Reactive nitrogen (NO y ) and ozone responses to energetic electron precipitation during Southern Hemisphere winter

. Energetic particle precipitation (EPP) affects the chemistry of the polar middle atmosphere by producing reactive nitrogen (NO y ) and hydrogen (HO x ) species, which then catalytically destroy ozone. Recently, there have been major advances in constraining these particle impacts through a parametrization of NO y based on high-quality observations. Here we investigate the effects of low (auroral) and middle (radiation belt) energy range electrons, separately and in combination, on reactive nitrogen and hydrogen species as well as on ozone during Southern Hemisphere winters from 2002 to 2010 using the SOCOL3-MPIOM chemistry-climate model. Our results show that, in the absence of solar proton events, low-energy electrons produce the majority of NO y in the polar mesosphere and stratosphere. In the polar vortex, NO y subsides and affects ozone at lower altitudes, down to 10 hPa. Comparing a year with high electron precipitation with a quiescent period, we found large ozone depletion in the mesosphere; as the anomaly propagates downward, 15 % less ozone is found in the stratosphere during winter, which is conﬁrmed by satellite observations. Only with both low-and middle-energy electrons does our model reproduce the observed stratospheric ozone anomaly.

High energy particles, i.e. solar protons (Jackman et al 2008) and radiation belt electrons (Semeniuk et al 2011, Arsenovic et al 2016 can penetrate directly into the mesosphere and stratosphere. Electrons of lower energies (< 30 keV, auroral) originate from the magnetosphere as well as the radiation belt electrons (Mironova et al 2015), but they get accelerated in the magnetotail and precipitate into the lower thermosphere in the auroral ovals (55 -70° geomagnetic latitude) (Baker et al 2001, Barth et al 2003. 5 There have been numerous attempts to include low energy electrons (LEE) in climate models. Chemistry-climate models with top in the thermosphere, e.g. HAMMONIA (Schmidt et al 2006), KASIMA (Reddmann et al 2010) and WACCM (Marsh et al 2007, Andersson et al 2018, have included effects of LEE directly because they deposit their energy within the model domain. For climate models that have an upper lid below the thermosphere, a prescription of LEE as NOx influx through the model top is recommended (Matthes et al 2017). Baumgaertner et al (2009) has developed a parameterization of this flux 10 based on the geomagnetic activity Ap index, a daily worldwide measure of the effects of solar wind on the Earth magnetic field.
When incorporated into several chemistry-climate models, results showed significant ozone depletion in the mesosphere and stratosphere (Baumgaertner et al 2011). For the SOCOL chemistry-climate model Rozanov et al (2012) also found a significant ozone decreases in the mesosphere and stratosphere, with peak values around 10 % in September around 36 km altitude over the Antarctic. between Ap index with observed NOy produced by EPP. This advance in the representation of LEE in climate models motivates us to investigate if LEE can have a larger impact on atmospheric chemistry than previously thought. Moreover, this LEE 20 parameterization is a part of the recommended solar forcing dataset for climate models within the upcoming Coupled Model Intercomparison Project Phase 6 (CMIP-6, Matthes et al 2017). It is therefore important to demonstrate that the particle impact is well represented in chemistry-climate models.
It is crucial to have a realistic representation of EPP in models as the introduced signal impacts atmospheric chemistry and potentially regional climate (Baumgaertner et al 2011, Rozanov et al 2012, Seppälä et al 2013, Maliniemi et al 2014. Here 25 we present results from a state of the art chemistry-climate model that employs the new Funke et al (2016) parameterization of LEE together with the previous representations of other energetic particles. We compare our results with the satellite observations. This paper focuses on evaluating NOx and ozone response to LEE precipitation in Antarctic winters (JJA: June, July and August), in order to avoid the more complicated Arctic polar vortex with its high variability and strong dependence on meteorological conditions (Hitchcock et al 2013). 30 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1123 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 23 November 2018 c Author(s) 2018. CC BY 4.0 License.

