# Rethinking environmental boundaries for contaminants of emerging concern

Ivan Kourtchev, Max R Mcgillen, John Wenger, Neil M Donahue

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Ivan Kourtchev, Max R Mcgillen, John Wenger, Neil M Donahue. Rethinking environmental boundaries for contaminants of emerging concern. Atmospheric Environment, 2025, 361, pp.121492. 10.1016/j.atmosenv.2025.121492. hal-05221796

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# Rethinking environmental boundaries for contaminants of emerging concern

Ivan Kourtchev<sup>a, *, †</sup>, Max R. McGillen<sup>b</sup>, John Wenger<sup>c</sup>, Neil M. Donahue<sup>d</sup>

## HIGHLIGHTS

* Contaminants of Emerging Concerns (CECs) can volatilise, aerosolise, and undergo long-range atmospheric transport

* Current regulations overlook air as a key vector for CEC dispersion and exposure

* Physicochemical properties and environmental conditions govern atmospheric mobility

* Many CECs cross environmental boundaries, moving between water, soil, and air

* Urgent research is needed on airborne CEC detection, modelling, and toxicity

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ABSTRACT

The global proliferation of synthetic chemicals has led to the widespread and continuous release of Contaminants of Emerging Concern (CECs) into the environment. CECs include pharmaceuticals, pesticides, personal care products and other industrial chemicals that pose a significant risk to both ecosystems and human health. Regulatory frameworks have predominantly targeted aquatic systems; however, mounting evidence reveals the capacity of many CECs to volatilise, aerosolise and undergo atmospheric transport. This perspective highlights the overlooked atmospheric dimension of CECs and analyses the key physicochemical parameters governing their transfer to the atmospheric domain. The results indicate that many CECs can mobilise from water or soil and undergo atmospheric transport in both the gas- and particle-phase, crossing between several environmental continua as a result. While intrinsic properties such as vapour pressure and partitioning coefficients are central to this analysis, environmental factors such as temperature, humidity, solar radiation, and transformation reactions further modulate the environmental fate and impact of CECs. We emphasise the need for environmental monitoring and regulatory frameworks to incorporate air as a critical vector for CEC dispersion and exposure. Key research priorities identified measurements of CECs in the atmosphere, further development of predictive models, and toxicity evaluation of airborne CECs to better inform policy for protecting public and environmental health.

<footnote><sup>a</sup> Centre for Agroecology Water and Resilience (CAWR), Coventry University, Wolston Lane, Ryton on Dunsmore, CV8 3LG, UK</footnote>

<footnote><sup>b</sup> Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE), CNRS, Orléans Cedex 2, 45071, France</footnote>

<footnote><sup>c</sup> School of Chemistry, University College Cork, Cork, T12 YN60, Ireland</footnote>

<footnote><sup>d</sup> Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, 15213, USA</footnote>

<footnote>* Corresponding author.
E-mail address: ivan.kourtchev@coventry.ac.uk (I. Kourtchev).</footnote>
# 1. Introduction

Technological advancement has led to the development, synthesis and application of millions of new chemical compounds. The Chemical Abstracts Service (CAS) now contains more than 200 million registered chemical substances, with about 4000 new entries added every day (CAS Registry, 2025). Many of these chemicals are released continuously into the environment and some of them, even when present at trace levels, may cause chronic toxicity, endocrine disruption, and the development of bacterial resistance (De Castro-Català et al., 2017; Larsson and Flach, 2022; Schwarzenbach et al., 2006). Some compounds can also persist in the environment long after release.

It is now more than a decade since the term "Contaminants of Emerging Concern" (CECs), also commonly referred to as Emerging Pollutants (EPs) in the literature, became a common term for the chemicals that are currently not subjected to a routine monitoring and/or emission control regime, but may be under scrutiny for future regulation (Dulio et al., 2018).

Due to their potential risks to ecosystems and health, some steps have been taken by regulatory bodies to identify and quantify CECs in aquatic media. For example, in 2013, the European Union (EU) created an amendment to the Environmental Quality Standards Directive (2008/105/EC), which cited the need for a mechanism that can provide targeted, high-quality, EU-wide information on the concentrations of substances of possible concern in aquatic environments (Council EU, 2013). The types of CECs monitored in water over the last few years typically includes, pharmaceuticals, pesticides, per- and polyfluoroalkyl substances (PFAS) and personal care products (Richardson and Ternes, 2022).

