Tailpipe emissions from road traffic contribute substantially to the burden of fine inhalable particulate matter (PM2.5) and deteriorate air quality. Exhaust emission standards, forcing improvements in combustion and exhaust after-treatment technology, considerably decreases combustion-related PM2.5 emitted by modern cars. A549 cancerous alveolar and BEAS-2B normal bronchial epithelial cells were exposed at the air-liquid interface to the total aerosol or gas phase of either fresh or photochemically aged tailpipe emissions from a gasoline EURO 6d car equipped with a gasoline particulate filter. Diluted fresh emissions contained particle number concentrations comparable to low ambient air levels and induced no detectable cytotoxicity. Photochemical aging led to the formation of secondary aerosols and caused significant cytotoxicity. While the aged aerosol induced significant DNA damage, oxidative stress was more associated with volatile secondary species. Our results call for the consideration of the exhaust emission atmospheric transformation processes in future emission standards toward health effect-driven emission regulations.

This paper presents the findings from a collaborative interlaboratory comparison exercise designed to assess oxidative potential (OP) measurements conducted by 20 laboratories worldwide. This study represents an innovative effort as the first exercise specifically aimed at harmonising this type of OP assay, setting a new benchmark in the field. Over the last decade, there has been a noticeable increase in OP studies, with numerous research groups investigating the effects of exposure to air pollution particles through the evaluation of OP levels. However, the absence of standardised methods for OP measurements has resulted in variability in results across different groups, rendering meaningful comparisons challenging. To address this issue, this study engages in an international effort to compare OP measurements using a simplified method (with a dithiothreitol (DTT) assay). Here, we quantify the OP in liquid samples to focus on the protocol measurement itself, while future international OP interlaboratory comparisons (ILCs) should aim to assess the whole chain process, including the sample extraction. We analyse the similarities and discrepancies observed in the results, identifying the critical parameters (such as the instrument used, the use of a simplified protocol, the delivery and analysis time) that could influence OP measurements and provide recommendations for future studies and interlaboratory comparisons even if other crucial aspects, such as sampling PM methods, sample storage, extraction methods and conditions, and the evaluation of other OP assays, still need to be standardised. This collaborative approach enhances the robustness of the OP DTT assay and paves the way for future studies to build on a unified framework. This pioneering work concludes that interlaboratory comparisons provide essential insights into the OP metric and are crucial to move toward the harmonisation of OP measurements.

The toxicity of particulate matter (PM) is highly related to the concentration of particle-bound reactive oxygen species (ROS). Chemical properties, including dissolved metals and the sources of PM, influence ROS production and ROS oxidative potential. Here, the photochemical aging of a secondary organic aerosol proxy (citric acid, CA) with metal complexes (iron–citrate, FeIII(Cit)) is assessed toward the production of particle-bound ROS with an online instrument (OPROSI). We studied the photochemically induced redox chemistry in iron/copper–citrate particles experimentally with an aerosol flow tube (AFT), mimicking atmospheric UV aging. Experiments were performed at different relative humidity (RH) levels, leading to variation in the physicochemical properties of the particles, e.g., viscosity. We found that UV-aged CA aerosol containing 10 mol % FeIII generated ROS concentrations on the order of 0.1 nmol H2O2 eq.µg-1, indicating the photochemically driven formation of peroxides. An increase in RH leads to only a slight but overall lower concentration of ROS, possibly due to a loss of volatile HO2 and H2O2 in the gas phase in the less viscous particles. The RH effect is enhanced in absence of oxygen. Compared to the FeIII(Cit)/CA particles, the iron/copper–citrate samples show a uniformly decreased ROS level. Interestingly, in the high-RH nitrogen experiment with copper, we found an enhanced drop in the ROS concentration down to 0.02 nmolH2O2eq.µg-1 compared to all other irradiation experiments. We suggest that copper may suppress radical redox reactions, and when particles are more viscous, ROS are still produced with photochemistry, but the levels are more sensitive to the presence of copper than under humid or lower-viscosity conditions.

