3.1 Temperature and water vapour

The boundary condition at the tropical tropopause for the reference simulation was evaluated by comparing simulated water vapour concentrations with observed ones from MLS. The top panels in Fig. 1 show daily mean water vapour at 80 hPa averaged between 21^∘ S and 21^∘ N for the two representative years of 2013 and 2014. The temperature for the same region is shown in the lower panels. The cold point, where SWV boundary conditions were prescribed, is just below the 80 hPa level. The temperature shows the typical annual cycle with minimum temperature in the Northern Hemisphere (NH) winter and maximum temperature in the NH summer. The temperature in the tropical tropopause layer (TTL) controls how much water vapour enters the stratosphere by freeze-drying the upwelling air (Fueglistaler et al., 2005). As a result the maximum water vapour concentration occurs in the NH autumn and minimum in the early NH spring. The effect of interannual variability and shorter term variations in the temperature on stratospheric water vapour can also be seen; e.g. the low temperature in early 2013 results in 0.5–1 ppmv less water vapour than during the same time in 2014.

Figure 2Vortex average temperature within the Arctic polar vortex between altitudes 170 and 10 hPa.

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The Interim simulation produces water vapour concentrations comparable to the amount seen by MLS (Fig. 1), which shows that the boundary condition is reasonable. However, Interim variability is ahead that of MLS by 3–4 weeks. The reason for the time lag between Interim and MLS is not clear, although it could at least partly be associated with too fast a Brewer–Dobson circulation in ERA-Interim, which is responsible for the upward transport of the water vapour anomalies in the tropics (Schoeberl et al., 2012). The Max simulation has 2–3 ppmv more water vapour in the tropics than the Interim simulation, while the Min simulation is about 1.5 ppmv drier than the Interim simulation. These differences correspond to the ratio between Max and Interim of approximately 1.55–1.6 and about 0.55–0.6 between Min and Interim; i.e. they are consistent with the prescribed boundary conditions.

Figure 3Water vapour concentration (ppmv) at 55 hPa within the Arctic polar vortex. The green line denotes the Interim simulation, blue denotes the Max simulation, red denotes the Min simulation and black denotes the MLS.

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We next describe the meteorological condition in the Arctic stratosphere during the analysed winters. Figure 2 shows the daily average temperature in the Arctic polar vortex in the winters of 2010/2011, 2012/2013, 2013/2014 and 2015/2016. The polar vortex was identified using the modified potential vorticity (Lait, 1994), with the 475 K potential temperature as a reference level. Here the polar vortex is defined as the area enclosed by the 36 PVU isoline separately for every model level. The 36 PVU contour approximately corresponds to the region of the maximum PV gradient, i.e the polar vortex edge (Rex et al., 1999; Streibel et al., 2006). The winter of 2010/2011 represents a cold winter, with vortex average temperatures below 200 K and minimum temperatures below 195 K, sufficient for the formation of NAT/STS PSCs, throughout most of the winter, from December to the beginning of April, with only a brief interruption by a warming in early January. Minimum temperatures in the vortex were record cold and below 190 K, even at the end of March (Manney et al., 2011). The winter of 2012/2013 is an example of a warm Arctic stratospheric winter. Vortex average temperatures below 195 K were seen for only a few days in December in the ERA-Interim data, and the minimum temperature was below 195 K until mid-January. A SSW occurred in early January, followed by a weakening and a break-up of the polar vortex in the lower stratosphere already in February. The winter of 2013/2014 was intermediate, with average temperatures inside the polar vortex being close to the long-term climatological mean through most of the winter, until late March when a final SSW occurred. There were only a few days in late December when the average temperature was below 195 K. The winter of 2015/2016 was as cold, or even colder, as the 2010/2011 winter during December–February with minimum vortex average temperatures below 195 K. However, a minor SSW occurred in early February and the final warming came in early March, ending the cold period and reducing ozone depletion potential much earlier than in the 2010/2011 winter.

