Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NPTC (New Perspectives in Tropospheric Chemistry)

Teaser

The last two hundred years have seen new anthropogenic emissions dramatically change the chemical composition and chemistry of the troposphere, creating an incredibly diverse set of atmospheric conditions based on location and level of human population. Alkenes are common to...

Summary

The last two hundred years have seen new anthropogenic emissions dramatically change the chemical composition and chemistry of the troposphere, creating an incredibly diverse set of atmospheric conditions based on location and level of human population. Alkenes are common to pristine environments (biogenic alkenes like isoprene); polluted or urban areas (smaller alkenes like ethene) and indoor environments (terpenes from cleaning products). The removal of alkenes from the troposphere primarily occurs through reaction with ozone, generating a wide range of molecular products. Despite the abundance of alkenes in the atmosphere, with yearly emissions on the order of 850 Tg, the atmospheric ozonolysis of alkenes is still relatively poorly understood. Only recently was a key intermediate in this ozonolysis process, a carbonyl oxide known as a Criegee intermediate (CI), detected directly.

Criegee intermediates are very important species in our atmosphere, and thus to society. At night-time their unimolecular decomposition is a dominant source of hydroxyl radicals, often coined an atmospheric detergent, helping remove trace pollutants. CI reactions with other trace atmospheric constituents (bimolecular reactions) can generate a wide range of important tropospheric chemicals. They can also generate aerosols, which influence the radiative forcing of the atmosphere, transport chemicals around the atmosphere and can be detrimental to our health.

In order to continue in the evaluation of the tropospheric impact of CIs, this project proposes to use advanced spectroscopy and computational chemistry to answer several key questions that still remain:

Can Criegee intermediates be detected under a range of atmospheric conditions, e.g. varying temperature and pressure?
Absorption spectroscopy will be employed to detect CIs under conditions consistent with those found within the atmosphere. We will develop new instrumentation to assist with this procedure.

How do size and chemical complexity of Criegee intermediates influence their tropospheric chemistry?
The fate of these intermediates can vary based on their size, chemical complexity, and where they are generated in the Earth\'s atmosphere. We will investigate a range of important atmospheric intermediates, from small CIs formed in urban environments, through to larger intermediates formed from the ozonolysis of alkenes emitted from foliage. We will develop relationships between CI size and shape with atmospheric reactivity.

How do Criegee intermediates facilitate aerosol nucleation?
Criegee intermediates have recently been implicated in the formation of aerosol particles in both indoor and outdoor environments. Many CI reactions with trace atmospheric gases lead to aerosol formation. We will begin to investigate this nucleation procedure, and investigate the use of UV and IR spectroscopy to detect the early stages of aerosol nucleation.

Work performed

New instrumentation has been developed for the detection of Criegee intermediates (CIs) under a wide range of temperature and pressure conditions. This instrumentation has been applied to the investigation of CI reactions with trifluoracetic acid and some small alcohols, both of which are present as trace atmospheric gases. This work has demonstrated that the rate of these reactions will change in a complex fashion as a function of tropospheric altitude. We have been able to use computer simulations to model this behaviour, and to predict the complex temperature dependencies of many different CIs with trace tropospheric constituents.

As an extension to this, we have used a combined experimental and computational approach to interrogate the chemistry of many different Criegee intermediates with ranging chemical properties, such as chemical substitution and chemical conjugation. We have used this approach to evaluate structure-activity relationships, for example, for CIs with atmospheric alcohols. Alcohol tropospheric concentrations vary by several orders of magnitude, with higher concentrations where biofuel use is prevalent. CI reactions with alcohols is a significant tropospheric source of alpha-alkoxylalkylhydroperoxides. Using this data we are building taxonomic groups for CIs.

Criegee intermediates react with SO2 to generate SO3, which can react with water to form sulphuric acid, ultimately resulting in aerosol formation. We have targeted and interrogated key CI reactions which lead to the formation of SO3 in the troposphere. Many of these SO3 forming reaction occur incredibly quickly, with effective reaction rates limited only by the rate at which the gases collide. Thus, these reactions, in particular urban environments, may be efficient at inducing aerosol formation. Future work is to experimentally identify key spectral fingerprints for precursors to CI-induced aerosol nucleation.

This work has led to two high quality scientific publications so far.
‘Criegee intermediate alcohol reactions, a potential source of functionalized hydroperoxides in the atmosphere’ M.R. McGillen, et al. J.M. Beames, N. Watson, A.J. Orr-Ewing ACS Earth Space Chem. 1 664 (2017)
‘Temperature Dependence of the Rates of Reaction of Criegee Intermediates with Trifluoroacetic Acid’ R. Chhantyal-Pun, et al. J.M. Beames, A.J. Orr-Ewing Angew. Chem. Int. Ed. 56 9044 (2017)

Data has been generated for two more scientific publications, one of which has been submitted for peer-review (not included in above text as they are still confidential at the time of this report).

This work has been disseminated through oral presentations at research conferences, and through invited seminars at a range of academic institutions including, but not limited to, the University of Cambridge and Xiamen University (China).

This work has led to the creation of new instrumentation at Cardiff University, which will continue to be used moving forward from the period of this grant, generating new and exciting scientific data.

Similarly, the PI has advanced his computational chemistry skills, and will use this knowledge in tackling similar atmospheric chemistry issues beyond the time period of this research grant.

Final results

As stated in the above section, we are truly beginning to understand the chemistry of Criegee intermediates in the troposphere, and beginning to be able to interrogate them under atmospherically relevant temperature and pressure ranges. The data garnered over the course of this project, and beyond, all of which is novel and beyond state of the art, will inform us of the fate of atmospheric alkenes, and in particular the chemistry of these CIs at a local environmental level and on a more global scale. This research can contribute to our understanding of the fate of these species by building classes or taxonomic groups for these intermediates for use in simulations of our atmosphere. It is this grouping that my research will continue to focus on in the future which is practical for computer simulations to model atmospheric chemistry. Some of this modelling can be seen in the published work highlighted above. The best groupings for these chemicals are yet to be determined. These simulations can truly unpick how our atmosphere works, and perhaps more importantly in the 21st century, how anthropogenic emissions impact upon it. This should have societal benefit as we learn more about our atmosphere, learn how to reduce levels of harmful pollutants and learn how to remove or limit global warming species.