Intercomparison of biomass burning aerosol optical properties from in-situ and remote-sensing instruments in ORACLES-2016

Abstract

<p><strong>Abstract.</strong> The total effect of aerosols, both directly and on cloud properties, remains the biggest source of uncertainty in anthropogenic radiative forcing on the climate. Correct characterization of intensive aerosol optical properties, particularly in conditions where absorbing aerosol is present, is a crucial factor in quantifying these effects. The Southeast Atlantic Ocean (SEA), with seasonal biomass burning smoke plumes overlying and mixing with a persistent stratocumulus cloud deck, offers an excellent natural laboratory to make the observations necessary to understand the complexities of aerosol-cloud-radiation interactions. The first field deployment of the NASA ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) campaign was conducted in September of 2016 out of Walvis Bay, Namibia.</p> <p> Data collected during ORACLES-2016 are used to derive aerosol properties from an unprecedented number of simultaneous measurement techniques over this region. Here we present results from six of the eight independent instruments or instrument combinations, all applied to measure or retrieve aerosol absorption and single scattering albedo. Most but not all of the biomass-burning aerosol was located in the free troposphere, in relative humidities typically ranging up to 60 %. We present the single scattering albedo (SSA), absorbing and total aerosol optical depth (AOD and AAOD), and absorption, scattering, and extinction Ångström exponents (AAE, SAE, EAE) for specific case studies looking at near-coincident and -colocated measurements from multiple instruments, and SSAs for the broader campaign average over the monthlong deployment. For the case studies, we find that SSA agrees within the measurement uncertainties between multiple instruments, though, over all cases, there is no strong correlation between values reported by one instrument and another. We also find that agreement between the instruments is more robust at higher aerosol loading (AOD<sub>400</sub> > 0.4).</p> <p> The campaign-wide average and range shows differences in the values measured by each instrument. We find the ORACLES-2016 campaign-average SSA at 500 nm (SSA<sub>500</sub>) to be between 0.85 and 0.88, depending on the instrument considered (4STAR, AirMSPI, or in situ measurements), with the inter-quartile ranges for all instruments between 0.83 and 0.89. This is consistent with previous September values reported over the region (between 0.84 and 0.90 for SSA at 550 nm). The results suggest that the differences observed in the campaign-average values may be dominated by instrument-specific spatial sampling differences and the natural physical variability in aerosol conditions over the SEA, rather than fundamental methodological differences.</p>

A La Niña‐Like Climate Response to South African Biomass Burning Aerosol in CESM Simulations

“…Several recent field campaigns have focused on improved understanding of absorbing aerosol off the coast of southern Africa, including Observations of Aerosols above Clouds and their interactions (ORACLES) (Zuidema et al, ) and Layered Atlantic Smoke Interactions with Clouds (LASIC) (Zuidema et al, ). The vertical heating profile in Figure is consistent with these surface and aircraft lidar observations, with smoke transport over the southeastern Atlantic mainly occurring between 2 and 4 km (800–600 hPa) (Mallet et al, ; Pistone et al, ).…”

Observation of absorbing aerosols above clouds over the South-East Atlantic Ocean from the geostationary satellite SEVIRI – Part 2: Comparison with MODIS and aircraft measurements from the CLARIFY-2017 field campaign

“…Regarding the assumed microphysical properties of the aerosol, the recent field campaigns (Zuidema et al, 2018;Pistone et al, 2019;Taylor et al, 2020;Wu et al, 2020) have examined the variability of the SSA of biomass burning aerosol over the SEAO. Although the aerosol model used for the retrieval is based on in situ observations 15 from CLARIFY-2017, using a single aerosol model to retrieve the above-cloud AOT is a limitation.…”

Unexpected Biomass Burning Aerosol Absorption Enhancement Explained by Black Carbon Mixing State

“…A number of field campaigns—including Dynamics‐Aerosol‐Chemistry‐Clouds Interactions in West Africa (DACCIWA), Aerosols, Radiation and Clouds in southern Africa (AEROCLO‐sA), Layered Atlantic Smoke Interactions with Clouds (LASIC), ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES), and Cloud and Aerosols Radiative Impact and Forcing (CLARIFY)—have recently been carried out over the eastern Atlantic Ocean, with the aim to better quantify the BBA radiative effects (Flamant, Knippertz, et al, 2018; Formenti et al, 2019; Zuidema et al, 2016, 2018). Both in situ and remote‐sensing observations acquired during these campaigns have all emphasized a strong shortwave (SW) absorption (low single scattering albedo, SSA ) in BBA transported from the coast of southern Africa to the far north over southern West Africa (SWA) (Chylek et al, 2019; Denjean et al, 2020; Pistone et al, 2019; Wu et al, 2020; Zuidema et al, 2018). This means that BBA over this climatically important region is more absorbing than is currently represented in climate models (Mallet et al, 2019; Peers et al, 2016; Stier et al, 2013).…”