Stoichiometry and Kinetics of Hydroxyl Radicals in Air Quality

Air quality is closely related to good health. Although in general terms more importance has been given to outdoors air quality, due to urbanization, humans now spend more than 90% of their time indoors (Figure 1), which corresponds to more than 10,000 liters of air breathed per day. FIGURE 1: Fraction of average time spent in each space in a day according to US Environmental Protection Agency data [1]

Indoor air can be up to ten times more polluted than outdoors resulting in poor Indoor Air Quality (IAQ), which poses a risk for the development or worsening of diseases as well as decreased productivity (resulting in economic charges) [1] [2]. Nearly 4 million people die each year from illnesses attributed to indoor air pollutants (IAP), which can exacerbate pre-existing acute allergy-type diseases such as dermatitis or rhinitis, and playa role in the development of respiratory diseases such as pneumonia, asthma or chronic obstructive pulmonary disease (COPD) and non-respiratory diseases such as heart attacks or cancer [3] [4]. IAPs include biological agents (viruses, bacteria, and fungi), as well as chemical agents and combustion products, the most relevant being particulate matter (PM) (e.g. smoke), volatile organic compounds (VOCs), carbon oxides (COx), nitrogen (NOx) or sulfur (SOx), radon andROS (Reactive Oxygen Species).

Ozone is one of the ROS and this gas can be emitted from any type of household appliance or device (e.g. microwave, television, photocopier, oven, discharge lamps such as tubular fluorescent lamps) by means of the so-called corona effect. This effect is an electrical phenomenon produced by the ionization of the gas surrounding a charged conductor. It occurs spontaneously on high-voltage lines and manifests itself in the form of aluminous halo [5] [6].

Due to the oxidation reactions generated by ozone inorganic compounds (especially terpenes, present in perfumes and cleaning products), secondary non-volatile organic compounds are produced in aerosols (SOA) even more dangerous than the primary ones [2] [7] [8] [9].

Although air does not contain indigenous microbiota, it acts as a dispersal medium for a large number of microorganisms such as may be the cause of the recent Sars-CoV2 pandemic. Most microorganisms (including bacteria, viruses and fungi) are transmitted by bioaerosols, airborne particles that are dispersed in the air and when expelled (e.g. by talking, coughing, sneezing, etc.) contain more than 90% water, which evaporates according to the temperature and relative humidity of the environment until it is reduced in size and consists of microorganisms or parts of microorganisms and inorganic ions [10]. The IAQ is highly dependent on three factors [8] [11]: (1) the activities carried out by the occupants (e.g. emission of tobacco smoke, fragrances, cleaning products, cooking odors). Individually, we can contribute to improving the IAQ at home by reducing the source of pollutants, i.e., controlling the activities we carryout to minimize their emissions, although this is not always possible. (2) the intrinsic characteristics of the building (e.g. building materials, furnishings, ventilation capacity, presence of air conditioners). There is a set of symptoms and diseases attributed to poor IAQ called "building related illnesses", which are divided into Building related illness (BRI) if the causative agent can be identified, usually biological (e.g. legionellosis) but can also be chemical (e.g. hypersensitivity pneumonitis) or, in Building related illness (BRI) if the causative agent can be identified, usually biological (e.g. legionellosis) but can also be chemical (e.g. hypersensitivity pneumonitis). Sick Building Syndrome (SBB) if general environmental contamination is the trigger for the symptoms, which include irritation of mucous membranes (e.g. eye), neurotoxic effects (e.g. fatigue), asthma and the like (e.g. chest tightness) among others. (3) Outdoor air since the quality of renewed air depends on it. Although indoor air is usually more polluted than outdoor air and therefore natural ventilation of spaces is recommended, 99% of the population lives in spaces where the recommended limits of pollutants in atmospheric air are exceeded, resulting in a problem to ensure good IAQ in these spaces [12]. In fact, recent studies have linked poor air quality in cities to increased infectious disease transmissibility, specifically on the risk of Sars-CoV2 infection [13].