Figure 3 shows the 5-day running mean concentration of water vapour at 55 hPa averaged over the Arctic polar vortex for winters 2010/2011, 2012/2013, 2013/2014 and 2015/2016. The gaps in the 2012/2013 MLS curve are due to an undefined vortex (or too small a vortex with only few observations) after the SSW. The water vapour concentration in the Interim simulation is comparable to the MLS data. However, the variability in water vapour is smaller in FinROSE than in the MLS data. Typically, there is a stronger increase in water vapour towards spring in the MLS observations compared to the FinROSE simulations. This is most evident in the winter of 2013/2014 when MLS concentrations increased by more than 1 ppmv between November and April while the simulated increase was only 0.3 ppmv. Although an increase by spring is expected due to downward transport of air with higher SWV concentration by the Brewer–Dobson circulation, the increase seen in MLS observations in 2014 is unusual. For example the observed increase in January 2013 after the SSW associated with downward transport of water-rich air from above was about 0.3 ppmv and that increase was reasonably well reproduced by FinROSE. Note that the MLS observations within the polar vortex are sparse, which adds some noise to the MLS vortex average. Also note that FinROSE vortex mean values are calculated using all data points inside the vortex even if MLS data are not available for each point. This approach increases the robustness of model estimates but at the same time complicates direct comparison with MLS. Interestingly, when looking at the 60–90^∘ N average, which also includes air from outside the polar vortex, there is no similar spring increase in MLS data as can be seen in Fig. 3, and the agreement between FinROSE and MLS improves (not shown). In all winters, the Max simulation has about 2 ppmv more water vapour in the Arctic polar vortex than the Interim simulation, and the Min simulation is about 1.5 ppmv drier than the Interim simulation. This indicates that the simulated differences in the polar vortex water vapour are about the same as the differences in the boundary conditions for the tropical tropopause (Fig. 1), despite the average increase in SSW between the TTL and the polar vortex of about 1.5 ppmv in each run.

Figure 4The areas of ICE PSCs (10⁶ km²) within the Arctic polar vortex in the FinROSE simulations at 55 hPa. The green line denotes the Interim simulation, blue denotes the Max simulation and red denotes the Min simulation.

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There are also several SWV decreases seen in Fig. 3, which are due to the formation of ICE PSCs and possibly also to dehydration due to sedimentation of ICE particles. The most pronounced one is in the winter of 2015/2016 when, during a very cold period (Fig. 2), the observed concentrations decreased from 5.2 to 4.7 ppmv and remained low until late February. A relatively small decrease of only about 0.2 ppmv was simulated in the Interim run. This decrease corresponds to the formation of ICE PSCs in the model (see Sect. 3.2 for discussion of PSC results), and therefore at least a part of the decrease could be explained by sedimentation. A much larger decrease of about 1 ppmv was seen in the Max simulation starting from late December, which is consistent with a larger number of ICE PSCs simulated in this run. Another, much smaller, decrease of about 0.2 ppmv can be seen in the MLS observations during mid-January 2011 corresponding to a cold period. The decrease is almost undistinguishable in the Interim simulation, but is pronounced in the Max simulation, which is a result of a larger number of ICE PSCs.

3.2 Polar stratospheric clouds

Figures 4 and 5 show PSC type 2 (ICE) and PSC type 1a (STS) and b (NAT) areas at 55 hPa (about 20 km) in the Arctic polar vortex for winters 2010/2011, 2012/2013, 2013/2014 and 2015/2016. The area was calculated by summing the areas of model grid boxes containing PSCs. There is no formation temperature for STS droplets, but they are formed gradually as water, and HNO₃ dissolves into binary aerosols with decreasing temperature. Here we have defined STS based on how much HNO₃ has dissolved into the liquid phase; we set the limit at 0.6 pptv liquid HNO₃ for any given grid point. The limit approximately corresponds to a formation threshold temperature of 202 K for STS.

Figure 5The area of NAT (thick lines) and STS PSCs (thin lines) (10⁶ km²) within the Arctic polar vortex in the simulations at 55 hPa. The green line denotes the Interim simulation, blue denotes the Max simulation and red denotes the Min simulation.