Methods
We used the coupled chemistry-climate model SOCOL3-MPIOM (Stenke et al 2013, Muthers et al 2014. The atmospheric dynamic component of the model is ECHAM5.4 (Roeckner and Bäuml 2003), coupled to the air chemistry module MEZON (Rozanov et al 1999, Egorova et al 2003 and the interactive ocean module MPIOM (Marsland et al 2002, Jungclaus et al 2006. We carried out the experiments with T31 spectral resolution on 39 vertical levels from the surface up to 0.01 hPa. 5 The model boundary conditions and parameterizations are identical to those described in Arsenovic et al (2016), except for the LEE parameterization.   ppbv is found in the upper mesosphere around 0.01 hPa (~80 km). There, the highest monthly values are observed in June. In the following months, this anomaly descends and reaches lower levels. In July, the NOy enhancement of around 10 ppbv reaches the upper stratosphere around 2 hPa, and the increase, although smaller, is visible all the way down to 10 hPa. In the 10 following months, the MIPAS nominal data were unavailable due special observation mode campaigns.

NOy enhancement propagation
The ALL experiment ( Figure 2b) shows a very similar pattern of NOy as the observations. The NOy increase of 500 -600 ppbv in the upper mesosphere around 0.01 hPa is similar as in MIPAS. However, the wintertime NOy peak below is slightly overestimated in the model compared to MIPAS. This is particularly visible in the lower mesosphere in June, as the modeled 100 ppbv NOy enhancement reaches 0.1 hPa. The mesospheric anomaly extends into the stratosphere, but remains confined to 15 the upper stratosphere, above 10 hPa, as in observations. The modeled NOy overestimation suggests that downward transport is somewhat too fast in the model, or the photochemical lifetime of NOy is too long, or horizontal mixing with mid-latitudes is underestimated. The modeled NOy enhancement in September stems from a SP event (NOAA, 2018). In contradiction to our results, the EMAC model slightly underestimates NOy even during polar summer, for two pressure levels, 0.1 and 1 hPa (Matthes et al 2017). Sinnhuber et al (2018) compared NOy observed by MIPAS with the results of 3dCTM, KASIMA and 20 EMAC chemistry-climate models and also showed overestimation of modeled NOy in the southern hemisphere.
The LEE simulation ( Figure 2c) shows very similar anomalies as ALL. The largest differences are in the upper mesosphere, where LEE anomalies reach around 400 ppbv, which is underestimated compared to 500-600 ppbv found in MIPAS and ALL.
A second interesting difference compared to ALL is the SP event in September. In LEE simulation, it reaches around 60 ppbv, while in ALL it exceeds 100 ppbv. This difference is coming from increased MEE precipitation that coincided with the SP 25 event (see Arsenovic et al 2016, Figure 1a).
The MEE simulation ( Figure 2d) is drastically different from MIPAS as well as the ALL and LEE simulations. Although NOy enhancement in the modeled geomagnetically active year exists, it is significantly decreased compared with the previous results. The pronounced modeled NOy anomaly maximum from the mesosphere is absent and enhancement of 10 ppbv does not reach the stratosphere. Nevertheless, although less intense, increased NOy is present throughout the mesosphere and 30 stratosphere, and the NOy increase in September due to the SP event exceeds again 100 ppbv, as in the ALL simulation. From the presented, we conclude that inclusion of only LEE was sufficient to reproduce most of the NOy enhancements. The 5 MEE contribution to NOy increases is minor and brings model closer to observations mainly in the upper mesosphere. As SP events can have impact on precipitation from outer Van Allen belt (Pierrard and Lopez Rosson 2016), MEE precipitation could significantly contribute to NOy increases in such events.

O3 anomaly propagation
In study of Matthes et al (2017), ozone responses were evaluated by comparing high and low geomagnetic activity years and 10 not by on/off experiments as done here and their estimate shows good agreement with satellite observations (Fytterer et al 2015). To evaluate our simulated ozone responses, we follow a similar approach as used in Matthes et al (2017), that is, we The ALL simulation ( Figure 3b) shows a negative ozone anomaly in the mesosphere as well. However, the magnitude is 25 generally higher (around 30 %) and it is present from May to September. The September 2005 SP event is visible in the model simulations as well and descends from around 1 hPa in late September, reaching 10 hPa in late October. A similar pattern, but less obvious, is seen in the observations. Ozone anomalies in the lower mesosphere (0.5 -0.1 hPa) are more pronounced in the model than in MLS observations. This is particularly evident in June when the modeled upper-mesosphere anomaly appears to relate to the upper-stratospheric anomaly, in contrast to the observations. This suggest that HOx production by MEE might 30 be overestimated. In the upper stratosphere model simulations agree well with observations. The decrease propagates downwards, reaching approximately 10 hPa in August, with a peak around 15 % in good agreement with the observations. Ozone anomalies in the LEE simulation are shown in Figure 3c. Negative ozone anomalies are present mostly in the upper mesosphere (above 0.3 hPa) and have similar magnitude to ALL. Although in the LEE simulation mesospheric ozone anomaly is overestimated compared to MLS observations, the stratospheric anomaly is almost completely absent. This is surprising, as there are very similar NOy anomalies in the ALL and LEE simulations (see Figure 2).
Our MEE simulation shows similar ozone anomalies to LEE (Figure 3d). The anomalies are confined to a region above 1 hPa 5 and are somewhat reduced compared to LEE and ALL. Similar to LEE, the stratospheric ozone anomaly seen in the observations and ALL simulation is almost absent.
In REF simulation (Figure 3e Compared with our ALL simulation, their ozone anomaly in case of all EEP of around 7 % is lower and occurs later (in October as opposed to August). However, their LEE simulation does not show significant ozone anomaly in the stratosphere, which is also the case in our results. 15

