Nitric acid (HNO3) is known to influence ozone (O3) concentrations in the polar regions, due to its role as a NOx (≡ NO + NO2) reservoir and its ability to form polar stratospheric clouds (PSCs) inside the vortex e.g.. In the stratosphere, HNO3 forms from the reaction between OH and NO2 (produced by the reaction N2O + O1D) and is destroyed by reaction with OH or photodissociation, both of these reactions being slow during daytime and virtually non-existent at night-time . This leads to photochemical lifetimes between 1 and 3 months up to 30 km altitude and around 10 days at higher altitudes , inducing similar general transport pathways for O3 and NOy (the sum of all reactive nitrogen species – including HNO3) . During the polar winter, with the arrival of low temperatures, PSCs, composed of HNO3, sulphuric acid (H2SO4) and water ice (H2O), form within the vortex e.g.. They act as sites for heterogeneous reactions, turning inactive forms of chlorine and bromine into active radicals, and leading to the depletion of O3 in the polar regions e.g.. Furthermore, the formation of these PSCs, particularly the nitric acid trihydrates (NAT), leads to the denitrification of the stratosphere (condensation of HNO3 followed by sedimentation towards the lower stratosphere), which prevents ClONO2 from reforming e.g. and further enhances the depletion of ozone.

HNO3 has been measured by a variety of instruments over the last few decades, of which the MLS (on the UARS, then the Aura satellite) provided the most complete data set. MLS measurements began in 1991 and allowed for the extensive analyses of seasonal and interannual variability, as well as the vertical distribution of HNO3 , however with a coarse horizontal resolution. Other instruments have also measured HNO3 in the atmosphere, such as MIPAS (ENVISAT, ), ACE-FTS (SCISAT, ) and SMR

in the order of the instruments cited above: Microwave Limb Sounder (MLS), Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), Sub-Millimetre Radiometer (SMR)

The HNO3 columns used here were retrieved from measurements taken by the IASI instrument on-board the Metop A satellite. IASI measures the upwelling infrared radiation from the Earth's surface and the atmosphere in the 645–2760 cm-1 spectral range at nadir and off-nadir along a broad swath (2200 km). The level 1C data set used for the retrieval consists of measurements taken twice daily (at 09:30 and 21:00, equatorial crossing time) at a 0.5 cm-1 apodized spectral resolution and with a low radiometric noise (0.2 K in the HNO3 atmospheric window) . The ground field of view of the instrument consists of four elliptical pixels (2 by 2) yielding a horizontal footprint (single pixel) that varies from 113 km2 (12 km diameter) at nadir to 400 km2 at the end of the swath.

To retrieve HNO3 atmospheric concentrations, we use the level 1C measurements available in near real-time at Université Libre de Bruxelles (ULB) and retrieved by the Fast Optimal Retrievals on Layers for IASI (FORLI) software, which uses the optimal estimation method . A complete description of the FORLI method can be found in and a summary of the retrieval parameters specific to HNO3 in . The retrieval initially yields HNO3 vertical profiles on 41 levels (from 0 to 40 km altitude) but with limited vertical sensitivity. The characterization of the retrieved profiles conducted by showed that the degrees of freedom for signal (DOFS) range from 0.9 to 1.2 at all latitudes. Because of this lack of vertical sensitivity, the HNO3 total column is the most representative quantity for the IASI measurements and is exploited here for the investigation of HNO3 time evolution. It is important to note, however, as thoroughly discussed in , that the information on the HNO3 profile comes mostly from the lower stratosphere (15–20 km) and the profile is therefore mainly indicative of stratospheric abundance. In order to compute the total column, the retrieved vertical profiles are integrated over the whole altitudinal range. Our previous study showed that the resulting total columns yield a mean error of 10 % and a low bias (10.5 %) when compared to ground-based FTIR measurements . The data set used spans from January 2008 to December 2016 with daily median HNO3 columns averaged on a 2.5∘×2.5∘ grid, for which both day and night measurements were used. Based on cloud information from the EUMETSAT operational processing, the cloud-contaminated scenes are filtered out, i.e. all scenes with a fractional cloud cover higher than 25 % are not taken into account. It should be noted that there was an abnormally small amount of IASI L2 data distributed by EUMETSAT between 14 September and 2 December 2010 , and that these data have been removed from the figures and analyses in this particular paper. For the purpose of this study the data are divided into several time series according to equivalent latitudes (sometimes referred to as “eqlat”), which allow one to consider dynamically consistent regions of the atmosphere throughout the globe and better preserve the sharp gradients across the edge of the polar vortex. The potential vorticity data are daily fields obtained from ECMWF ERA Interim reanalyses, taken at the potential temperature of 530 K. Following the analysis of the potential vorticity contours, we consider five equivalent latitude bands in each hemisphere (30–40, 40–55, 55–65, 65–70, 70–90∘), plus the intertropical band (30∘ N–30∘ S), with the corresponding potential vorticity contours, in units of 10-6 K m2 kg-1 s-1, being 2.5 (30∘), 3 (40∘), 5 (55∘), 8 (65∘) and 10 (70∘) (Fig. ).

