the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Mar 2024
Tropospheric Ozone Precursors: Global and Regional Distributions, Trends and Variability
Abstract. Ozone formation is nonlinear, and results from the photochemical oxidation of methane and non-methane hydrocarbons (NMHCs) in the presence of nitrogen oxide (NOx=NO+NO2). Previous studies showed that O3 short- and long-term trends are nonlinearly controlled by near-surface anthropogenic emissions of carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides. In this review article, we investigate tropospheric ozone spatial variability and trends from 2005 to 2019 and relate those to ozone precursors on global and regional scales. We also investigate the spatiotemporal characteristics of the ozone formation regime in relation to ozone chemical sources and sinks. Our analysis is based on remote sensing products of the Tropospheric Column of Ozone (TrC-O3) and its precursors, nitrogen dioxide (TrC-NO2), formaldehyde (TrC-HCHO), and total column of CO (TC-CO) as well as ozonesonde data and model simulations. Our results indicate a complex relationship between tropospheric ozone column levels, surface ozone levels, and ozone precursors. While the increasing trends of near-surface ozone concentrations can largely be explained by variations in VOC and NOx concentration under different regimes, TrC-O3 may also be affected by other variables such as tropopause height. Decreasing trends in TrC-NO2 have varying effects on the TrC-O3, which is related to the different local chemistry in each region. The concomitant increase or decrease in TrC-O3 and TrC-NO2 over the eastern US, and central Europe is due to dominant NO-sensitive conditions resulting from the strict measures to control NOx emissions over the last two decades. The decreasing trends of TrC-NO2 but increasing trends of TrC-O3 in some regions in the central US and parts of eastern Asia are due to high NOx conditions leading to VOC sensitivity in these regions. We also shed light on the contribution of NOx lightning and soil NO and nitrous acid (HONO) emissions to trends of tropospheric ozone on regional and global scales.
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RC1: 'Comment on egusphere-2024-720', Anonymous Referee #1, 18 Apr 2024
This is a very detailed paper using a large amount of satellite, ozonesonde and modelling data to investigate the precursor drivers to the trends in total column tropospheric ozone between 2005-2019 on regional and global scales. The use of NO2, VOC, HCHO tropospheric column data is novel for this type of analysis, is within the scope of ACP and should be published subject to a slight restructure and few minor corrections.
General comments
It is unclear to me the relevance of the model analysis for different parts of the troposphere (Figure 5) if it isn’t referred to in the later interpretation of the measurements or in the conclusion. There is some clear parallel in the model analysis where the contribution and trends are separated by region (Figure 11) but can this be brought into the discussion more and perhaps section 3,3. (and Figure 5,11) be moved to after the measurements are presented (3.4.4, 3.4.5, 3.4.6, 3.4.7 ). A summary could then also include a discussion about which parts of the column are driving the trends in different regions of the globe, otherwise why include the model information? Are the model trends inline with the measurements? Please include some reference to the model findings in the precursor measurement discussions and conclusions.
The figures are a little confusing. It would make sense to use similar figures for each of the ozone precursors to allow easier interpretation. The discussion on CO trends includes more plots (including anomaly trend plots) for which the other precursor species there is a summary (also nice but maybe don’t need both). Can the plots used be the same for all species in sections 3.4.4, 3.4.5, 3.4.6, 3.4.7, either anomaly time-series or summary of anomalies?
As stated above Figure 5 perhaps relates more to the O3 discussion and should sit with Figure 11 as they are related (contribution to each part of the column from model and actual trend). Also perhaps Figure 4 should sit later with Figure 12 (see later specific points).
I think it is just a misunderstanding on my part but there should be a consistency with the time period that you are discussing, for the most part the time period 2005-2019 is shown but the main O3 trend figure 6 is possibly until 2021. There is also some reference to COVID years and this seems a bit out of place in the paper unless related to the above point?
Specific comments
Figure 2: Please include time range (2005-2019) in the figure caption.
Line 263: Use ‘peroxyl’ not ‘proxy’.
Line 272: Self referencing, can you choose other references for these fundamental reactions?
Lines 281-282 and Figure 4: NO2 column is stated to be decreasing in North America, Europe and Australia. Please include a reference for this relating to air pollution controls or alternatively Figure 4 should show the trend of NO2 rather than the mean if you are referring to it? The trends are shown later in Figure 12 so discussion about them should be there unless you have another reference for it?
