Periodic Reporting for period 2 - ADAPT (Autoxidation of Anthropogenic Volatile Organic Compounds (AVOC) as a Source of Urban Air Pollution)
Reporting period: 2022-08-01 to 2024-01-31
The poor state of the urban air quality shortens the life spans of millions of people around the world every year. It’s closely connected to climate change, with direct and indirect couplings to the atmosphere’s radiative transfer. For example, deposition of polluting black carbon on top of ice sheets decreases their albedo and contributes to climate warming, while clouds seeded over an ocean surface can help to reflect the sunlight back to space, effectively cooling the climate. The molecular level details of the rapid gas-phase chemical reactions responsible for air pollutant generation and removal, especially those relating to gas to particle conversion and urban particulate load generation, are still poorly charted, and severely hinder predicting the outcome of emissions to the atmosphere. The current project aims to significantly reduce the remaining uncertainties by resolving the particulate formation potential of gaseous hydrocarbon emissions relating to anthropogenic activities. The project has an experimental-theoretical approach that seems necessary in considering the multi-faceted nature of the targeted research problem.
The ADAPT project has two overarching research objectives that guide all the undertaken activities: (1) Making ambient sampling chemical ionization mass spectrometry (Api-CIMS) an all-powerful detection method across all media and phases, and (2) to detail the direct, rapid aerosol precursor formation from anthropogenic volatile organic compounds (AVOCs) by autocatalytic oxidation processes, the so-called autoxidation. The first objective is born by realization that the current research methodologies are not capable of retrieving enough information from the studied reaction systems to build definitive conclusions, and thus a further development of the experimental abilities is required. This part specifically relates to the development and application of the multi-scheme chemical ionization platform (i.e. MION), which allows first to inspect and at a later stage utilize several ion schemes in a concomitant fashion, increasing the throughput of the work significantly. The work targeting the second objective takes full advantage of the developments in the first objective and applies the comprehensive multi-ion detection for better understanding the detailed molecular pathways to condensable AVOC products. Absolutely crucial is the complementary theoretical characterization of individual reaction mechanistic steps by high-level quantum chemical computations, as they can follow the molecular structure evolution step-by-step in a complex multi-pathway reaction. Similar capability is not possible still for a long while with the experimental detection methodologies.
The ADAPT project has two overarching research objectives that guide all the undertaken activities: (1) Making ambient sampling chemical ionization mass spectrometry (Api-CIMS) an all-powerful detection method across all media and phases, and (2) to detail the direct, rapid aerosol precursor formation from anthropogenic volatile organic compounds (AVOCs) by autocatalytic oxidation processes, the so-called autoxidation. The first objective is born by realization that the current research methodologies are not capable of retrieving enough information from the studied reaction systems to build definitive conclusions, and thus a further development of the experimental abilities is required. This part specifically relates to the development and application of the multi-scheme chemical ionization platform (i.e. MION), which allows first to inspect and at a later stage utilize several ion schemes in a concomitant fashion, increasing the throughput of the work significantly. The work targeting the second objective takes full advantage of the developments in the first objective and applies the comprehensive multi-ion detection for better understanding the detailed molecular pathways to condensable AVOC products. Absolutely crucial is the complementary theoretical characterization of individual reaction mechanistic steps by high-level quantum chemical computations, as they can follow the molecular structure evolution step-by-step in a complex multi-pathway reaction. Similar capability is not possible still for a long while with the experimental detection methodologies.
The work performed up to date can be divided by the two main objectives, understanding AVOC autoxidation and developing Api-CIMS towards comprehensive detection methodology. In concerning autoxidation research, we have uncovered crucial new pathways concerning AVOC autoxidation in the gas-phase. The most significant single advancement has been the realization of a new carbon structure rearrangement reaction during aromatic compound oxidation, which breaks the functionalization hindering internal ring structures and flexes the molecule, consequently enabling looser intramolecular motions, accelerating further oxidation steps. Already in the early meters of gas-phase VOC autoxidation research it was observed how the intact rigid ring structures prevent oxidation propagation, and thus are a dead-end for the autoxidation sequence. The aromatic ring is exceptionally stabile, and thus breaking it seemed even more harder puzzle to solve. Now, with our newly found rearrangement reaction mechanism of prototypical bicyclic peroxy radical intermediate breaking the ring structures in the critical reaction intermediates, the aerosol forming characteristics of single-ring aromatic compounds can be predicted.
In aliphatic AVOC autoxidation we have recently shown how even the pristine acylperoxy radicals (i.e. primary peroxy radicals where the internal H-abstraction can only happen from C-H bonds with no adjacent functional groups) can autoxidize by rates competitive with other peroxy radical loss processes in the atmosphere. This realization was borne from our investigations on aldehyde autoxidation, where we noted that significant portion of the oxidation initiation starts from the longer carbon chain, notwithstanding the much higher reactivity of the aldehydic carbon atoms. Further investigations targeting cyclic and functionalized aliphatic hydrocarbon autoxidation reactions are ongoing and have already provided interesting observations that will to an extent challenge our current understanding of hydrocarbon oxidation under ambient atmospheric conditions.