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In the winter of 2010/2011 the polar vortex was stable and cold, but not extremely cold. The ICE PSC area (Fig. 4) in the Interim simulation was mostly moderate except for a period in late January with cold temperatures and large ICE PSC areas. The ICE PSCs lasted longer in the spring than in other winters. It is unusual that ICE PSCs occur after January, but in 2011 ICE PSCs were seen through February, even in the Interim simulation. In the Max simulation the ICE PSCs lasted until mid-March. CALIPSO (Pitts et al., 2007, 2018; Spang et al., 2018) also observed PSCs in the 2010/2011 winter. The observed ICE PSC areas are comparable to the FinROSE modelled ICE PSC areas (Thölix et al., 2016). The duration of ICE clouds being present is also comparable. However, Khosrawi et al. (2018) reported that a comparison showed that the PSC volume densities simulated with the EMAC model were several orders of magnitude smaller than the ones observed with MIPAS.

Table 1Sum of the ICE, NAT and STS PSC areas (10⁶km² day) at 55 hPa.

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In the winter of 2012/2013 the polar vortex was very cold in December, and some ICE PSCs were simulated. However, after the SSW in early January no ICE PSCs were simulated, not even in the Max simulation. The winter of 2013/2014 was moderately cold, with some ICE PSC occurrence in late January. The winter of 2015/2016 started as being very cold in December and January, and the ICE PSC area was large throughout January. The maximum ICE PSC areas in the Interim simulation were about 70 % larger compared to the other cold winter of 2010/2011. Dörnbrack et al. (2017) and Khosrawi et al. (2017) also reported unprecedented and widespread ICE PSC formation, seen in CALIPSO observations in 2015/2016. This was also the only winter with a significant ICE PSC area in the Min simulation, with a water vapour concentration of less than 4 ppmv.

The water vapour concentration has a strong effect on the ICE PSC formation: in the Max simulations, the ICE PSC area increases significantly in all winters. For instance, in 2010/2011, the largest PSC area is more than twice as large in the Max as in the Interim simulation. In the warm winters (2012/2013 and 2013/2014) the relative increase in the ICE PSC area due to additional water vapour was even larger than in the cold winters (2010/2011 and 2015/2016). The amount of water vapour was also an important factor for the extent of ICE PSC occurrence in the winter of 2015/2016; however, the relative increase between the Interim and Max simulation was smaller than in other studied winters that were warmer. PSCs start to form about 2 weeks earlier in the Max simulation compared to the Interim simulation. In the Min simulation the stratosphere is too dry for ICE PSC formation in nearly all years.

Figure 5 shows the area of NAT and STS PSCs in the Arctic vortex. Both NAT and STS areas are always significantly larger than the ICE areas because type 1 PSCs form at warmer temperatures than ICE PSCs. Type 1 PSCs typically start to form in early November and ICE PSCs in mid-December to late December. The simulated peak values in the STS area range from 18 to 24 million km², peaks in the NAT area range from 11 to 16 million km², while the ICE area peaks range from 1.5 to 5.5 million km² in the Interim simulation. As expected the type 1 PSCs occur later in the spring than ICE PSCs; e.g. in the winter of 2010/2011, both NAT and STS PSCs were simulated until late April, more than a month later than the ICE PSCs.

