Multifunctional roles of water in the ozonolysis of limonene aerosols

Aerosols are suspensions of tiny solid particles or liquid droplets, which significantly impact Earth’s atmosphere by influencing the planet’s energy balance, global climate, and public health. Secondary organic aerosols can be formed due to various physicochemical processes involving natural and human-made aerosols, or their gas-phase precursors. Limonene, a type of monoterpene, often found in citrus peel oils and many household and cosmetic products, readily undergoes ozonolysis to form secondary organic aerosols. Professor Chia C Wang from Taiwan’s National Sun Yat-sen University employs spectroscopic measurements and quantum mechanical calculations to unravel water’s multiple roles in intervening in the ozonolysis of limonene and altering the formation yield and physicochemical properties of the resultant secondary organic aerosols.

Aerosols are blends of minute solid, semi-solid particles or liquid droplets dispersed in air or other gaseous medium. They can be of natural or human-made origin. Natural aerosols encompass phenomena such as fog, mist, dust, and emissions from forests and oceans. Anthropogenic aerosols include particulate pollutants in the air, irrigation spray, atomised perfumes, smoke, dust, aerosols generated during kettle steam, pesticide sprays, and medical treatments for respiratory conditions.

Aerosol emission has profound effects on the chemical and physical processes that occur in the atmosphere, and it is considered one of the key factors affecting the stability of the Earth’s system – its resilience to environmental perturbations of human origin and its ability to sustain life. Understanding how aerosols interact with the atmosphere, what changes in their chemical structure this interaction brings about, and the potential risks that aerosols can pose to the environment and to human health on both regional and global scales is important in defining the environmental limits within which human societies can operate safely.
Professor Chia C Wang, founding director of the Aerosol Science Research Center at National Sun Yat-sen University, Taiwan, R.O.C., has collaborated with fellow researchers to answer some key questions about aerosols.

Secondary organic aerosols formed during ozonolysis of monoterpenes

Secondary organic aerosols can emerge through a multi-generational transformation of their parent organic molecules or primary organic aerosols, often driven by oxidation reactions. Unlike primary organic aerosols, which are directly released into the atmosphere from the sources, secondary organic aerosols come to life through two primary processes. Firstly, they can form through the gradual oxidation of organic compounds in the gas phase. These gas-phase compounds are highly volatile and remain stable. However, as oxidation progresses, their polarity increases, causing a decrease in volatility and vapour pressure. After sufficient oxidation, the vapour pressure drops low enough for the gas-phase compounds to partition into the condensed phase, giving birth to secondary organic matter. Alternatively, they can also condense onto existing particles, or undergo chemical transformation from primary organic aerosols. Secondary aerosols constitute 70–80% of the total concentration of aerosols in the atmosphere.

One of the most important sources of secondary organic aerosols in the atmosphere is a class of chemical reactions, known as monoterpene ozonolysis. Monoterpenes are a family of chemical compounds found in the essential oils extracted from numerous plants, including fruits, vegetables, spices, and herbs. These molecules contribute to enhancing the taste and fragrance of the source plant. In the atmosphere, they readily react with ozone, a powerful gaseous oxidant which is generated naturally by the interaction of oxygen molecules with the Sun’s UV radiation and undergo a complex series of chemical changes. This leads to the formation of several transient intermediate compounds, which originate from the cleavage of double bonds in the monoterpenes by reaction with ozone and can be rationalised in terms of a sequence of elementary reactions, known as ‘Criegee mechanism’. The decomposition of these intermediates eventually leads to the formation of secondary organic aerosols.

Mechanistic study of monoterpene ozonolysis

Several spectroscopic approaches have been used to study the mechanistic details of the ozonolysis of monoterpenes, including vibrational spectroscopy, ultraviolet spectroscopy, and mass spectrometry. However, little attention has been devoted so far to two important aspects of the fundamental physics and chemistry of this process. One of them is the electronic structure of the monoterpenes, both in the gas phase and in the aerosol form, in particular, the binding energy of the so-called valence electrons. As the monoterpene aerosol undergoes oxidation, the valence electrons must first be removed from it. This oxidation process is more likely to occur when the binding energy of the valence electrons is lower, which means that less energy is required to remove the valence electron to undergo oxidation. Valence electron removal triggers changes in the bonding structure of the molecule or aerosol, promoting its conversion into oxidation intermediates.

