Project No: 16215524

Title: Atmospheric brown carbon: a novel quantifying algorithm with aethalometer data, updated mechanisms for numeric simulation, and assessment of its regional impacts

Principal Investigator: Dr. Kezheng LIAO

Co-Investigator: Prof. Christian GEORGE, Prof. Cheng WU, Prof. Qi YING, Prof. Jianzhen YU

Abstract:

Atmospheric brown carbon (BrC) aerosol refers to a group of organic compounds with absorption spectra in which the mass absorption efficiency decreases sharply from shorter to longer wavelengths within the UV-visible light range, resulting in a yellow to brownish appearance. Therefore, BrC has been recently recognized as an important factor in aerosol radiative forcing on the global scale, with its absorption-related contribution estimated to be a quarter of that of more notorious black carbon (BC). Unlike BC or total organic carbon, there exists no standard protocol in determining total BrC aerosol in ambient samples. So far, the most discussed BrC species in labs are model molecules with large, conjugated π systems (e.g., 4-nitrocatechol and imidazole derivatives), whereas the chemical complexity and variation in concentration of ambient BrC make it difficult to characterize its molecular composition. Much like its chemical diversity, the emission sources and atmospheric fates of BrC can be highly complicated as well. BrC could be either primarily emitted from biomass burning sources (e.g., forest fires) or secondarily formed from airborne precursors (e.g., glyoxal, methylglyoxal, and aromatic compounds). In ambient air, BrC could undergo either aging or photobleaching processes depending on its chemical structure and the environmental conditions. Latest studies also suggest that some BrC species can serve as photosensitizers that are capable of enhancing the formation of secondary components in aerosol. Despite the growing interest in BrC, modelling research that attempts to incorporate these experimental discoveries is still in its infancy. Nor are we able to assess the impacts of BrC while considering its optical property, dynamic nature, and photosensitizing capability in a comprehensive manner. To fill these research gaps, we hereby propose the following research plans.
First, a Bayesian inference (BI) approach will be developed for quantifying total BrC and its absorption Angstrom exponent (AAE) value in ambient aerosol using Total Carbon Analyzer and Aethalometer data. These two pieces of instrument are financially affordable and able to achieve real-time online measurement, and then our novel statistical algorithm can resolve BC, BrC, and white carbon (WtC) concentrations simultaneously, as well as its AAE value. Preliminary work on a case study has demonstrated such a method to be accurate, robust, and produce results comparable to references. Information of BrC ambient concentration and its AAE is essential for any further studies. Second, we plan to apply the BI approach to a multi-year aethalometer measurement dataset made at 9 monitoring stations in the Greater Bay Area (GBA), China. This dataset will be used to examine the long-term trends and spatial distribution of atmospheric BrC in GBA. Further application of receptor model like Positive Matrix Factorization will help in resolving the emission sources of BrC at a Hong Kong site, where co-located measurements of molecular and elemental PM source tracers are available. Third, we will analyze and summarize the latest advancements in lab experiments about the photochemistry of brown carbon, and compile them into an integrated reaction mechanism module for numeric models. A simple zero-dimension box model has been developed and targeted at compiling an updated reaction mechanism for BrC species, once sufficient information is gathered. Last but not least, we plan to implement the updated reaction mechanism of BrC into a regional air quality model, i.e., the Community Multiscale Air Quality Modeling System, to conduct a preliminary assessment of the environmental impacts of BrC in the GBA in recent years. Emphasis will be placed on the evolution of its optical properties and its roles in the enhancement of secondary formation reactions as photosensitizers. Meanwhile, the previously mentioned quantification method will provide observation data for model validation. In conclusion, this research project will produce not only a valuable quantification method for ambient BrC, but also a new modelling scheme to evaluate its environmental influences.