Assessment of the impacts of brown carbon (BrC) requires accurate determination of its physicalproperties, but a model must be invoked to derive these from instrument data. Ambient measurements were made in London at a site influenced by traffic and solid fuel (principally wood) burning, apportioned by single particle soot photometer data and optical properties measured using multiwavelength photoacoustic spectroscopy. Two models were applied: a commonly used Mie model treating the particles as single-coated spheres and a Rayleigh-Debye-Gans approximation treating them as aggregates of smaller-coated monomers. The derived solid fuel BrC parameters at 405 nm were found to be highly sensitive to the model treatment, with a mass absorption cross section ranging from 0.47 to 1.81 m 2 /g and imaginary refractive index from 0.013 to 0.062. This demonstrates that a detailed knowledge of particle morphology must be obtained and invoked to accurately parameterize BrC properties based on aerosol phase measurements. BackgroundOptically absorbing soot in the atmosphere is characterized as being composed of a mixture of black carbon (BC) [Bond et al., 2013] and brown carbon (BrC) [Andreae and Gelencsér, 2006]. These are both important to the radiative properties of the atmosphere but have different wavelength-dependent characteristics. Attempts to model the effect of BC and BrC are hindered by uncertainties in their fundamental properties. A key parameter governing the effect of BrC is the imaginary part (k BrC ) of the refractive index at short wavelengths, and a large range of values have been derived through field and laboratory studies, resulting in uncertainty surrounding its impacts [Wang et al., 2014]. The absorption of brown carbon (σ abs,BrC ) from a variety of biomass burning sources has been assessed recently [e.g., Lack et al., 2012;Saleh et al., 2014] by subtracting the black carbon absorption (σ abs,BC ) from the total measured absorption (σ abs,total ). The explicit calculation of σ abs,BC is crucial for deriving σ abs,BrC , and therefore its mass absorption cross section (MAC) and k BrC . Therefore, the accuracy of the derived BrC parameters depends on the accuracy by which the BC properties can be estimated, which requires the invocation of an optical model treatment of the measured physical properties of the BC.Mie-based models [Bohren and Huffman, 1983] has been widely used to model the absorption properties of particles by treating them as concentric spheres; however, this model may not apply for BC particles close to source regions as they often more resemble aggregates of many small monomers [e.g., Xiong and Friedlander, 2001;China et al., 2013]. These aggregates can be described [Filippov et al., 2000] by using parameters such as the fractal dimension (D f ), monomer diameter (D s ), and number of monomers (N s ), which forms the basis for a number of advanced models considering particle morphology, such as superposition T-matrix [Mishchenko et al., 2013] and discrete dipole [Adachi et al., 20...