Effect of ammonium sulfate on the absorbance spectra of acoustically levitated brown carbon containing droplets

Light absorbing organic aerosol such as brown carbon can impact the climate through warming. Atmospheric processing including evaporation and photochemical ageing can modify the microphysical and optical properties of aerosol which in turn affects how aerosols affect the climate. Currently the change in microphysical and optical properties induced by atmospheric processing is not well understood.

Previously we have measured the UV-visible absorbance spectra of single acoustically levitated droplets of brown carbon as they evaporate. We measured a shift in the absorbance maximum towards the visible region of the spectrum for evaporating droplets of humic acid and water-soluble extracts of wood smoke aerosol. The shift in the absorbance maximum has important implications for the climate as it results in an increased overlap of the aerosol absorbance with actinic radiation. Building on this work and acknowledging the complex composition of real atmospheric aerosols, here we report absorbance measurements of evaporating droplets containing brown carbon and ammonium sulfate. Initial results show differences in absorbance spectra compared to spectra without ammonium sulfate that could indicate the formation of new chemical species as the droplets evaporate and are illuminated.