Photochemical Aging of Levitated Aqueous Brown Carbon Droplets

The contribution of brown carbon aerosol to climate forcing remains poorly understood. Atmospheric processes, such as photochemical aging and evaporation of brown carbon aerosol, can result in a change in their physicochemical properties; previous studies have focused on the effects of such processes in the bulk phase rather than in individual aerosol particles. We measured direct ultraviolet-visible absorbance measurements of acoustically levitated aqueous brown carbon droplets. These show a reproducible red shift in the absorbance maximum as the droplets evaporate under illumination. This shift does not occur in the bulk phase. The findings illustrate that common atmospheric processes, such as evaporation and photochemical aging of brown carbon aerosol, can cause a red shift in absorbance, leading to a greater overlap with the solar spectrum. This has implications for the contribution of brown carbon to radiative forcing of the climate.