Organonitrogen and Organosulfur Compounds in Archean-Analogue Hazes: Impact of CO ₂ on Chemical Composition

The Earth’s atmosphere during the Archean eon (4.0–2.5 billion years ago) likely contained an intermittent organic haze. This haze, formed through photochemical reactions in the atmosphere, is mostly composed of molecular nitrogen (N2), carbon dioxide (CO2), and methane (CH4) and could have served as a protective layer shielding Earth’s surface from harmful ultraviolet (UV) radiation. However, the chemical composition of these organic haze particles remains poorly understood, due in part to the lack of knowledge of how trace atmospheric constituents, such as sulfur gases from volcanic activity, affect haze chemistry. Determining organic/inorganic S speciation from atmospheric chemistry is important for interpreting Earth’s geologic record. Here, we chemically characterized organic haze particles that form under Archean-relevant atmospheric conditions (0.1% CH4, 5 ppm of H2S) with either 0.5 or 0.1% CO2 in a N2 background by using hydrophilic interaction liquid chromatography coupled to electrospray ionization and high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). HILIC/ESI-HR-QTOFMS was able to achieve chromatographic resolution of isomers and provide accurate molecular formula determinations of organic haze particulate constituents. We show how the relative abundances of 121 compounds vary between these two CO2 concentrations. The observed molecules fall into 6 categories of elemental composition (i.e., CHN, CHS, CHNS, CHNO, CHOS, and CHNOS). Compounds composed of CHN, CHS, and CHNS formed preferentially under low-CO2 conditions and CHNOS molecules under high-CO2 conditions. Important biomolecules such as urea were detected, highlighting the fact that atmospheric photochemistry may have been an important source of complex organic molecules that could help the proliferation of early life.