Preprints
https://doi.org/10.5194/acp-2021-483
https://doi.org/10.5194/acp-2021-483

  08 Jul 2021

08 Jul 2021

Review status: this preprint is currently under review for the journal ACP.

The number fraction of iron-containing particles affects OH, HO2 and H2O2 budgets in the atmospheric aqueous phase

Amina Khaled, Minghui Zhang, and Barbara Ervens Amina Khaled et al.
  • Université Clermont Auvergne, CNRS, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, 63000 Clermont-Ferrand, France

Abstract. Reactive oxygen species (ROS), such as OH, HO2, H2O2 affect the oxidation capacity of the atmosphere and cause adverse health effects of particulate matter. The role of transition metal ions (TMIs) in impacting the ROS concentrations and conversions in the atmospheric aqueous phase has been recognized for a long time. Model studies usually assume that the total TMI concentration as measured in bulk aerosol or cloud water samples is distributed equally across all particles or droplets. This assumption is contrary to single-particle measurements that have shown that only a small number fraction of particles contain iron and other TMIs (FN,Fe < 100 %) which implies that also not all cloud droplets contain TMIs. In the current study, we apply a box model with an explicit multiphase chemical mechanism to simulate ROS formation and cycling in (i) aqueous aerosol particles and (ii) cloud droplets. Model simulations are performed for the range of 1 % ≤ FN,Fe ≤ 100 % for constant pH values of 3, 4.5 and 6 and constant total iron concentration (10 or 50 . Model results are compared for two sets of simulations with FN,Fe < 100 % (FeN < 100) and 100 % (FeBulk). We find largest differences between model results in OH and HO2/O2 concentrations at pH = 6. Under these conditions, HO2 is subsaturated in the aqueous phase because of its high effective Henry's law constant and the fast chemical loss reactions of the O2 radical anion. As the main reduction of process of Fe(III) is its reaction with HO2/O2, we show that the HO2 subsaturation leads to predicted Fe(II)/Fe(total) ratios for FN,Fe < 100 % that are lower by a factor of ≤ 2 as compared to bulk model approaches. This trend is largely independent of the total iron concentration, as both chemical source and sink rates of HO2/O2 scale with the iron concentration. The chemical radical (OH, HO2) loss in particles is usually compensated by its uptake from the gas phase. We compare model-derived reactive uptake parameters γ(OH) and γ(HO2) for the full range of FN,Fe. While γ(OH) is not affected by the iron distribution, the calculated γ(HO2) range from 0.0004 to 0.03 for FN,Fe = 1 % and 100 %, respectively. Implications of these findings are discussed for the application of lab-derived γ(HO2) in models to present reactive HO2 uptake on aerosols. As the oxidant budget in aerosol particles and cloud droplets is related to the oxidative potential, we also conclude that the iron distribution FN,Fe should be taken into account to estimate the ROS concentrations and health impacts of particulate matter that might be overestimated by bulk sampling and model approaches. Our study suggests that the number concentration of iron-containing particles may be more important than the total iron mass concentration in determining ROS budgets and uptake rates in cloud and aerosol water.

Amina Khaled et al.

Status: open (until 19 Aug 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Amina Khaled et al.

Amina Khaled et al.

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Short summary
Chemical reactions with iron in clouds and aerosol form and cycle reactive oxygen species (ROS). Previous model studies assumed that all cloud droplets (particles) contain iron, while single-particle analyses showed otherwise. By means of a model, we explore the bias in predicted ROS budgets by distributing a given iron mass to either all or only a few droplets (particles). Implications for oxidation potential, radical loss and iron oxidation state are discussed.
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