Exposure to airborne particulate matter (PM) has been attributed to millions of deaths annually. However, the PM components responsible for observed health effects remain unclear. Oxidative potential (OP) has gained increasing attention as a key property that may explain PM toxicity. Using online measurement methods that impinge particles for OP quantification within seconds, we reveal that 60 to 99% of reactive oxygen species (ROS) and OP in secondary organic aerosol and combustion-generated PM have a lifetime of minutes to hours and that the ROS activity of ambient PM decays substantially before offline analysis. This implies that current offline measurement methods substantially underestimate the true OP of PM. We demonstrate that short-lived OP components activate different toxicity pathways upon direct deposition onto reconstituted human bronchial epithelia. Therefore, we suggest that future air pollution and health studies should include online OP quantification, allowing more accurate assessments of links between OP and health effects.

Air pollution is one of the largest environmental health risks and one of the leading causes of adverse health outcomes and mortality worldwide. The possible importance of the oxidative potential (OP) as a metric to quantify particle toxicity in air pollution is increasingly being recognised. In this work, the OP and reactive oxygen species (ROS) activity of particles from fresh and aged petrol passenger car emissions and residential wood combustion (RWC) emissions were investigated using two novel instruments. Applying online instruments using an ascorbic acid (AA) and 2′,7′-dichlorodihydrofluorescein (DCFH) assay provides a much higher temporal resolution compared with traditional filter-based methods and allows for new insights into the highly dynamic changes in the OP and ROS activity of these sources. Due to the efficiency of the particulate filter in the Euro 6d car, almost no primary particles were emitted and, thus, no particle OP or ROS activity was detected in primary exhaust. However, a substantial and highly dynamic OP and ROSs were observed after photochemical ageing due to the formation of secondary particles. Increasing OP and ROS activity due to ageing was also observed when comparing fresh and aged RWC emissions. Overall, RWC emissions had higher OP and ROS signals compared with car emissions. This suggests that aged RWC emissions could be a major contributor to air pollution toxicity and may be an intrinsically more harmful emission source than car exhaust, although the formation potential for secondary particles from car emissions was still high. These measurements illustrate the strong differences and highly dynamic nature of toxicity-relevant particle properties from two air pollution sources and could contribute to more efficient air pollution mitigation policies.

Large-scale epidemiological studies have consistently shown that exposure to ambient particulate matter (PM) is responsible for a variety of adverse health effects. However, the specific physical and chemical properties of particles that are responsible for the observed health effects, as well as the underlying mechanisms of particle toxicity upon exposure, remain largely uncertain. Studies have widely suggested that the oxidative potential (OP) of aerosol particles is a key metric to quantify particle toxicity. OP is defined as the ability of aerosol particle components to produce reactive oxidative species (ROSs) and deplete antioxidants in vivo. Traditional methods for measuring OP using acellular assays largely rely on analyzing PM collected in filters offline. This is labor intensive and involves a substantial time delay between particle collection and OP analysis. It therefore likely underestimates particle OP because many reactive chemical components which contribute to OP are short-lived and therefore degrade prior to offline analysis. Thus, new techniques are required to provide a robust and rapid quantification of particle OP, capturing the chemistry of oxidizing and short-lived, highly reactive aerosol components and their concentration dynamics in the atmosphere. To address these measurement shortcomings, we developed a portable online instrument that directly samples particles into an ascorbic acid-based assay under physiologically relevant conditions of pH 6.8 and 37 degrees C, providing continuous, accurate OP measurements with a high time resolution (5 min). The instrument runs autonomously for up to 3 d and has a detection limit of about 5 mu g m(3) in an urban environment, which allows the characterization of particle OP even in lowpollution areas.

Wildfires are a major source of biomass burning aerosol to the atmosphere, with their incidence and intensity expected to increase in a warmer future climate. However, the toxicity evolution of biomass burning organic aerosol (BBOA) during atmospheric aging remains poorly understood. In this study, we report a unique set of chemical and toxicological metrics of BBOA from pine wood smoldering during multiphase aging by gasphase hydroxyl radicals (OH). Both the fresh and OH-aged BBOA show activity relevant to adverse health outcomes. The results from two acellular assays (DTT and DCFH) show significant oxidative potential (OP) and reactive oxygen species (ROS) formation in OH-aged BBOA. Also, radical concentrations in the aerosol assessed by electron paramagnetic resonance (EPR) spectroscopy increased by 50% following heterogeneous aging. This enhancement was accompanied by a transition from predominantly carboncentered radicals (85%) in the fresh aerosol to predominantly oxygen-centered radicals (76%) following aging. Both the fresh and aged biomass burning aerosols trigger prominent antioxidant defense during the in vitro exposure, indicating the induction of oxidative stress by BBOA in the atmosphere. By connecting chemical composition and toxicity using an integrated approach, we show that short-term aging initiated by OH radicals can produce biomass burning particles with a higher particle-bound ROS generation capacity, which are therefore a more relevant exposure hazard for residents in large population centers close to wildfire regions than previously studied fresh biomass burning emissions.

