Photochemical Production of Organic Haze from Inorganic Gases

HANALEI LEWINE, Nathan Reed, Zachary Schiffman, Jeffrey Price, Margaret Tolbert, Eleanor Browne, University of Colorado Boulder & CIRES

     Abstract Number: 674
     Working Group: Planetary Aerosols: From Earth to Exoplanets

Abstract
Atmospheric hazes, the complex mixture of gases and aerosols produced by atmospheric chemistry and frequently initiated by photolysis, are common in planetary atmospheres, both in our solar system and on exoplanets. Photochemical hazes are important for climate and habitability. Organic aerosol (OA) is particularly important for prebiotic chemistry, as it is a potential abiotic source of complex organic molecules. Understanding atmospheric compositions that lead to OA is critical to elucidate how hazes impact radiative balance and for understanding how we interpret exoplanet spectra. Laboratory experiments are important for connecting atmospheric compositions to OA formation potential because the mechanisms for OA formation in exoplanetary atmospheres are unknown. Nitrogen/carbon dioxide (N2/CO2) atmospheres are of particular interest because of their connection to early Earth’s habitable, anoxic atmosphere. There is geochemical evidence of organic haze in the Archean atmosphere, formed via methane (CH4) photochemistry. Traditional thought is that CH4 is required for OA formation; however, recent work suggests that OA formation is possible in CO2-rich atmospheres in the absence of CH4 if water and/or hydrogen sulfide (H2S) are present. Here, we performed laboratory experiments of haze formation in Archean atmospheric analogs formed via far-UV irradiation of N2/CO2/H2S mixtures. We measured aerosol particles using aerosol mass spectrometry and gas-phase products using iodide chemical ionization mass spectrometry in real-time. We find that addition of trace H2S (5 ppmv) enables organic haze production in CO2-rich atmospheres without CH4. The aerosol was composed of OA and inorganic sulfate and the amount of OA exceeded that of a CH4/CO2 ratio of 0.5 (0.1% CH4/0.2% CO2) with no H2S. We hypothesize that formaldehyde and sulfuric acid are produced from CO2 and H2S photolysis, the latter of which catalyzes polymerization of the former, forming lower volatility organic species that condense to form OA.