the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Oct 2024
Influence of nitrogen oxides and volatile organic compounds emission changes on tropospheric ozone variability, trends and radiative effect
Abstract. Ozone in the troposphere is a prominent pollutant whose production is sensitive to the emissions of nitrogen oxides (NOx) and volatile organic compounds (VOC). In this study, we assess the variation of tropospheric ozone levels, trends, ozone photochemical regimes, and radiative effects using the ECHAM6–HAMMOZ chemistry-climate model for the period 1998–2019 and satellite measurements. The global mean simulated trend in Tropospheric Column Ozone (TRCO) during 1998–2019 is 0.89 ppb decade−1. The simulated global mean TRCO trends (1.58 ppb decade−1) show fair agreement with OMI/MLS (2005–2019) (1.4 ppb decade−1). The simulations for doubling emissions of NOx (DNOx), VOCs (DVOC), halving of emissions NOx (HNOx) and VOCs (HVOC) show nonlinear responses to ozone trends and tropospheric ozone photochemical regimes. The DNOX simulations show VOC–limited regimes over Indo-Gangetic Plains, Eastern China, Western Europe, and the eastern US, while HNOx simulations show NOx–limited regimes over America and Asia. Emissions changes in NOx (DNOx/HNOx) influence the shift in tropospheric ozone photochemical regimes compared to VOCs (DVOC/HVOC).
Further, we provide estimates of tropospheric ozone radiative effects (TO3RE). The estimated global mean TO3RE during 1998–2019 from the CTL simulations is 1.21 W m−2. The global mean TO3RE shows enhancement by 0.36 W m−2 in DNOx simulations than CTL. While TO3RE shows reduction in other simulations compared to CTL (DVOC: by -0.005 W m−2; HNOx: by -0.12 W m−2; and HVOC: by -0.03 W m−2). The impact of anthropogenic NOx emissions is higher on TO3RE than VOCs emissions globally.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3050', Anonymous Referee #1, 11 Nov 2024
Recommendation: Publication after major revision.
This study evaluates the variation in tropospheric ozone levels, trends, photochemical regimes, and radiative effects using the global chemistry-climate model ECHAM6–HAMMOZ and satellite data from 1998 to 2019. It examines how anthropogenic emissions of nitrogen oxides and volatile organic compounds affect ozone production. The global trend in tropospheric ozone is increasing, with simulations showing strong agreement with satellite data, which is in accordance with previous studies. The study also explores how changes in pollution emissions impact ozone trends and photochemical regimes. Doubling emissions of nitrogen oxides and volatile organic compounds leads to different ozone trends compared to halving these emissions, with region-specific responses observed in different parts of the world.
The manuscript explores an important and timely topic regarding the climate and health impacts of tropospheric ozone, which holds great relevance. To further enhance the clarity and impact of the work, however, it is recommended revisiting some language elements to improve readability. Additionally, some of the figures would benefit from improvements in quality; they appear somewhat small, with low resolution, and the axis labels and legends could be more legible.
Content-wise, there are several key points that also need to be revised to bring the manuscript to publication standard. Addressing those specifically will be essential to enhance the clarity and impact of the results:
- The introduction lacks structure. Model and observational data from the literature are presented in a mixed way, as are global and regional findings. Chemical symbols such as NOx, NOy, and VOC need to be defined or explained right from the beginning. In addition, the abbreviations used for the sensitivity experiments are not well chosen, as they could be mistaken for names of chemical species.
- The manuscript does not specify which emission scenario from ACCMIP (Representative Concentration Pathway - RCP) is used. A rationale or justification for the selection of sensitivity experiments regarding NOx and VOC emissions is currently missing, and providing this context would strengthen the study’s approach. Are there real-world examples for this? What exactly is being investigated beyond the well-known fact that these are the primary drivers of tropospheric ozone?
- It would be important to differentiate regionally more when describing the relationships between NOx and VOC development. In addition, a more detailed discussion of the effects of different VOC species would also be helpful. And, it is worth considering whether natural VOC emissions in different geographical regions might play a significant role. Despite their considerable contribution natural VOCs are not addressed at all.