However, some CECs have been found to mobilise throughout the various environmental compartments (Hill et al., 2019; Mordechay et al., 2022; Wojsławski et al., 2019). Some of these chemicals can volatilise or partition into the atmosphere in substantial quantities raising questions about the effectiveness of regulating these chemicals within a single environmental compartment. Moreover, CECs can be transported via atmospheric dispersion, surface and groundwater flow, and marine currents across national borders, resulting in transboundary pollution. This poses additional challenges for regulatory frameworks, which are often jurisdictionally confined and may be insufficient to address the global nature of chemical distribution and impacts.

# 2. Evidence for atmospheric mobilisation of CECs

Pollutants can migrate to the atmosphere through a variety of mechanisms, which are influenced by their chemical properties and the environmental conditions. These mechanisms include, but are not limited to volatilisation, aerosol formation (including secondary organic aerosol formation), and chemical sinks. For example, it is well established that certain CECs, such as pesticides, readily undergo volatilisation from soil and plants and can be detected in the air for many days after application (Figueiredo et al., 2021). In addition to soil-air transfer, there is growing empirical evidence that some CECs, long thought to be confined to aquatic media, can undergo significant transfer into the atmosphere. For example, perfluoroalkyl acids (PFAAs), a subset of PFAS compounds, which have been observed in oceans (Dunn et al., 2024) and soils (Brusseau et al., 2020), were also recently detected in atmospheric aerosol particles (Kourtchev et al., 2024) even in a pristine environment. Furthermore, experimental studies demonstrate that sea spray aerosol (SSA) formation is a key mechanism enabling the atmospheric mobilisation of waterborne PFAAs, with enrichment factors suggesting that their concentration in aerosols can exceed bulk water levels by an order of magnitude (Johansson et al., 2019). This implies that SSA can act as an efficient vector for transferring persistent and water-soluble pollutants into the atmosphere.

These findings challenge the traditional compartmentalisation of CECs and raise concerns about under-recognised atmospheric transport

and human exposure pathways. Given the shared physicochemical properties among various CECs, such as surfactant behaviour, water solubility, and surface activity, it is plausible that other polar and semi-volatile CECs could undergo similar air-water exchange processes, particularly under dynamic environmental conditions. While direct emissions of CECs from industrial activities, combustion, agriculture, and indoor sources to the atmosphere are well documented (e.g., Barroso et al., 2019; Enyoh et al., 2020; Liu et al., 2021; Robichaud, 2020; Tanvir et al., 2023), secondary emissions from regulated environmental reservoirs deserve greater attention in the context of multiphase pollutant transfer. In this work, we focus specifically on volatilisation and aerosolisation of CECs from water and soil compartments as atmospheric sources. This emphasis reflects a deliberate effort to highlight a relatively underexplored pathway that remains poorly addressed in current air quality frameworks.

# 3. Current policy

In the literature, the terms CECs are often used interchangeably with persistent organic pollutants (POPs), while the latter are generally regulated worldwide by the Stockholm Convention and the Aarhus Protocol (UNEP, 2001). Current air quality regulations are limited to a small number of pollutants. In Europe, for example, the EU Ambient Air Quality (AAQ) Directive defines standards for ambient air quality, regulating thirteen air pollutants e.g., SO₂, NO₂, NOx, CO, O₃, benzene, PM₁₀ and PM₂.₅, along with some particulate-bound species such as benzo[a]pyrene, which apply to all member states (EC, 2008). In the United States, the United States Environmental Protection Agency (US EPA) has developed the National Ambient Air Quality Standards (NAAQS) which are based on the Clean Air Act that was last amended in the 1990s (USEPA, 2025a), at which point attainment thresholds were assigned for six so-called principal pollutants (SO₂, CO, NOx, lead), PM₁₀ and PM₂.₅ in outdoor air throughout the United States. In addition, the U.S. regulates a distinct group of 188 substances classified as Hazardous Air Pollutants (HAPs), which are either known or suspected to cause cancer, serious health issues, or environmental damage. Examples of these pollutants include benzene, perchloroethylene, methylene chloride, dioxins, toluene, and heavy metals such as cadmium, mercury, chromium, and lead (USEPA, 2025b).

