Analysis of inorganic chlorine chemistry in the midlatitude summer stratosphere using aircraft and satellite observations

Wilmouth, David; Hare, Jennifer; Howar, Laila; Salawitch, Ross; Hintsa, Eric; Smith, Jessica; Sayres, David; Anderson, James; St. Clair, Jason; Delaria, Erin; Hannun, Reem; Hanisco, Thomas; Czizco, Daniel; Shen, Xiaoli; Homeyer, Cameron; Santee, Michelle; Bui, T. Paul; Newman, Paul; Keutsch, Frank; Bowman, Kenneth

doi:10.5194/egusphere-2026-3480

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https://doi.org/10.5194/egusphere-2026-3480

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25 Jun 2026

| 25 Jun 2026

Analysis of inorganic chlorine chemistry in the midlatitude summer stratosphere using aircraft and satellite observations

David Wilmouth, Jennifer Hare, Laila Howar, Ross Salawitch, Eric Hintsa, Jessica Smith, David Sayres, James Anderson, Jason St. Clair, Erin Delaria, Reem Hannun, Thomas Hanisco, Daniel Czizco, Xiaoli Shen, Cameron Homeyer, Michelle Santee, T. Paul Bui, Paul Newman, Frank Keutsch, and Kenneth Bowman

Abstract. In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO₂). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO₂ from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO₂, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO₂ is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO₂ production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO₂ + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.

How to cite. Wilmouth, D., Hare, J., Howar, L., Salawitch, R., Hintsa, E., Smith, J., Sayres, D., Anderson, J., St. Clair, J., Delaria, E., Hannun, R., Hanisco, T., Czizco, D., Shen, X., Homeyer, C., Santee, M., Bui, T. P., Newman, P., Keutsch, F., and Bowman, K.: Analysis of inorganic chlorine chemistry in the midlatitude summer stratosphere using aircraft and satellite observations, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-3480, 2026.

Received: 14 Jun 2026 – Discussion started: 25 Jun 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.

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RC1:
'Comment on egusphere-2026-3480', Anonymous Referee #1, 17 Jul 2026
Reviewer Report on egusphere-2026-3480
Title: Analysis of inorganic chlorine chemistry in the midlatitude summer stratosphere using aircraft and satellite observations

Authors: David M. Wilmouth et al.

This manuscript presents a valuable and carefully compiled dataset of high-precision in situ aircraft measurements of ClO and ClONO₂ from 29 research flights during the two-year NASA DCOTSS mission. The precision achieved by the HAL instrument is unprecedented for this altitude range and enables analyses that satellite observations alone cannot support. This study indicates that no significant chlorine activation was observed despite targeted sampling of both convectively influenced and biomass-burning-influenced air masses, which is scientifically important and provides a valuable observational constraint on previous modeling studies. The agreement between measured and steady-state calculated ClONO₂ also provides an important quantitative test of stratospheric chlorine photochemistry. Overall, the manuscript is well written, and the conclusions are generally well supported by the observations. The comments below concern clarification, interpretation, or additional discussion rather than substantial new analyses. I support publication in Atmospheric Chemistry and Physics subject to minor revisions.
Major Comments
In Section 3.3, it would be helpful to report more explicitly whether the sampled convective events actually reached the temperature, aerosol conditions, and water vapor levels under which the Anderson et al. (2012, 2017) mechanism predicts chlorine activation, and to compare these conditions directly with those assumed in the cited modeling studies. This would help clarify whether the absence of observed activation reflects a genuine absence of the proposed mechanism, insufficiently extreme sampled conditions, or differences in reaction timescales relative to those assumed in the models.

In Section 3.4, Figure 5b is fitted using ordinary least-squares regression, which assumes negligible uncertainty in the x-axis. However, both measured and steady-state calculated ClONO₂ carry substantial uncertainties (approximately 20% and 45%, respectively), making an errors-in-variables regression (e.g., York or Deming regression) more appropriate. I recommend repeating this fit using such a method before suggesting that the recommended value of k_ClO+NO2 may be biased high, and briefly discussing whether uncertainties in the NO₂ measurement or the JClONO₂ calculation could also contribute to the observed discrepancy.