The HNO3 time series for the mid to high latitudes are displayed in Fig.  for the years 2008–2016. Total columns are represented for both north (green) and south (blue curves) hemispheres, for equivalent latitudes bands 40–55, 55–65, 65–70 and 70–90∘. Also highlighted by shaded areas are the periods during which the northern and southern polar temperatures, taken at 50 hPa (light green and light blue for the 70–90 eqlat band, N and S respectively, and purple for the 65–70∘ S eqlat band), were equal to or below the polar stratospheric clouds formation threshold (195 K, based on ECMWF temperatures). It should be noted that while this temperature is a widely accepted approximation for the formation threshold for NAT (type I), its actual value can differ depending on the local conditions . Also, other forms of PSCs, particularly type II PSCs (ice clouds), form at a lower temperature of 188 K, corresponding to the frost point of water, or 2–3 K below the frost point (e.g. ).

As a general rule, we find larger concentrations in the Northern Hemisphere, for the entire latitudinal range shown here (40–90 eqlat). The hemispheric difference in HNO3 maximum concentrations can be partly attributed to the hemispheric asymmetry of the Brewer–Dobson circulation associated with the many topographical features in the Northern Hemisphere compared to the Southern Hemisphere. As a result, the Northern Hemisphere has a more intense planetary wave activity, which strengthens the deep branch of the Brewer–Dobson circulation. This also has a direct effect on the latitudinal mixing processes, which usually extend into the Arctic polar region, but less so into the Antarctic due to a stronger polar vortex .

Beyond hemispheric asymmetry, we also find that HNO3 columns are generally larger at higher latitudes, with total column maxima between 3.0 × 1016 and 3.7 × 1016 molec cm-2 in the equivalent latitudes bands 70–90, 65–70 and 55–65∘, and lower at around 2.2 × 1016 molec cm-2 in the 40–55∘ band, especially for the Southern Hemisphere. This latitudinal gradient of HNO3 has been previously documented (e.g. ) and can mainly be explained by the larger amounts of NOy at high latitudes due to a larger age of air and the NOy partitioning favouring HNO3. An interesting feature observable in Fig.  is the different behaviour of the three highest latitude regions with regard to the polar stratospheric cloud formation threshold in the Southern Hemisphere. The denitrification process that occurs with the condensation and sedimentation of PSCs (see e.g. and for further details) is obvious in the 70–90∘ S region (blue curve in Fig. a), with a systematic and strong decrease in HNO3 total columns (from 3.3 × 1016 to 1.5 × 1016 molec cm-2) starting within 12 to 25 days after the stratospheric temperature reaches the threshold of 195 K (start of the blue shaded areas). The loss of HNO3 therefore usually starts around the beginning of June in the Antarctic and the concentrations reach their minimum value within one month. They stay low at 1.4 × 1016 molec cm-2 until mid-November (with a slight gradual increase to 1.7 × 1016 molec cm-2 quite often seen during the two following months), and start to increase again during January, i.e. between 2.5 and 3 months after the polar stratospheric temperatures are back above the NAT formation threshold. The same pattern can be observed in the 65–70∘ S equivalent latitude region. However, a delay of approximately 1 month exists for the start of the steepest decrease in HNO3 columns, which appears to be more gradual than in the 70–90∘ S regions (3 months to reach minimum values, starting in July). The minimum and plateau column values are thus reached by the end of September; they remain higher than at the highest latitudes, with values staying at around 1.7 × 1016 molec cm-2. The delayed and less severe loss of HNO3 in the 65–70∘ S band confirms that the denitrification process spreads from the centre of the polar vortex, where the lowest temperatures are reached first . This spreading from the centre also leads to slightly higher concentrations for the maxima in the 65–70∘ S eqlat band (mean of maxima of 3.26 × 1016 versus 3.11 × 1016 molec cm-2 in the 70–90∘ S eqlat band). The delayed decrease in HNO3 in the outer parts of the vortex (i.e. in the 65–70∘ S eqlat band) can thus be attributed to the later appearance of PSCs in this region (see Fig. b purple shaded areas). By the end of December, when the vortex has started breaking down (e.g. ), the total columns in both eqlat bands become homogenized and reach the same range of values (1.7 × 1016 molec cm-2).