Although WHO recognises that there is no safe level of exposure to PM₂.₅, it has established a guideline value of 10 µg m⁻³ as the annual mean concentration above which the risk of mortality increases. There is significant evidence that health risks of PM₂.₅ are not completely represented by mass concentration alone because the chemical composition is highly complex and the associated toxicity and health impacts can vary widely (Park et al., 2018). Therefore, understanding the contribution of airborne CECs to PM₂.₅ is essential to fully assess and mitigate the health risks posed by fine particulate pollution.

Several studies have attempted to model the behaviour of organic substances that fulfil the persistence and bioaccumulation criteria established at the Stockholm Convention on POPs (Scheringer et al., 2012; Socorro et al., 2016; Strempel et al., 2012). These studies either considered specific classes of pollutants (e.g. pesticides) or exceedingly high numbers of chemicals (e.g. 93,000 of industrial chemicals) making it very difficult to implement strategies for monitoring these potential pollutants in the environment. NORMAN, the network of reference laboratories, research centres and related organisations for monitoring of emerging environmental substances, has created a list of CECs that already have been observed in the environment, but which are still not included in routine monitoring programmes at the EU level and whose fate, behaviour and (eco)toxicological effects are not well understood (NORMAN, 2025). This list contains ~1000 substances, which were already detected across a range of environments (mainly in water). These substances can be classified into 20 subclasses and include chemicals found in pharmaceuticals, personal care products, pesticides, industrial and household products, and solvents (Dulio et al., 2018).
## 4. Modelling approach

Vapour pressures were predicted using EPI Suite's MPBPWIN v1.43 program (USEPA, 2012), calculated at 25 °C, based on the method described in Lyman et al. (1990).

The evaluated databases of CECs included: (1) NORMAN, (2) REACH (the Registration, Evaluation, Authorisation, and Restriction of Chemicals) SVHC (Substances of Very High Concern) (ECHA, 2025) and UCMR (US EPA Unregulated Contaminant Monitoring Rule) lists 4 and 5 datasets (USEPA, 2025c). Where possible, output from this method was compared with other techniques available in UManSysProp (Topping et al., 2016) as well as experimental data. However, given that methods therein are more tuned towards CHONS compounds, and given the diversity contained within the datasets, the generally robust MPBPWin was preferred in our study. In the case of Henry's law calculations, these were also performed at 25 °C using EPI Suite's HenryWin v3.20 software. In this instance, bond-estimated Henry's law coefficients were preferred (Hine and Mookerjee, 1975), as they generally offer broader coverage across diverse chemical data. As with vapour pressure, estimated data were compared against experimental determinations where available (Sander, 2015). Finally, hydroxyl radical rate coefficients were estimated at 25 °C using the EPI Suite's AopWin v1.92, which is based on the methodology of Kwok and Atkinson (1995). As with the other outputs, this was compared against experimental data (McGillen et al., 2020) to confirm the overall robustness of the method for calculating rate coefficients for these datasets.

## 5. Atmospheric transfer potential of CECs

Understanding the environmental fate and transport of CECs requires consideration of key physicochemical properties such as vapour pressure, Henry's law constant, partition coefficients (e.g., $K_{ow}$, $K_{oc}$), and water solubility. These properties play a critical role in determining how compounds distribute across environmental compartments, how persistent they are, and their potential for long-range transport or bio-accumulation. However, for the vast majority of CECs, parameters

describing these properties have not been experimentally measured. Using estimation programs, we have examined CECs from NORMAN, REACH- SVHC and US EPA UCMR lists 4 and 5. These lists include substances that were already observed mainly in aquatic environments across EU, UK and USA providing a solid empirical basis and confidence to extend investigations into their occurrence and behaviour in other environmental compartments such as air. The latter two lists are particularly important as they reflect a level of regulatory scrutiny and selection based on specific criteria such as persistence, toxicity, potential for human exposure, or occurrence in drinking water that make them strong candidates for further assessment beyond aquatic compartments.

