Chemical Characterization of African Biomass Burning Over the Southeast Atlantic Ocean.

M.S. Thesis. University of Hawaiʻi at Mānoa 2018

Empirically derived parameterizations of the direct aerosol radiative effect based on ORACLES aircraft observations

In this paper, we use observations from the NASA ORACLES (ObseRvations of CLouds above Aerosols and their intEractionS) aircraft campaign to develop a framework by way of two parameterizations that establishes regionally representative relationships between aerosol-cloud properties and their radiative effects. These relationships rely on new spectral aerosol property retrievals of the single scattering albedo (SSA) and asymmetry parameter (ASY). The retrievals capture the natural variability of the study region as sampled, and both were found to be fairly narrowly constrained (SSA: 0.83&thinsp;&plusmn;&thinsp;0.03 in the mid-visible, 532&thinsp;nm; ASY: 0.54&thinsp;&plusmn;&thinsp;0.06 at 532&thinsp;nm). The spectral retrievals are well suited for calculating the direct aerosol radiative effect (DARE) since SSA and ASY are tied directly to the irradiance measured in the presence of aerosols &ndash; one of the inputs to the spectral DARE. The framework allows for entire campaigns to be generalized into a set of parameterizations. For a range of solar zenith angles, it links the broadband DARE to the mid-visible aerosol optical depth (AOD) and the albedo (&alpha;) of the underlying scene (either clouds or clear sky) by way of the first parameterization: P(AOD, &alpha;). For ORACLES, the majority of the case-to-case variability of the broadband DARE is attributable to the dependence on the two driving parameters of P(AOD, &alpha;). A second, extended, parameterization PX(AOD, &alpha;, SSA) explains even more of the case-to-case variability by introducing the mid-visible SSA as a third parameter. These parameterizations establish a direct link from two or three mid-visible (narrowband) parameters to the broadband DARE, implicitly accounting for the underlying spectral dependencies of its drivers. They circumvent some of the assumptions when calculating DARE from satellite products or in a modeling context. For example, the DARE dependence on aerosol microphysical properties is not explicit in P or PX because the asymmetry parameter varies too little from case to case to translate into appreciable DARE variability. While these particular DARE parameterizations only represent the ORACLES data, they raise the prospect of generalizing the framework to other regions. </div

Daytime aerosol optical depth above low-level clouds is similar to that in adjacent clear skies at the same heights: airborne observation above the southeast Atlantic

To help satellite retrieval of aerosols and studies of their radiative effects, we demonstrate that daytime aerosol optical depth over low-level clouds is similar to that in neighboring clear skies at the same heights. Based on recent airborne lidar and sun photometer observations above the southeast Atlantic, the mean aerosol optical depth (AOD) difference at 532&thinsp;nm is between 0 and &minus;0.01, when comparing the cloudy and clear sides, each up to 20&thinsp;km wide, of cloud edges. The difference is not statistically significant according to a paired t&nbsp;test. Systematic differences in the wavelength dependence of AOD and in situ single scattering albedo are also minuscule. These results hold regardless of the vertical distance between cloud top and aerosol layer bottom. AOD aggregated over &sim;2∘ grid boxes for each of September 2016, August 2017 and October 2018 also shows little correlation with the presence of low-level clouds. We posit that a satellite retrieval artifact is entirely responsible for a previous finding of generally smaller AOD over clouds (Chung et al., 2016), at least for the region and time of our study. Our results also suggest that the same values can be assumed for the intensive properties of free-tropospheric biomass-burning aerosol regardless of whether clouds are present below. &nbsp;</p

Above-cloud aerosol radiative effects based on ORACLES 2016 and ORACLES 2017 aircraft experiments