In the cold winter of 2010/2011, STS PSCs persisted for more than 5 months, and NAT almost 5 months, from December to mid-April. An increase in moisture (Max simulation) only had a minor effect on the NAT and STS areas. In the Min simulation the maximum STS (and NAT) area was about 2 million km² smaller than in the Interim simulation, while the difference between the Max and Interim simulations was much smaller. In the winter of 2012/2013, both NAT and STS PSCs were only simulated in the beginning of the winter and by early January all the NAT PSCs disappeared due to warm conditions. The maximum value of STS area in the winter of 2012/2013 was the largest among the simulated years, 24 million km². The maximum NAT area was also the largest, about 13 million km². The increase of water vapour in the Max simulation did not change the PSC area as much as the decrease of water vapour in the Min simulation. In the early winter of 2012/2013 the NAT and STS areas were even larger than in 2010/2011, but warmer temperatures in the vortex in February caused the PSCs to diminish more rapidly. The effect of water vapour in 2012/2013 was the largest among the simulated years; the increase in NAT and STS areas between the Min and Interim simulations was about 3 million km². In the winter of 2013/2014 both NAT and STS maximum areas were nearly as high as in 2012/2013 (about 23 and 11 million km², respectively), but large areas of PSCs persist much longer than in the winter of 2012/2013. The maximum difference between Interim and Min simulation was about 2 million km². In the winter of 2015/2016 the NAT and STS areas were also larger than in 2010/2011, but the PSCs did not persist as late as in 2010/2011. The increase in type 1 PSC area due to increased water vapour was smaller than in 2013/2014, about 1.5 million km².

Table 1 shows cumulative ICE, NAT and STS PSC areas at 55 hPa. The largest cumulative ICE areas are always seen in the Max simulations. In the Min simulations there are only very small or no ICE PSC areas, with the exception of the winter of 2015/2016, when a considerable ICE PSC area was present in all runs. However, in 2015/2016 the ICE PSCs occurred mainly in December and January, while in 2010/2011 the ICE PSCs occurred from January until the end of February. The timing of the PSCs is important for chlorine activation and ozone loss as discussed later. The effect of water vapour on the cumulative ICE PSC area is larger in warm years than in cold years. The NAT and STS areas also strongly depend on winter temperatures – the maximum NAT and STS areas are simulated in the coldest winter of 2015/2016 in every simulation, while the smallest areas are simulated during the warmest winter of 2012/2013. However, unlike ICE PSC, the NAT and STS clouds are formed every winter. The formation of type 1 PSCs is less sensitive to changes in water vapour concentration than the ICE PSCs. The relatively large changes in water vapour between different simulations result in relatively small changes in the cumulative NAT and STS area.

Table 2Vortex-mean mixing ratio of ClO_x integrated over the whole winter (ppbv day) and as monthly mean concentration (ppbv) in the Min, Max and Interim simulations. Percentages in parentheses indicate the effect of SWV concentration changes compared to the Interim simulation.

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3.3 Chlorine activation

In early winter chlorine is present as reservoir compounds (HCl and ClONO₂), which do not destroy ozone. In the cold conditions within the polar vortex the chlorine species are transformed through heterogeneous reactions into intermediate species such as Cl₂. When sunlight reaches the polar vortex these species are easily dissociated to form active chlorine species that participate in the catalytic ozone depletion cycles, i.e. ClO_x (ClO, Cl₂O₂ and Cl). Active chlorine is transformed back to reservoir species through reactions with NO₂ and CH₄; however if PSCs are present the regeneration of ClO_x is sustained.

Figure 6Chlorine partitioning (%) within the Arctic polar vortex at 55 hPa in the Min and Max simulations. Active form (green) is $\mathrm{Cl}+\mathrm{ClO}+\mathrm{2}\times {\mathrm{Cl}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{2}}$. Intermediate (magenta) contains $\mathrm{2}\times {\mathrm{Cl}}_{\mathrm{2}}+\mathrm{HOCl}+\mathrm{OClO}+\mathrm{BrCl}+{\mathrm{ClNO}}_{\mathrm{2}}$ and reservoir chlorine (black) HCl+ClONO₂.

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Figure 6 shows the fraction of reservoir, intermediate and active chlorine species at 55 hPa in the Min and Max simulations. The results from the Max simulation are represented by the upper limit for the intermediate (magenta) and active species (green) and by the lower limit for the reservoir species (black). The chlorine fractions from the Interim simulation always fit within the range of the Min and Max simulations. The timing of the changes in the partitioning of the chlorine species correlates well with the occurrence of PSCs; e.g. the chlorine reservoir species start to transform into intermediate species when the STS PSCs appear (Fig. 5).