Due to public awareness of the detrimental effects of breathing polluted air, technologies have emerged to reduce contaminants (both biological and chemical) in these spaces, such as HEPA filters. The recent pandemic has exacerbated this concern and makes it imperative to find ways to ensure good indoor air quality in a manner that is safe for occupants and independent of outdoor air renewal. 1.1 THE OH· The generation of OH· as well as other superoxides by different methods, called advanced oxidation processes (AOP), was first studied by NASA in the 1990s, in order to be used to avoid the accumulation of organic compounds on spacecraft during space travel [14]. The evaluation of the role of OH· in the decomposition of these compounds showed that it also participates in the inactivation of microorganisms and the elimination of inorganic pollutants. Since then, a large amount of scientific literature has corroborated the use of AOPs for the treatment of pollutants and microorganisms, first in water and later in air and surfaces. AOPs include all those methods of OH-generation that aim to generate an oxidizing environment allowing the mineralization of pollutants (organic and inorganic) and the inactivation of microorganisms [15] [16]. These methods generate OH· and other oxidizing agents mainly by reactions between O , H O , UVand/or a catalyst. 3 2 2 OH· is the most important natural oxidant in tropospheric chemistry, often called a "detergent" because it oxidizes pollutants and microorganisms in the environment. Due to its chemical instability and high oxidative potential of 2,80V (second only to flourine and 2,05 times higher than the potential of chlorine) OH-is the most reactive species in biology, with an ephemeral half-life of 10-9 seconds [17]. Therefore, its synthesis, degradation and consequent local concentration depend solely on the species present in situ around it, with which it reacts rapidly and non-selectively, acting like a spark. OH· is part of a large number of intercommunicating and dynamic reactions involving species naturally present in the air. Both OH· itself and many of the species in these reactions are Reactive Oxygen Species (ROS), unstable species derived from oxygen that react with others to return to equilibrium. Within ROS we find ions such as hydroxide anion (OH-), free radicals such as superoxide anion (O -) and precursor species of instability, such as hydrogen peroxide (H O ) (Figure 2). These species 2 2 2 generate cascades of biochemical electron transfer reactions (oxidation and reduction) where other ROS and secondary species are generated [7] [15] [18] [19] [20]. All this contributes to a generalized oxidizing environment in space, as instability spreads from molecule to molecule.

FIGURE 2: Simplified representation of the reactions between ROS taking place in atmospheric air. [Own source] In the 60s it was determined that rural air had powerful germicidal properties, termed Open Air Factor (OAF). Druett et al. attributed this ability to the ozone-olefin complex, an organic compound previously identified in the composition of air that we now refer to as an alkene (hydrocarbon with double bonds) [21]. A couple of years later, Dark et al. verified that peroxide hybrid ions (now known as Criegee biradicals) formed in the ozonolysis of various alkenes were responsible for the significant but variable inactivation of two microorganisms and thus the active agent of OAF [22]. Very little work has been done on AOF since then, and although the disinfection capacity of AOF (greater than that of ozone) has been confirmed, no further information on the identity of the active agent has been provided [23] [24] [25].

