the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Global sensitivities of reactive N and S gas and particle concentrations and deposition to precursor emissions reductions
Yao Ge
Massimo Vieno
David S. Stevenson
Peter Wind
Abstract. The reduction of fine particles (PM2.5) and reactive N (Nr) and S (Sr) species is a key objective for air pollution control policies because of their major adverse effects on human health, ecosystem diversity, and climate. The sensitivity of global and regional Nr, Sr, and PM2.5 to 20 % and 40 % individual and collective reductions in anthropogenic emissions of NH3, NOx, and SOx (with respect to a 2015 baseline) is investigated using the EMEP MSC-W atmospheric chemistry transport model with WRF meteorology. Regional comparisons reveal that the individual emissions reduction has multiple co-benefits and small disbenefits on different species, and those effects are highly geographically variable. Reductions in NH3 emissions are effective at decreasing NH3 concentrations and deposition but much less so for NH4+. A 40 % NH3 emissions reduction decreases regional average NH3 concentrations by 47–49 %, while sensitivities of NH4+ concentrations decrease in the order Euro_Medi (Europe and Mediterranean, 18 %), East Asia (15 %), North America (12 %), and South Asia (4 %), reflecting the increasing regional ammonia-richness. A disbenefit is the increased SO2 concentrations in these regions (10–16 % for 40 % NH3 emissions reductions) because reduced NH3 levels decrease SO2 deposition by altering atmospheric acidity. The 40 % NOx emissions reductions decrease NOx concentrations in East Asia by 45 %, Euro_Medi and North America by ~38 %, and South Asia by 22 %, whilst decreases in fine NO3- are regionally reversed, which is related to enhanced O3 levels in East Asia (and also, but by less, in Euro_Medi), and decreased O3 levels in South Asia (and also, but by less, in North America). Consequently, the oxidation of NOx to NO3- and of SO2 to SO42- is enhanced in East Asia but decreased in South Asia, which in East Asia causes a more effective decrease in NOx and SO2 but a less effective decrease in NO3- and even an increase in SO42-; in South Asia it causes a less effective decrease in NOx and an increase in SO2 but a more effective decrease in NO3- and SO42-. For regional policy making, it is thus important to reduce NH3, NOx and SOx emissions together and/or go for stronger reductions to minimise such adverse effects in East Asia and Euro_Medi. Reductions in SOx emissions are slightly more effective for SO2 than SO42-. A disbenefit is that SOx emissions reductions increase NH3 total deposition and ecosystem eutrophication (~12 % increase for 40 % emissions reduction). PM2.5 mitigation in South Asia is most sensitive to 40 % SOx reduction (3.10 μg m-3, 10 %) and least sensitive to NH3 reduction (0.29 μg m-3, 1 %), which is because South Asia is so ammonia-rich that reducing NH3 has little impact. The most effective measure for North America is reducing NOx emissions with an 8 % (0.63 μg m-3) decrease in PM2.5 in response to a 40 % reduction. In Euro_Medi, the sensitivities of PM2.5 to 40 % individual emissions reductions range 5–8 % (0.55–0.82 μg m-3). In the UK and Scandinavia PM2.5 is more sensitive to NH3, in central Europe it is more sensitive to NOx, while in the Mediterranean it is more sensitive to SOx. In East Asia, reductions in SOx, NOx and NH3 emissions are almost equally effective with PM2.5 sensitivities to 40 % reductions of 7–8 % (1.89–2.33 μg m-3). Due to the varying contributions of SIA, PM2.5 sensitivities to 40 % collective reductions in all 3 precursors decrease in the order East Asia (20 %), Euro_Medi and North America (17 %), South Asia (13 %). The geographically-varying non-linear chemical responses of Nr, Sr, and PM2.5 to emissions reductions revealed by this work show the importance of both prioritising emissions strategies in different regions and combining several precursor reductions together to maximise the policy effectiveness.
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Yao Ge et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-657', Anonymous Referee #1, 20 Nov 2022
Fine particles (PM2.5) and reactive N (Nr) and S (Sr) are important air quality mitigation targets due to their negative effects on human health, ecosystem diversity, and climate. This study uses the EMEP MSC-W model coupled with WRF meteorology for 2015 to explore the sensitivity of global and regional Nr, Sr, and PM2.5 to 20% and 40% reductions in anthropogenic emissions of NH3, NOx, and SOx. They found the multiple co-benefits and small disbenefits of individual emissions on different species are highly geographically variable, which reveals the importance of prioritizing emissions strategies and order of precursor reduction. The subject matter is appropriate for Atmospheric Chemistry and Physics. The manuscript is also generally well-written, but I recommend a major revision before the paper is accepted for publication.
General comments:
The paper systematically describes an evaluation of PM and N/S concentrations and deposition sensitivity to precursor emissions, but overall, I feel a too weak connection with any empirical evidence:
- Although the paper is already very long, I suggest adding a brief section and 1-2 new figures that describe a “base” scenario that shows the simulated concentrations of related chemical species compared against available observations.
- Such a 20-40% emission reduction occurred in many parts of the world, for example, the United States/Canada, EMEP, and East Asia regions. There are also long-term air quality observations available in these regions. How are your sensitivity results compared with this observational evidence?
