EC-04. Optical Studies of Formation of Brown Carbon

Abstract
The climate impact of aerosols is difficult to predict because some aerosols absorb solar radiation and result in warming, while other aerosols are dominantly reflective and result in cooling. The uncertainty in radiative impact of atmospheric aerosols, especially organic aerosol, drives the overall uncertainty in models predicting climate change. Although organic-containing aerosol has historically been considered to be either extremely reflecting (“white carbon”) or strongly absorbing (“black carbon”), many organic aerosols exhibit wavelength-dependent light absorption across the UV-visible region; we call such compounds "brown carbon". While previous laboratory experiments of brown carbon (BrC) formation have been performed, such studies do not regularly consider acidity, even though atmospheric aerosols are typically acidic and aerosol pH varies. Thus, studies of BrC formation as a function of pH are needed. We form BrC aerosol from aqueous mixtures of methylglyoxal and ammonium salts at varying pH levels. A cavity ringdown spectrometer (CRD) measures the light extinction caused by the aerosol using a laser in a cavity capped with reflective mirrors, and a photoacoustic spectrometer (PAS) measures the light absorption of the aerosol with a highly sensitive microphone that can detect molecular absorption. Once absorption and extinction have been measured, the optical properties, or “refractive indices,” for the particles can be determined. Because the refractive index is a fundamental property of the aerosol, these values can be used to calculate optical effects of BrC under a variety of atmospheric conditions. Our studies reveal that bulk acidic BrC solutions absorb less than basic solutions in the UV-visible region. In addition, we find absorption increases overall as the solutions age in the dark. Ongoing studies are measuring the refractive indices of the methylglyoxal ammonium sulfate in the aerosol phase as a function of pH. By retrieving the refractive indices of aerosols produced and reacted under varying conditions of atmospheric composition, we can improve prediction of aerosols’ effect on radiative balance, air quality, and climate variability and change.