the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Influence of energetic particle precipitation on Antarctic stratospheric chlorine and ozone over the 20th century
- 1Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, Norway
- 2Risk Management Solutions, London, UK
- 3Department of Physics, University of Otago, Dunedin, New Zealand
- 1Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, Norway
- 2Risk Management Solutions, London, UK
- 3Department of Physics, University of Otago, Dunedin, New Zealand
Abstract. Chlorofluorocarbon (CFC) emissions in the latter part of the 20th century reduced stratospheric ozone abundance substantially, especially in the Antarctic region. Simultaneously, polar stratospheric ozone is also destroyed catalytically by nitrogen oxides (NOx = NO + NO2) descending from the mesosphere and the lower thermosphere during winter. These are produced by energetic particle precipitation (EPP) linked to solar activity and space weather. NOx and active chlorine (ClOx = Cl + ClO) also react mutually and transform both reactive agents into reservoir gas chlorine nitrate, which buffers ozone destruction by both NOx and ClOx. We study the interaction between EPP produced NOx, ClO and ozone over the 20th century by using free running climate simulations of the chemistry-climate model SOCOL3-MPIOM. Substantial increase of NOx descending to polar stratosphere is found during winter, which causes ozone depletion in the upper and mid-stratosphere. However, in the Antarctic mid-stratosphere the EPP induced ozone depletion becomes less efficient after 1960s, especially during springtime. Simultaneously, significant decrease in stratospheric ClO and increase in chlorine nitrate between 10–30 hPa can be ascribed to EPP forcing. Hence, interaction between NOx and ClO likely suppressed the ozone depletion due to both EPP-NOx and ClO at these altitudes. Furthermore, at the end of the century significant ClO increase and ozone decrease is obtained at 100 hPa altitude during winter and spring. This lower stratosphere response is likely due to activation of chlorine from reservoir gases on polar stratospheric clouds. Our results show that EPP has been a significant modulator of reactive chlorine in the Antarctic stratosphere during the CFC era. With the implementation of the Montreal Protocol, stratospheric chlorine is estimated to return to pre-CFC era levels after 2050. Thus, we expect increased efficiency of chemical ozone destruction by EPP-NOx in the Antarctic upper and mid-stratosphere over coming decades. The future lower stratosphere ozone response by EPP is more uncertain.
Ville Maliniemi et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-151', Anonymous Referee #1, 31 Mar 2022
This paper investigates the influence of energetic particle precipitation on ozone and chlorine in the SH stratosphere over the 20th century by means of SOCOL3-MPIOM chemistry climate model simulations. EPP-induced NOx increases and associated ozone decreases were found to be in agreement with results of previous studies. A new finding is that EPP also induces substantial ClO decreases in the upper and mid stratosphere which reduces the ozone-depleting efficiency of EPP. In the lower stratosphere, EPP-induced ClO increases and ozone decreases were obtained at the end of the century while the opposite occurred before the period of high chlorine load. These results suggest a significant modulation of EPP-induced ozone loss by atmospheric chlorine which has implications for the future evolution of polar stratospheric ozone. This is a relevant topic and the paper is certainly suitable for publication in ACP.
The paper is well written, however, it fails short in convincingly identifying the chemical processes that are responsible for the EPP-induced ClO changes. Regarding the upper atmospheric response, the authors note in the abstract that the ClO decreases go along with increases in chlorine nitrate. A closer look at the absolute changes in the chlorine partitioning, however, suggests that most of the ClO is converted into HCl rather than into ClONO2.
Regarding the lower stratospheric response, the only explanation for the encountered ClO response is that "ClO is increased by activation of chlorine from the reservoirs" in the presence of PSCs. This is well known but does not explain why the ClO increase is enhanced by EPP. A possible reason for enhanced chlorine activation under EPP could be that the ClONO2-limited heterogeneous processing on PSCs in the Antarctic lower stratosphere is accelerated by the availability of more NOx and hence ClONO2.
