Wavelength-, pH- and Composition-Dependence of Light Absorption by Common Brown Carbon Chromophores in Aerosol Particles

In this study, the real and imaginary refractive index (n and k) of oxidized nitroaromatic brown carbon (BrC) chromophores were determined using a linear quadrupole electrodynamic balance (LQ-EDB) coupled with broadband light scattering (BLS) spectroscopy as a function of pH and relative humidity (RH). The direct radiative forcing induced by aerosols containing these chromophores is largely unconstrained due to uncertainties in their dynamic microphysical properties. Bulk solution UV/vis spectroscopy, as a method to determine k, cannot fully describe aerosol particles that exist in metastable states. However, BLS measurements on levitated particles allow for the determination of k at much higher solution concentrations typical of aqueous atmospheric aerosol. Although previous studies have characterized k at single or limited wavelengths, in this work the complex refractive index of BrC-containing aerosol particles was determined across a range of wavelengths (350 to 700 nm). The measured k was found to increase with chromophore volume fraction and decrease with RH, consistent with concentration effects, with an additional strong dependence on molecular structure and pH that controlled both the magnitude and composition dependence of k. Mass and volume fraction weighted predictions of k were assessed based on bulk UV/vis and droplet measurements, with the bulk measurements showing limited ability to predict the absorption due to the need to extrapolate over a significant concentration gap. Finally, we discuss how particle phase separation can impact observations, leading to a breakdown of the assumption that the particles are uniformly absorbing.