Development and first application of a new online system for continuous measurement of oxidative potential

Audoux, Thomas; Delater, Ambre; Ghanem, Manuella; Lily, Jean-Baptiste; Cazaunau, Mathieu; Pangui, Edouard; Picquet-Varrault, Benedicte; Coll, Patrice; Perdrix, Esperanza; Alleman, Laurent; Rousset, Davy

doi:10.5194/egusphere-2026-4293

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https://doi.org/10.5194/egusphere-2026-4293

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30 Jul 2026

| 30 Jul 2026

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Development and first application of a new online system for continuous measurement of oxidative potential

Thomas Audoux, Ambre Delater, Manuella Ghanem, Jean-Baptiste Lily, Mathieu Cazaunau, Edouard Pangui, Benedicte Picquet-Varrault, Patrice Coll, Esperanza Perdrix, Laurent Alleman, and Davy Rousset

Abstract. This study introduces a new online system for measuring oxidative potential (OP), using the dithiothreitol (DTT) assay with a direct-to-reagent approach. The instrument enables real-time OP^DTT assessments with 5-minute resolution and a competitive instrumental detection limit (0.066 nmol_DTT min^-1). Its performance was evaluated using model compounds and under controlled experiments in the CESAM atmospheric simulation chamber. The system exhibited clear and consistent responses to major OP-active species, including quinones, copper, manganese, secondary organic aerosols (SOA), and black carbon. In chamber experiments conducted without photochemistry, the OP_v^DTT values remained consistently lower than during experiments involving photochemistry, despite the higher concentrations of metals and similar particulate masses. This finding underscores the critical role of photochemically produced species in driving OP. Overall, the instrument demonstrated its capacity to monitor OP_v^DTT at a high temporal resolution, paving the way for a deeper understanding of the influences on OP from emission sources and atmospheric processes, particularly transient ones.

Received: 17 Jul 2026 – Discussion started: 30 Jul 2026

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Thomas Audoux, Ambre Delater, Manuella Ghanem, Jean-Baptiste Lily, Mathieu Cazaunau, Edouard Pangui, Benedicte Picquet-Varrault, Patrice Coll, Esperanza Perdrix, Laurent Alleman, and Davy Rousset

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RC1:
'Comment on egusphere-2026-4293', Anonymous Referee #1, 10 Aug 2026 reply
This manuscript describes a new online DTT system based on a continuous-flow, direct-to-reagent design. Aerosols collected by the PILS are mixed with DTT shortly after collection, followed by reaction in a flow loop and continuous absorbance measurement. The system was characterized using several model compounds and then tested in the CESAM chamber under photochemical and non-photochemical conditions. I found the continuous-flow design interesting, particularly because it shortens the time between aerosol collection and reaction with DTT. However, several points regarding the method validation and interpretation of the chamber results need further clarification.
Major comments
The lack of an ambient field test is an important limitation. The chamber experiments are useful for showing that the complete system can follow dynamic changes in OP. However, the intended application of this instrument is clearly ambient measurement, while no outdoor measurements are presented here. Previous online DTT systems have already been operated under ambient conditions for several weeks.

I do not necessarily think that a new field campaign is required for this manuscript, but the authors should discuss this limitation more clearly. In particular, I would like to better understand whether the current sensitivity, baseline stability, reagent consumption, and potential matrix effects are expected to allow reliable measurements at typical ambient OP levels. At present, the manuscript sometimes reads as if field applicability has already been demonstrated, whereas the current study is more appropriately viewed as an initial laboratory/chamber evaluation.
The reported LOD needs some context when compared with previous instruments. The instrumental LOD of 0.066 nmol min-1 is based on 11 HEPA-filtered blank periods measured over two consecutive experimental days. This is a reasonable way to estimate short-term instrumental noise, but it is different from a field detection limit obtained from repeated blanks during a longer ambient deployment. The manuscript should make this distinction clear.

Also, the present assay uses a substantially lower DTT concentration than most previous online DTT systems. Since the authors themselves note that lower DTT concentration can increase analytical sensitivity, direct comparison of absolute LOD values among systems should be made cautiously. I would be interested to see a clearer discussion of what the reported LOD means for actual ambient measurements.
Some conclusions from the chamber experiments appear stronger than the experimental design supports. The chamber study is useful as a test of the instrument under a changing and chemically complex atmosphere. However, Mn, OA, BC, particle mass, and other species often changed at the same time. The photochemical and non-photochemical experiments also differed in more than just irradiation.

For this reason, I do not think the current data clearly demonstrate independent responses to Mn, SOA, or BC. In particular, the “BC” was generated from wood-pellet combustion and therefore represents a complex combustion aerosol, and the ACSM measurement of OA should not automatically be interpreted as SOA. Statements such as “clear and consistent responses,” “main driver,” or “directly attribute” should therefore be reconsidered. The results are convincing as time-resolved measurements of the integrated OP of evolving chamber mixtures, but component-specific attribution is less certain.
The calculation and interpretation of the continuous-flow DTT signal need a little more explanation. The present method differs from conventional DTT assays because it uses a fixed residence time rather than measuring the DTT decay slope at several reaction times. It would be helpful for the authors to explain more clearly why the endpoint signal can be treated as a DTT consumption rate across different samples.

I am also unclear about the air-volume normalization in Eq. (2). The 6.3 mL reaction loop and the combined liquid flow of 0.50 mL min-1 correspond to a nominal residence time of about 12.6 min, whereas the air volume is calculated using the 5 min system response time. Please provide a clearer explanation or mass-balance derivation showing why the 5 min response time is the appropriate quantity for the calculation.
Minor comments
Please clarify whether the OP time series in Figure 4 has been corrected for this delay before comparison with metals, OA, BC, and ozone.

The target reaction temperature was 37°C, but the measured temperature was around 30°C. The possible influence on DTT kinetics should be briefly discussed.

The reaction-loop volume is given as 6.3 mL in the text but appears as 6.5 mL in Figure 1. Please check for consistency.

It would be useful to report how frequently HEPA blanks were performed and how much valid sample data are lost during the blank and transition periods

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Citation: https://doi.org/10.5194/egusphere-2026-4293-RC1

Thomas Audoux, Ambre Delater, Manuella Ghanem, Jean-Baptiste Lily, Mathieu Cazaunau, Edouard Pangui, Benedicte Picquet-Varrault, Patrice Coll, Esperanza Perdrix, Laurent Alleman, and Davy Rousset

Supplement

https://doi.org/10.5194/egusphere-2026-4293-supplement

Thomas Audoux, Ambre Delater, Manuella Ghanem, Jean-Baptiste Lily, Mathieu Cazaunau, Edouard Pangui, Benedicte Picquet-Varrault, Patrice Coll, Esperanza Perdrix, Laurent Alleman, and Davy Rousset

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Short summary

Air pollution can affect human health, but not all particles are equally harmful. We developed a new instrument that measure the oxidative properties of the atmosphere continuously. Laboratory experiments showed that pollutants formed through sunlight-driven chemical reactions have a greater oxidative potential than particles containing higher levels of metals under night-time conditions. This will help us to better understand how rapid changes in air pollution may affect health.


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