Measurement data: Refractive index of engine-emitted black carbon and the influence of organic coatings on optical properties.

Black carbon (BC) and brown carbon (BrC) aerosols are extensively investigated components of atmospheric aerosol due to their ability to absorb solar radiation and contribute to atmospheric heating, resulting in a positive radiative forcing on climate. Although BC and BrC are very important for climate, they are poorly represented in atmospheric models. This is in part due to the lack of accurate refractive index (RI) descriptions for both BC and BrC. Previous studies have used mobility selection approaches to select BC/BrC particles upstream of optical spectroscopy instruments to allow RI characterisations, but these retrievals suffer from issues caused by multiple charging. We solved this issue by using a new aerosol classification technique, enabling optical measurements for an aerosol sample classified according to a single physical size without multiple charge artefacts, which improves the subsequent RI retrieval. In addition, non-absorbing and weakly absorbing organic materials were condensed onto BC to form coated soot particles, allowing different optical models for mixed particles to be evaluated. We found that the absorption of coated BC particles may not be predicted with sufficient accuracy from knowledge of only the equivalent diameter, coating composition, and RI, and considering additional factors such as morphology may be necessary for accurate predictions.