Investigation of the summer 2018 European ozone air pollution episodes using novel satellite data and modelling
Abstract. In the summer of 2018, Europe experienced an intense heat wave which coincided with several persistent large-scale ozone (O3) pollution episodes. Novel satellite data of lower tropospheric column O3 from the Global Ozone Monitoring Experiment-2 (GOME-2) and Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp satellite showed substantial enhancements in 2018 relative to other years since 2012. Surface observations also showed ozone enhancements across large regions of continental Europe in summer 2018 compared to 2017. Enhancements to surface temperature and the O3 precursor gases carbon monoxide and methanol in 2018 were co-retrieved from MetOp observations by the same scheme. This analysis was supported by the TOMCAT chemistry transport model (CTM) to investigate processes driving the observed O3 enhancements. Through several targeted sensitivity experiments we show that meteorological processes, and emissions to a secondary order, were important for controlling the elevated O3 concentrations at the surface. However, mid-tropospheric (~500 hPa) O3 enhancements were dominated by meteorological processes. We find that contributions from stratospheric O3 intrusions ranged between 15–40 %. Analysis of back trajectories indicates that the import of O3-enriched air masses into Europe originated over the North Atlantic substantially increasing O3 in the 500 hPa layer during summer 2018.
Richard J. Pope et al.
Status: final response (author comments only)
- RC1: 'Comment on acp-2022-827', Anonymous Referee #1, 22 Feb 2023
- RC2: 'Comment on acp-2022-827', Anonymous Referee #2, 02 Mar 2023
Richard J. Pope et al.
Richard J. Pope et al.
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Pope et al. use a combination of satellite measurements, surface observation, and modeling to investigate the severity and the drivers of ozone pollution during the heatwave over Europe in 2018. They find that meteorological processes were mostly responsible for the increased surface ozone, with emissions changes playing a smaller role. Increases in ozone at 500 hPa was associated with increased ozone transport from the stratosphere and the troposphere above the North Atlantic.
Heatwaves are getting more frequent and severe with climate change and it is important to understand its effect on air quality. This paper addresses this issue but has significant shortcomings, as described below.
1. The study uses "novel" satellite observations of tropospheric ozone from the GOME-2 and IASI instruments, but the manuscript has few details about the retrievals. While the GOME-2 retrieval have been described in Miles et al., 2015, a few more details about it in the manuscript would be useful. I could not find details of the IASI ozone retrievals in the cited manuscripts (Pope et al., 2021, Palmer et al., 2022). It is important to discuss the vertical sensitivity of the retrievals (averaging kernel profiles), its the vertical resolution in the troposphere, how the retrieval compares to independent (ozonesonde) observations, etc.
2. The model used for investigating the ozone pollution episodes in 2018 does not seem well-suited for this application. In particular, it fails to capture the observed increase in ozone in 2018 compared to 2017, suggesting that the sensitivity of the model to emission or meteorological charges is too low. Also, a 2.8°x2.8° horizontal resolution seems too coarse for regional modeling.
3. The back trajectory analysis to determine changes in ozone transport is confusing and unconvincing. The back trajectories are weighted by the modeled ozone to determine the amount of advected ozone. However, the frequency with which air arriving at a receptor site passes a particular location does not seem to be accounted for. Also, the choice of Paris and Berlin as receptor sites seems arbitrary. How does the back trajectory model handle convective transport from the boundary layer to the free troposphere? I wonder if it would be much easier just to derive the net influx of ozone into Europe from the 3D chemical transport model using the ozone concentrations and the wind fields.
4. Could the authors include in a discussion of what the results of this work imply for our understanding of ozone air pollution during heat waves? The results, as presented currently, seem to apply only to the particular episode in the summer of 2018, which limits the scientific significance of the work. Do any of the results apply more broadly?
1. The introduction lacks a discussion of previous work on this topic.
2. Line 100: "cost function less than 200" - This needs some context.
3. Line 104: How were the 83 sites selected out of the hundreds of sites in the EMEP network?
4. Line 129: Please clarify that the fixed emission run includes fixed biogenic VOC emissions.
5. How do biogenic and lightning NOx emissions differ between the two years?
6. Line 211: Please verify whether GOME-2 actually has peak sensitivity in the lower troposphere. Miles et al. 2015, which describes the GOME-2 retrieval, states that it peaks at 500 hPa. It would be useful to include the averaging kernel profiles for GOME-2 and IASI retrievals in the supplement.
7. Line 242: Do you see any spatial consistency between the charges observed at the surface and from the satellite data?
8. Line 248: Again, I do not think that the tendency of the model to underestimate ozone charges between 2018 and 2017 can be brushed aside.
9. Section 3.4 can be considerably shortened. Much of text just repeats what is shown in the figures. Better to emphasize the main takeaways instead.
10. Line 455: The influence of stratospheric ozone intrusions is shown to be higher in 2018 than in 2017. Is this a coincidence or are more stratospheric intrusions linked to meteorological processes associated with heat waves? Also, this does not seem to be supported by the back trajectory analysis, which do not show any trajectories originating from the stratosphere.
11. Figure 5: What do the dotted lines show?
12. Figure 5: Wouldn't MDA8 ozone be a better metric for this?
Miles, G.M., Siddans, R., Kerridge, B. J., Latter, B. G., and Richards, N. A. D.: Tropospheric ozone and ozone profiles retrieved from GOME‐2 and their validation, Atmospheric Measurement Techniques, 8, 385–398, doi:10.5194/amt-8-385-2015, 2015.
Palmer, P., I., Marvin, M., R., Siddans, R., et al.: Nocturnal survival of isoprene linked to formation of upper tropospheric organic aerosol, Science, 375 (6580), 562-566, doi:10.1126/science.abg4506, 2022.
Pope, R. J., Kerridge, B. J., Siddans, R., et al.: Large enhancements in southern hemisphere satellite-observed trace gases due to the 2019/2020 Australian wildfires, Journal of Geophysical Research: Atmospheres, 1–13, doi:10.1029/2021jd034892, 2021.