EEP effect on NOy, HOx and O3
To estimate the total effect of energetic electron precipitation on NOy, HOx and ozone, we calculated the differences of The zonal mean of austral winter (JJA) average NOy differences between ALL and REF is shown in Figure 4a. In polar night, NOy is transported to lower altitudes by descending air motion. Significant modeled NOy enhancements are present in the whole mesosphere and upper stratosphere above 10 hPa. Around 0.01 hPa, EPP produced NOy increases from 50 ppbv at around 60° S to more than 500 ppbv at the pole. The differences in HOx between those two experiments are shown on Figure  25 4b. Increases are mostly confined to the upper mesosphere and they reach the maximum of around 5 ppbv. However, smaller (< 1ppbv) but statistically significant HOx increase appears in lower mesosphere and upper stratosphere around 60° S. Increases of NOy and HOx impact the ozone chemistry. Figure 4c shows changes in ozone concentrations due to electron precipitation.
Ozone is significantly reduced throughout the whole polar region above 10 hPa. There are two peaks of ozone anomaly. The maximum decrease of up to 65 % (350 -400 ppbv) is located in the upper mesosphere. This decrease is more severe than in 30 previous modeling studies (Rozanov et al 2012), but this is because we focus on the geomagnetically active winters, when EPP effects are much more pronounced. The magnitude of ozone depletion is gradually decreasing with height reaching ~15 % (>200 ppbv) at the stratopause. The second ozone depletion peak is located between 10 and 1 hPa, reaching 15 % (>400 ppbv).
A similar ozone response as in ALL has been shown by Semeniuk et al. (2011). Figure 4d shows the difference between modeled NOy in LEE and REF simulation. Similarly, as in Figure 2, modeled NOy in LEE simulation is very similar as in ALL, confirming the fact that the most of the NOy is coming from LEE. Slight reduction to ALL still exists, visible mostly at 0.1 hPa at 90° S. Here, the value of NOy is 100 ppbv while it is somewhat more in Figure  5 4a. Second difference is the absence of the enhancement equatorward of 30° S which is present in Figure 4a. Increase of HOx in case of LEE is illustrated on Figure 4e. Changes of HOx are very small and statistically insignificant, except for small (<1 ppbv) increase in the polar upper mesosphere. This is expected as LEE do not produce HOx. The small increase could be explained by increase of NOy which causes small increase of background HOx through the Verronen and Lehmann (2015) mechanism. As Verronen and Lehmann (2015) pointed out, enhanced NO coming from EEP leads to HOx repartitioning 10 increasing HOx concentrations. Figure 4f shows ozone changes due to the LEE. Similar ozone decrease pattern as in Figure   4c exists but with a reduced intensity. The upper-mesospheric reduction reaches 35 % (~200 ppbv) and the upper-stratospheric anomaly is halved compared to ALL (200 ppbv ≙ 10 %). The absence of HOx increases and reduced ozone anomalies compared to ALL illustrates the importance of MEE. Figure 4g shows increase of NOy due to the MEE. Although MEE cause increase of NOy, modeled NOy is significantly reduced 15 in the whole area compared to LEE and ALL simulation. In the upper mesosphere, this increase is around 50 ppbv, or tenth of total produced NOy in ALL simulation. Equatorward from 30° S NOy enhancement is present again, as in ALL simulation.
This enhancement is coming from the fact that MEE do not necessarily precipitate inside the polar vortex, as they precipitate in the sub-auroral ovals, which are centered around the geomagnetic pole. In contrast, NOy coming from LEE descends into the mesosphere in the down-welling air motion inside of the polar vortex. The sum of NOy increases (not shown) due to the 20 LEE (Fig 4d) and due to the MEE (Fig 4g) closely reassembles NOy increase as in ALL case (Fig 4a).
Increases of HOx due to MEE are presented in Figure 4h. Enhancements are present mostly in the upper mesosphere reaching 4 ppbv. The position and intensity of HOx is very similar to ALL, but somewhat reduced. Because MEE produce OH, neglecting MEE in climate models would lead to an underestimation of HOx; neglecting LEE would also lead to an underestimation of HOx through the changed HOx partitioning (Verronen and Lehmann, 2015). Changes in ozone 25 concentrations due to MEE are shown in Figure 4i. Negative ozone anomalies are present in the mesosphere and in the upper stratosphere, albeit stratospheric anomaly is statistically not significant. Biggest reduction with 35 % (~200 ppbv) is visible in the upper mesosphere. The anomaly in the upper stratosphere (10 -1 hPa) does not exceed 100 ppbv. Interestingly, summing stratospheric ozone anomaly from LEE (Fig 3f) and from MEE (Fig 3i) does not reproduce ALL ozone anomaly (Fig 3c). The sum of the LEE and MEE ozone anomaly accounts for around 300, while ALL shows about 400 ppbv between 10 and 1 hPa. 30 Since sum of enhanced NOy due to LEE and MEE corresponds to ALL NOy and HOx enhancements occur in mesosphere, this discrepancy in ozone anomaly cannot be chemically explained. It could be caused by changes in dynamics (polar vortex strength) and temperature (which affects reaction rates). Our results indicate that LEE and MEE are equally responsible for ozone anomaly in the mesosphere. LEE deplete ozone through the production of large amounts of NOy, while MEE contribute to the anomaly mostly through production of HOx, which is more efficient ozone destructor (Brasseur and Solomon 2005). Both LEE and MEE produce stratospheric anomaly; however, LEE, through production of large amounts of NOy are more important.

