The sky is burning: Elucidating brown carbon composition using CSI:FingerID

Black and brown carbon in the atmosphere

Black carbon is a component of fine particulate matter formed through the incomplete combustion of fossil fuels, biofuel, and biomass. It is well-known for its contributions to climate change by warming the Earth through absorbing sunlight and heating the atmosphere, reducing albedo when deposited on snow and ice, as well as interaction with clouds. Brown carbon is brown smoke primarily produced by the burning of organic matter. Brown carbon also has a significant impact on atmospheric radiation and global climate^​1,2​. It absorbs light^​3,4​ and disturbs the temperature pattern of the atmosphere. Brown carbon in cloud and fog water can accelerate water evaporation and cloud dissolution through light absorption^​5​. It is essential to comprehensively characterize the light-absorbing brown carbon molecules to understand their roles in atmospheric processes.

Nontargeted tandem mass spectrometry for measuring brown carbon

To identify and study brown carbon chromophores, researchers have employed various analytical techniques. Concurrent UV/visible light absorption measurements and LCMS have been used to analyze brown carbon samples, but tandem MS experiments that benefit the structural elucidation of brown carbon chromophores are still sparse. 

Researchers from the China University of Geosciences and the Nanjing University of Information Science and Technology^​6​ collected five types of atmospheric samples to study brown carbon chromophores. They collected ambient aerosol samples on the rooftop of an academic building, rainwater during a 2-day rain event, road tunnel aerosol samples at the outlet of a local road tunnel, a biomass burning aerosol sample from a traditional iron stove that burned camphor tree wood, and generated a toluene secondary organic aerosol sample in a Teflon smog chamber^​7​. The researchers developed a non-targeted tandem mass spectrometry experiment workflow to identify the most probable structures of brown carbon chromophores.

The analysis involved Ultra-High Performance Liquid Chromatography coupled with a UV/vis photodiode array detector, electrospray ionization and high-resolution mass spectrometer with data-dependent acquisition for collecting MS2 spectra. MS1 data analysis was carried out using MZmine^​8​ and molecular structures of the brown carbon chromophores were assigned using SIRIUS^​9​ with CSI:FingerID^​10​ based on the MS2 spectra.

What absorbs the light?

They identified 100 brown carbon chromophores, all of them possessing an aromatic conjugated system, such as a benzene ring or poly-aromatic ring, with at least one functional group, such as -OH, -COOH, -NO2, or an amino group. The chromophores were classified into nine groups: nitrophenols, benzoic acids, oxygenated polycyclic aromatic hydrocarbons (OPAHs), phenols, aryl amides/amines, phenylpropene derivatives, coumarins and flavonoids, pyridines, and nitrobenzoic acids. Notably, 33 new chromophores were identified in this study, which have not been reported before. The aryl amides/amines group, in particular, was observed for the first time, and they were predominantly found in rainwater samples.

The implications of these findings in the atmosphere are substantial. These chromophores play a significant role in light absorption. Interestingly, only one chromophore, nitrophenol, was identified in both rainwater and ambient aerosols. This suggests that the chromophores in rainwater may originate from atmospheric aqueous-phase reactions or distant regional sources, rather than being predominantly contributed by local sources to ambient aerosols. The identification and characterization of brown carbon chromophores in various atmospheric samples provide valuable insights into the understanding of their sources, behavior, and effects on radiative forcing, cloud formation, and global climate change.