally low accumulation mode aerosol numbers at ASI do not
necessarily indicate the relative lack of other trace pollutants, this
suggests the importance of regional variations in what constitutes an
“ultra-clean” marine boundary layer. Finally, surface drizzle rates,
frequencies and accumulation – as well as retrievals of liquid water path
– all consistently tend toward higher values on ultra-clean days. This
implicates enhanced coalescence scavenging in low clouds as the key driver
of ultra-clean events in the southeast Atlantic marine boundary layer. These
enhancements occur against and are likely mediated by the backdrop of a
seasonal increase in daily mean cloud fraction and daily median liquid water
path over ASI, peaking in September and October in both LASIC years.
Therefore the seasonality in ultra-clean day occurrence seems directly
linked to the seasonality in SEATL cloud properties. These results highlight
the importance of two-way aerosol–cloud interactions in the region.

/pdf/ultra-clean-and-smoky-marine-boundary-layers-frequently-52acgdzcyy.pdf

Ultra-clean and smoky marine boundary layers frequently occur in the same season over the southeast Atlantic

. The mean altitude of the smoke loading over the southeast Atlantic moves from the boundary layer in July to the free troposphere by October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass burning aerosol loading at Ascension Island (8 ∘  S, 14.5 ∘  W) from July to October, based on island measurements, satellite retrievals, and reanalysis. In July and August, the smoke loading predominantly varies within the boundary layer. During both months, the
low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol–cloud interactions explain the cloudiness changes. September marks a transition month during which midlatitude disturbances can intrude into the Atlantic subtropics, constraining the free tropospheric aerosol closer to the African coast. Stronger boundary layer winds on cleaner days help deepen, dry, and cool much of the marine boundary layer compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck edge. The September free troposphere is better mixed on smoky days compared to October. Longwave cooling rates, generated by a sharp water vapor gradient at the aerosol layer top, encourage a small-scale vertical mixing that could help maintain the well-mixed smoky September free troposphere. The October meteorology primarily varies as a function of the strength of the free tropospheric winds advecting aerosol offshore. The free tropospheric aerosol loading is less than in September, and the moisture variability is greater. Low-level clouds increase and are more stratiform in October when the smoke loadings are higher. The increased free tropospheric moisture can help sustain the clouds through a reduction in evaporative drying during cloud-top entrainment. Enhanced subsidence above the coastal upwelling region, increasing cloud droplet number concentrations, may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free tropospheric winds at Ascension Island supports slightly higher cloud tops during smokier conditions. Overall, the monthly changes in the large-scale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology. This is climatically important, as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

/pdf/sunlight-absorbing-aerosol-amplifies-the-seasonal-cycle-in-3o8l1jlxc1.pdf

Sunlight-absorbing aerosol amplifies the seasonal cycle in low-cloud fraction over the southeast Atlantic

Instrumentation on the Met Office C-130 aircraft measured aerosol physical and optical properties during the Southern African Regional Science Initiative (SAFARI 2000) in September 2002 while flying from Windhoek, Namibia. Filter measurements of aged regional haze suggest a ratio of apparent elemental carbon (EC a ) to organic carbon (OC) of 0.12 ± 0.02 and mass fractions of 5% EC a , 25% inorganic compounds, and 70% organic matter (OC plus associated elements). The submicron size distribution of aged regional haze may be fitted with three lognormal distributions with geometric mean radii (r n ) of 0.12 ± 0.01, 0.26 ± 0.01, and 0.80 ± 0.01 μm and geometric standard deviations (a) of 1.3 ± 0.1, 1.5 ± 0.1, and 1.9 ± 0.4. Measurements over 2500 km from the emission region show similar r n and σ for the smallest two modes, while the third mode is absent presumably as a result of sedimentation. At a wavelength (λ) of 0.55 μm, effective medium approximations suggest a refractive index of 1.54 - 0.018i for aged regional haze aerosol. The single scattering albedo (ω oλ ) derived using this refractive index and measured size distributions are consistent with those from the nephelometer and Particle Soot Absorption Photometer (PSAP). The optical parameters for aged regional haze a few days old are specific extinction coefficient (k eλ=055 ) of 5.0 ± 0.8 m 2 g -1 , asymmetry factor (g λ=0.55 ) of 0.59 ± 0.02, and ω oλ=0.55 of 0.91 ± 0.04. Measurements of fresh biomass burning aerosol a few minutes old show smaller more absorbing particles. Vertical profiles of carbon monoxide, aerosol concentration, and aerosol scattering show a good correlation. Over land, aerosols become well mixed in the vertical from the surface to approximately 500 hPa. Over ocean, the aerosols can be separated from underlying stratocumulus cloud by a clear gap and a strong inversion, which may limit the indirect effect.

The mean physical and optical properties of regional haze dominated by biomass burning aerosol measured from the C-130 aircraft during SAFARI 2000 : SAFARI 2000-Southern African Regional Science Initiative

Southern Africa is the world’s largest emitter of biomass-burning (BB) aerosols. Their westward transport over the remote southeast Atlantic Ocean colocates some of the largest atmospheric loadings of absorbing aerosol with the least examined of the Earth’s major subtropical stratocumulus decks. Global aerosol model results highlight that the largest positive top-of-atmosphere forcing in the world occurs in the southeast Atlantic, but this region exhibits large differences in magnitude and sign between reputable models, in part because of high variability in the underlying model cloud distributions. Many uncertainties contribute to the highly variable model radiation fields: the aging of shortwave-absorbing aerosol during transport, how much of the aerosol mixes into the cloudy boundary layer, and how the low clouds adjust to smoke-radiation and smoke-cloud interactions. In addition, the ability of the BB aerosol to absorb shortwave radiation is known to vary seasonally as the fuel type on land changes.

/pdf/layered-atlantic-smoke-interactions-with-clouds-lasic-2htndmu6b2.pdf

Layered Atlantic Smoke Interactions with Clouds (LASIC) Science Plan

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Layered Atlantic Smoke Interactions with Clouds (LASIC) Science Plan