According to current information, the cycle addition of ozone to an alkene (both in the gas phase) attacks the double bond and forms an unstable primary ozonide, which evolves generating radical intermediates called Criegee biradicals [26]. Although biradicals can theoretically decompose via three pathways, evidence rules out two of them occurring naturally in the environment: the removal of an oxygen atom from their structure is not feasible at room temperature and pressure, and radical formation, a consequence of the rearrangement of the biradical into an unstable ester, has not been detected [27] [28]. The decomposition of the biradical by the remaining pathway, through its rearrangement into a hydroperoxide, is considered an important source of generation of hydroxyl radicals (OH·), the major natural oxidant in the troposphere [29]. Could OH· then be the active agent of OAF? 1.2 THE OXIDATION Chemically OH· is a free radical, which together with ions are the two main sources of chemical instability. Both species present anomalies in electrons and to return to equilibrium they need to react with others, which generates a cascade of instability in which other species, called instability precursors, participate. Ions present charge imbalance in their chemical structure, they have either an excess or a deficit of electrons: those species that have lost electrons (the cations) are electropositively charged (oxidized), while those that have gained electrons (the anions) are electronegatively charged (reduced). It is a dynamic and compensated system: due to the theft of an electron, one species is in the reduced (-) state when it leaves another in the oxidized (+) state. On the other hand, free radicals have a spin imbalance in their structure, i.e. they have some unpaired electrons, so they are much more reactive than ions. Radicals are electrically neutral by itself, so they do not seek to compensate their charge, but quickly steal an electron to compensate their spin, oxidizing without selectivity. OH· react with organic compounds in 3 different ways: (1) by electron transfer, generating charge imbalance in the molecule with which it reacts and allowing the formation of new bonds and links. Finally, these structures lose their conformation and three-dimensional structure. (2) by addition to aromatic rings or double or triple bonds in unsaturated hydrocarbons, which breaks these bonds and generates an alcohol radical (-OH) in the chain. These unstable radicals inorganic compounds are incorporated in the classical oxidation pathway, until they become mineralized: the structures lose organic matter in the form of carbon dioxide (CO ) 2 (Figure 3) [30]. (3) by abstraction of a hydrogen in saturated hydrocarbons, resulting in a water molecule and a free alkyl radical (R·) in the organic compound [31]. The presence of an alkyl radical initiates the peroxidation process, whereby incorporation and interaction of species a positive feedback of irreversible damage is generated in the molecule. Other radicals can also form in the chain (e.g. alkoxy radicals (RO·)) that are transformed by decomposition, isomerization or hydrolysis, leading to the formation of oxygenated compounds (e.g. alcohols, carbonyls (aldehydes or ketones), carboxylic acids) that are incorporated into the classical oxidation pathway (Figure 3) [9] [32].

The remaining CO present at the end of these mineralizations is eventually transformed into carbonate or bicarbonate, oxygen 2 and water, depending on the ambient humidity. This degradation is related to an indirect reaction between OH· and CO , which 2 occurs by the instantaneous uptake of an electron by the radical (part of the Fenton process) in an environment contaminated with NO . The resulting hydroxyl anion (OH-) is responsible for the formation of carbonate ions upon reaction with CO [33]. 2 2 𝑁𝑂− →𝑁𝑂 ·+𝑒− 2 2 𝑂𝐻·+𝑒− →𝑂𝐻− 2𝑂𝐻−+𝐶𝑂 →𝐶𝑂2−+𝐻 𝑂 2 3 2 FIGURE 3. The incorporation of an oxygen molecule (O 2 ) into the alkyl radical (R·) gives rise to the peroxyl radical (ROO·) in the organic compound, which can subsequently isomerize by stealing the hydrogen from a nearby carbon, transforming it into a hydroperoxide radical (ROOH) and generating another alkyl radical in the chain (R·). [Own source] 1.2.1 Mineralization, transformation, and precipitation of pollutant compounds Due to the relevance of IAQ control, WHO developed guidelines recommending safe exposure limits for selected HAPs: formaldehyde and other VOCs (benzene, naphthalene, benzo[a]pyrene, trichloroethylene and tetrachloroethylene), radon, particulate matter (PM), carbon monoxide and nitrogen dioxide [4].

VOCs include all those hydrocarbons ("organic compounds") that occur in a gaseous state ("volatile") at room temperature. OH· reacts with them by abstraction and addition, mineralizing them and decreasing their concentration indoors. Among the VOCs, the most important are alkenes, due to the danger that their accumulation poses to health, as well as the SOAs generated by their reaction with ozone, which is attacked by OH· through addition or abstraction of a hydrogen, initiating its mineralization; formaldehyde, one of the SOAs generated by ozonolysis, especially of terpenes and highly flammable, with a strong and penetrating odor, soluble in water and very volatile, which is incorporated in phase 3 of the oxidation cascade of organic compounds (Figure 3); and odoriferous molecules, lipophilic and small chemical compounds perceptible by our sense of smell that usually contain nitrogen in addition to hydrogen and carbon, which due to the presence of nitrogen we will also obtain ammonia or another similar compound as a product of mineralization [34]. Due to its high reactivity, the reaction time of OH· for the mineralization of most VOCs is generally shorter than the corresponding time for other oxidizing agents such as NO radicals and ozone, reducing the mineralization time of some organic compounds from years to days, even hours (Table 3 1). TABLE 1 HALF-LIFE OF SOME VOCS WHEN REACTING WITH OH·, NO 3-AND O 3 IN THE GASEOUS STATE [35].

