Abstract:
The southeast Atlantic region, characterized by persistent stratocumulus clouds, has one of the highest uncertainties in aerosol radiative forcing and significant variability across climate models. In this study, we
analyze the seasonally varying role of marine aerosol sources and identify key uncertainties in aerosol composition at cloud-relevant altitudes over the southeast Atlantic using the GEOS-Chem chemical transport model.
We evaluate simulated aerosol optical depth (AOD) and speciated aerosol concentrations against those collected
from ground observations and aircraft campaigns such as LASIC, ORACLES, and CLARIFY, conducted during 2017. The model consistently underestimates AOD relative to AERONET, particularly at remote locations
like Ascension Island. However, when compared with aerosol mass concentrations from aircraft campaigns during the biomass burning period, it performs adequately at cloud-relevant altitudes, with a normalized mean bias
(NMB) between −3.5 % (CLARIFY) and −7.5 % (ORACLES). At these altitudes, in the model, organic aerosols
(63 %) dominate during the biomass burning period, while sulfate (41 %) prevails during austral summer, when
dimethylsulfide (DMS) emissions peak in the model. Our findings indicate that marine sulfate can account for
up to 69 % of total sulfate during the high-DMS period. Sensitivity analyses indicate that refining DMS emissions and oxidation chemistry may increase sulfate aerosol produced from marine sources, highlighting that
there remains large uncertainty as to the role of DMS emissions in the marine boundary layer. Additionally,
we find marine primary organic aerosol emissions may substantially increase total organic aerosol concentrations, particularly during austral summer. This study underscores the imperative need to refine marine emissions
and their chemical transformations, as aerosols from marine sources are a major component of total aerosols at
cloud-relevant altitudes and may impact uncertainties in aerosol radiative forcing over the southeast Atlantic.
Description:
DATA AVAILABITY STATEMENT: The GEOS-Chem model used here
is publicly available at https://doi.org/10.5281/zenodo.5748260
(The International GEOS-Chem User Community, 2021).
The ORACLES campaign data from 2017 are available at
https://doi.org/10.5067/Suborbital/ORACLES/P3/2017_V2
(ORACLES Science Team, 2020). The CLARIFY campaign
data are available at http://data.ceda.ac.uk/badc/faam/data/
2017/c056-sep-09 (last access: 22 June 2024, CEDA, 2021).
The ACSM dataset from LASIC campaign is available at
https://doi.org/10.5439/1763029 (Atmospheric Radiation Measurement (ARM) user facility, 2016).