In the 2010/2011 winter chlorine activation starts in the latter half of December and the fraction of ClO_x is large through the January–March period, reaching a maximum of about 85 %. The active chlorine starts to transform back to reservoir species (mainly ClONO₂) in the beginning of March. In early April when the PSCs disappear, the active chlorine rapidly decreases to background values.

In the winter of 2012/2013 chlorine activation starts slightly earlier than in the other years, but already in the beginning of February most of the chlorine has converted back to the reservoir species due to a SSW. The maximum fraction of ClO_x is about 75 %, and is already reached at the end of December. The active chlorine decreased during January, and in the beginning of February the concentration reached nearly background values.

The beginning of the winter of 2013/2014 was very cold, the chlorine activation started in mid-December and the maximum chlorine activation was already reached at the end of January. After that, the vortex temperature increased and chlorine transformed back to reservoir species. The maximum fraction of ClO_x was slightly lower than in cold winters, about 70 %.

The winter of 2015/2016 started similar to the cold winter of 2010/2011 and nearly all of the chlorine was activated at the beginning of January, but the deactivation already started at the end of January, making the period with high ClO_x shorter than in the winter of 2010/2011. At the end of February the vortex temperature increased and chlorine transformed back to reservoir species. The maximum fraction of the activated chlorine of about 80 % was reached by the beginning of January.

The water vapour concentration seems to strongly affect the transformation of chlorine from the reservoir species to the intermediate ones in the beginning of the Arctic winter. The fractions of intermediate and reservoir chlorine species change significantly with water vapour concentration during November and December. The water vapour concentration affects the composition of binary aerosols and STS. When more water condenses to the particles the uptake coefficients for the heterogeneous reactions and the surface area increase; i.e., the reaction rates increase. The difference between Min and Max simulations can be up to 30 % when about half of the reservoir chlorine have transformed to intermediate species, just before the concentration of active chlorine species starts to increase. The difference in concentration of active chlorine and reservoir species between the Min and Max simulations is smallest during the cold periods, due to heterogeneous chemistry on the PSCs (Fig. 6). The cold winter of 2015/2016 shows a very small range, and the intermediately cold winter of 2013/2014 shows a wider range in concentrations. The water vapour content has less effect on the chlorine partitioning in cold winters. In the cold spring of 2011 the difference in chlorine activation between Min and Max simulations was about 5 % on average; it reached nearly 20 % at the beginning of April, when the chlorine deactivation was fast. In the warm winter of 2012/2013 the change was less significant, about 5 % during the winter. In the winter of 2013/2014 the difference in chlorine activation between Min and Max simulations reached 10 % in the latter half of January; from mid-February to mid-March the difference was 15–18 %. The chlorine activation in the winter of 2015/2016 seemed to be less dependent on water vapour content, probably due to the ICE PSCs that appear in all simulations. Therefore, the conditions were favourable for high chlorine activation in all simulations. The difference in the fraction of activated chlorine between simulations is only a few percents; only when the deactivation starts (in the end of February) is the difference more than 5 %.

Figure 7Chemical total ozone loss (DU) and difference between ozone loss in the Min and Max simulations within the Arctic polar vortex. The green line denotes the Interim simulation, blue denotes the Max simulation and red denotes the Min simulation. The difference is in black.

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The effect of increased water vapour seems to be large in moderately cold years, i.e. when the chlorine activation is not entirely complete. The start and end of chlorine activation correlate with the appearance of STS and NAT. The ICE PSCs did not significantly increase the chlorine activation. For example in the winters of 2012/2013 and 2013/2014 there were no ICE PSCs in the Min simulation, but the chlorine activation was nearly as high as in the Max simulation, which had ICE PSCs.