Section 3.2, lines 290-295 generally needs more references when discussing sources from certain regions, not just ‘e.g.’ in brackets.
Figure 5: Model shows lower, middle, upper differences in precursor contribution. At this point the reader is interested in the trends within the column and whether any specific part of the troposphere has been increasing/decreasing over this time and in which regions, actually shown later in Figure 11. Can these be put together, perhaps in a later discussion?
Figure 6: What timescale are the trends over, 2005-2019 or 2005-2021?
Figure 7: Does the right plot show the ‘zonal mean trend’ of column depths or should the caption say ‘mean O3’ by latitude for different column depths for 2005-2019?
Figure 9: Can latitude of the observations be included on this plot?
Line 431: What is a partial column, is it defined?
Line 547: Include reference for biomass burning activity
Figure 16: This is a nice figure showing average CO anomalies in column mean 2005-2021 but with trends only until 2019 which is good. Can we have similar plots for NO2?
Figure 18, Summary of trends, for HCHO, why don’t we have this type of plot for CO?
Figure 24: Monthly anomalies of HONO from soil by region, please include trend data on these.
Citation: https://doi.org/10.5194/egusphere-2024-720-RC1 - AC1: 'Reply on RC1', Yasin Elshorbany, 14 Jun 2024
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RC2: 'Comment on egusphere-2024-720', Anonymous Referee #2, 22 Apr 2024
This manuscript describes global and regional trends of tropospheric ozone and its precursors (NO2, CO, and HCHO) and aims to investigate the spatio-temporal characteristics of the ozone formation regime. Satellite observations of the tropospheric column have been used as the main data source, while ozone sonde data and GEOS-GMI model simulations have also been analyzed to study the sub-columns. The topic is central to the scope of the journal, and the trend analysis from OMI/MLS seems sufficiently valid.
However, particularly for the part analyzing ozone formation regimes, I find the authors’ discussion rough and even flawed: my main argument is that without considering 1) the trends of "long-range transported" ozone, 2) the seasonality, or 3) the major components from lower/middle/upper tropospheric sub-columns that drive the trends (although treated in the model for Figure 11), the assessment of the regimes is not correct. For example, in lines 652-653, it is doubtful to conclude that most regions in the southern hemisphere are VOC sensitive regions (lines 365 and 652), simply from the fact that the trends of O3 and HCHO are decreasing while the trend with NO2 is increasing. In the atmospheric chemistry theory, VOC-limited conditions must occur where NOx is abundant (and thus OH loss is controlled by its reaction with NO2), which is unlikely for "most regions in the southern hemisphere". Another crude statement is made in lines 343-344, that the positive trends in the 30-60degS band are mainly driven by oceanic emissions, without any supporting results. All other parts discussing regimes need to be reviewed and reconsidered.
The discussion on the TrC-HCHO/TrC-NO ratio (section 3.4.7) seems to be the opposite. If the ratio decreases, the chemical status must be becoming more VOC sensitive (rather than NOx sensitive, line 675). The increasing trend should indicate more NOx sensitive conditions (rather than ROx sensitive, line 678). I also did not understand why the positive emission trends with soil HONO in the southern hemisphere lead to a decrease in O3 (line 1074). Overall, I could not see that the authors have a good understanding of ozone chemistry.
When presenting the satellite data used in Section 2.2.1, the authors need to describe what kind of data screening was applied, in particular with respect to cloud fraction and solar zenith angle. It is also necessary to describe which emission inventory was used for the GEOS-GMI model simulations has to be described (leading to an erroneous positive CO trend over East Asia).
In section 3.2, only the first authors papers are cited. It is needed to provide a more balanced citation.
Considering the importance of understanding the chemical regimes and providing valid information for the abatement strategy (line 141), I do not recommend publication of this manuscript in its current form.
Citation: https://doi.org/10.5194/egusphere-2024-720-RC2 - AC3: 'Reply on RC2', Yasin Elshorbany, 14 Jun 2024
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CC1: 'Comment on egusphere-2024-720', Owen Cooper, 02 May 2024
This comment can be found in the attached pdf.
- AC2: 'Reply on CC1', Yasin Elshorbany, 14 Jun 2024
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