Conclusions 5
We used the period 2005-2010 comprising intervals of high and low geomagnetic activity, which is well characterized by stratospheric and mesospheric measurements of NOy and O3, to investigate the accuracy of representations of energetic particle forcing in a chemistry-climate model. We assessed the impact of employing a new parameterization of LEE (< 30 keV) recommended for CMIP-6 in combination with the AIMOS parameterization for MEE (30 -300 keV) on the simulated NOy, HOx and ozone variability. We used the SOCOL3-MPIOM climate model and focused on the Southern Hemispheric winter 10 season. We compared NOy with stratospheric and mesospheric MIPAS observations. The model captures the main features very well, but shows some differences in the winter maxima. LEE can reproduce most of the NOy features, without including MEE. However, increased MEE precipitation coincident with SP events may contribute to reproduce the observed NOy amounts.
Simulated ozone depletion has been compared to MLS satellite observations, showing that patterns of ozone anomalies during 15 the high EPP year 2005 compared to 2006-2010 match reasonably well. The model overestimates mesospheric ozone anomalies, but in the stratosphere a good match is accomplished. Ozone depletion of up to 15 % is found during July and August and reaches into the lower stratosphere. In essence, without including both LEE and MEE, the stratospheric anomaly cannot be accurately modeled. In addition to chemical changes, indirect changes in temperature and dynamics also play a role in the EPP-induced stratospheric ozone variation. 20 Most of the NOy in the mesosphere and stratosphere is produced by LEE in the upper mesosphere and lower thermosphere (<0.01 hPa) and transported downwards. A smaller fraction, namely ~10 %, is generated in-situ by ionization due to precipitating electrons of higher energies. These electrons play an important role because they produce HOx, which depletes ozone near HOx source region in the mesosphere. Although not producing HOx directly, LEE increase NOy concentrations, which then causes repartitioning of HOx and increase HOx lifetime (Verronen and Lehmann 2015). 25 In summary, LEE and MEE lead to a reduction of ozone throughout the mesospheric and stratospheric polar region with a maximum percentage ozone depletion in the mesosphere (-65 %) and a second peak anomaly in the upper stratosphere (-15 %) with respect to the simulation where they are omitted. These chemical EPP signals can cause dynamical changes in the stratosphere that propagate into the lower atmosphere, which eventually affect regional climate (Rozanov et al 2012).