If the decrease is slower at 65–70 eqlat, this is not the case for the recovery, with the build-up of concentrations starting roughly at the same time as for the 70–90∘ S eqlat band, hence resulting in a shorter period of denitrified atmosphere in the 65–70∘ S band. These results agree well with previous studies by and for earlier years. However, the recovery of the HNO3 total columns is very slow compared to other species, namely O3, for which concentrations return to usual values within 2 months (i.e. in December) after PSCs have disappeared. In fact, the HNO3 columns stay low until well after the September equinox and are only subject to a slow increase starting 2 months later (in early December) with concentrations back to pre-denitrification levels by May. While more persistent local temperature minima staying below 195 K could explain part of this late recovery, we hypothesize that it is mainly due to a combination of two factors: (1) the significant sedimentation of PSCs towards the lower atmosphere during the winter, such that few PSCs remain available to release HNO3 under warmer temperatures ; and (2) the effective photolysis of HNO3 and NO3 in spring and summer under prolonged sunlight conditions, mainly at the highest latitudes, which respectively increase the HNO3 sink and reduce the chemical source (because NO3 cannot react with NO2 to produce N2O5, ). The increase observed in March, at the start of the winter, can in turn be explained by a reduction in the number of hours of sunlight (implying less photodissociation), as well as by diabatic descent, which brings HNO3-rich air to lower altitudes.

It is worth noting that the two regions previously mentioned (inner and outer vortex) have been observed to behave differently; the inner vortex (70–90∘ S) undergoes strong internal mixing whereas the outer vortex (65–70∘ S), isolated from the vortex core, experiences little mixing of air. This, combined with a cooling of the stratosphere, could lead to increased PSC formation and further ozone depletion .

Regarding the HNO3 columns in the 55–65∘ S eqlat band, which comprises the vortex rim, or “collar” , it is evident from Fig. c that they are not affected by denitrification, which is in agreement with previous observations (e.g. ). In fact, we show that columns in this particular band keep increasing when temperatures at higher latitudes start decreasing, to reach maximum values of about 3.4 × 1016 molec cm-2 in June–July; this is due to a change in NOy partitioning towards HNO3, which is in turn due to less sunlight in this period compared to summer. Also inducing increased concentrations during the winter at high latitudes is the diabatic descent occurring inside the vortex when temperatures decrease. This downward motion of air enriches the lower stratosphere with HNO3 coming from higher altitude , yielding higher column values which are, in this eqlat band, not affected by denitrification. The slow decrease in HNO3 starting in August and leading to minimum values in January is related to the combined effect of increased photodissociation and mixing with the denitrified polar air masses which are no longer confined to the polar regions. Finally, as previously mentioned, the 40–55∘ S eqlat band records lower column values throughout the year (generally below 2 × 1016 molec cm-2) and a much less pronounced seasonal cycle.

