Gas-phase and aqueous-surface reaction mechanism of Criegee radicals with serine and nucleation of products: A theoretical study

Highlights

• The reactions of CIs with three functional groups of Ser were compared.

• Water molecule has different effects on the reactions of CIs with Ser.

• The gas-liquid interface reactions were simulated by Born-Oppenheimer molecular dynamics (BOMD).

• All of the reaction products can form clusters with sulfuric acids, ammonias, water molecules.

Abstract

Criegee intermediates (CIs) are short-lived carbonyl oxides, which can affect the budget of OH radicals, ozone, ammonia, organic/inorganic acids in the troposphere. This study investigated the reaction of CIs with serine (Ser) in the gas phase by using density functional theory (DFT) calculations and at the gas-liquid interface by using Born-Oppenheimer molecular dynamics (BOMD). The results reveal that the reactivity of the three functional groups of Ser can be ordered as follows: COOH > NH₂ > OH. Water-mediated reactions of CIs with NH₂ and OH groups of Ser on the droplet follow the proton exchange mechanism. The products, sulfuric acids, ammonia, and water molecules form stable clusters within 20 ns. This study shows that hydroperoxide products can contribute to new particle formation (NPF). The result deepens the understanding of the reaction of CIs with multifunctional pollutants and atmospheric behavior of CIs in polluted areas.

Introduction

The ozonolysis reaction is a main removal pathway of alkenes in troposphere (Khan et al., 2018). Concerted cycloaddition of ozone to alkene double bond leads to the formation of primary ozonide. The primary ozonide rapidly decomposes into Criegee intermediates (CIs) and carbonyl compounds (Johnson and Marston, 2008). CIs are critical atmospheric carbonyl oxides, also known as reactive species that plays an important role in pollutant removal (Chhantyal-Pun et al., 2020; Khan et al., 2018; Osborn and Taatjes, 2015). Unimolecular decomposition of nascent CIs leads to the formation of OH radicals, which is the main source of OH radicals at night and accounts for approximately one-fourth of OH radicals at daytime (Kidwell et al., 2016; Qu et al., 2018; Taatjes et al., 2014). Alternatively, excited CIs lose energy by collisions and form stabilized CIs (Drozd and Donahue, 2011). Stabilized CIs can exist for 100–500 ms and react with various trace species in the atmosphere, which affects the atmospheric budgets of CIs (Anglada et al., 2011; Osborn and Taatjes, 2015; Vereecken et al., 2012, 2015). Due to the large concentration of water vapor (1.3 × 10¹⁷ to 8.3 × 10¹⁷ molecule cm⁻³) in the troposphere, the reaction with water is the dominant removal pathway of small CIs, while larger CIs react slowly with water and are limited by their unimolecular reactions (Huang et al., 2015; Osborn and Taatjes, 2015; Vereecken et al., 2017). The primary reaction product of CIs with water is hydroxymethyl hydroperoxide, which is toxic to plant cells and inactivates enzymes (Marklund, 1972; Sheps et al., 2017). CIs have remarkable oxidability in the troposphere. For example, CIs can oxidize SO₂ to SO₃, which can reacts with water molecule to form H₂SO₄, a vital component of acid rain (Mauldin et al., 2012). Therefore, the study of CI reactions in the atmosphere is of great significance to reveal the sources and sinks of atmospheric pollutants.

The CI reactions play an important role in the formation of secondary organic aerosols (SOAs) and nucleation in the atmosphere (Donahue et al., 2011; Johnson and Marston, 2008; Taatjes et al., 2014). It has been proved that the reactions between CH₂OO/anti-CH₃CHOO and HO₂/H₂O₂ can form oligomers, the addition of CIs into the oligomers leads to the generation of chain structure oligomers, which can participate in the SOA formation (Chen et al., 2017). Criegee intermediate C₆H₅CHOO reacts easily with HCHO (the energy barrier is 0.8 kcal/mol), and the product 3,5-diphenyl-1,2,4-trioxolane is one of the components of aerosols (Ma et al., 2018). In the tropical rainforests, the concentration of CIs is large, and the concentration of organic hydroperoxides is up to 8 ppbv, so the oxidation of organic hydroperoxides by CIs is prominent in these areas and has a significant contribution to the formation of aerosols and atmospheric nucleation (Khan et al., 2018).

The reactions of CIs with various other pollutants containing functional groups are also important pathways of SOA generation in the atmosphere. Reactions of CIs with carboxylic acids lead to the formation of low volatile organic compounds. These compounds significantly contribute to the formation of SOAs in areas with high concentrations of biogenic alkenes (Chhantyal-Pun et al., 2018). On the surface of ozonized particles, CIs react with amines and produce low volatile amides (Zahardis et al., 2008). Reactions of CIs with alcohols are one of the sources of functionalized hydroperoxides in the atmosphere. For example, the reactions between CIs and methanol or ethanol lead to the formation of α-alkoxyalkyl hydroperoxides that have a production rate of ~30 Gg year⁻¹, and the secondary oxidation of primary alkoxyalkyl hydroperoxides plays a subtle role in the formation of atmospheric aerosols (McGillen et al., 2017). Recently, the reactions of CIs with multi-functional pollutants have aroused extensive interest, but research on the reaction mechanism has been relatively limited.

Amino acids are a kind of multi-functional substances whose fate and role in the atmosphere are still unclear. Amino acids are ubiquitous atmospheric species and have been detected in the air of the California coast, the Western Pacific and the Atlantic Ocean (Matsumoto and Uematsu, 2005; Wedyan and Preston, 2008; Zhang and Anastasio, 2003). They exist in the form of free or combined amino compounds in particles and fog water (Gorzelska et al., 1994; Matos et al., 2016; Zhang and Anastasio, 2001; Zhang et al., 2002). Due to the hygroscopic property, they can affect the formation of cloud (Chan et al., 2005; Kristensson et al., 2010) and ice-forming (Szyrmer and Zawadzki, 1997) condensation nuclei in the atmosphere. Amino acids are important organic carbon and organic nitrogen fraction of aerosols, and are likely to influence chemical composition of aerosol particles (Zhang and Anastasio, 2003; Zhang et al., 2002). It has been proved that threonine (Thr), glycine (Gly) alanine (Ala), and serine (Ser) play significant roles in promoting new particle formation (NPF) (Ge et al., 2018). Ser represents a kind of amino acids with three functional groups (-COOH, –NH₂, –OH), and has the potential to react with CIs (Barbaro et al., 2011). The products contain functional groups, which have the possibility to form clusters with atmospheric molecules by hydrogen bonds.

In this study, the density functional theory (DFT) and ab initio dynamic simulation were used to investigate the reaction mechanism of CIs (CH₃CHOO) with Ser. Based on reaction energy barriers, the reactivity of the COOH, NH₂ and OH groups was compared. The reactions of CIs with Ser at the droplet interface also were simulated. On the droplet interface, the effect of water molecule on the reactions was explored. The nucleation ability of the products was revealed by molecular dynamics (MD). In the systems, products containing two functional groups were put into simulated atmosphere environment to investigate aggregation with atmospheric molecules.