Atmospheric implications of untested carbonyl photochemistry: an evaluation using chemically explicit and global 3D modelling

From the millions of gaseous species present on Earth’s atmosphere, those containing the carbonyl group (C=O) are some of the most atmospherically important. While all organic species react with OH and other radicals, the C=O bond in carbonyls provides a chromophore for photolysis reactions. Consequently, carbonyls react quickly and are an important source of atmospheric radicals. Recent laboratory findings on the photochemistry of several aldehydes have yielded new photochemical and photophysical mechanisms that are of unknown importance in atmospheric chemistry, which paves the way for this thesis. Here, two atmospheric models, GEOS-Chem and AtChem2, were used to understand the implications and relevance of these untested and new carbonyl photochemistry findings on atmospheric sources and composition. The results of implementing the two models to explore carbonyl photochemistry are described in this thesis in five chapters. Two chapters explore the previously untested primary production of molecular hydrogen (H2) from a range of aldehydes. Both in AtChem2 and in GEOS-Chem, a 1% quantum yield for H2 was tested, based on experimental data for acetaldehyde photolysis. Globally, and under three different atmospheric settings (urban, pristine oceanic and pristine forested), both models showed that biogenic-related aldehydes (e.g., methylgyoxal and glycolaldehyde) make an important contribution to the chemical H2 production; GEOS-Chem showed that it is up to 10% throughout the troposphere (most influential in tropical regions). Results showed that including the previously unaccounted photolytic sources of H2 does not improve model biases but calls for more experimental research into the true H2channel quantum yields for the most important aldehydes. Following, in a collaborative project with the New South Wales state government framed within Australia’s potential migration to a H2 economy, we implemented the AtChem2 model to test the sensitivity of radicals (OH and HO2) to changes in H2 levels. Radicals and other gases (e.g., CH4, CH3OH) displayed sensitivity to increases in H2 levels, highlighting the necessity of careful consideration of a potential new source of H2 to the Australian atmosphere. In the next chapter, the atmospheric fate of trifluoroacetaldehyde (CF3CHO), a fluorinated carbonyl produced from the oxidation of a fourth-generation refrigerant (HFO-1234ze, CF3CF=CF2), was modelled. CF3CHO photolysis has been shown to be a source of fluoroform (HFC-23) in small yields. The degradation and deposition pathways of CF3CHO were modelled using AtChem2 to provide a range of values on the production of HFC-23. The modelling estimates show that CF3CHO could contribute up to 14.7% to the current HFC-23 global growth rate. Finally, a new mechanism called photophysical oxidation (PPO) in carbonyls was also studied using acetaldehyde as a benchmark compound. In acetaldehyde, PPO forms the peroxyacetyl radical, which affects peroxyacetylnintrate (PAN) formation. Results using GEOS-Chem showed that PPO in acetaldehyde could be responsible for 3% of the global chemical production of PAN. However, PPO is unlikely to be important when photolysis is not the dominant carbonyl degradation pathway. This thesis provides new insight into carbonyl photochemistry through modelling applications: A new H2 simulation in GEOS-Chem was developed; in which further research related to H2 can be performed, the possible generation of HFC-23 from a new secondary source was explored, and a new photophysical mechanism for carbonyls was assessed. Ultimately, the results from exploring carbonyl photochemistry throughout this PhD are intended to be used as guide for new laboratory research.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2023

Author dc:creator

• Perez Pena, Maria Paula

• Carbonyl photochemistry
• Atmospheric modelling
• anzsrc-for: 3407 Theoretical and computational chemistry
• anzsrc-for: 3701 Atmospheric sciences
• anzsrc-for: 370104 Atmospheric composition, chemistry and processes

Statement dc:rights

• open access
• CC BY 4.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

en

Identifier

https://doi.org/10.26190/unsworks/25209

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/101502