The oxidative potential (OP) of particulate matter has been widely suggested as a key metric for describing atmospheric particle toxicity. Secondary organic aerosol (SOA) and redox-active transition metals, such as iron and copper, are key drivers of particle OP. However, their relative contributions to OP, as well as the influence of metal–organic interactions and particulate chemistry on OP, remains uncertain. In this work, we simultaneously deploy two novel online instruments for the first time, providing robust quantification of particle OP. We utilize online AA (OPAA) and 2,7-dichlorofluoroscein (ROSDCFH) methods to investigate the influence of Fe(II) and Cu(II) on the OP of secondary organic aerosol (SOA). In addition, we quantify the OH production (OPOH) from these particle mixtures. We observe a range of synergistic and antagonistic interactions when Fe(II) and Cu(II) are mixed with representative biogenic (β-pinene) and anthropogenic (naphthalene) SOA. A newly developed kinetic model revealed key reactions among SOA components, transition metals, and ascorbate, influencing OPAA. Model predictions agree well with OPAA measurements, highlighting metal–ascorbate and −naphthoquinone–ascorbate reactions as important drivers of OPAA. The simultaneous application of multiple OP assays and a kinetic model provides new insights into the influence of metal and SOA interactions on particle OP.

Regardless of decades of convincing epidemiological evidence, large uncertainty remains regarding the physical and chemical characteristics of aerosols, as well as the toxicity mechanisms upon exposure to human health. Oxidative potential (OP) is defined as the capability of particles to catalytically produce reactive oxygen species (ROS) with subsequent depletion of anti-oxidants, naturally present in the human lung. OP has been widely suggested as a measure of the potential toxicity of aerosols, but suitable instruments, especially for continuous field deployment are lacking. Due to the circumstance that ROS (i.e. inorganic and organic peroxides and radicals) are highly reactive, they are therefore short-lived. Thus, classical offline analysis, where aerosol particles are typically collected on a filter for 24h or longer prior to analysis, may lead to an underestimation of the oxidative potential. Therefore, we developed an online instrument that can continuously measure particle oxidative potential with immediately sampling of particles, and a high time resolution (10 minutes). We further developed an online instrument described in Wragg et al. (2016) and implemented a physiologically relevant assay to assess aerosol oxidative potential, based on the chemistry of ascorbic acid (Campbell et al. (2019)). Ascorbic acid (AA) is a prevalent naturally occurring anti-oxidant present in the lung and can therefore be used as a proxy to measure the oxidative potential and thus toxicity of aerosol particles. In this work, we present the overall design and operation of the OOPAAI (Campbell et al., 2019, Anal. Chem.). Recent improvements are also discussed where we further developed the OOPAAI into a field-deployable instrument with an improved continuous flow cell for fluorescence detection, a particle to liquid sampler with a higher efficiency and optimizations of the chemical reaction system to ensure that the ascorbic acid assay is stable and buffered at pH 6.8. These technical developments of the OOPAAI improved its detection limit, operational stability and physiological relevance. A range of laboratory flow tube studies were conducted to identify the OP of secondary organic aerosols (SOA) from naphthalene and beta-pinene and metal particles (Fe and Cu). A range of synergistic and antagonistic effects was observed when the OP of mixtures of metal and SOA particles was quantified. With the improvements of having a more physiological relevant assay and an improved detection method, this instrument is capable of providing a real time and more realistic estimation of the oxidising aerosol properties and their potential effect on human health. Moreover, in lab-based experiments the OOPAAI helps to gain better understanding of fundamental interaction between different aerosol types like organic aerosols and transition metals and their influence on OP and hence their potential toxicity. Wragg, F. P. H. et al. (2016), Atmospheric Measurement Techniques, 9(10), pp. 4891–4900. Campbell, S. J. et al. (2019), Analytical Chemistry, 91, 20, 13088-13095.