Determining the direct aerosol radiative effect (DARE) of absorbing aerosols above clouds from satellite observations alone is a challenging task, in part because the radiative signal of the aerosol layer is not easily untangled from that of the clouds below. In this study, we use aircraft measurements from the NASA ObseRvations of CLouds above Aerosols and their intEractionS (ORACLES) project in the southeastern Atlantic to derive it with as few assumptions as possible. This is accomplished by using spectral irradiance measurements (Solar Spectral Flux Radiometer, SSFR) and aerosol optical depth (AOD) retrievals (Spectrometer for Sky-Scanning, Sun-Tracking Atmospheric Research, 4STAR) during vertical profiles (spirals) that minimize the albedo variability of the underlying cloud field &ndash; thus isolating aerosol radiative effects from those of the cloud field below. For two representative cases, we retrieve spectral aerosol single scattering albedo (SSA) and the asymmetry parameter (g) from these profile measurements and calculate DARE given the albedo range measured by SSFR on horizontal legs above clouds. For mid-visible wavelengths, we find SSA values from 0.80 to 0.85 and a significant spectral dependence of g. As the cloud albedo increases, the aerosol increasingly warms the column. The transition from a cooling to a warming top-of-aerosol radiative effect occurs at an albedo value (critical albedo) just above 0.2 in the mid-visible wavelength range. In a companion paper, we use the techniques introduced here to generalize our findings to all 2016 and 2017 measurements and parameterize aerosol radiative effects.</p

Intercomparison of airborne and surface-based measurements during the CLARIFY, ORACLES and LASIC field experiments

This is the final version. Available on open access from the European Geosciences Union via the DOI in this recordCode availability: Processing code for the FAAM core measurements suite is available from GitHub (Sproson et al., 2020).Data availability Airborne data for the CLARIFY campaign are available from the Centre for Environmental Data Analysis (Facility for Airborne Atmospheric Measurements et al., 2017) and for the ORACLES campaign from NASA Earth Science Project Office (ORACLES Science Team, 2020). The LASIC ground-based data sets are publicly available from the Atmospheric Radiation Measurement Climate Research Facility (Zuidema et al., 2017) with specialist data sets available for the following: SP2 – https://iop.archive.arm.gov/arm-iop/2016/ (last access: 25 October 2022, Sedlacek, 2017), CO – https://doi.org/10.5439/1046183 (Springston, 2018b), CAPS PMSSA – https://adc.arm.gov/discovery/#/results/s::caps-ssa (Onasch et al., 2015), ACSM – https://doi.org/10.5439/1763029 (Zawadowicz and Howie, 2021).Data are presented from intercomparisons between two research aircraft, the FAAM BAe-146 and the NASA Lockheed P3, and between the BAe-146 and the surface-based DOE (Department of Energy) ARM (Atmospheric Radiation Measurement) Mobile Facility at Ascension Island (8∘ S, 14.5∘ W; a remote island in the mid-Atlantic). These took place from 17 August to 5 September 2017, during the African biomass burning (BB) season. The primary motivation was to give confidence in the use of data from multiple platforms with which to evaluate numerical climate models. The three platforms were involved in the CLouds–Aerosol–Radiation Interaction and Forcing for Year 2017 (CLARIFY-2017), ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), and Layered Atlantic Smoke and Interactions with Clouds (LASIC) field experiments. Comparisons from flight segments on 6 d where the BAe-146 flew alongside the ARM facility on Ascension Island are presented, along with comparisons from the wing-tip-to-wing-tip flight of the P3 and BAe-146 on 18 August 2017. The intercomparison flight sampled a relatively clean atmosphere overlying a moderately polluted boundary layer, while the six fly-bys of the ARM site sampled both clean and polluted conditions 2–4 km upwind. We compare and validate characterisations of aerosol physical, chemical and optical properties as well as atmospheric radiation and cloud microphysics between platforms. We assess the performance of measurement instrumentation in the field, under conditions where sampling conditions are not as tightly controlled as in laboratory measurements where calibrations are performed. Solar radiation measurements compared well enough to permit radiative closure studies. Optical absorption coefficient measurements from all three platforms were within uncertainty limits, although absolute magnitudes were too low (<10 Mm−1) to fully support a comparison of the absorption Ångström exponents. Aerosol optical absorption measurements from airborne platforms were more comparable than aircraft-to-ground observations. Scattering coefficient observations compared adequately between airborne platforms, but agreement with ground-based measurements was worse, potentially caused by small differences in sampling conditions or actual aerosol population differences over land. Chemical composition measurements followed a similar pattern, with better comparisons between the airborne platforms. Thermodynamics, aerosol and cloud microphysical properties generally agreed given uncertainties.Natural Environment Research Council (NERC)NERC/Met Office Industrial Case studentshipResearch Council of NorwayUS Department of Energy, Office of ScienceNASAUS Department of Energy Atmospheric Systems Research (ASR) programm