Lifetime due to reaction with: VOC OH· radicals NO radicals O 3 3 Acetaldehyde 8.8 hr 17 d >4.5 yr Acrolein 6.9 hr 4.2 d 57 d Benzene 9.4 d >4 yr >4.5 yr Ethane 47 d >12 yr >4500 yr Ethanol 3.6 d 23 d -- Ethene 1.4 d 225 d 10 d Formaldehyde 1.2 d 80 d >4.5 yr n-Octane 1.3 d 240 d >4500 yr Phenol 5.1 hr 9 min ~50 d Propane 10 d 7 yr >4500 yr Propene 5.3 hr 4.9 d 1.6 d Styrene 2.4 hr 22 min 1.0 d Toluene 1.9 d 1.9 yr >4.5 yr On the other hand, OH· interacts with rhodon gas and PM (suspended volatile particles generally of undetermined composition of sulfates, nitrates, ammonia, sodium chloride, elemental carbon, mineral dust, water and even inorganic compounds such as dust or sand) by electron transfer, being electrostatically captured and agglomerated by its strong electrostatic charge, and subsequently precipitated to the ground with a sufficient size to be collected. In addition, the accumulation of carbon, nitrogen and sulfur oxides is prevented by promoting their natural cyclic oxidation [34]. 1.2.2 Inactivation of microorganisms Microorganisms, as well as all living matter, are made up of organic molecules (mainly hydrocarbon chains (H-C)) that are mineralized by a sequence of free oxidative reactions to obtain mainly CO , O and H O. Although OH· acts non-selectively 2 2 2 on all microorganisms present in water, air and surface, the morbid effect of OH-on each of themis due to their sensitivity to oxidation [36]. This sensitivity depends on which organic macromolecules makeup its outermost structures and which, therefore, interact with the environmental OH·. There are 4 groups of macromolecules: the nucleic acids that makeup the genetic material and the glucids, lipids and proteins that makeup the structures that surround it. Of these last three, lipids and proteins are the macromolecules sensitive to oxidation.

According to their outermost layer, viruses can be non-enveloped (they present a protein capsid) or enveloped (over the capsid there is a lipid bilayer with glycoproteins); bacteria are classified as gram-positive (they present a thick proteoglycan wall) and gram-negative (over the wall there is a lipid bilayer with lipopolysaccharides); while fungi and yeasts (unicellular fungi) present in their outermost layer a wall formed by chitin and glucans from which glycoproteins protrude (Figure 4) [37]. FIGURE 4: Structural composition of pathogens. [Own source]

Lipids are mainly composed of unsaturated hydrocarbon chains that react with OH· either by addition breaking the double and triple bonds generating alcohol radicals (-OH) that will be incorporated into the oxidation pathway or by abstraction of a hydrogen generating alkyl, peroxyl and finally hydroperoxides radicals. Mineralization and peroxidation of lipid membranes modify their spatial distribution and consequently their structure. On the other hand, proteins are composed of amino acids and OH· can alter their charges by electron transfer. This modifies their 3D structure and leads to protein fragmentation and can form disulfide (covalent) bonds between amino acids, resulting in intra-and interprotein bonds, so that the protein loses its conformation. In both cases, proteins form aggregates and lose their correct structure.

These irreversible structural alterations created in lipids and proteins through OH· promoted oxidation lead to the loss of the integrity of the structures that envelop the microorganisms, and consequently the loss of their viability and functionality.