For the CIMS ionization reagent development the progress has been somewhat slower than hoped for, as only a fraction of the planned ionization schemes has been tested up to date. However, the workflow has now been significantly accelerated by adopting machine learning (ML) techniques for spectral screening. Specifically, a large group of pesticide standards (i.e. over 700 compounds) have been measured concomitantly by several reagent ion schemes, and this dataset is used as the starting point for the ML development. This data was originally collected to gauge the usefulness of the multi-ion detection for rapid pre-screening of goods and food items, and the work was also published recently in the journal ACS Omega. However, it was soon after realized how this data could be applied as an initial training dataset for the ML approach. This dataset is now also augmented with new chemical targets, the main idea being to bring a large collection of more atmospherically relevant compounds into the database to facilitate a more valid training dataset for the ML method.
In aliphatic AVOC autoxidation we have recently shown how even the pristine acylperoxy radicals (i.e. primary peroxy radicals where the internal H-abstraction can only happen from C-H bonds with no adjacent functional groups) can autoxidize by rates competitive with other peroxy radical loss processes in the atmosphere. This realization was borne from our investigations on aldehyde autoxidation, where we noted that significant portion of the oxidation initiation starts from the longer carbon chain, notwithstanding the much higher reactivity of the aldehydic carbon atoms. Further investigations targeting cyclic and functionalized aliphatic hydrocarbon autoxidation reactions are ongoing and have already provided interesting observations that will to an extent challenge our current understanding of hydrocarbon oxidation under ambient atmospheric conditions.
For the CIMS ionization reagent development the progress has been somewhat slower than hoped for, as only a fraction of the planned ionization schemes has been tested up to date. However, the workflow has now been significantly accelerated by adopting machine learning (ML) techniques for spectral screening. Specifically, a large group of pesticide standards (i.e. over 700 compounds) have been measured concomitantly by several reagent ion schemes, and this dataset is used as the starting point for the ML development. This data was originally collected to gauge the usefulness of the multi-ion detection for rapid pre-screening of goods and food items, and the work was also published recently in the journal ACS Omega. However, it was soon after realized how this data could be applied as an initial training dataset for the ML approach. This dataset is now also augmented with new chemical targets, the main idea being to bring a large collection of more atmospherically relevant compounds into the database to facilitate a more valid training dataset for the ML method.
We have uncovered the crucial pathways to condensable products from aromatic oxidation reactions. The theoretically found and experimentally verified reaction mechanism is currently the only viable explanation for the rapid aerosol precursor generation seen in laboratory flow reactor experiments. We expect that during the project timeframe we can cover the full range of important reaction paths leading to in-situ aerosol formation from single ring aromatic systems. Furthermore, we are also making the pioneering oxidation experiments with prototypical polyaromatic systems, gauging if similar carbon structure rearrangements are the path to particulate phase products in them as well.
We have already shown how the pristine hydrocarbons are also able to autoxidize, though with lesser yields than the functionalized starting points. Nevertheless, the results indicate that almost all VOCs can undergo hydrogen shift rearrangements (i.e. autoxidation) under atmospheric conditions, in steep contrast to the common text-book chemistry, which attains that such reactions are possible only at elevated temperatures approaching combustion conditions. Currently it seems that the two reasons why such phenomena were not observed previously were: (1) used too high reactant concentrations prevented observing the relatively slow reaction product formation by autoxidation (i.e. in the laboratory generally much higher reagent conversions are utilized), and (2) apparently no one ever questioned if such a chemistry could be happening also at room temperature, most likely motivated by the common observation that material does not spontaneously ignite in open air. Now we know better and can appreciate the analogy of organic aerosol generation being a very slow burning of hydrocarbons by exposure to oxidants and ambient air.
Structure elucidation is the holy grail of mass spectrometry, and usually it requires at least another dimension of detection, such as chromatographic separation or information from sequential fragmentation reactions. The current development of multi-ion detection aims to ultimately identify compounds by their different gas-phase ionization characteristics in a semi-online manner. Such a technique could offer far greater insight of the studied phenomena in reduced experiment time, and would make it possible to rapidly characterize complex gas- and aerosol-phase samples at an unprecedented accuracy.
We have already shown how the pristine hydrocarbons are also able to autoxidize, though with lesser yields than the functionalized starting points. Nevertheless, the results indicate that almost all VOCs can undergo hydrogen shift rearrangements (i.e. autoxidation) under atmospheric conditions, in steep contrast to the common text-book chemistry, which attains that such reactions are possible only at elevated temperatures approaching combustion conditions. Currently it seems that the two reasons why such phenomena were not observed previously were: (1) used too high reactant concentrations prevented observing the relatively slow reaction product formation by autoxidation (i.e. in the laboratory generally much higher reagent conversions are utilized), and (2) apparently no one ever questioned if such a chemistry could be happening also at room temperature, most likely motivated by the common observation that material does not spontaneously ignite in open air. Now we know better and can appreciate the analogy of organic aerosol generation being a very slow burning of hydrocarbons by exposure to oxidants and ambient air.
Structure elucidation is the holy grail of mass spectrometry, and usually it requires at least another dimension of detection, such as chromatographic separation or information from sequential fragmentation reactions. The current development of multi-ion detection aims to ultimately identify compounds by their different gas-phase ionization characteristics in a semi-online manner. Such a technique could offer far greater insight of the studied phenomena in reduced experiment time, and would make it possible to rapidly characterize complex gas- and aerosol-phase samples at an unprecedented accuracy.