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

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 m2 g−1 for OA-MAC at 440 nm. Using the combination of AERONET and OMI UV absorption observations we estimate that the AAE388∕440 nm for BrC is generally ∼ 4 worldwide, with a smaller value in Europe (