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Implication of Organic Compounds in Atmospheric New Particle Formation - Molecular Insight Utilising Quantum Mechanics

Internationalisation Fellowship | 27/07/2016

Airborne atmospheric nanoparticles known as aerosols counteract the warming effect of greenhouse gases by scattering incident sunlight back into space, thereby cooling the global climate. However, the participating vapours and underlying mechanisms for forming particles, remain unknown. The aim of this project, which is supported by the Carlsberg Foundation, is to identify how new particles are formed at the molecular level, using quantum chemical methods. Very little is known about the participation of oxidised organic compounds in new particle formation. Previously it has been believed that the ability of an oxidised organic compound to participate in new particle formation is directly related to its oxygen-to-carbon content. This is under the assumption that the higher the oxygen-to-carbon content of a given compound is, the lower is its saturation vapour pressure and thereby its ability to form new particles. My current work has shown that the ability of an oxidised organic compound to form new particles cannot alone be correlated with the oxygen-to-carbon content and that the specific molecular structure of the compounds involved is very important.

Introduction

Atmospheric aerosols influence the global climate by interacting with solar radiation. Aerosol particles directly scatter incident sunlight back into space. Indirectly aerosol particles can influence cloud microphysics by changing the properties and lifetime of clouds. Aerosol particles have also been shown to have a negative impact on human health. Inhalation of ultrafine aerosol particles yield increased risk for lung cancer and cardiovascular diseases and are currently responsible for approximately 5 million premature deaths every year. Aerosol particles thereby have a large global impact for humanity and it is our social responsibility as scientists to get a better fundamental understanding of aerosol formation and processes such that the best cause of action can be taken to counteract the negative effects.

Formation of new particles is known to involve sulfuric acid, but the participation of other vapours is  still very poorly understood. The participation of organic compounds in new particle formation is unclear and has only been elucidated in the recent years. It has been indicated that 1-4 extremely low volatile organic compounds (ELVOCs) and 1-3 sulfuric acid molecules might be sufficient to account for a stable cluster. The phrase “ELVOCs’” covers a large range of highly oxidised organic compounds, with a high oxygen-to-carbon (O/C)-ratio of around or even above 1. There is, however, limited knowledge about the specific molecular structure of individual ELVOCs. Most studies of the role of organic compounds in new particle formation have involved oxidation products of the monoterpene α-pinene, a volatile organic compound emitted from pine trees. One of the identified α-pinene oxidation products close to the “ELVOC” region is 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) with an (O/C)-ratio of 6/8, see Figure 1a). Highly oxidised organic compounds are likely to be formed via an autoxidation mechanism. The autoxidation process leads to an array of possible products. Using the simpler cyclohexene system as a proxy for larger monoterpene oxidation, the complete autoxidation pathways have shown to yield highly oxidized C6H8O7 compounds, see Figure 1b).

Figure 1: a) MBTCA, b) C6H8O7

The exact involvement of organic compounds in new particle formation at the molecular level is thereby still an unresolved issue and the assistance of quantum mechanical simulations is an important tool to elucidate the formation of a critical cluster. Calculated quantum mechanical Gibbs free energy values can be used directly to estimate cluster stabilities and calculate cluster evaporation rates.

(H2SO4)a(C6H8O7)b Clusters

I have studied the interaction between the cyclohexene autoxidation product C6H8O7 and sulfuric acid. Using quantum chemical methods, I have calculated the formation free energy of the clusters up to (H2SO4)1-2(C6H8O7)1-2. I have found that C6 H8 O7 interacts very weakly with itself and sulfuric.

High O/C ratio ≠ low vapour pressure ≠ participation in new particle formation

This is due to the presence of intramolecular hydrogen bonds in the peroxyacid groups of the C6H8O7 compounds, which stabilises the isolated molecule with respect to its clusters, and lead to unfavorable interaction energies. This leads to the conclusion that autoxidation products containing solely or mainly hydroperoxide and carbonyl functional groups cannot participate in new particle formation. In a more general sense, it explicitly shows that the oxygen to carbon ratio cannot be used as a metric for determining if an organic compound is involved in new particle formation.

Effect of Bases

Atmospheric bases such as dimethylamine are capable of forming strongly bound clusters with sulfuric acid. The inclusion of bases in the system could thereby be able to enhance the interaction with the C6H8O7 compound. I have studied (H2 SO4 )1-2 (C6 H8 O7 )1-2 (X) clusters, with X being either water, ammonia or dimethylamine.

Bases do not enhance the interaction between sulfuric acid and autoxidation products

The interaction of C6H8O7 with existing clusters is however, not able to compete with the corresponding uptake of another sulfuric acid molecule, even at a high loading of organic compounds. Atmospheric bases are thereby not able to enhance the process of cluster formation between sulfuric acid and C6H8O7.

(H2SO4)a(MBTCA)b Clusters

Carboxylic acids are more promising candidates for being involved in atmospheric new particle formation, I have studied (H2SO4)1-3(MBTCA)1-3 clusters. The molecular interaction between MBTCA and sulfuric acid is found to be stronger than previously studied carboxylic acids. Especially clusters consisting of 2-3 MBTCA and 2-3 sulfuric acid molecules are found to be particularly stable, see Figure 2.

A high cluster stability is obtained by maximizing direct sulfuric acid - carboxylic acid interactions

A large stabilisation is found when the amount of direct sulfuric acid - carboxylic acid interactions are maximised. Tricarboxylic acid compounds such as MBTCA could potentially be key species in the initial steps in atmospheric new particle formation involving highly oxidised organic compounds and sulfuric acid.

Figure 2: (H2SO4)3(MBTCA)2 cluster

Conclusions

This article has outlined part of my current research progress for the first year of my Postdoc position at the University of Helsinki. My current research has shown that contrary to current believes the O/C-ratios cannot exclusively be used as a metric for involvement of organic compounds in atmospheric new particle formation. With this research project, I will be able to take a large step in understanding the formation of new particles from a molecular point of view and will lead to a significantly improved description of current climate models. 

How the Grant from the Carlsberg Foundation Has Affected My Career

The vast scope of this project would not have been possible without the support from the Carlsberg Foundation. The granted Internationalisation Fellowship has allowed me to work in one of the globally leading institutions for researching atmospheric nucleation both experimentally and theoretically. The opportunity of working in the Computational Aerosol Physics group led by Prof. Hanna Vehkamäki in Helsinki has led to new research ideas and significantly strengthened my international profile. The results carried out during the project, will significantly aid my pursuit of returning to Denmark, striving for an independent researcher career.

References 

J. Elm, N. Myllys, N. Hyttinen and T. Kurtén, "Computational Study of the Clustering of a Cyclohexene Autoxidation Product C6H8O7 with Itself and Sulfuric Acid.", 2015, J. Phys. Chem. A, 119, 8414 -8421.

J. Elm, N. Myllys, J-N Luy, T. Kurtén and H. Vehkamäki, "The Effect of Water and Bases on the Clustering of a Cyclohexene Autoxidation Product C6H8O7 with Sulfuric Acid", 2016, 120, 2240-2249.

J. Elm, N. Myllys, T. Kurtén and H. Vehkamäki, "Formation of Atmospheric Molecular Clusters Consisting of Sulfuric Acid and a C8H12O6 Tricarboxylic Acid", 2016, Manuscript draft