The open review process under ACP is a great opportunity to have a fair and public discussion of the core element of this paper:  the concept of tagging of chemical species like O3 that has been developed by Volker Grewe and his colleagues. 

First, in terms of review, this paper does an excellent job of calculating the global impacts of three different SSP scenarios with the MESSY model.  That alone wit a little more documentation of the current model is publishable. 

Where I have a problem is with the tagging methodology.  I had to re-read the core Mertens 2020 paper (Atmos. Chem. Phys., 20, 7843–7873) to try again to understand why one would want tagging versus sensitivity studies.  At the time, that 2020 paper had some difficulty in convincing the reviewers of the usefulness of tagging for a chemical system in which there are many feedbacks (as for CH4 and O3). The Mertens Table 1 helped explain the difference between sensitivity and tagging, but it did not give me confidence in the usefulness of the tagging "attribution" value.  I am not sure that 100% of the O3 in the troposphere must be attributed to something.  If this is a misreading, please let me know.  My point of view is that given the indistinguishable nature of O3 molecules – from whichever source – labeling such a molecule is simply not useful.  The essence of any calculation for policy options should be simply what happens if a policy is invoked.  For that purpose, I can understand how sensitivity runs give the correct answer, but tagging may or may not.

As a lesson, one can look at the idea of labeling/tagging CH4.  If one emits a Tg of CH4 and colors it uniquely, we find it decays with the lifetime timescale (e.g., 9 yr), but if we model CH4 as a whole, we find that 99.5% of that Tg perturbation decays with the perturbation time scale (e.g., 12 yr).  Well, our colored/tagged CH4 does decay in 9 yr because the perturbation to OH is small, but the remaining atmospheric methane responds to the added Tg and alters the abundance of the untagged CH4, so as to make the overall perturbation decay in 12 yr.  Thus, the colored/tagged CH4 does not represent the system behavior, nor even the attributable response to the emission. This example is one of the fundamentals of atmospheric chemistry that we had to develop to “explain” the seemingly nonsensical behavior of a small CH4 perturbation, and it is why a linear attribution that sums to 100% is troublesome to me.

, Deconstruction of tropospheric chemical reactivity using aircraft measurements: the Atmospheric Tomography Mission (ATom) data, Earth Syst. Sci. Data, 15, 3299–3349, doi:10.5194/essd-15-3299-2023For O3, the situation is the same, but in opposite sense.  With the ATom data (), we showed that increases in O3 result in significant drops in production (log sensitivity = -0.4) in addition to increased loss (log sensitivity ~ 1).  Thus, an O3 perturbation reduces net P-L for all tropospheric O3 and the perturbation decays much faster than expected (opposite to CH4).  We recently showed that the impact of the stratosphere-troposphere exchange (STE) flux of O3 was much less than expected because of these chemical feedbacks (2024. Lifetimes and timescales of tropospheric ozone. Elem. Sci. Anth., 12: 1. doi: 10.1525/elementa.2023.00112).  In particular, the use of tagged O3S tracers for attributing the role of STE in tropospheric O3 is found to be mistaken because the simple, linear loss does not include the reduced production for O3 of tropospheric 'origin'.  The idea that an O3 molecule has an origin is flawed.  The O3S tagged tracer is typically 30-40% of tropospheric O3, but the perturbation to tropospheric O3 caused by the total STE O3 flux is only about 8%. 

The authors are very worried about the non-linear O3-NOx relationship, but that is exaggerated since most of the results here are far from the pollution centers where the NOx-limited vs VOC-limited issues are fought.  One of the Mertens 2020 reviews notes that "the response of ozone to perturbation of precursor emissions in remote regions has been shown to be approximately linear," which I believe is true.  Production of O3 is almost linear in NOx over the oceans as found in ATom.  What we have globally for O3 and CH4 are chemical feedbacks caused by the non-linearity of chemistry – specifically, the reaction of two species always has 2^nd-order Taylor expansion terms that produce a Jacobian with off-diagonal elements, which give us timescales that differ from lifetimes and indirect greenhouse gases (i.e., NOx and CO alter the CH4 timescale). 

My review is not intended to prevent publication of this manuscript in ACP, but I would like to be able to understand how tagging helps us understand how to alter emissions to produce a better result in such a coupled world. 

Michael Prather

This comment can be found in the attached pdf.