18 Jan 2022
18 Jan 2022
Status: a revised version of this preprint is currently under review for the journal ACP.

Evaluating the contribution of the unexplored photochemistry of aldehydes on the tropospheric levels of molecular hydrogen (H2)

Maria Paula Pérez-Peña1, Jenny A. Fisher2, Dylan B. Millet3, Hisashi Yashiro4, Ray L. Langenfelds5, Paul B. Krummel5, and Scott H. Kable1 Maria Paula Pérez-Peña et al.
  • 1School of Chemistry, University of New South Wales, Sydney, NSW, Australia
  • 2Centre for Atmospheric Chemistry, School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, NSW, Australia
  • 3Department of Soil, Water and Climate. University of Minnesota, Saint Paul, MN, USA
  • 4Earth System Division, National Institute for Environmental Studies, Tsukuba, Japan
  • 5Climate Science Centre, CSIRO Oceans & Atmosphere, Aspendale, Australia

Abstract. Molecular hydrogen, H2, is one of the most abundant trace gases in the atmosphere. The main known chemical source of H2 in the atmosphere is the photolysis of formaldehyde and glyoxal. Recent laboratory measurements and ground-state photochemistry calculations have shown other aldehydes photo-dissociate to yield H2 as well. This aldehyde photochemistry has not been previously accounted for in atmospheric H2 models. Here, we used two atmospheric models to test the implications of the previously unexplored aldehyde photochemistry on the H2 tropospheric budget. We used the AtChem box model implementing the nearly chemically explicit Master Chemical Mechanism at three sites selected to represent variable atmospheric environments: London, Cape Verde and Borneo. We conducted five box model simulations per site using varying quantum yields for the photolysis of 16 aldehydes and compared the results against a baseline. The box model simulations showed that the photolysis of acetaldehyde, propanal, methylglyoxal, glycolaldehyde and methacrolein yield the highest chemical production of H2. We also used the GEOS-Chem 3-D atmospheric chemical transport model to test the impacts of the new photolytic H2 source on the global scale. A new H2 simulation capability was developed in GEOS-Chem and evaluated for 2015 and 2016. We then performed a sensitivity simulation in which the photolysis reactions of six aldehyde species were modified to include a 1 % yield of H2. We found an increase in the chemical production of H2 over tropical regions where high abundance of isoprene results in the secondary generation of methylglyoxal, glycolaldehyde and methacrolein, ultimately yielding H2. We calculated a final increase of 0.4 Tg yr−1 in the global chemical production budget, compared to a baseline production of ~41 Tg yr−1. Ultimately, both models showed that H2 production from the newly discovered photolysis of aldehydes leads to only minor changes in the atmospheric mixing ratios of H2, at least for the aldehydes tested here when assuming a 1% quantum yield across all wavelengths. Our results imply that the previously missing photochemical source is a less significant source of model uncertainty than other components of the H2 budget, including emissions and soil uptake.

Maria Paula Pérez-Peña et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-1052', Anonymous Referee #1, 20 Mar 2022
  • RC2: 'Comment on acp-2021-1052', Anonymous Referee #2, 21 Mar 2022
  • AC1: 'Comment on acp-2021-1052', Maria Paula Perez-Pena, 20 May 2022

Maria Paula Pérez-Peña et al.

Maria Paula Pérez-Peña et al.


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Short summary
We used two atmospheric models to test the implications of previously unexplored aldehyde photochemistry on the atmospheric levels of molecular hydrogen (H2). The modelling study showed that this newly tested photochemistry produces more H2 over densely forested areas were is also removed more strongly compared to the rest of the world. The results highlight that other processes, like emissions or sinks, that contribute to atmospheric H2 levels should be studied further.