### 5.1. Vapour pressure

Vapour pressure is a fundamental physicochemical property that describes the tendency of a substance to transition from the liquid or solid phase into the gas phase under given conditions. Some atmospheric organic compounds occur predominantly in the gas phase, some are present as liquids or solids in aerosols, while others can partition between the gas and particle phases. Based on vapour pressure, organic pollutants can be divided into three main groups, i.e. volatile, semi-volatile and non-volatile organic compounds. Organic compounds with vapour pressures more than $10^{-4}$ atm (at 20 °C) are referred to as volatiles, between $10^{-14}$ and $10^{-4}$ atm are semi-volatiles and less than $10^{-14}$ are non-volatiles (Olsen and Nielsen, 2001). At a given temperature, substances with higher vapour pressure will evaporate more readily than their lower vapour pressure counterparts. Based on the estimated vapour pressure of the CECs listed in the NORMAN database, up to 85 % of total CECs can be attributed to semi-volatile and volatile compounds (see Fig. 1).

### 5.2. Volatility Basis Set

An alternate volatility classification known as the Volatility Basis Set (VBS) uses the partitioning of species between the particle phase and the vapour phase under ambient conditions as well as saturation

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Distribution of volatile, semi-volatile and non-volatile CECs in NORMAN, SVHC for authorisation and USEPA (UCMR4-5) lists based on the estimated compound vapour pressure using EPI Suite's MPBPWIN v1.43 model and UManSysProp. A substantial fraction of these compounds corresponds to semi-volatile and volatile substances, indicating a significant potential for atmospheric transport and environmental dispersion.</figcaption></figure>
concentration (c°) as opposed to vapour pressure (c = (V/RT)P) (Donahue et al., 2019). The classifications are:

$$VOC (c^o > 3 \times 10^7 \mu g \text{ m}^{-3}; \sim 1 \times 10^{-3} \text{ atm}) \text{ are "volatile"}$$

IVOC (3 × 10³ < c° < 3 × 10⁷ µg m⁻³; ~1 × 10⁻⁷ – 1 × 10⁻³ atm) are “intermediate volatility”

SVOC (0.3 < c° < 3 × 10³ µg m⁻³; ~3 × 10⁻¹¹ – 1 × 10⁻⁷ atm) are “semi volatile”

LVOC (3 × 10⁻⁵ < c° < 0.3 µg m⁻³; ~3 × 10⁻¹⁵ – 3 × 10⁻¹¹ atm) are “low volatility”

ELVOC (3 × 10⁻⁹ < c° < 3 × 10⁻⁵ µg m⁻³) are “extremely low volatility”

ULVOC (c° < 3 × 10⁻⁹ µg m⁻³) are “ultra low volatility”

Volatility is important for all of these classifications, even for the ULVOC class which is conventionally defined as “non volatile”, because it is the key property governing vapour pressure and gas-aerosol partitioning. The SVOC class is so named because these compounds are found at equilibrium with significant fractions in both the gas and particle phases. The fraction found in the particle phase (Fparticle) depends on the ambient concentration of organic aerosol (cOA), which typically ranges from 1 to 100s of µg m⁻³, and is represented by (Donahue et al., 2009):

$$F_{\text{particle}} = 1 / (1 + c^{\circ} / c_{\text{OA}})$$

The implication is that any contaminant in the SVOC range that aerosolises will partition significantly into the gas phase. Those in the IVOC range will vaporise almost immediately (Donahue et al., 2019). This includes up to 43 % of the CECs from NORMAN list classified as “semi-volatile” in Fig. 1.

Although VOCs and SVOCs have the highest potential to volatilise into the atmosphere from aquatic or soil environments, nominally non-volatile compounds can also transfer to the atmosphere through shifts in thermal equilibria or mechanical processes. These processes may include aeration, intense mixing and turbulence where CECs can be dissolved into a liquid aerosol or ‘transported’ on the surface of the suspended particles in a process like that observed with bioaerosols. One of the potential sources of these pollutants is open air liquid waste facilities, e.g. wastewater treatment plants (WWTP) and manure lagoons. WWTPs can serve as a repository of CECs, where the pollutants become concentrated and then escape to the atmosphere during WWT processes (i.e. open aeration, intense mixing and turbulence). For example, previous studies indicated that PFAS can aerosolise from aqueous solutions enriched with contaminants, with the extent of aerosolisation strongly dependent on the PFAS carbon chain length and pH of the aqueous solution (Pandamkulangara Kizhakkethil et al., 2024). As a result, some of these compounds were detected in aerosol at WWTPs with concentrations 15 times higher than in background areas (Ahrens et al., 2011).