An Integrated Analysis of African Biomass Burning Aerosol Properties in the Marine Boundary Layer and Free Troposphere over the Remote Southeast Atlantic Ocean

Biomass burning, the burning of living and dead vegetation, contributes significant amounts of particulate matter and trace gases into the atmosphere. The copious organic, nitrate, sulfate, and black carbon aerosol amounts can dominate the atmospheric aerosol composition over regions far larger than the original emission source. The aerosol&rsquo;s ability to scatter and absorb sunlight influences most of the radiative forcing terms considered by the 6th IPCC Assessment. Despite the smoke&rsquo;s importance to climate radiative forcing, how smoke characteristics change during aging and long-range transport is still poorly known. This is in large part because of a lack of in-situ measurements, especially in the Southern Hemisphere where biomass-burning aerosols from southern Africa contribute over 30% to the particulate matter released into Earth&rsquo;s atmosphere each year. NASA, the Department of Energy, and the UK Met Office recognized the lack of research in the Southern Hemisphere and conducted field campaigns spanning 2016-2018 over the southeast Atlantic Ocean (SEA) to study the chemical, optical, and physical properties of biomass-burning aerosol during long-range transport, aerosol-cloud interactions, and the impacts of aerosols and aerosol-cloud interactions on the Earth&rsquo;s radiative budget. In this dissertation, the results from three major field campaigns in the southeast Atlantic region are integrated to contribute to a greater understanding of the chemical, optical, and physical properties of biomass burning aerosol, as it is transported across the SEA. We find that chemical aging processes in the free troposphere and marine boundary layer contribute to a net loss of aerosol mass, which leads to enhanced aerosol absorption in this region.&nbsp;Throughout this dissertation, we&nbsp;seek to provide a simple, yet straightforward relationship between the aerosol&rsquo;s optical and chemical properties that can someday be integrated into regional and global climate models, as the unique properties of biomass-burning aerosol are not yet appropriately represented.</p

Marine Shallow cloud Adjustments to the Presence of Shortwave-Absorbing Aerosols: Advancing Understanding Through a Combined Analysis of Lasic Datasets and Process Modeling (Final Report)

This is a final report for this award, whose purpose was to support the analysis of data emanating from the LASIC (Layered Atlantic Smoke Interaction with Clouds) campaign. (Layered Atlantic Smoke Interactions with Clouds) is a strategy to improve our understanding of aged carbonaceous aerosol, its seasonal evolution, and the mechanisms by which clouds adjust to the presence of the aerosol. The observational strategy centers on deploying the AMF1 cloud, aerosol, and atmospheric profiling instrumentation to Ascension Island, located within the trade-wind shallow cumulus regime (14.50W, 80S) 3000 km offshore of continental Africa. The location is within the latitude zone of the maximum outflow of aerosol, with the deepening boundary layer known to entrain free-tropospheric smoke. The primary activities for LASIC are: 1) to improve current knowledge on aged biomass burning aerosol and its radiative properties as a function of the seasonal cycle; 2) to use surface-based remote sensing to sensitively interrogate the atmosphere for the relative vertical location of aerosol and clouds; 3) to improve our understanding of the cloud adjustments to the presence of shortwave-absorbing aerosol within the vertical column, both through aerosol-radiation and through aerosol-cloud interactions; 4) to aid low cloud parameterization efforts for climate models. This award serves to further these activities and includes a sub-award to Dr. Pablo Saide at UCLA for activities 3) and 4)

An attribution of the low single-scattering albedo of biomass burning aerosol over the southeastern Atlantic