It is also unclear why the lower stratospheric ClO response changes sign around the 80ths with increasing chlorine load. Is it possible that associated ozone depletion alters the Cly partitioning which could then modulate the ClO and O3 responses? Low ozone favors HCl formation and reduces ClONO2 by increasing the NO/NO2 ratio through the NO+O3 reaction (which then increases the rate of the ClO+NO reaction).
In summary, a more detailed analysis of the chorine partitioning in absolute terms (i.e. by use of line plots of the seasonal evolution of ClO, HCl, and ClONO2 from both EXP and REF simulations at 10 and 100 hPa levels for both low and high chlorine load conditions) would be very useful for identifying the responsible processes which, in turn, would significantly enhance the strength of this paper.
Therefore, I strongly encourage the authors to address these points in a revised version, together with the specific comments listed below.
l162-163: I agree that the ClONO2 decrease under EPP in mid-winter, seen in Fig. 8, suggests a Cly partitioning in favor of HCl by reaction R9. However, there is essentially no NO in the dark polar mid-winter stratosphere which could react with ClO. Although there could be a minor NO contribution from the the sunlit region, it is still striking that the ClONO2 decrease occurs in mid-winter and not in spring when sunlight (and hence NO) is available in the entire region.
l168-169: Webster et al. (1993) looked at an Arctic winter which might not be representative for the Southern hemisphere. In any case, an explanation about *how* EPP reduces the HCl amount is missing.
l180 "ClO-ClO catalytic cycle". Maybe "ClO dimer cycle" is more common.
l181ff / Fig. 9d: The EXP-RED TCO difference is negative throughout the winter/spring. This is in contradiction to the observational results of Gordon et al. (2021) who showed a TCO increase during SH spring (Oct-NOv) in high EPP years.
l190-193: The change of sign in the ClO response around the 80s is particularly interesting in Fig 10b. However, this is not discussed in the manuscript.
l193-198: It is unclear how the discussion on GCR/EEP/SPE helps to understand the negative ozone response in the last two decades of the century. All these types of EPP produce NOx. The key questions are: Why is the lower stratospheric ClO response positive at the end of the century (it is negative in the middle and upper stratosphere...)? Why does it change sign with the onset of enhanced chlorine load?
l198: "ClO seen in the lower stratosphere". Do you mean "ClO response seen in the lower stratosphere"?
l213: "is more than expected". Do you mean "is more than unexpected"?
l221: "We propose that this ClO increase can be explained by activation of chlorine from reservoir species ClONO2 and HCl." This is well known. What needs to be explained here is the positive ClO *response to EPP* after 1980.
l231: What do you mean with "ideal simulations"? Idealized model experiment? If yes, what kind of experiments?
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RC2: 'Comment on acp-2022-151', Anonymous Referee #2, 02 Apr 2022
This paper investigates the impact of energetic particle precipitation (EPP) forcing of chlorine species, and its consecutive impact on EPP-NOx driven stratospheric ozone loss. Ensemble model runs with and without EPP are carried out over the whole 20th century (1900-2008), a period with high solar activity and high chlorine loading in the second half of the 20th century. The impact of particle precipitation on NOy, HOx and ozone in the middle atmosphere has been studied in detail in a number of publications, but analyses of the impact of EPP on chlorine species are rare; the publication thus provides a new aspect. Of particular note is their observation that the high chlorine loading apparently had an impact on stratospheric ozone loss due to EPP, presumably by restricting both NOx- and ClOx-driven catalytic cycles due to the reaction of ClO with NO2. The inference is that in the coming decades, when the atmospheric chlorine loading will decrease, EPP ozone loss via NOx catalytic cycles will likely become more efficient. The paper is generally very well written, and the conclusions appear sound. However, conclusions could become more robust with a few more analyses, see suggestions below. Also it seems to me that the EPP ClOx mostly transfers into HCl, not ClONO2, and a more detailed discussion of this, and of possible pathways, would be useful.
