Deriving brown carbon from multiwavelength absorption measurements: method and application to AERONET and Aethalometer observations

Abstract

Abstract. 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 wavelengthdependent 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 2 g −1 for OA-MAC at 440 nm. Using the combination of AERONET and OMI UV absorption observations we estimate that the AAE 388/440 nm 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 toPublished by Copernicus Publications on behalf of the European Geosciences Union.

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…5e). This observation implies that the BrC aerosol particles measured at the ATTO site are mixed down into the boundary layer in the early morning and are then quickly photo-degraded during the day (Forrister et al, 2015;Wang et al, 2016b;Wong et al, 2017). This pattern is not observed during the wet season, when σ ap BrC 370 exhibits no significant diel variability.…”

“…10). We found that BrC contributes 24 % is ~1 day due to photolysis and oxidation, which destroy the chromophores (Forrister et al, 2015;Wang et al, 2016b;Wong et al, 2017). Therefore, BrC emitted from fires in the southern borders of the Amazon rain forest, which require ~3 days to be transported to the ATTO site, is likely to be significantly photodegraded and to contribute only weakly to total aerosol light absorption after atmospheric processing.…”

“…However, this assumption is theoretically wrong and the BC-related å abs is very sensitive to the size of the particles (Moosmüller et al, 2011). Wang et al (2016b) proposed a method to calculate the wavelength dependence of the Ångström exponent (WDA) in order to estimate the BrC contribution to total light absorption by aerosol particles. They use the difference between two å abs calculated for two different wavelength pairs (440 -870 nm, and 675 -880 nm) using aerosol robotic network (AERONET) and Aethalometer data.…”

“…Several studies have used the absorption spectral dependence to apportion the fossil fuel and BB contributions to total absorption (Favez et al, 2010;Massabò et al, 2015;Sandradewi et al, 2008). However, the å abs values do not always reflect the combustion type and using it as a source apportionment parameter could lead to erroneous results (Garg et al, 2016;Lewis et al, 2008;Wang et al, 2016b).…”

“…In order to retrieve the ambient BC wavelength dependence, Wang et al (2016b) proposed the use of the wavelength dependence of å abs instead of å abs itself. The so-called wavelength dependence of å abs (WDA) is calculated as the difference of two wavelength pairs; one for shorter to long wavelengths (e.g., 440 -870 nm) and another for medium to long wavelengths (e.g., 675 -880 nm).…”