It is being suggested that particle-bound or particle-induced reactive oxygen species (ROS), which significantly contribute to the oxidative potential (OP) of aerosol particles, are a promising metric linking aerosol compositions to toxicity and adverse health effects. However, accurate ROS quantification remains challenging due to the reactive and short-lived nature of many ROS components and the lack of appropriate analytical methods for a reliable quantification. Consequently, it remains difficult to gauge their impact on human health, especially to identify how aerosol particle sources and atmospheric processes drive particle-bound ROS formation in a real-world urban environment. In this study, using a novel online particle-bound ROS instrument (OPROSI), we comprehensively characterized and compared the formation of ROS in secondary organic aerosols (SOAs) generated from organic compounds that represent anthropogenic (naphthalene, SOANAP) and biogenic (β-pinene, SOAβPIN) precursors. The SOA mass was condensed onto soot particles (SP) under varied atmospherically relevant conditions (photochemical aging and humidity) to mimic the SOA formation from a mixing of traffic-related carbonaceous primary aerosols and volatile organic compounds (VOCs). We systematically analyzed the ability of the aqueous extracts of the two aerosol types (SOANAP-SP and SOAβPIN-SP) to induce ROS production and OP. We further investigated cytotoxicity and cellular ROS production after exposing human lung epithelial cell cultures (A549) to extracts of the two aerosols. A significant finding of this study is that more than 90 % of all ROS components in both SOA types have a short lifetime, highlighting the need to develop online instruments for a meaningful quantification of ROS. Our results also show that photochemical aging promotes particle-bound ROS production and enhances the OP of the aerosols. Compared to SOAβPIN-SP, SOANAP-SP elicited a higher acellular and cellular ROS production, a higher OP, and a lower cell viability. These consistent results between chemical-based and biological-based analyses indicate that particle-bound ROS quantification could be a feasible metric to predict aerosol particle toxicity and adverse human effects. Moreover, the cellular ROS production caused by SOA exposure not only depends on aerosol type but is also affected by exposure dose, highlighting a need to mimic the process of particle deposition onto lung cells and their interactions as realistically as possible to avoid unknown biases.

Many large-scale epidemiological studies have shown a close correlation between adverse human health effects and ambient PM2.5 exposure. A report by the World Health Organisation estimates that 1 out of 8 deaths globally are linked to air pollution. Even though various epidemiological studies underline this argument, the chemical components and physical properties of particulate matter that leads to the observed health effects remains highly uncertain. Aerosol oxidative potential defined as the capability of particles to produce reactive oxygen species (ROS) with subsequent depletion of anti-oxidants, naturally present in the human lung, has been widely suggested as measure of their potential toxicity. Due to the fact that ROS (i.e. inorganic and organic peroxides and radicals) are highly reactive, they are therefore short-lived. Subsequently, classical offline analysis, where aerosol particles are typically collected on a filter for 24h, may lead to an underestimation of the oxidative potential. Therefore, we developed an online instrument that can continuously measure particle oxidative potential with a high time resolution (10 minutes). We further developed an online instrument described in Wragg et al. (2016) and implemented a physiologically relevant assay to assess aerosol oxidative potential, based on the chemistry of ascorbic acid (Campbell et al. (2019)). Ascorbic acid (AA) is a prevalent naturally occurring anti-oxidant present in the lung and can therefore be used as a proxy to measure the oxidative potential of aerosol. In this work, we further developed the AA online assay based on Campbell et al. (2019), implementing more physiologically relevant chemical conditions such as pH7 and we improved components of the instrument to increase its detection limit. With the current instrument AA oxidation can be quantified via two different spectroscopic methods: one based on fluorescence as described in Campbell et al. (2019) and a newly developed UV-absorption detection system using a liquid waveguide capillary cell (LWCC) which is a very sensitive long pathway (100cm) absorption cell. For the fluorescence approach, a limit of detection (LOD) of 0.22 µg/m3 was determined for copper (Campbell et al. (2019)). In comparison, the current detection limit for the UV-absorption based setup is an order of magnitude lower (0.02 µg Cu/m3). This LOD is close to observations of copper concentrations at urban European locations, which are in the range of 0.001-0.009 µg/m3. Using both detection methods, we gain an improved understanding of the oxidation process, because the absorbance method measures AA depletion whereas in the fluorescence method the formation of the AA oxidation product dehydroascorbic is quantified. The online ascorbic acid assay as described will be applied in lab experiments (i.e. flow tubes or smog chamber) as well as for field measurements. With the improvements of having a more physiological relevant assay and an improved detection method, this instrument is capable of providing a real-time and more realistic estimation of the oxidizing aerosol properties and their potential effect on human health compared to traditional offline methods. Wragg, F. P. H. et al. (2016), Atmospheric Measurement Techniques, 9(10), pp. 4891–4900. Campbell, S. J. et al. (2019), Analytical Chemistry, 91, 20, 13088-13095.

