Modeling of Carbonyl/Ammonium Sulfate Aqueous Brown Carbon Chemistry via UV/Vis Spectral Decomposition

Abstract

The proper characterization of aqueous brown carbon (BrC) species, their formation, and their light absorbance properties is critical to understanding the aggregate effect that they have on overall atmospheric aerosol climate forcing. The contribution of dark chemistry secondary organic aerosol (SOA) products from carbonyl-containing organic compounds (CVOCs) to overall aqueous aerosol optical properties is expected to be significant. However, the multiple, parallel pathways that take place within CVOC reaction systems and the differing chromophoricity of individual products complicates the ability to reliably model the chemical kinetics taking place. Here, we proposed an alternative method of representing UV-visible absorbance spectra as a composite of Gaussian lineshape functions to infer kinetic information. Multiple numbers of curves and different CVOC/ammonium reaction systems were compared. A model using three fitted Gaussian curves with magnitudes following first-order kinetics achieved an accuracy within 65.5% in the 205–300-nm range across multiple organic types and solution aging times. Asymmetrical peaks that occurred in low-200-nm wavelengths were decomposed into two overlapping Gaussian curves, which may have been attributable to different functional groups or families of reaction products. Component curves within overall spectra exhibited different dynamics, implying that the utilization of absorbance at a single reference wavelength to infer reaction rate constants may result in misrepresentative kinetics for these systems.

Competing Photochemical Effects in Aqueous Carbonyl/Ammonium Brown Carbon Systems

“…The modeled major absorbance peak of the Gly/AS system was represented as a single peak at 209 nm, which exhibited negligible dynamics across the observed time frame. Observed and modeled peaks in this region have been previously attributed to α-carbonyls, pyrazine derivatives, and imidazole derivatives. ,,− The minor peak was decomposed into a growing peak centered on 258 nm and a shrinking peak centered on 272 nm. The wavelength offset of these two peaks coupled with their opposing magnitudes captures the broad overall increase then decrease observed in the minor peak region.…”

“…Individual modeled lineshapes were shown to increase, decrease, or stay the same, depending on the location and reaction system, implying both the enhancement and loss of chromophoricity as potential effects of direct irradiation. As in previous studies using this modeling approach, , the total and decomposed model lineshapes used to infer kinetic information are not meant to represent any specific chromophoric compound but rather the overall behavior of light-absorbing compounds under those conditions. As such, the absolute magnitude of any singular decomposed peak may yield nonphysical (i.e., negative) values in representing the loss of overall chromophoricity due to a particular mechanism.…”

“…The number of lineshapes employed for each reaction system were selected such that the features of the time- and wavelength-resolved MAC values were captured with as few lineshapes as possible; more complex observed dynamics require additional lineshapes to appropriately model their behavior. As observed in Fan et al, the implementation of further lineshapes into model equations provides significant increases to computational resources with diminishing returns of model fidelity . A case-by-case consideration for the most appropriate number of curves is therefore necessary to reliably estimate kinetic behavior without over- or under-fitting the dynamics in a given reaction system.…”

Atmospheric Aqueous-Phase Chemistry

“…The articles of this Special Issue can be divided into three groups: (i) the first is mostly based on field measurements and/or combined field and modeling studies giving insights into the chemical characterization of different atmospheric liquid water samples from various environments [1][2][3][4]; (ii) the second group is focused on studies of aqueousphase reactivity of some important atmospheric organic compounds [5][6][7][8]; (iii) and the final group comprises articles based on predictive modeling and/or combined modeling and laboratory studies providing insight into aqueous secondary organic aerosol (SOA) formation [9,10].…”

“…The multiple pathways taking place in such systems and different chromophoricity of individual products make it complicated to reliably model the kinetics. In their work, Fan et al [9] proposed an alternative method of representing UV-Vis absorbance spectra as a composite of Gaussian lineshape functions to infer kinetic information. Their data and model parameters were focused on the light-absorbing properties of glyoxal, methylglyoxal, and glycolaldehyde in ammonium sulfate environments, selected due to their notable contributions to aqueous SOA mass.…”