Abstract View
Quantifying the Contributions of Functional Groups to Light Absorptivity of Brown Carbon by a Two-layer Mapping Algorithm
KUNPENG CHEN, Jin Chen, King-Fai Li, Manuel Valdivia, Roya Bahreini, Ying-Hsuan Lin, University of California, Riverside
Abstract Number: 476
Working Group: Aerosol Chemistry
Abstract
Brown carbon (BrC), defined as organic compounds with significant light absorption in ultraviolet (UV) and near-UV region, has been identified as an important warming source in the troposphere. Laboratory and field studies have provided extensive information about BrC composition (e.g., chemical formula), but the climate forcing by individual BrC constituents is not fully understood. To bridge the chemical variability with optical response, evaluation of light absorptivity for BrC at molecular level is required. Since the authentic chemical standards of most detected BrC compounds are not commercially available, recent research has used time-dependent density functional theory (TD-DFT) to simulate the ultraviolet-visible (UV-Vis) spectra. However, the simulated spectra may be subject to various degree of bias, depending on different computational models and assumptions within TD-DFT. Here, we developed a two-layer vector decomposition structure to map the bias between experimental and theoretical spectra into chemical information, including molecular structures and functional groups. We used the experimental spectra of conjugated compounds from the MPI-Mainz UV/VIS Spectra Atlas of Gaseous Molecules of Atmospheric Interest and calibrated the simulated spectra against experimental data in an attempt to reduce the bias. A supervised learning was carried out to fit the weight coefficients in our algorithm, which obtains a semiempirical operator to parameterize the bias. For new detected BrC compounds, its UV-Vis spectrum can be evaluated by the TD-DFT simulated spectrum plus the bias estimated by the operator. With bias-corrected UV-Vis spectra, we can attribute the alternation of light-absorbing properties in different ranges of wavelength to specific functional groups and estimate the imaginary refractive index corresponding to BrC molecules. Further, the assembled patterns of conjugated functional groups which are known to induce light absorption in the troposphere will also be presented.