Final Report Summary - ISOMASSKIN (Laboratory study of in-cloud reactions of isoprene oxidation products with sulphate radical-anions by mass-spectrometric and kinetic analysis)
Secondary organic aerosol (SOA) is omnipresent in the Earth troposphere over urban, rural and remote regions. It participates actively in transformation of atmospheric trace components and affects climate, weather, air quality, human health and condition of the biosphere. For a decade or so, scientists have intensively tried to quantify the composition of SOA and to unveil the chemical pathways it originates and ages in the atmosphere. In spite of the efforts, the knowledge of SOA is still limited and needs work. High stakes in the game include weather control, better air quality and healthy world.
Among SOA precursors, isoprene plays many important roles - either directly or through the products of its oxidation in the atmosphere. Isoprene is a major non-methane hydrocarbon emitted to the atmosphere mainly by terrestrial vegetation. Primary products of its gas-phase oxidation include methacrolein (MAC), methyl vinyl ketone (MVK) and methacrylic acid (MAA). The three compounds are sufficiently well soluble in water to exploit the opportunity of unique and fast chemical transformations offered by the atmospheric aqueous phases - clouds, rains, fogs, hazes and deliquescent aerosols.
Organosulfates - organic molecules containing at least one sulfate group - were recently discovered in samples of ambient aerosols and rainwater, as well as in aerosols produced in large laboratory facilities - smog chambers. They were also produced in aqueous-phase laboratory experiments, by reacting dissolved isoprene with sulfate radical-anions. Sulfate radical-anions are highly reactive transients that form during the aqueous-phase autoxidation (i.e. oxidation with molecular oxygen) of sulfur dioxide catalysed by ions of transition metals, such as iron, manganese, cobalt or copper. Sulfur dioxide and transition metals are common trace components of the atmosphere, so they have chance to meet there with isoprene and its aforementioned descendants.
The project ISOMASSKIN aimed to elucidate the chemical reactions of MAC, MVK and MAA acid with sulfate radical-anions and the formation of organosulfate compounds therefrom. Objectives of the project included the identification and characterisation of products from these reactions, estimation of kinetic parameters of the reactions and unveiling the chemical mechanisms involved. For these purposes, advanced mass spectrometric methods and synthesis of necessary reference compounds were planned as well.
The project work done and the results obtained included:
1. chemical-kinetic experiments on reactions of MAC, MVK and MAA with sulfate radical anions, based on the principle that the radical-anions are generated by the autoxidation of bisulfite ions (a form of dissolved sulfur dioxide) catalysed by ferric ions, which in turn is slowed down when organic reactant is added to consume the radical-anions. Computational analysis of the slow-down process allows the derivation of the rate constant for the reaction of the added compound with sulfate radical-anions.
2. reactions of MAC and MVK with sulfate radical anions were found very fast. The rate constants were estimated at several temperatures. Negative energies of activation for the reactions calculated therefrom indicated the reactions were more complicated than expected. The possible explanation was the reaction was preceded by the formation of MAC and MVK adducts with bisulfite ions. Rate and equilibrium constants for the addition reactions were determined experimentally. The reactions appeared faster than reported in the literature. MAA escaped the procedure used, because it accelerated the autoxidation of bisulfite ions instead of slowing it down.
3. photochemical and dark experiments on reactions of MAC, MVK and MAA with sulfate radical-anions generated by several methods (photolytic decomposition of peroxydisulfate ions using various lamps and natural sunlight, thermal decomposition of peroxydisulfate ions, decomposition of peroxydisulfate ions catalysed by ferric ions, decomposition of oxone catalysed by cobalt and copper ions, autoxidation of sulfur(IV) catalysed by ferric ions. The experiments revealed formation of several organosulfates that retained the carbon skeleton of their precursors, including sulfated and high molecular weight oligomers of MAA. The 2-methyl-glyceric acid is often detected in samples of ambient aerosol.
4. Analysis of chemical mechanisms for reactions of MAC, MVK and MAA with sulfate radical-anions which showed that the sulfate radical-anions react with MAC and MVK via the addition to a C=C double bond, while with MAA - also through the abstraction of proton from the acid molecule. Plausible reaction pathways were derived for a few products identified, including sulfated 2-methyl glyceric acid and high molecular weight oligomers of MAA.
5. Development and optimisation of several advanced mass-spectrometric methods necessary to carry out the product studies of the project and synthesis of a reference compound necessary for the unambiguous structural characterisation of novel reaction products.
The results obtained from the project increase the general knowledge of the chemistry of atmospheric trace components, including the formation of SOA. They emphasise the generally underestimated role the aqueous-phase processes can play in the formation of secondary atmospheric pollutants, including organosulfates. In addition, the project shows that the interaction of organic compounds originating from biogenic emissions (MAC, MVK and MAA as isoprene descendants) with anthropogenic pollutants (sulfate radical-anions descending form sulfur dioxide) strongly contributes to the complexity of chemical processes in the atmosphere.