- It is well-known that temperature and humidity have a significant impact on the life cycle of ozone. The most obvious explanation for the positive ozone trend - climate warming - is not discussed in this study, which is a major shortcoming.
- 5. The conclusions section is more of a listed summary of findings rather than a true conclusion. What would be the interpretation of your findings, for example regarding current and future mitigation measures in a warming climate and changing natural sources?
Citation: https://doi.org/10.5194/egusphere-2024-3050-RC1 - CC1: 'Comment on egusphere-2024-3050', Owen Cooper, 10 Dec 2024
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RC2: 'Comment on egusphere-2024-3050', Anonymous Referee #2, 26 Feb 2025
The manuscript by Fadnavis et al. presents an analysis of transient simulations of ozone over 1998 – 2019 simulated by the ECHAM6-HAMMOZ model. To assess the simulations, the authors compare absolute amounts and trends for tropospheric column amounts of ozone, NO2 and HCHO with a number of satellite observations and find reasonable agreement. The analysis focuses on a set of four sensitivity experiments where the emissions from anthropogenic sources of either NOx or VOCs are alternatively increased by 100% or decreased by 50%. The response of simulated surface ozone and tropospheric column from these four sensitivity experiments are used to calculate whether photochemical generation of ozone in different regions around the globe is limited by the availability of NOx or VOCs, and the derived sensitivity is then used to calibrate the transition between NOx and VOC sensitivity given by the widely used ratio of HCHO/NO2. The authors also present estimates of the radiative effect of tropospheric ozone (TO3RE) and the changes in TO3RE across the different sensitivity simulations.
Given the horizontal resolution of typical global chemistry climate models I would expect significant limitations in being able to resolve the full spectrum of NOx and VOC sensitivity that would occur in a particular region. While that is a limitation that will affect this work, it is also true that chemistry climate models are widely used to understand interactions of climate change and air quality, and project future regional or global scale concentrations of ozone. On that consideration I find the research presented here to be an interesting addition.
My most significant concern is the focus on deriving NOx and VOC sensitivity using, what I must assume are, annual average fields of ozone and the change in ozone in response to imposed emission changes. Given how different the response to NOx and VOC changes would be between summer and winter, why were the results not broken down by season? The winter season at mid-latitudes is a period with very weak photochemical activity and very limited ozone photochemical production. A significant part of the chemistry in the winter at mid-latitudes is dominated by ozone titration near regions with strong emissions of NOx, followed by NOx oxidation and removal. I am not completely sure how one derives NOx and VOC sensitivity under conditions with only very weak local ozone photochemical production. Given how weak ozone photochemical production is in the winter the analysis of annual averages really dilutes and, arguably, confounds the separation into NOx or VOC limited regimes. I believe the authors must rework the analysis to separate the summer season in each hemisphere from the full year.
While not as central to the main findings of the paper, for the comparison of HCHO and, especially, NO2 vertical columns with satellite observations, was time of day accounted for? The Boersma et al. (2016) paper mentioned in the manuscript does investigate different strategies for comparing models and measurements of total column amounts from UV-Vis instruments, but all the different cases investigated in Boersma et al. included matching time of day. From the text it would appear that full-day averages were used from the model to compare with the satellite observations.
Additional minor comments are given below.
Lines 39 – 41: ‘The global mean simulated trend in Tropospheric Column Ozone (TRCO) during 1998 – 2019 is 0.89 ppb decade−1. The simulated global mean TRCO trends (1.58 ppb decade−1) show fair…’ The text here is confusing because there are two near identical references to the TRCO trend but with two different values given. Reading on a bit, I assume the second trend (1.58 ppb per decade) is for the 2005 – 2019 period of the OMI/MLS observations but the text should be clearer.
Lines 54 – 55: ‘The impact of anthropogenic NOx emissions is higher on TO3RE than VOCs emissions globally.’. Given how different VOCs and NOx are I am not sure how one can interpret this statement. Is it per unit mass of emissions or a fractional perturbation? And, given how non-linear the chemistry is, I would worry about comparing the effects of NOx and VOCs when the perturbations are as large as they are here.
