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An extreme biomass burning event occurred in Indonesia from September through October 2015 due to severe drought conditions, partially caused by a major El Niño event, thereby allowing for significant burning of peatland that had been previously drained. This event had the highest sustained aerosol optical depths (AODs) ever monitored by the global Aerosol Robotic Network (AERONET). The newly developed AERONET Version 3 algorithms retain high AOD at the longer wavelengths when associated with high Ångström exponents (AEs), which thereby allowed for measurements of AOD at 675 nm as high as approximately 7, the upper limit of Sun photometry. Measured AEs at the highest monitored AOD levels were subsequently utilized to estimate instantaneous values of AOD at 550 nm in the range of 11 to 13, well beyond the upper measurement limit. Additionally, retrievals of complex refractive indices …