Abstract. The adverse health effects associated with ambient aerosol particles have been well documented, but it is still unclear which aerosol properties are most important for their negative health impact. Some studies suggest the oxidative effects of particle-bound reactive oxygen species (ROS) are potential major contributors to the toxicity of particles. Traditional ROS measurement techniques are labour-intensive, give poor temporal resolution and generally have significant delays between aerosol sampling and ROS analysis. However, many oxidising particle components are reactive and thus potentially short-lived. Thus, a technique to quantify particle-bound ROS online would be beneficial to quantify also the short-lived ROS components.We introduce a new portable instrument to allow online, continuous measurement of particle-bound ROS using a chemical assay of 2 7 -dichlorofluorescein (DCFH) with horseradish peroxidase (HRP), via fluorescence spectroscopy. All components of the new instrument are attached to a containing shell, resulting in a compact system capable of automated continuous field deployment over many hours or days.From laboratory measurements, the instrument was found to have a detection limit of ∼ 4 nmol [H 2 O 2 ] equivalents per cubic metre (m 3 ) air, a dynamic range up to at least ∼ 2000 nmol [H 2 O 2 ] equivalents per m 3 air and a time resolution of ≤ 12 min. The instrument allows for ∼ 16 h automated measurement if unattended and shows a fast response to changes in concentrations of laboratory-generated oxidised organic aerosol. The instrument was deployed at an urban site in London, and particulate ROS levels of up to 24 nmol [H 2 O 2 ] equivalents per m 3 air were detected with PM 2.5 concentrations up to 28 µg m −3 .The new and portable Online Particle-bound ROS Instrument (OPROSI) allows fast-response quantification; this is important due to the potentially short-lived nature of particlebound ROS as well as fast-changing atmospheric conditions, especially in urban environments. The instrument design allows for automated operation and extended field operation with twice-daily presence of an operator. As well as having sensitivity suitable for ambient level measurement, the instrument is also suitable at concentrations such as those required for laboratory and chamber toxicological studies.

Abstract. The chemical composition of organic aerosols influences their impacts on human health and the climate system. Aerosol formation from gas-to-particle conversion and in-particle reaction was studied for the oxidation of limonene in a new facility, the Cambridge Atmospheric Simulation Chamber (CASC). Health-relevant oxidising organic species produced during secondary organic aerosol (SOA) formation were quantified in real time using an Online Particlebound Reactive Oxygen Species Instrument (OPROSI). Two categories of reactive oxygen species (ROS) were identified based on time series analysis: a short-lived component produced during precursor ozonolysis with a lifetime of the order of minutes, and a stable component that was longlived on the experiment timescale ( ∼ 4 h). Individual organic species were monitored continuously over this time using Extractive Electrospray Ionisation (EESI) Mass Spectrometry (MS) for the particle phase and Proton Transfer Reaction (PTR) MS for the gas phase. Many first-generation oxidation products are unsaturated, and we observed multiphase aging via further ozonolysis reactions. Volatile products such as C 9 H 14 O (limonaketone) and C 10 H 16 O 2 (limonaldehyde) were observed in the gas phase early in the experiment, before reacting again with ozone. Loss of C 10 H 16 O 4 (7-hydroxy limononic acid) from the particle phase was surprisingly slow. A combination of reduced C=C reactivity and viscous particle formation (relative to other SOA systems) may explain this, and both scenarios were tested in the Pretty Good Aerosol Model (PG-AM). A range of characterisation measurements were also carried out to benchmark the chamber against existing facilities. This work demonstrates the utility of CASC, particularly for understanding the reactivity and health-relevant properties of organic aerosols using novel, highly time-resolved techniques.

Review for "Multiphase composition changes and reactive oxygen species formation during limonene oxidation in the new Cambridge Atmospheric Simulation Chamber (CASC)" by Peter J. Gallimore et al. General comments: this manuscript showed interesting results about the online composition changes of gas and particle phase products during the photolysis of limonene by using mass spectrometry. Meanwhile, they also measured the reactive oxygen species (ROS) formation by limonene SOA in water by using a fluorescent assay. Based on these experiments and mathematic modelling, the authors claimed that C1

Abstract. The adverse health effects associated with ambient aerosol particles have been well documented, but it is still unclear which aerosol properties are most important for their negative health impact. Some studies suggest the oxidative effects of particle bound reactive oxygen species (ROS) are potential major contributors to the toxicity of particles. Traditional ROS measurement techniques are labour intense, give poor temporal resolution, and generally have significant delays between aerosol sampling and ROS analysis. However, many oxidizing particle components are reactive and thus potentially short lived. Thus, a technique to quantify particle-bound ROS online would be beneficial to quantify also the short-lived ROS components. We introduce a new portable instrument to allow on-line, continuous measurement of particle-bound ROS using a chemical assay of 2'7'-dichlorofluorescein (DCFH) with horseradish peroxidase (HRP), via fluorescence spectroscopy. All components of the new instrument are attached to a containing shell, resulting in a compact system capable of automated continuous field deployment over many hours to days. From laboratory measurements, the instrument was found to have a detection limit of ~4 nmol[H2O2]equivalents per m3 air, a dynamic range up to at least ~2000 nmol[H2O2]equivalents per m3 air, and a time resolution under 12 minutes. The instrument allows for ~12 hours automated measurement if unattended, and shows a fast response to changes in concentrations of laboratory-generated oxidised organic aerosol. The instrument was deployed at an urban site in London and particulate ROS levels of up to 24 nmol[H2O2]equivalents per m3 air were detected with PM2.5 concentrations up to 28 μg m-3. The new and portable On-line Particle-bound ROS Instrument (OPROSI) allows fast-response quantification; this is important due to the potentially short-lived nature of particle-bound ROS as well as fast changing atmospheric onditions, especially in urban environments. The instrument design allows for automated operation and extended field operation. As well as having sensitivity suitable for ambient level measurement, the instrument is also suitable at concentrations such as those required for toxicological studies.

Supplementary material for "Multiphase composition changes and reactive oxygen species formation during limonene oxidation in the new Cambridge Atmospheric Simulation Chamber (CASC)" by P.J. Gallimore et al.