Ascorbic acid is among the most abundant antioxidants in the lung, where it likely plays a key role in the mechanism by which particulate air pollution initiates a biological response. Because ascorbic acid is a highly redox active species, it engages in a far more complex web of reactions than a typical organic molecule, reacting with oxidants such as the hydroxyl radical as well as redox-active transition metals such as iron and copper. The literature provides a solid outline for this chemistry, but there are large disagreements about mechanisms, stoichiometries and reaction rates, particularly for the transition metal reactions. Here we synthesize the literature, develop a chemical kinetics model, and use seven sets of laboratory measurements to constrain mechanisms for the iron and copper reactions and derive key rate constants. We find that micromolar concentrations of iron(III) and copper(II) are more important sinks for ascorbic acid (both AH 2 and AH − ) than reactive oxygen species. The iron and copper reactions are catalytic rather than redox reactions, and have unit stoichiometries: Fe(III)/Cu(II) + AH 2 /AH − + O 2 → Fe(III)/Cu(II) + H 2 O 2 + products. Rate constants are 5.7 × 10 4 and 4.7 × 10 4 M −2 s −1 for Fe(III) + AH 2 /AH − and 7.7 × 10 4 and 2.8 × 10 6 M −2 s −1 for Cu(II) + AH 2 /AH − , respectively.

Epidemiological studies have consistently linked exposure to PM2.5 with adverse health effects. The oxidative potential (OP) of aerosol particles has been widely suggested as a measure of their potential toxicity. Several acellular chemical assays are now readily employed to measure OP; however, uncertainty remains regarding the atmospheric conditions and specific chemical components of PM2.5 that drive OP. A limited number of studies have simultaneously utilised multiple OP assays with a wide range of concurrent measurements and investigated the seasonality of PM2.5 OP. In this work, filter samples were collected in winter 2016 and summer 2017 during the atmospheric pollution and human health in a Chinese megacity campaign (APHH-Beijing), and PM2.5 OP was analysed using four acellular methods: ascorbic acid (AA), dithiothreitol (DTT), 2,7-dichlorofluorescin/hydrogen peroxidase (DCFH) and electron paramagnetic resonance spectroscopy (EPR). Each assay reflects different oxidising properties of PM2.5, including particle-bound reactive oxygen species (DCFH), superoxide radical production (EPR) and catalytic redox chemistry (DTT/AA), and a combination of these four assays provided a detailed overall picture of the oxidising properties of PM2.5 at a central site in Beijing. Positive correlations of OP (normalised per volume of air) of all four assays with overall PM2.5 mass were observed, with stronger correlations in winter compared to summer. In contrast, when OP assay values were normalised for particle mass, days with higher PM2.5 mass concentrations (μgm-3) were found to have lower mass-normalised OP values as measured by AA and DTT. This finding supports that total PM2.5 mass concentrations alone may not always be the best indicator for particle toxicity. Univariate analysis of OP values and an extensive range of additional measurements, 107 in total, including PM2.5 composition, gas-phase composition and meteorological data, provided detailed insight into the chemical components and atmospheric processes that determine PM2.5 OP variability. Multivariate statistical analyses highlighted associations of OP assay responses with varying chemical components in PM2.5 for both mass- and volume-normalised data. AA and DTT assays were well predicted by a small set of measurements in multiple linear regression (MLR) models and indicated fossil fuel combustion, vehicle emissions and biogenic secondary organic aerosol (SOA) as influential particle sources in the assay response. Mass MLR models of OP associated with compositional source profiles predicted OP almost as well as volume MLR models, illustrating the influence of mass composition on both particle-level OP and total volume OP. Univariate and multivariate analysis showed that different assays cover different chemical spaces, and through comparison of mass- and volume-normalised data we demonstrate that mass-normalised OP provides a more nuanced picture of compositional drivers and sources of OP compared to volume-normalised analysis. This study constitutes one of the most extensive and comprehensive composition datasets currently available and provides a unique opportunity to explore chemical variations in PM2.5 and how they affect both PM2.5 OP and the concentrations of particle-bound reactive oxygen species.