Lines 54-56, R8 and R9: include and discuss pathways of HCl formation in the introduction, as this appears to be important as well: HOCl + Cl --> HCl + ClO; HO2 + ClO --> HCl + O3; OH + ClO --> HCl + O2, anything else? This works via an increase in HOx; EPP HOx is available during the particle precipitation in the (upper) mesosphere, but also possibly due to storage of EPP NOx and EPP HOx in the form of HNO3, which is transported down into the stratosphere during winter and there slowly photolyses, releasing both NOx and HOx (Verronen and Lehmann, GRL, 2015)
Line 89: wouldn’t it be more exact to use only data from REF for the estimation of the significance? Than (EXP-REF) would be tested against the variability of REF, which seems to be more to the point.
Line 104: maybe you could say a few more words about the content and meaning of Fig 2.
Figure 3 – for better readability, please include ticks for 10 hPa and 0.1 hPa for the vertical axis. Same for Fig 4 and following.
Line 117: if the mesospheric ozone depletion is due solely to in-situ EEP HOx production, why is it stronger in the Southern hemisphere? Doesn’t the difference between Northern and Southern hemisphere imply a dynamical/long-lived component in the mesospheric ozone depletion as well? Possibly HNO3 formation/photolysis?
Line 120-121 and following discussion of lower stratosphere ozone anomaly: the positive ozone anomaly covers nearly the whole lower stratosphere, from high Southern to high Northern latitudes, with the exception of the polar winters, when the anomaly turns sign. I mid-and low-latitudes, this positive anomaly is interpreted by the authors as a GCR impact (line 123), and this appears likely. However, I would argue based on the spatial/temporal evolution of this signal that this GCR signal extends from pole to pole, but is overwritten by the auroral signal indirect effect during polar winter.
Line 129: Are these corresponding to the negative NOx anomalies? And, is there a corresponding anomaly of ClONO2?
Line 135: Over hundred percent --> more than a hundred percent
Line 143: … as can be seen in Fig 2a --> by comparing with the Ap index shown in Fig 2a. However, it would be better to provide some hard numbers here to substantiate this statement, e.g., by providing a correlation coefficient (preferably from some ordered, non-linear method – rank? – not Pearson) between NOx and Ap.
Fig 7 b and e, as well as following figures – can you provide error bars due to ensemble variability for the timeseries?
Fig 7 c and f: the lines appear to be anti-correlated – are they? E.g., provide correlation coefficient
Line 147: Loss of … as this is a decrease relative to the reference scenario, I wouldn't call it "loss", which would imply chemical loss
Line 151: seems to be anticorrelated --> just provide the correlation coefficient
Line 154: response --> response to EPP forcing
Lines 154-156: and possibly because NOx is bound in PSCs in the form of HNO3?
Line 158-159: Substantial increase compared to what – EXT to REF, or to the beginning of the model periods?
Line 161: the ClONO2 amount is not negative in your model runs (one hopes), it is less than in the REF scenario without EPP.
Line 163: also strengthened by the fact that the ClONO2 difference in absolute numbers seems to be much smaller than the HCl difference. I think you could explore this in more detail. Do you really think this is due mainly to CH4 + Cl?
Line 173: again, just provide a correlation coefficient
Figure 8 c and f: are the lines anticorrelated? Are they correlated/anticorrelated to the Ap index shown in Fig 2a?
Line 215: We find a significant decrease of stratospheric ClO … relative to a model run without EPP impact / due to the EPP impact
Line 219: ClO abundances decrease … relative to a model run without EPP .. by …
Line 220: Why is this negative before 1980?
Line 230: In principle, I agree with this conclusion, but find “crucially” maybe a bit too strong / confident.
- AC1: 'Author responses to reviewer comments on acp-2022-151', Ville Maliniemi, 19 May 2022
Ville Maliniemi et al.
Ville Maliniemi et al.
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