From the journal Environmental Science: Atmospheres Peer review history

Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions

Round 1

Manuscript submitted on 10 May 2021


Dear Dr Lewis:

Manuscript ID: EA-PER-05-2021-000037
TITLE: Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions.

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Reviewer 1

This is a well-written, highly pertinent, and timely perspective piece. Future hydrogen use will involve combustion, and this will generate NOx, with implications for air quality. This has typically been overlooked in past analyses of impacts from future hydrogen economies. Policymakers should be aware that optimal NOx standards for hydrogen combustion should be set sooner rather than later, and that they should be suitably ambitious, in order to improve air quality.

My only substantial, but relatively minor, comment is that when the climate impacts of NOx are discussed (line 112), in addition to tropospheric ozone formation, the wider perturbations to atmospheric chemistry from NOx also need to be mentioned. NOx emissions alter (typically increase) OH and hence reduce the lifetime of methane, and hence its concentration and climate impact. The latest major IPCC report on climate change (Myhre et al., 2013; Section found that it was difficult to be definitive about whether the net impact of NOx emissions on climate was warming or cooling.
If this and the few minor typos noted below are fixed, I am very happy to recommend publication.

L94: higher -> high
L203: travels -> travel
L223: emissions NOx -> NOx emissions


Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forcing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Reviewer 2

Review of “Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-
specific standards for NO x emissions.”
This paper attempts to answer an interesting question that is whether the use of hydrogen is of significant side-effects due to the thermal formation of nitrogen oxides (NO x – the sum of NO + NO 2 ), which is harmful in its own right as a key air pollutant and constitutes to a precursor to other pollutants of concern such as fine particulate matter and ozone. As summarized in the paper, the authors suggested that use of hydrogen combustion appliances within existing frameworks for air quality emissions control will almost certainly mean that NO x emissions will not get worse even with a widespread adoption, but it also mean they do not improve either. The also suggested that it is possible that other air pollutant emissions arising from hydrogen are better than fossil fuel equivalents, for example of carbon monoxide or directly emitted particular matter, and these benefits would need to be included in various cost-benefit analyses of new energy. As a result, the authors concluded that the optimal time to develop those standards is before a net zero hydrogen roll-out has become substantially advanced and major investments in new manufacturing and infrastructure become locked-in. The major points in this paper are correct and deserves notices in the development of hydrogen energy.
I recommend that this perspective can be published if they can clearly explain how they obtained the three curves in Figure 1.


Response to reviewers.

I would like to thank both reviewers for their very positive comments and have included their suggestions and corrections in a revised version.

Reviewer 1.

As requested the uncertain impacts of NOx on changing OH budgets and methane lifetime are now included, including the IPCC reference suggested. Text now added:

At line 115:

“The contribution of NOx to the formation of ozone and wider atmospheric oxidant levels also has climatic effects. In the mid and upper troposphere ozone is a short-lived climate forcer which leads to warming, however the presence of NOx can also generate higher concentrations of hydroxyl radicals (OH), which shorten the lifetime of methane via atmospheric oxidation. The IPCC 5th Assessment Report from 2013 considered it uncertain whether NOx emissions led overall to a warming or a cooling effect21.”

The three minor typographic errors highlighted are now corrected.

Reviewer 2.

Text now added to explain the source of the Figure:

At line 174:

“These trade-offs can be summarised in an illustrative schematic for an internal combustion engine use of H2, in Figure 1. This figure shows a stylised behaviour of NOx emissions, temperature and efficiency, drawn from previously published real-world test-bed data – e.g. reference 31 for a hydrogen internal combustion engine. Similar curves of NOx emissions and power output can be found for an aircraft gas turbine engine in reference 24.”

Round 2

Revised manuscript submitted on 01 Jun 2021


Dear Dr Lewis:

Manuscript ID: EA-PER-05-2021-000037.R1
TITLE: Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NOx emissions.

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form.

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With best wishes,

Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by sister journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at:


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