In Section 3.5, the HCl values are derived from a regional-mean MLS HCl:O₃ relationship and applied to all DCOTSS observations. Please discuss whether this relationship remains valid for the convectively influenced air masses that are central to this study, where chlorine partitioning may differ from the regional background. As the same calculated HCl is subsequently used in the Cl_y budget analysis (Fig. 7b), please clarify the extent to which this budget closure represents an independent validation of the ClO and ClONO₂ measurements rather than a self-consistency check. This concern is amplified by the fact that HCl accounts for over 95% of total inorganic chlorine, and both the calculated HCl and the independently derived Cl_y are expected to correlate strongly with O₃ through their shared dependence on air mass age.

In Section 3.6, the negative correlations shown in Fig. 8 may largely reflect the common dependence of both chlorine species and aerosol composition on potential temperature (i.e., age of air), as the authors note. I recommend accounting for this confounding effect, for example by examining residuals after removing the θ dependence or by presenting the Cl_y-normalized comparison currently shown in Supplement Fig. S5 as the primary analysis in the main text. Such analyses would provide a more rigorous test of whether biomass-burning aerosol independently influences chlorine partitioning.

Also in Section 3.6, the Solomon et al. (2023) mechanism specifically concerns aged organic-rich biomass-burning aerosol. Where available please provide additional information on the transport history or processing state of the sampled biomass-burning air masses and discuss how representative these air masses are of the aged smoke plumes for which the mechanism was originally proposed. This would help clarify how broadly the reported null result can be generalized.

Minor Comments
The anomalously high ClONO₂ values from three May 2022 flights (lines 241–247) are attributed to "large-scale dynamics" rather than convection, but no trajectory or potential vorticity analysis is presented to support this attribution. Please provide supporting evidence or soften the claim.

The aerosol composition analysis (Fig. 8) uses only the 16 flights from 2022 with PALMS-NG data. This limitation should be stated earlier and more prominently in the main text rather than only in the figure caption.

Please clarify the specific division of analysis and conclusions between this manuscript and the companion paper (Howar et al., 2025), as the latter is cited repeatedly to explain the absence of observed chlorine activation. Readers would benefit from a clearer statement of the distinct contribution of each paper.
Citation: https://doi.org/10.5194/egusphere-2026-3480-RC1
RC2: 'Comment on egusphere-2026-3480', Anonymous Referee #2, 27 Aug 2026

Reviewer Report on egusphere-2026-3480
Analysis of inorganic chemistry in the midlatitude summer stratosphere using aircraft and satellite observations
Wilmouth et al.
General comments
The manuscript by Wilmouth et al. presents unprecedented airborne data of the inorganic chlorine species chlorine monoxide and chlorine nitrate from the DCOTSS mission covering two stratospheric summer periods over Northern America. No chlorine activation was observed in the surrounding of convective injections and also no inorganic chlorine activation in wildfire influenced air masses. Steady-state calculated ClONO2 agreed well with measured ClONO2. Further, HCl was calculated along the flight track using satellite data. The sum of ClO, HCl and ClONO2 showed good agreement with the total inorganic chlorine, calculated by independently measured organic chlorine species. By covering the different aspects of the inorganic chlorine species, the manuscript adds relevant knowledge to the research community. I support publication in Atmospheric Chemistry and Physics after minor revisions.

Specific comments
L429: “interpolating between the three available MLS pressure levels” Do you mean linear interpolation or exponential decay or something else?
L487/488: “… and expected differences between the studies due to different latitudes being sampled,” Would also different seasons and years play into the differences?
L490: “Cly determined from organics might be biased low at low mixing ratios.” Could detection limits play a role in here?
L588-594: How severe had been the fire seasons in 2021 and 2022 in Northern America? The ANY fires had for example strong overshooting convection? Do you mean this with “significant interannual variability in stratospheric conditions” (L590)?

Technical corrections
L187: suggestion for easier reading and understanding for people outside US: “southern IL” to “southern Illinois”
Figure 5: in 5a the x-axes range from 50° to >110° SZA, in the caption and the text (L365) you state filtered for SZA >= 60°. Please unify
Figure 6b: please enlarge the dots in the legend, that different colours are easier to identify per molecule. Additionally, cross check also other Figures. Additionally, I would recommend to not use red and green in one plot, where possible, to make it more readable for people with colour-blindness.

Citation: https://doi.org/10.5194/egusphere-2026-3480-RC2

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Short summary

Inorganic chlorine chemistry in the stratosphere is extensively examined over two summers using high precision in situ aircraft data and satellite data. Measured values both agree and enable refinement to current understanding. Chlorine measurements obtained under perturbed stratospheric conditions of enhanced water vapor from convective storms or aged organic aerosol from wildfires do not appear significantly different from chlorine measurements obtained in the background stratosphere.


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