Mass Spectrometry of Atmospheric Nitrophenols

Brown carbon atmospheric aerosol particles are a significant source of uncertainty in climate change models as the effects of their composition and absorptivity on the atmosphere are not well described. Nitrophenols, which show strong absorption in the UV-visible spectrum, may have a disproportionate impact on the balance of atmospheric radiative forcing as components of these aerosol particles. Fog water absorption spectra from Los Angeles, an area with high levels of nitrophenols and nitrophenol precursors, exhibit both a general increase in absorption and an additional broad absorbance peak extending into the visible range. This added absorption peak is consistent with the spectrum of several nitrophenols, and the contribution of these compounds to the spectrum can be quantitatively determined when nitrophenol levels are known. Mass spectrometer analysis of aerosol particles and fog water in parallel with UV-visible absorption measurements allows for identification and quantitation of different nitrophenols and determination of their effect on the overall absorptivity of atmospheric particulate matter. Nitrophenols display easily identifiable characteristic fragmentation patterns and a linear response in mass spectrometer counts, and are present in high enough concentrations to eliminate preconcentration of fog water samples. Our work shows that nitrophenols are major components of Los Angeles fog water and that the contribution of nitrophenols to the real-time absorption of atmospheric samples can be quantified and isolated.