Lines 62 – 63: ‘short-term climate forcer’, I think, is more widely accepted as ‘short-lived climate forcer’.
Line 80: To avoid confusion IAGOS should really be ‘In-service Aircraft for a Global Observing Network’
Lines 98 – 100: The reference to Archibald et al. (2020) is to the TOAR paper on tropospheric ozone budget and burden, but the text supported by the reference discusses only UKESM1 results. Should the reference be to Archibald et al. Description and evaluation of the UKCA stratosphere–troposphere chemistry scheme (StratTrop vn 1.0) implemented in UKESM1. Geosci Model Dev 13: 1223–1266?
Line 117 – 118: I am not sure it is accurate to suggest that in a NOx-limited regime there is ‘no impact from VOC perturbations’. Only that the ozone production is more sensitive to changes in the concentration o NOx than for changes in VOCs.
Line 518: The year 1955 in ‘during 1955–2017 over South and East Asia’ should be 1995.
Lines 572 – 573: ‘The surface ozone trend shows a large negative trend over Europe and South Asia, while a positive trend over the US, China, and Australia (Fig. 5g).’ It is not always clear if the trends in the sensitivity experiments, shown in Figure 5, are the trends in the experiment or the difference in the trend between the sensitivity experiment and the control. Phrases like lines 572 – 573 seem to suggest it is the trend calculated directly from the sensitivity experiment which can be compared with the trend in the control. But other places, such as the titles on the panels of Figure 5, the text seems to imply it is the difference in trends that is plotted and discussed.
Lines 577 – 579: ‘Similarly, the trend from anomalies of NOx in the HNOx-CTL simulations is positive over the US and Europe while negative over India and China (Fig. S1a-b).’ Is this because in the control simulation NOx emissions are decreasing in the US and Europe, but increasing in India and China? Everything else remaining the same, decreasing NOx emissions by 50% will have produced a weaker trend, but of the same sign as the underlying trend. But I am not completely in agreement with the argument that is being made here. The trend of NOx emissions in the HNOx simulation would still be negative over the US and Europe, just not as negative as in the control simulation. And the resulting trend in ozone may have been negative as well, only not as negative as in the control simulation. So it is not quite an accurate representation of the situation to state (Lines 579 -580) ‘The strong positive trend in both VOC and NOx might have resulted in the observed positive trend in surface ozone over the US…’ because there is not necessarily a positive trend in surface ozone in the HNOx simulation, but a less negative trend. This confusion connects with the comment on Lines 572 – 573, that it is not clear what quantities are being shown in Figure 5 and discussed here.
Lines 656 – 662: The box and whisker plot shows the range of emissions over different regions. Are these emission fluxes at the grid resolution of ECHAM or from a higher resolution dataset such as the original ACCMIP dataset? The figure caption should state this.
Lines 770 – 771: I am having trouble seeing how the definition of NOx limited is formulated correctly with d[O3]/dENOx in ‘for the conditions d[O3]/dENOx < 0) (NOx limited) and (d[O3]/dENOx > d[O3]/dEVOC > 0) (VOC–limited) (Fig. 8b)’. For the NOx limited case, an increase in NOx emissions should increase ozone, dENOx> 0 results in d[O3] > 0 giving d[O3]/dENOx >0. Likewise, for NOx limited conditions, a decrease in NOx emissions (dENOx<0) should produce a decrease in ozone (d[O3]<0) so that d[O3]/dENOx > 0 as well. Reading a bit more, I see the cases are reversed in the Figure 8 caption, which may be the problem here.
Lines 971 – 974: The caption for Figure 12 needs a brief description of what each panel is presenting.
Lines 1054 – 1055: Can the statement ‘The minor differences in the estimated global mean TO3RE from the model and satellites are due to different time periods of observations/simulations.’ be supported? The TO3RE for 1998 – 2019 is 1.21 W/m^2, which is very close to the observational estimates. I would think moving towards the period of the satellite observations (2008 – 2017) would result in a larger value of TO3RE since TRCO has a positive trend over the 1998 – 2019 period.
Citation: https://doi.org/10.5194/egusphere-2024-3050-RC2
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