Another potential source of airborne CECs includes open air anaerobic or manure lagoons that are designed to manage and treat refuse created by concentrated animal feeding operations. Although these liquid wastes are mainly linked to emissions of volatile inorganic substances (e.g. ammonia, hydrogen sulfide, carbon dioxide) and methane into atmosphere, they may also be enriched with antibiotics, estrogens, bacteria and pesticides (Yost et al., 2014) that can run off into surrounding land and water, but due to the factors outlined above have a potential to diffuse into the atmosphere as well.

### 5.3. The Henry's law constant ($H_v$)

While vapour pressure is a key parameter in assessing the atmospheric transfer potential of pollutants, other environmental and molecular factors must also be considered. For instance, the volatilisation of pesticides and other organic contaminants is influenced by their Henry's law constant, sorption properties, and octanol-air partition coefficient (Koa), as well as environmental conditions such as temperature,

humidity, and wind speed.

The Henry's law constant ($H_v$) defines the equilibrium partitioning of a compound between the aqueous phase or soil moisture and the gas phase, influencing how readily a substance transitions from environmental matrices into the air. It is the activity coefficient corrected vapour pressure of a compound in a dilute aqueous solution.

In general, a compound with $H_v > 10^{-3}$ atm m³ mol⁻¹ is considered volatile whereas a compound with $H_v < 5 \times 10^5$ atm m³ mol⁻¹ is considered soluble and will remain in water (Olsen and Davis, 1990). Based on these assumptions a significant fraction of CECs from the NORMAN, REACH SVHC and EPA UCMR 4 and 5 lists are expected to volatilise into the atmosphere (Fig. 2).

Based solely on estimates of their Henry's law constant, up to 155 of CECs in the NORMAN list have the potential to volatilise into the atmosphere. Although the use of $H_v$ as the key parameter to assess the potential for atmospheric transfer ignores factors such as the pH of the aquatic media, CEC concentration, and the presence of dissolved oxidants, it does give comparable results to those obtained using vapour pressure only.

The atmospheric transport of volatile and semi-volatile compounds is governed by dynamic partitioning between the gas and particle phases (Pankow, 1994), with subsequent incorporation into precipitation through mechanisms such as aerosol scavenging, in-cloud condensation, and below-cloud washout (Rosenfeld et al., 2014).

For example, organophosphate insecticides such as dichlorvos, and malathion (both emerging pollutants associated with water and soil pollution) have been detected in atmospheric samples, particularly in regions with extensive agricultural activities or pest control operations. A further example, in a study by the U.S. Geological Survey highlighted that diazinon (another CEC) is among the organophosphate insecticides most frequently detected in the air, rain, and fog in California's Central Valley, a region known for intensive agriculture (Majewski et al., 2006).

### 5.4. Other potential factors

The octanol–air partition coefficient (Koa) is a critical parameter governing the gas–particle partitioning (Pankow, 1987) of CECs in the atmosphere. Compounds with higher Koa values are more likely to associate with atmospheric particles, which significantly influences their atmospheric lifetime, transport range, and deposition pathways. For low volatility chemicals with high Koa, particle-bound deposition becomes a dominant removal process, affecting where and how these compounds are ultimately deposited in the environment (Baskaran and Wania, 2023). Among the key transport and fate processes, namely volatilisation from surfaces such as soil, water, and waste; gas-particle partitioning as measured by Koa; and deposition, some CECs, (e.g. organophosphate esters (OPEs) and some organophosphate flame retardants (OPFRs)), can also undergo atmospheric transformation via photooxidation and degradation. These transformations dynamically alter their gas-particle partitioning behaviour and consequently their environmental fate. Recent studies have emphasised the important role of aqueous-phase oxidation chemistry in controlling the atmospheric persistence and transformation of CECs such as OPEs (Lv et al., 2025). Furthermore, transformation products formed through these atmospheric processes can be globally distributed and may pose greater overall risks due to their combined toxicity and persistence compared to the parent compounds alone (Liu et al., 2021).