Table 2 shows the vortex-averaged ClO_x as a cumulative sum over the whole winter and as monthly mean concentration. The sums are integrated from November to April. The cumulative sum provides information about both the duration of the chlorine activation period and the concentration of ClO_x, while the monthly average concentration shows the timing of chlorine activation. The cumulative chlorine activation was largest in the winter of 2010/2011 and smallest in 2012/2013. The activation started in November every year, but remained small until December. The winter of 2010/2011 differs from the others, with high chlorine activation from January to March, giving the largest cumulative sum of the studied winters. Even in April, the ClO_x concentration remains elevated. The warm winter of 2012/2013 had the smallest cumulative chlorine activation; significant chlorine activation was only seen in December and January. The changes in water vapour between the Min/Interim/Max simulations had the largest effect on the cumulative ClO_x in moderately cold winters (2010/2011 and 2013/2014), for which the increase of ClO_x from Interim to Max was 3 % to 6 % and the change of ClO_x from Interim to Min was −1 % to −11 %. In the cold winter of 2015/2016 the respective changes were only 3 % and −2 %, and in the warm winter of 2012/2013 the changes were 4 % and −1 %.

3.4 Ozone loss

Figure 7 shows the mean chemical total ozone loss within the polar vortex for all the studied winters of 2010/2011, 2012/2013, 2013/2014 and 2015/2016. The total column chemical ozone loss was calculated by subtracting the passive transported total ozone from the modelled total ozone. In Fig. 7 the polar vortex is defined using the potential vorticity limit of 36 PVU, only at the 475 K level. The figure shows the chemical ozone depletion in the Interim, Min and Max simulations as well as the difference in the loss between the Min and Max simulations. The passive ozone tracer was initialized every year on 1 December, when it was set equal to the ozone distribution in the model on that day. Chemical processes start to reduce ozone already in December, but they have a minor effect on the total wintertime ozone loss. In January the chemical processes become more intensive when the chlorine activation increases (see Fig. 6).

In general the ozone loss is larger in cold years. The largest ozone loss was simulated in the beginning of April 2011 when about 90 DU ozone had been destroyed according to our model. FinROSE seems to underestimate the ozone loss; for example, Sinnhuber et al. (2011) and Manney et al. (2011) simulated 120 DU ozone loss and Pommereau et al. (2013) even simulated 170 DU in the winter of 2010/2011. If we look at maximum ozone losses instead of the polar vortex mean losses, then the numbers are larger. The maximum ozone loss in 2010/2011 within the polar vortex was 128 DU, which is comparable to the value given by Sinnhuber et al. (2011).

The ozone loss in the warm winter of 2012/2013 differs from the loss in colder winters (2010/2011 and 2015/2016). The maximum average ozone loss in the polar vortex in the winter of 2012/2013 was only 23 DU because the polar vortex was unstable and small. By mid-April 2014 the simulated vortex mean ozone loss was 79 DU in the Interim simulation. Before mid-February, i.e. during the coldest period, there was very little effect from the changes in SWV. A relatively small ozone loss of 56 DU was simulated in 2015/2016, which was due to the unstable polar vortex, which split and warmed, stopping the catalytic ozone cycles and ozone loss in the beginning of March, i.e. earlier than in 2010/2011 and 2013/2014.

Figure 7 also shows the difference of polar vortex averaged chemical ozone loss between Min and Max simulation. It shows how much the water vapour concentration change affects the ozone loss. The difference is largest (about 15 DU) in 2013/2014, a moderately cold winter, with significant ozone depletion. Another winter, 2010/2011, with significant ozone loss and cold, but not extremely cold conditions showed the second largest effect from addition of water vapour, about 10 DU. In 2012/2013, when the ozone loss stopped very early, the difference between Min and Max simulations was about 5 DU. The winter of 2015/2016 started as very cold, but was ended early by a SSW. The difference in ozone loss between the simulations remained very small up to mid-February; by mid-March the difference was about 7.5 DU. A reduction in the water vapour decreased ozone loss every winter. In the Interim simulation the deepest ozone losses were about 2–9 DU (6 %–11 %) deeper than in the Min-simulation. The effect from an increase in water vapour from Interim to Max was about the same. In the 2010/2011 winter the loss increased by about 7 %, while Sinnhuber et al. (2011) and Vogel et al. (2011) reported an increase of ozone loss by 20 % and 10 % (respectively) with a water vapour increase of about the same magnitude as considered here. Thus, our estimates are slightly smaller than those by Vogel et al. (2011). An additional sensitivity experiment showed that the difference compared to other studies can be, at least partly, due to the coarse horizontal resolution in FinROSE (${\mathrm{3}}^{\circ }\times {\mathrm{6}}^{\circ }$), which is not sufficient to fully capture the deepest ozone loss. Specifically, repeating Interim simulation for winter of 2010/11 with higher resolution (${\mathrm{1.5}}^{\circ }\times {\mathrm{3}}^{\circ }$) than in the original simulation showed larger ozone loss by 15 DU.