The abstract and conclusion sections need to be significantly shortened, less repetitive, and with a higher-level summary. The results section is generally too long, while the discussion section is too short and weak. The discussion should be expanded significantly with subsections. The research implication of the article is not emphasized enough, giving the readers the impression that it is a purely technical report. Besides, the discussion on the sources of uncertainty needs to be elaborated in more detail.
The equation to calculate the sensitivity and the definition of sensitive areas need to be described in more detail within the method section. Why are only these four regions analyzed? Why not include more regions in the world, such as Middle/South America, Africa, and the Middle East? These regions may have sensitive ecosystems, emerging economic growth, and also large populations. The omission of these regions needs better reasoning.
Specific comments:
- Line 49-50. There is a grammatical error, consider changing “also its form” to “also by its form”.
- Line 57-58. This sentence is vague. It is not enough to support your point on emphasizing the global dominance of emissions in East and South Asia.
- Line 89-95. The description of the single reference cited here is too long, you may consider summarizing it.
- Line 99-100. Consider changing “global” to “the global”.
- Line 175-178. These sentences may be moved to the discussion section.
- Line 367. The method of the specific definition of sensitivity regime through the precursor includes sensitivities of NH3, NOx, and SOx, which could be elaborated in your method section.
- Line 382. It seems a bit obscure to be a separate paragraph here, are you trying to explain the sensitivity of SOx in the marine area?
- Line 514-515. This sentence may be not proper here, please consider moving it to the discussion section.
- Line 717. Is it possible that the significant seasonality in these few areas is related to the vegetation on the land surface?
- Line 737-738. Consider elaborating and expanding it by citing some related references.
- Line 778-779. This sentence seems a bit vague. Please check it.
12 . Line 789-790. There are several grammatical errors here. Please check and rewrite it.
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AC1: 'Reply on RC1', Yao GE, 01 Feb 2023
We thank the reviewer for their time spent reading our manuscript and for their recommendation for publication upon addressing their comments. In the attached response file, we include all the reviewer’s comments and provide in blue text our point-by-point responses. Please note that the line numbers mentioned in our responses refer to the clean revised manuscript (not the track-changed version).
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RC2: 'Comment on acp-2022-657', Anonymous Referee #2, 06 Dec 2022
The manuscript presented a suite of model sensitivity simulations, which investigated the global responses of atmospheric reactive nitrogen and sulfur levels and their deposition fluxes to their emission reductions. Global 20% and 40% emission reductions for NH3, NOx, and SO2 (individually and jointly) were simulated to analyze the resulting changes in global nitrogen and sulfur concentrations and deposition, as well as annual mean PM2.5 concentrations. The results show that the individual emissions reduction (NH3, NOx, or SO2) has multiple co-benefits and small disbenefits on different species. Such changes reflect geographically-varying non-linear chemical responses of nitrogen, sulfur, and PM2.5 to emissions reductionsI think the manuscript, by comprehensively analyzing the response of multiple indicators (air pollutants levels and deposition fluxes), includes important findings to understand the underlying drivers or mechanisms in response to emission reductions regionally and globally. This study is thus well-suitable for the scope of ACP, and I would like to see its publication. Before that, some moderate comments shall be addressed.
Specific comments:
1) Model set-up
Global 20% and 40% emission reductions were applied in this study. Some sentences are needed to justify the selection of 20% and 40%. Why a greater emission reduction (i.e., 60%) was not considered?2) Section 3.1.2, Line 260
What are the sources of fine and coarse nitrate aerosol? Can you explain why coarse nitrate would increase associated with NH3 emission reduction? Please clarify.3) Page 18, Figure 8
Figure 8 showed the spatial sensitivity regimes based on 40% emission reductions and annual mean PM2.5 concentrations. Such sensitivity regimes shall have large seasonal variations. Do you have the model datasets to generally the seasonal maps? That shall provide valuable information to understand the SIA formation regimes.4) For the green/red circles and stars in Figures 2, 4, 6, 9, and 11, in the main text, the symbols that are discussed as the maximum reductions (e.g., Page 11, Line 330-335) were denoted as “Min” in these figures. Please be consistent.
5) Page 25, Line 708-710
It is not clear what “non-linearity” mean here in the text. We can see from Figures 3, 5, 7, and 10, the responses with respect to 20% vs. 40% emission reductions are rather linear. They deviate from the 1:1 line, however, the responses are linear. Please clarify.6) The writing of the manuscript is rather intensive. Many results are described in parallel, which makes the manuscript less focused. I understand that many results can be derived from the set of sensitivity simulations, still, the key findings of the study shall be better emphasized in the abstract and conclusions.
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AC2: 'Reply on RC2', Yao GE, 01 Feb 2023
We thank the reviewer for their time spent reading our manuscript and for their recommendation for publication upon addressing their comments. In the attached response file, we include all the reviewer’s comments and provide in blue text our point-by-point responses. Please note that the line numbers mentioned in our responses refer to the clean revised manuscript (not the track-changed version).
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AC2: 'Reply on RC2', Yao GE, 01 Feb 2023
Yao Ge et al.
Yao Ge et al.
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