Aerosol over the remote southeastern Atlantic is some of the most sunlight-absorbing aerosol on the planet: the in situ free-tropospheric single-scattering albedo at the 530 nm wavelength (SSA530nm) ranges from 0.83 to 0.89 within ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) aircraft flights from late August–September. Here we seek to explain the low SSA. The SSA depends strongly on the black carbon (BC) number fraction, which ranges from 0.15 to 0.4. Low organic aerosol (OA)-to-BC mass ratios of 8–14 and modified combustion efficiency values >0.975 point indirectly to the dry, flame-efficient combustion of primarily grass fuels, with back trajectories ending in the miombo woodlands of Angola. The youngest aerosol, aged 4–5 d since emission, occupied the top half of a 5 km thick plume sampled directly west of Angola with a vertically consistent BC:ΔCO (carbon monoxide) ratio, indicating a homogenization of the source emissions. The younger aerosol, transported more quickly off of the continent by stronger winds, overlaid older, slower-moving aerosol with a larger mean particle size and fraction of BC-containing particles. This is consistent with ongoing gas condensation and the coagulation of smaller non-BC particles upon the BC-containing particles. The particle volumes and OA:BC mass ratios of the older aerosol were smaller, attributed primarily to evaporation following fragmentation, instead of dilution or thermodynamics. The CLARIFY (CLoud–Aerosol–Radiation Interaction and Forcing: Year 2017) aircraft campaign sampled aerosols that had traveled further to reach the more remote Ascension Island. CLARIFY reported higher BC number fractions, lower OA:BC mass ratios, and lower SSA yet larger mass absorption coefficients compared to this study's. Values from one ORACLES 2017 flight, held midway to Ascension Island, are intermediate, confirming the long-range changes. Overall the data are most consistent with continuing oxidation through fragmentation releasing aerosols that subsequently enter the gas phase, reducing the OA mass, rather than evaporation through dilution or thermodynamics. The data support the following best fit: SSA530nm=0.801+0055⋅(OA:BC) (r=0.84). The fires of southern Africa emit approximately one-third of the world's carbon; the emitted aerosols are distinct from other regional smoke emissions, and their composition needs to be represented appropriately to realistically depict regional aerosol radiative effects

Biomass Burning Aerosol as a Modulator of Droplet Number in the Southeast Atlantic Region

Abstract. The southeastern Atlantic (SEA) and its associated cloud deck, off the west coast of central Africa, is an area where aerosol-cloud interactions can have a strong radiative impact. Seasonally, extensive biomass burning (BB) aerosol plumes from southern Africa reach this area. The NASA ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) study focused on quantitatively understanding these interactions and their importance. Here we present measurements of cloud condensation nuclei (CCN) concentration, aerosol size distribution, and vertical updraft velocity in and around the marine boundary layer (MBL) collected by the NASA P-3B aircraft during the August 2017 ORACLES deployment. BB aerosol levels vary considerably but systematically with time; high aerosol concentrations were observed in the MBL (800-1000 cm −3 ) early on, decreasing mid-campaign to concentrations between 500-800 cm −3 . By late August and early September, relatively clean MBL conditions were sampled (< 500 cm −3 ). These data then drive a state-of-the-art droplet formation parameterization, from which the predicted cloud droplet number and its sensitivity to aerosol and dynamical parameters are derived. Droplet closure was achieved to within 20 %. Droplet formation sensitivity to aerosol concentration, vertical updraft velocity, and the hygroscopicity parameter, κ, vary and contribute to the total droplet response in the MBL clouds. When aerosol concentrations exceed ~ 900 cm −3 and maximum supersaturation approaches 0.1 %, droplet formation in the MBL enters a velocity-limited droplet activation regime, where cloud droplet number responds weakly to CCN concentration increases. Below ~ 500 cm −3 , in a clean MBL, droplet formation is much more sensitive to changes in aerosol concentration than to changes in vertical updraft. In the competitive regime, where the MBL has intermediate pollution (500-800 cm −3 ), droplet formation becomes much more sensitive to hygroscopicity (κ) variations than for clean and polluted conditions. Higher concentrations increase the sensitivity to vertical velocity by more than ten-fold. We also find that characteristic vertical velocity plays a very important role in driving droplet formation in a more polluted MBL regime, in which even a small shift in w* may make a significant difference in droplet concentrations. Identifying regimes where droplet number variability is primarily driven by updraft velocity and not aerosol concentration is key for interpreting aerosol indirect effects, especially with remote sensing. Droplet number responds proportionally to changes in characteristic velocity, offering the possibility of remote sensing of w* under velocity-limited conditions