The changes in the amount of water vapour are in the range that was tested here and are not very important for ozone loss in cold years. In the 2010/2011 winter the chlorine activation was nearly complete in the Arctic polar vortex, and additional water vapour did not increase chlorine activation and thus did not increase the ozone depletion. Increasing water vapour concentration (compared to the Interim simulation) strengthens ozone loss by at least 4 DU for other winters, except for the 2011 winter when the increase was not significant.

Table 3Maximum polar vortex mean ozone loss produced by full chemistry, heterogeneous chemistry and separately by the STS, NAT and ICE part in Min, Max and Interim simulations (DU). Percentages show the fraction due to each part relative to the full chemistry.

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To better understand the mechanism of SWV influence on ozone loss, simulations without heterogeneous chemistry were performed. From those simulations ozone loss caused by heterogeneous chemistry can be separated by subtracting the total ozone simulated without heterogeneous chemistry from that simulated in the full chemistry run. Two different set-ups were used for testing the effect of the heterogeneous chemistry. In the first gas-phase chemistry simulation the heterogeneous chemistry was not included at all. In the second simulation the formation of PSCs was limited by setting the air temperature passed to the heterogeneous chemistry module to 200 K, similarly to what was done in Karpechko et al. (2013). The increase of water vapour (Max simulation) did not increase the ozone loss, but in the Min simulation ozone depletion was reduced by 6 DU. This setting allows some heterogeneous processing on binary aerosols and some STS droplets that are very dilute in HNO₃, and due to the temperature limit, the surface area densities will remain quite small. Table 3 summarizes ozone loss characteristics during the studied years and shows the loss produced by full chemistry, heterogeneous chemistry and separately by the NAT, STS and ICE PSCs in the Interim, Min and Max simulations.

In the Interim simulation with full chemistry in 2010/2011 about 90 DU ozone was depleted, of which the heterogeneous chemistry caused depletion of 56 DU, i.e. about 62 % of the total ozone loss. Heterogeneous chemistry due to PSCs destroyed 30 DU ozone, which was about 33 % of the total loss. The result indicates that chlorine activation on the binary aerosols has a significant role in ozone depletion. Some studies suggest that binary aerosols are more important for chlorine activation than PSCs (Drdla and Müller, 2012; Kirner et al., 2015; Wohltmann et al., 2013). The increase of water vapour (Max simulation) did not increase the ozone loss, but in the Min simulation there was 6 DU less ozone depletion. This is consistent with the results by Kirner et al. (2015), who argue that the contribution of ICE PSCs to the ozone loss is always less than 5 % in the Antarctic spring, where the chlorine activation is nearly complete.

In the warm winter of 2012/2013 the ozone loss is only 23 DU, and the heterogeneous part is 63 % of it. NAT, STS and ICE PSCs only caused a small part of the total heterogeneous-chemistry-driven ozone loss (24 %). The loss caused by heterogeneous chemistry increased with increasing water vapour, but remained small even in the Max simulation. In 2013/2014 the heterogeneous chemistry caused about 40 DU ozone destruction, which is about 51 % of the ozone loss and NAT, STS and ICE about 23 DU (30 %), when the total ozone loss was 79 DU in the Interim run. The increase in SWV from Interim to Max increased the ozone loss by about 6 DU and the decrease in SWV from Min to Interim decreased the ozone loss by 9 DU. Thus, water vapour changes have a larger effect on ozone loss in moderately cold years than in cold ones. The ozone depletion due to heterogeneous chemistry increased with water vapour, even though the fraction due to heterogeneous chemistry was smaller than in 2010/2011 and 2015/2016.

