the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2024
The contribution of transport emissions to ozone mixing ratios and methane lifetime in 2015 and 2050 in the Shared Socioeconomic Pathways (SSPs)
Abstract. We quantify the contributions of emissions from the transport sector to tropospheric ozone and the hydroxyl radical (OH) by means of model simulations with a global chemistry-climate model equipped with a source attribution method. For the first time we applied a method which allows to quantify also contributions to OH which is invariant upon disaggregation or recombination and additive. Based on these quantified contributions, we analyse the ozone radiative forcings (RF) and methane lifetime reductions attributable to emissions from the transport sectors. The contributions were analysed for each transport sector separately and for 2015 as well as for 2050 under the Shared Socioeconomic Pathways (SSPs) SSP1-1.9, SSP2-4.5, and SSP3-7.0. In line with previous publications using the source attribution approach, we quantify an ozone RF attributable to emissions from land transport, shipping, and aviation for the year 2015 of 121 mW m-2, 60 mW m-2, and 31 mW m-2, respectively. At the same time, we diagnose a relative reduction in methane lifetime due to transport emissions of 14.3 % (land transport), 8.5 % (shipping), and 3.8 % (aviation). These reductions are significantly larger as reported by previous studies due to the application of the source attribution method. Compared to 2015, only SSP1-1.9 shows a strong decrease in ozone RF and methane lifetime reduction attributable to the entire transport sector in 2050. For the projections SSP2-4.5, we find similar effects of the total transport sector as for 2015, while the effects in SSP3-7.0 increase compared to 2015. This small change of the effects for the two projections compared to 2015 is caused by two main factors. Firstly, aviation emissions are projected to increase in SSP2-4.5 (increase of 107 %) and SSP3-7.0 (+86 %) compared to 2015, resulting in a projected ozone RF of 55 mW m-2 (+78 %) and 50 mW m-2 (+61 %) for the year 2050 from aviation emissions. Secondly, the non-linear effects of atmospheric chemistry in polluted regions such as Europe and North America lead to rather small reductions in ozone and OH in response to emission reductions, especially from land transport emissions. In addition, the increase in emissions from land transport in other parts of the world, particularly in South Asia, leads to an increased contribution of ozone and OH. In particular, ozone formed by land transport emissions from South Asia causes a strong RF that partially offsets the reductions in Europe and North America. Moreover, our results show that besides the non-linear response, lack of international co-operations, as in the SSP3-7.0 projection, hinder mitigation of ground-level ozone.
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Status: open (until 28 Apr 2024)
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RC1: 'Comment on egusphere-2024-324', Anonymous Referee #1, 27 Feb 2024
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The study quantifies the contributions of emissions from the transport sector to tropospheric ozone and the hydroxyl radical (OH) using a global chemistry-climate model equipped with a source tagging method. The contributions are estimated for present-day level and several future scenarios.The novelty largely lies in the tagging techniques used to account for the non-linear source contribution to ozone and OH, as well as the analysis of different scenarios, which can provide new insights into controlling emissions from the transport sector. Overall, the study is well-designed and falls within the scope of ACP. I have a few suggestions.
1.Introduction: The rationale for emphasizing emissions from the transport sector in this study needs to be clarified. Is it due to the large scale of emissions from the transport sector, or is it because these emissions are expected to undergo significant changes in the future and across different scenarios, offering a potential means to mitigate air pollution?
2. While the EMAC model has been widely used, there is a need for an evaluation of the model results, particularly regarding ozone in the free troposphere, where it has a stronger radiative impact. Additionally, it is important to assess how well the EMAC model captures present-day OH levels and methane lifetime.
3. Figure 10 is an excellent illustration of the non-linear nature of ozone chemistry and the higher ozone production efficiency from aviation emissions. Would it be feasible to perform a comparable calculation for the radiative efficiency from land transport and aviation? Can we expect that aviation-emitted NOx has a significantly higher radiative efficiency, as indicated by Wang et al. (2022)?
Wang, H., et al. Global tropospheric ozone trends, attributions, and radiative impacts in 1995–2017: an integrated analysis using aircraft (IAGOS) observations, ozonesonde, and multi-decadal chemical model simulations, Atmos. Chem. Phys., 22, 13753–13782,
4. Section 4: While the impacts of NOx and ozone on OH are discussed (Line 535), changes in CO and VOCs emissions also influence OH and methane lifetime, yet they are not addressed in this section. This discussion should be included.
5. Figure caption in Figure 1: “Please not” should be “please note”?
Citation: https://doi.org/10.5194/egusphere-2024-324-RC1 -
CC1: 'Comment on egusphere-2024-324 - what use is tagging?', Michael Prather, 16 Mar 2024
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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 2nd-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
Citation: https://doi.org/10.5194/egusphere-2024-324-CC1 -
CC2: 'Comment on egusphere-2024-324', Owen Cooper, 15 Apr 2024
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This comment can be found in the attached pdf.
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RC2: 'Comment on egusphere-2024-324', Michael Prather, 16 Apr 2024
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To Mariano Mertens and fellow co-authors,
My apologies for what seemed like an intrusive, unnecessary community comment #1 on your manuscript. It was meant to be marked as a review that I was obligated to deliver. Editor Qiang Zhang asked me to review egusphere-2024-324 and I went through the process and downloaded the paper and finished reviewing it last week. Given the severity of the comments on tagged tracers, I recognized that it should be signed. I now see that it not a review comment because I forgot to click the accept box on the form. I stand by my "community comment #1 and submit it as my review.
My view remains that tagged tracers simply cannot deal with the chemical feedbacks that alter the lifetime of emissions for major gases like CH4 and O3. We have know it for 20+ years for CH4, and now it is clearly seen for O3. As expected from the chemical equations: a tagged CH4 has 40% too little impact on the atmosphere; and a tagged O3, like O3S, has about 4x too much impact.
Michael Prather
Citation: https://doi.org/10.5194/egusphere-2024-324-RC2 -
RC3: 'Comment on egusphere-2024-324', Anonymous Referee #3, 20 Apr 2024
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-324/egusphere-2024-324-RC3-supplement.pdf
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