The volatilisation potential of CECs is significantly influenced by temperature fluctuations and seasonal variations. Warmer temperatures generally enhance the kinetic energy of molecules, increasing the rate of volatilisation, whereas colder temperatures will shift the equilibrium towards the condensed phases. The enthalpy of vaporisation determines this sensitivity, and for compounds in the SVOC range, $\Delta H_{vap}$ is approximately 110 kJ mol⁻¹, meaning that a temperature change of 15 °C will cause a factor of 10 change in the volatility (Epstein et al., 2010). This phase transition can lead to an increased atmospheric
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** CECs in NORMAN, SVHC for authorisation and US EPA (UCMR4) lists that have potential to volatilise or remain in water based on their estimated compound Henry's constants using EPI Suite HenryWin v3.20.</figcaption></figure>

residence time for certain CECs, particularly those with intermediate volatility, as they may preferentially adsorb onto preexisting aerosols under lower temperature conditions. In some cases, such as perfluorinated pollutants in the Arctic, atmospheric transport of low volatility pollutants can be an important route for contamination (Martin et al., 2004; Young et al., 2007).

Furthermore, pH variations in aquatic and soil systems play a crucial role in determining the speciation and partitioning behaviour of CECs (e. g. Nguyen et al., 2020; Oste et al., 2002; Pandamkulangara Kizhakkethil et al., 2024). CECs that exist in weak acid and weak base forms exhibit pH-dependent ionisation, which can strongly influence their volatilisation rates. Compounds that exist predominantly in their neutral form under specific pH conditions are more likely to volatilise, while those that ionise into charged species tend to remain in the aqueous phase due to their increased water solubility. For instance, herbicides such as 3, 6-dichloro-2-methoxybenzoic acid (also known as dicamba) can exist as a semi-volatile acid under acidic conditions but remains largely dissociated at neutral or alkaline pH, which significantly reduces its atmospheric transfer potential (Carbonari et al., 2022; Zaccaro-Gruener et al., 2022). PFAS such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and the later generation PFAS, hexafluoropropylene oxide-dimer acid (GenX) at lower pH levels, mainly exist in their neutral form, making them more susceptible to aerosolisation or volatilisation from water surfaces. However, under higher pH conditions, they predominantly exist as anions, which dramatically reduces their volatility but enhances their sorption to solid particles and organic matter in water, altering their environmental fate (Pandamkulangara Kizhakkethil and Kourtchev, 2025; Vierke et al., 2013).

Another key factor to consider is the role of surfactants and organic matter present in water bodies. Surfactants, such as those found in detergents and personal care products, can form micelles or surface films that either suppress or enhance volatilisation depending on their interactions with CECs. Some pollutants may adsorb onto organic matter, altering their availability for volatilisation or resuspension. This is particularly relevant in heavily contaminated sites such as air force

bases, naval stations, and firefighter training grounds where Aqueous Film-Forming Foam (AFFF) is frequently used, and contamination is often widespread (Maizel et al., 2021).

It has been well established that waste incineration and electronic waste (e-waste) recycling using pyrometallurgy and hydrometallurgy can lead to emissions of POPs (Van Caneghem et al., 2010). Similarly to POPs, CECs can be emitted directly from these processes. Waste incineration generally operates at temperatures in the range 800 °C to 1200 °C in controlled facilities (Gimţauskaitė et al., 2022), while e-waste burning often takes place at much lower temperatures, around 200 °C–600 °C (Andooz et al., 2024). At lower temperatures, incomplete combustion can lead to the release of semi-volatile and persistent compounds, including brominated, chlorinated and fluorinated pollutants (Ni et al., 2016). Some CECs, such as PFAS and certain brominated flame retardants, can survive high temperatures or degrade into equally persistent and toxic byproducts (Altarawneh et al., 2019; Björklund et al., 2023). A study on Waste-to-Energy (WtE) plants, where waste is incinerated at a minimum of 850 °C with grate temperatures reaching up to 1100 °C, found that PFAS, particularly short-chain perfluorocarboxylic acids (C₄–C₇), were present in all examined residues. Notably, PFAS were also detected in flue gas for the first time, demonstrating that some compounds are not fully degraded during WtE conversion and can be emitted into the environment through ash, gypsum, treated process water, and flue gas (Björklund et al., 2023).

