Aerosol absorption in global models from AeroCom phase III
Authors: Sand Maria , Samset Bjørn H. , Myhre Gunnar , Gliß Jonas , Bauer Susanne E. , Bian Huisheng , Chin Mian , Checa-Garcia Ramiro , Ginoux Paul , Kipling Zak , Kirkevåg Alf , Kokkola Harri , Le Sager Philippe , Lund Marianne T. , Matsui Hitoshi , van Noije Twan , Olivié Dirk J. L. , Remy Samuel , Schulz Michael , Stier Philip , Stjern Camilla W. , Takemura Toshihiko , Tsigaridis Kostas , Tsyro Svetlana G. , Watson-Parris Duncan 
Published: October 2021

Abstract. Aerosol-induced absorption of shortwave radiation can modify the climate through local atmospheric heating, which affects lapse rates,
precipitation, and cloud formation. Presently, the total amount of aerosol
absorption is poorly constrained, and the main absorbing aerosol species
(black carbon (BC), organic aerosols (OA), and mineral dust) are diversely
quantified in global climate models. As part of the third phase of the
Aerosol Comparisons between Observations and
Models (AeroCom) intercomparison initiative (AeroCom phase III), we here
document the distribution and magnitude of aerosol absorption in current
global aerosol models and quantify the sources of intermodel spread,
highlighting the difficulties of attributing absorption to different
species. In total, 15 models have provided total present-day absorption at 550 nm
(using year 2010 emissions), 11 of which have provided absorption per
absorbing species. The multi-model global annual mean total absorption
aerosol optical depth (AAOD) is 0.0054 (0.0020 to 0.0098; 550 nm), with the range given as the minimum and maximum model values. This is 28 % higher compared to the 0.0042 (0.0021 to 0.0076) multi-model mean in AeroCom phase II (using year 2000 emissions), but the difference is within 1 standard
deviation, which, in this study, is 0.0023 (0.0019 in Phase II). Of the summed component AAOD, 60 % (range 36 %–84 %) is estimated to be due to BC, 31 % (12 %–49 %) is due to dust, and 11 % (0 %–24 %) is due to OA; however, the components are not independent in terms of their absorbing efficiency. In models with internal mixtures of absorbing aerosols, a major
challenge is the lack of a common and simple method to attribute absorption
to the different absorbing species. Therefore, when possible, the models
with internally mixed aerosols in the present study have performed
simulations using the same method for estimating absorption due to BC, OA,
and dust, namely by removing it and comparing runs with and without the
absorbing species. We discuss the challenges of attributing absorption to
different species; we compare burden, refractive indices, and density; and
we contrast models with internal mixing to models with external mixing. The
model mean BC mass absorption coefficient (MAC) value is 10.1 (3.1 to 17.7) m2 g−1 (550 nm), and the model mean BC AAOD is 0.0030 (0.0007 to
0.0077). The difference in lifetime (and burden) in the models explains as
much of the BC AAOD spread as the difference in BC MAC values. The
difference in the spectral dependency between the models is striking. Several models have an absorption Ångstrøm exponent (AAE) close to 1, which likely is too low given current knowledge of spectral aerosol optical properties. Most models do not account for brown carbon and underestimate the spectral dependency for OA.