Deriving brown carbon from multiwavelength absorption measurements: method and application to AERONET and Aethalometer observations
Other literature type, Article
Sedlacek, Arthur J.
de Sá, Suzane S.
Martin, Scot T.
Alexander, M. Lizabeth
Watson, Thomas B.
Aiken, Allison C.
Springston, Stephen R.
Heald, Colette L.
- Publisher: Copernicus GmbH
(issn: 1680-7324, eissn: 1680-7324)
Chemistry | QD1-999 | Physics | QC1-999
The radiative impact of organic aerosols (OA) is a large
source of uncertainty in estimating the global direct radiative effect (DRE)
of aerosols. This radiative impact includes not only light scattering but
also light absorption from a subclass of OA referred to as brown carbon
(BrC). However, the absorption properties of BrC are poorly understood, leading
to large uncertainties in modeling studies. To obtain observational
constraints from measurements, a simple absorption Ångström exponent
(AAE) method is often used to separate the contribution of BrC absorption
from that of black carbon (BC). However, this attribution method is based on
assumptions regarding the spectral dependence of BC that are often violated
in the ambient atmosphere. Here we develop a new AAE method which improves
upon previous approaches by using the information from the wavelength-dependent measurements themselves and by allowing for an
atmospherically relevant range of BC properties, rather than fixing these at
a single assumed value. We note that constraints on BC optical properties and
mixing state would help further improve this method. We apply this method to
multiwavelength absorption aerosol optical depth (AAOD) measurements at
AERONET sites worldwide and surface aerosol absorption measurements at
multiple ambient sites. We estimate that BrC globally contributes up to
40 % of the seasonally averaged absorption at 440 nm. We find that the
mass absorption coefficient of OA (OA-MAC) is positively correlated with the
BC ∕ OA mass ratio. Based on the variability in BC properties and
BC ∕ OA emission ratio, we estimate a range of
0.05–1.5 m<sup>2</sup> g<sup>−1</sup> for OA-MAC at 440 nm. Using the combination of
AERONET and OMI UV absorption observations we estimate that the
AAE<sub>388∕440 nm</sub> for BrC is generally ∼ 4 worldwide, with a
smaller value in Europe (< 2). Our analyses of observations at two
surface sites (Cape Cod, to the southeast of Boston, and the GoAmazon2014/5
T3 site, to the west of Manaus, Brazil) reveal no significant relationship
between BrC absorptivity and photochemical aging in urban-influenced
conditions. However, the absorption of BrC measured during the biomass
burning season near Manaus is found to decrease with photochemical aging with
a lifetime of ∼ 1 day. This lifetime is comparable to previous
observations within a biomass burning plume but much slower than estimated
from laboratory studies. Given the large uncertainties associated with
AERONET retrievals of AAOD, the most challenging aspect of our analysis is
that an accurate, globally distributed, multiple-wavelength aerosol absorption
measurement dataset is unavailable at present. Thus, achieving a better
understanding of the properties, evolution, and impacts of global BrC will
rely on the future deployment of accurate multiple-wavelength absorption
measurements to which AAE methods, such as the approach developed here, can