Beyond direct volatilisation, indirect pathways such as photochemical transformations can lead to the formation of volatile or semi-volatile secondary products. For instance, UV radiation or hydroxyl radical reactions may degrade non-volatile CECs into more volatile derivatives, which can then be transported in the atmosphere via both the gas phase and adsorption onto particulate matter. For example, the fluorotelomer sulfonate 6:2 FTS (an emerging pollutant), may undergo degradation via UV/H₂O₂, into shorter-chain perfluoroalkyl carboxylic acids (C₇–C₂) (Yang et al., 2014).
# 6. Conclusions

Water and soil are the primary focus of many environmental studies, as pollutants tend to accumulate in these media due to their chemical properties and contamination pathways. Consequently, regulations and mitigation strategies have been largely developed for surface water, groundwater, and soil, with stringent monitoring in place. However, the potential for certain compounds to volatilise, adsorb onto airborne particles, or undergo long-range atmospheric transport is often overlooked, leading to an underestimation of their presence in the air. This monitoring bias affects environmental risk assessments and public health policies. Many persistent and semi-volatile compounds, including pesticides, PFAS, and industrial byproducts, transition between environmental compartments, making air both a transport medium and a secondary contamination source. Yet, air monitoring of CECs remains underdeveloped compared that of water and soil, with limited regulations addressing airborne contaminants. This regulatory gap makes it harder to implement mitigation strategies and reduces the accuracy of pollutant fate and transport models.

Despite extensive modelling efforts covering a wide range of chemicals, translating the resulting information into effective monitoring and regulatory frameworks remains challenging. The sheer number of pollutants, combined with uncertainties regarding their actual contribution to environmental pollution, complicates prioritisation and implementation. Here, we focus on compounds that have already been scrutinised through regulatory assessments, scientific investigations, or environmental monitoring programs and detected in compartments other than air. By selecting chemicals with documented environmental presence, we focus on those with a higher likelihood of atmospheric occurrence, rather than on compounds that have not been previously observed. This list is not exhaustive, as certain chemicals may remain undetected in water or soil due to their physicochemical properties, favouring atmospheric transport over deposition. Some compounds may escape detection simply because current monitoring efforts do not target them, rather than their absence in the environment. This suggests the need for broader air monitoring to identify pollutants that are currently overlooked and better understand their environmental behaviour. Expanding monitoring frameworks and integrating atmospheric pathways into environmental policies will provide a more accurate picture of contaminant transport and exposure risks, ensuring that air pollution is addressed alongside water and soil contamination in regulatory efforts.

# 7. Future research directions

To improve our understanding of CECs and their potential impacts on air quality, several research directions should be pursued. First, further investigation is needed into the phase transfer mechanisms of CECs, particularly under varying environmental conditions such as temperature, pH, and organic matter content. This includes direct volatilisation, aerosolisation and indirect pathways, such as photochemical transformation into more mobile compounds. Second, there is a pressing need to characterise the presence and fate of CECs in atmospheric compartments, including gas phase, aerosols, and precipitation. Studies should aim to quantify emissions from diffuse and point sources, such as wastewater treatment plants, manure lagoons, and waste incineration facilities. Third, research should focus on developing robust screening models to predict which CECs are likely to transfer into the atmosphere based on physicochemical properties (e.g., vapour pressure, Henry's law constant, octanol-air partitioning coefficient). Additionally, improved analytical techniques are required to detect CECs in the air at trace levels, including methods for capturing both particle-bound and gaseous species. This requires going beyond targeted approaches, recognising the complexity of the matrix and the potential differences in the inherited properties of the CECs, which may necessitate the use of several complementary analytical tools. A further direction involves assessing the toxicological significance of atmospheric CECs, especially

their role in PM<sub>2.5</sub> toxicity and potential to affect human health upon inhalation. Finally, an interdisciplinary effort is required to integrate CEC atmospheric transport and transformation pathways into regulatory frameworks, particularly in the context of multi-compartment environmental exposure and long-range transport potential.

# CRediT authorship contribution statement

**Ivan Kourtchev:** Writing – review & editing, Writing – original draft, Visualization, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Max R. McGillen:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **John Wenger:** Writing – review & editing, Writing – original draft, Conceptualization. **Neil M. Donahue:** Writing – review & editing, Writing – original draft, Conceptualization.

# Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

# Data availability

Data will be made available on request.

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