Figure 8Averaged chemical ozone loss (ppmv) in the Interim simulation (a) and the difference between Max and Min simulations (b) within the Arctic polar vortex.

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In the winter of 2015/2016 the heterogeneous contribution was largest when compared to other simulated years, reaching even 63 % of the ozone loss, and the STS, NAT and ICE part was also large, at 30 %. The total ozone loss is however only 56 DU. When the water vapour content was increased from Interim to Max simulations, the fraction due to the heterogeneous chemistry remained the same, but the fraction due to STS, NAT and ICE PSCs increased.

Based on the results in Table 3 it can be concluded that nearly all SWV impact on ozone loss is due to heterogeneous chemistry. For example in 2010/2011 the ozone loss without heterogeneous chemistry was 34 DU in Interim, 34 DU in Min and 35 DU in Max simulation and only the heterogeneous part changed from model run to model run. In 2013/2014 the non-heterogeneous contribution is about 39 DU, and in 2015/2016 about 21 DU; i.e. in warm years it is larger than in cold years. In 2012/2013 the non-heterogeneous contribution was only 9 DU, but also the total ozone loss is very small. However, the heterogeneous contribution is about 62 % of the total ozone loss in all other winters than 2013/2014 and also with both increased and decreased water vapour. The fraction of STS-, NAT- and ICE-driven chemistry changes with water vapour concentration.

Finally we analyse the vertical distribution of the ozone loss and the effect of SWV on ozone loss, which is shown in Fig. 8. The largest ozone loss was simulated in 2010/2011, when the ozone destruction in the Interim run with normal SWV was about 1.4 ppmv between 60 and 30 hPa. The ozone depletion increased by 0.2 ppmv between the Min and Max simulations. In 2012/2013 the maximum ozone reduction was almost the same as in 2010/11, but it occurred at higher altitude and lasts for a shorter period than in 2011. The effect of the increase in water vapour from Min to Max simulations only had a minor effect on the ozone depletion in 2012/2013. The heterogeneous chemistry and chlorine activation did not have an important role in the warm conditions, and the ozone loss between 60 and 30 hPa remains very weak. In winters when the polar vortex is unstable and small or disturbed, the Brewer–Dobson circulation brings more NO_x-rich air into the polar vortex than usual. Hence the ozone loss in the winter of 2012/2013 was mostly produced by NO_x chemistry as shown previously by, e.g. Sagi et al. (2017), and can be seen from FinROSE results by comparing the simulations with and without the heterogeneous chemistry. The total ozone column loss in this winter remained smaller than in cold years, when the ozone depletion is driven by halogens. In the 2014 spring the conditions in the polar vortex remained favourable, but the temperature was not as low as in 2011. The ozone loss developed steadily, but remained moderate. The two winters of 2010/2011 and 2013/2014 with the most favourable conditions for halogen-driven ozone depletion showed the largest increase in ozone loss with water vapour. The effect was more pronounced in 2013/2014, which was the warmer of the two winters. The winters of 2010/11 and 2015/2016 look similar during January–February, but the ozone loss became much more severe in 2010/2011 due to favourable conditions in March–April. In 2015/2016 there was a very cold period, but it occurred too early to have a large impact on the ozone depletion, and therefore the water vapour increase only had a moderate effect later in the spring. In 2013/2014 the largest ozone loss was about 1.1 ppmv between 60 and 30 hPa, while in 2015/2016 it was about 1 ppmv at the same altitude. Livesey et al. (2015) and Sagi et al. (2017) showed results from the 450 K level, and their ozone losses were about 2 ppmv in the winter of 2011. In the winter of 2013 Sagi et al. (2017) recorded about 1.5 ppmv ozone loss, which is about the same as what we found.