The negative effects of air pollution on human health has been subject to a number of epidemiological studies that consistently link respiratory and cardiovascular diseases to exposure to particulate matter (PM) (Englert, 2004). It is estimated that up to 0.3 million premature deaths per year in Europe and 2.1 million deaths worldwide are the result of exposure to particles with an aerodynamic diameter less than 2.5 μm (PM2.5) (Andersson, 2009). However, identifying the specific particle properties responsible for these health effects, such as their physical and physicochemical characteristics, as well as their chemical composition, remains a challenge. One of the leading hypotheses for how particles cause harm is by inducing oxidative stress and inflammation, which can subsequently lead to disease (Øvrevik, 2015). In particular, reactive oxygen species (ROS), which typically refer to a range of species including hydrogen peroxide (H2O2) possibly including organic peroxides, the hydroxyl radical (.OH) and superoxide radical (O2.-), may substantially contribute to the oxidative potential (OP) of PM and hence influence their toxicity. An excess of ROS in the lung, introduced or generated via particle exposure, leads to an imbalance of the oxidant-antioxidant ratio in favour of the former, which can subsequently promote oxidative stress. There are a number of acellular methods used routinely to measure aerosol OP, including the dithiothreitol assay (DTT), ascorbic acid assay (AA), 2,7-dichlorofluoroscein/hydrogen peroxidase assay (DCFH/HRP), and electron paramagnetic resonance (EPR) spectroscopy. In this work, the OP of aerosol collected in Beijing, China, in the winter 2016 and summer 2017 during the Atmospheric Pollution and Human Health in a Chinese Megacity (APHH) campaign is quantified, with 30 24-hr aerosol filter samples analysed for each season. We use the four aforementioned methods to measure OPAA, OPDTT, OPDCFH and OPEPR, and to extensively characterise the seasonal variation of aerosol OP in a megacity. All OP measurements show a significantly stronger correlation with PM2.5 mass in the winter compared to summer. Furthermore, the OPAA, OPDTT, OPDCFH and OPEPR were correlated using univariate and multivariate analysis with a variety of other measurements such as meteorological data, trace gas measurements and aerosol composition measurements including organic aerosol components and x-ray fluorescence elemental analysis. These results emphasise that the four OP methods applied in this study capture different aspects of aerosol OP between the seasons. As an example, OPAA normalised to account for aerosol mass show that aerosol OPAA in the winter is higher on average and more variable compared to the summer, whereas OPDCFH is more consistent between the winter and summer seasons. OPAA also showed a strong correlation with PM2.5 mass in the winter (r2 = 0.91) but correlated poorly in the summer months (r2 = 0.09), suggesting different aerosol components affect OPAA in summer and winter. Englert, N. Toxicol. Lett. 149, 235–242 (2004). Andersson, C., et al., Atmos. Environ. 43, 3614–3620 (2009). Øvrevik, J., et al., Biomolecules 5, 1399–1440 (2015).

Large-scale epidemiological studies have shown a close correlation between adverse human health effects and exposure to ambient particulate matter (PM). The oxidative potential (OP) of ambient PM has been implicated in inducing toxic effects associated with PM exposure. In particular, reactive oxygen species (ROS), either bound to PM or generated by particulate components in vivo, substantially contribute to the OP and therefore toxicity of PM by lowering antioxidant concentrations in the lung, which can subsequently lead to oxidative stress, inflammation, and disease. Traditional methods for measuring aerosol OP are labor intensive and have poor time resolution, with significant delays between aerosol collection and ROS analysis. These methods may underestimate ROS concentrations in PM because of the potentially short lifetime of some ROS species; therefore, continuous online, highly time-resolved measurement of ROS components in PM is highly advantageous. In this work, we develop a novel online method for measuring aerosol OP based on ascorbic acid chemistry, an antioxidant prevalent in the lung, thus combining the advantages of continuous online measurement with a physiologically relevant assay. The method limit of detection is estimated for a range of atmospherically important chemical components such as Cu(II) 0.22 ± 0.03 μg m-3, Fe(II) 47.8 ± 5.5 μg m-3, Fe(III) 0.63 ± 0.05 μg m-3, and secondary organic aerosol 41.2 ± 6.9 μg m-3, demonstrating that even at this early stage of development, the online method is capable of measuring the OP of PM in polluted urban environments and smog chamber studies.