A second important aspect of ozonolysis processes of monoterpenes in the atmosphere is the potential involvement of water, either water molecules or water clusters. Water is ubiquitous in the atmosphere, where it can appear in various forms, including vapour, clusters, liquid droplets, and icy particles. The chances that aerosols encounter water molecules or clusters once released in the atmosphere are therefore exceedingly high. Recent work suggests that water plays only a minor role in the long-term ageing of the monoterpene ozonolysis products. However, increasing evidence has also been put forward for a more crucial involvement of water in the initial stages of the ozonolysis process, particularly in modulating the reactivity of some of the ozonolysis transient intermediates.

Indoor secondary organic aerosols

Wang and colleagues have used a combination of experimental and computational techniques to study the generation of secondary organic aerosols from the ozonolysis of limonene aerosols. Limonene is an important naturally occurring monoterpene with remarkably high biogenic emission rates. About 8.6 billion kilograms of limonene are released in the atmosphere every year from natural sources alone, which accounts for about 9% of the global monoterpene emission.

Limonene also has a variety of anthropogenic sources, since it is a key component of many household cleaning products, essential oils, air fresheners, and cosmetic products. When limonene-based cleaning products are used indoors, an increase of two orders of magnitude has been reported in the indoor air concentration of limonene compared to its outdoor levels (Rosales et al, 2022). Limonene has been considered one of the most important sources of indoor secondary organic aerosols in domestic environments.

How water promotes ozonolysis

For the first time, the research team has examined the valence electronic structure and reactivity of limonene aerosols using a technique called valence photoelectron spectroscopy. This approach uses vacuum ultraviolet (VUV) radiation to induce the emission of valence electrons from a molecule or an aerosol. By measuring the kinetic energy of the emitted photoelectrons, vital information concerning the binding energy of these electrons inside the molecule or aerosol can be gained. Since valence electrons are responsible for the chemical reactivity of a molecule or an aerosol, and for its ability to function as an electron donor in chemical reactions, valence photoelectron spectroscopy provides a direct link between electronic energetic structures and the tendency of a molecule to undergo chemical transformations in different environments.

Using this approach, the team was able to show that the overall yield of secondary organic aerosols from the ozonolysis of limonene aerosols is dramatically enhanced by the presence of water in the primary organic aerosol. In the experimental conditions considered in their study, the team measured a 4.8-factor increase in secondary organic aerosol production in the presence of water compared to anhydrous conditions.

To understand the origin of the observed water-induced enhancement of secondary organic aerosols in limonene ozonolysis, Wang and colleagues used a combination of mass spectrometry, which provides information on the chemical composition and identity of the reaction products, and density-functional theory, a powerful computation approach that makes it possible to predict the molecular structure of the chemical species involved in the reaction, as well as the energetics associated with these molecules.

The results have confirmed that the ozonolysis of limonene occurs through the Criegee mechanism involving the formation of multiple generations of reaction intermediates, including primary ozonides, Criegee intermediates, hydroperoxides, and secondary ozonides, which eventually leads to the formation of secondary organic aerosols. The team was also able to demonstrate that water acts as a catalyst promoting the formation of Criegee intermediates by reducing the energy barriers required to produce vinyl hydroperoxides. In addition, water can also react directly with the Criegee intermediates to form hydroxyl hydroperoxides. This is a key factor affecting the final chemical composition of the secondary organic aerosols.

Environmental and health implications

In addition to providing novel insight into the chemistry of secondary organic aerosol formation, the researchers’ work has important atmospheric and environmental implications. ‘Water’, explains Wang, ‘has been conventionally considered mostly as an aqueous medium, playing only minor roles in affecting the aerosol chemistry. This study provides new experimental and theoretical evidence to show that water may, in fact, contribute greatly to affect not only the yield of secondary organic aerosols in certain systems, but also their physicochemical properties.’

Considering that ‘atmospheric aerosol loading’ (aerosols’ effect on cloud formation, etc), one of the most crucial processes affecting the Earth stability, remains to be quantified, it is important and urgent to revise present atmospheric chemical modelling approaches to accurately estimate the formation yield of aerosols and to predict their impact on the climate.

Another important finding of this research is that ozone concentration in the air is a key factor promoting the conversion of limonene aqueous aerosols into secondary organic aerosols. Environments with high ozone concentrations, for instance, places in which ozone-based air purifiers are used, can experience enhanced production of secondary organic aerosols from ozonolysis of limonene aerosols, with numerous adverse health effects on human and animals. Avoiding the usage of limonene-containing products alongside these purifiers could potentially reduce the formation of harmful indoor secondary organic aerosols. To fully understand the impact of aerosols in human environmental settings, it is particularly crucial to pay close attention to the possible indoor and outdoor sources of aerosols, their chemical composition and formation routes, and their physicochemical and biochemical properties.