The socioeconomic impact of the project is remote in time but clear. The expected influence is in the area of improving the air quality and reducing the adverse effects of pollution on human health and condition of biosphere.
Among SOA precursors, isoprene plays many important roles - either directly or through the products of its oxidation in the atmosphere. Isoprene is a major non-methane hydrocarbon emitted to the atmosphere mainly by terrestrial vegetation. Primary products of its gas-phase oxidation include methacrolein (MAC), methyl vinyl ketone (MVK) and methacrylic acid (MAA). The three compounds are sufficiently well soluble in water to exploit the opportunity of unique and fast chemical transformations offered by the atmospheric aqueous phases - clouds, rains, fogs, hazes and deliquescent aerosols.
Organosulfates - organic molecules containing at least one sulfate group - were recently discovered in samples of ambient aerosols and rainwater, as well as in aerosols produced in large laboratory facilities - smog chambers. They were also produced in aqueous-phase laboratory experiments, by reacting dissolved isoprene with sulfate radical-anions. Sulfate radical-anions are highly reactive transients that form during the aqueous-phase autoxidation (i.e. oxidation with molecular oxygen) of sulfur dioxide catalysed by ions of transition metals, such as iron, manganese, cobalt or copper. Sulfur dioxide and transition metals are common trace components of the atmosphere, so they have chance to meet there with isoprene and its aforementioned descendants.
The project ISOMASSKIN aimed to elucidate the chemical reactions of MAC, MVK and MAA acid with sulfate radical-anions and the formation of organosulfate compounds therefrom. Objectives of the project included the identification and characterisation of products from these reactions, estimation of kinetic parameters of the reactions and unveiling the chemical mechanisms involved. For these purposes, advanced mass spectrometric methods and synthesis of necessary reference compounds were planned as well.
The project work done and the results obtained included:
1. chemical-kinetic experiments on reactions of MAC, MVK and MAA with sulfate radical anions, based on the principle that the radical-anions are generated by the autoxidation of bisulfite ions (a form of dissolved sulfur dioxide) catalysed by ferric ions, which in turn is slowed down when organic reactant is added to consume the radical-anions. Computational analysis of the slow-down process allows the derivation of the rate constant for the reaction of the added compound with sulfate radical-anions.
2. reactions of MAC and MVK with sulfate radical anions were found very fast. The rate constants were estimated at several temperatures. Negative energies of activation for the reactions calculated therefrom indicated the reactions were more complicated than expected. The possible explanation was the reaction was preceded by the formation of MAC and MVK adducts with bisulfite ions. Rate and equilibrium constants for the addition reactions were determined experimentally. The reactions appeared faster than reported in the literature. MAA escaped the procedure used, because it accelerated the autoxidation of bisulfite ions instead of slowing it down.
3. photochemical and dark experiments on reactions of MAC, MVK and MAA with sulfate radical-anions generated by several methods (photolytic decomposition of peroxydisulfate ions using various lamps and natural sunlight, thermal decomposition of peroxydisulfate ions, decomposition of peroxydisulfate ions catalysed by ferric ions, decomposition of oxone catalysed by cobalt and copper ions, autoxidation of sulfur(IV) catalysed by ferric ions. The experiments revealed formation of several organosulfates that retained the carbon skeleton of their precursors, including sulfated and high molecular weight oligomers of MAA. The 2-methyl-glyceric acid is often detected in samples of ambient aerosol.
4. Analysis of chemical mechanisms for reactions of MAC, MVK and MAA with sulfate radical-anions which showed that the sulfate radical-anions react with MAC and MVK via the addition to a C=C double bond, while with MAA - also through the abstraction of proton from the acid molecule. Plausible reaction pathways were derived for a few products identified, including sulfated 2-methyl glyceric acid and high molecular weight oligomers of MAA.
5. Development and optimisation of several advanced mass-spectrometric methods necessary to carry out the product studies of the project and synthesis of a reference compound necessary for the unambiguous structural characterisation of novel reaction products.
The results obtained from the project increase the general knowledge of the chemistry of atmospheric trace components, including the formation of SOA. They emphasise the generally underestimated role the aqueous-phase processes can play in the formation of secondary atmospheric pollutants, including organosulfates. In addition, the project shows that the interaction of organic compounds originating from biogenic emissions (MAC, MVK and MAA as isoprene descendants) with anthropogenic pollutants (sulfate radical-anions descending form sulfur dioxide) strongly contributes to the complexity of chemical processes in the atmosphere.
The socioeconomic impact of the project is remote in time but clear. The expected influence is in the area of improving the air quality and reducing the adverse effects of pollution on human health and condition of biosphere.