High latitudes in the Northern Hemisphere do not usually experience denitrification, mostly because the temperatures, while frequently showing local minima below 195 K, rarely reach the PSC formation threshold over broad areas and for long time spans (see Fig.  for average temperatures, light green vertical areas). A few years stand out, however, with exceptionally low stratospheric temperatures. This is especially the case of the 2011 , 2016 and, to some extent, 2014 Arctic winters. During these three winters, temperatures dropped below the 195 K threshold over a broader area and stayed low for a longer period than usual. Lower concentrations of HNO3 were recorded as a consequence, especially in the northernmost equivalent latitude band (see Fig. ). The winter 2016 recorded exceptionally low temperatures in particular, which led to large denitrification and significant ozone depletion . The denitrification that occurred in the northern polar regions affected a smaller area than is generally observed in the Southern Hemisphere; the columns in the 65–70∘ N eqlat band in particular do not show a significant decrease.

Figure , which consists of the time series of the zonally averaged distribution of HNO3 retrieved total columns, illustrates all of these features particularly well: it highlights the low and constant columns between -40 and 40 degrees of latitude, the marked annual cycle at mid to high latitudes and the systematic and occasional (2011, 2014, 2016) loss of HNO3 during denitrification periods in the high latitudes of the Southern and Northern hemispheres respectively, which are highlighted by the iso-contours of potential vorticity at ±10 × 10-6 K m2 kg-1 s-1 (dark blue).

In order to give further insights into the interannual variability of HNO3 in polar regions, Fig.  shows the seasonal cycle for each individual year from 2008 to 2016 for eqlat 70–90 in the northern (Fig. 4a) and the southern (Fig. 4b) hemispheres. July and August of 2010 stand out in the Antarctic, with high and variable columns recorded by IASI. This is a consequence of a mid-winter (mid-July) minor sudden stratospheric warming (SSW) event, which induced a downward motion of air masses and modified the chemical composition of the atmosphere between 10 and 50 hPa until at least September . The principal effect of this sudden stratospheric warming was to reduce the formation of PSCs (which stayed well below the 1979–2012 average ) and hence reduce denitrification. This is shown by an initial drop in HNO3 columns in June, as is usually observed in other years but then by an increase in HNO3 columns when the SSW occurs. These results confirm those previously obtained by the Aura MLS during that particular winter and reported in the World Meteorological Organization (WMO) Ozone Assessment of 2014 (see Fig. 6-3 in ). Apart from these peculiarities for the year 2010, all years seem to coincide quite well in terms of seasonality in the Southern Hemisphere (bottom panel, Fig. 4). The timing of the HNO3 steep decrease in particular is consistent from one year to another.

The Northern Hemisphere high latitudes (top panel, Fig. 4) show more interannual variability than in the south, especially during the winter because of the unusual denitrification periods observed in 2011 (purple), 2014 (blue) and 2016 (black) in January (concentrations as low as 2.2 × 1016 molec cm-2 in 2016). In contrast to the winter, the summer columns are more uniform from one year to another with values around 2.1 × 1016 to 2.8 × 1016 molec cm-2.

Time series of HNO3 total columns retrieved from IASI/Metop between 2008 and 2016 have been presented and analysed in terms of seasonal cycle and global variability. The analysis was conducted in terms of equivalent latitudes (here calculated on the basis of potential vorticity) and focused mainly on high latitude regions. We have shown that the IASI instrument captures the broad patterns of the seasonal cycles at all latitudes but also year-to-year specific behaviours. The systematic denitrification process occurring every winter–spring in the Southern Hemisphere shows up unambiguously in the time evolutions and the use of equivalent latitudes enables one to isolate the regions affected based on the dominating stratospheric dynamical regimes. Three distinct zones within the polar regions were separated in particular: (1) the inner polar region (70–90∘ S), where the denitrification starts earliest and where the HNO3 columns reach their lowest values for the longest period; (2) the outer part of the polar vortex (65–70∘ S), where the HNO3 columns drop occurs 1 month later and the minimum concentrations do not reach such low levels; (3) the polar vortex edges (55–65∘ S), where the columns follow a more normal annual cycle, with maxima around July, forming a collar of high columns around the denitrified vortex. The IASI-derived HNO3 distributions also reflect the denitrification periods in the Northern Hemisphere, during the exceptionally cold winters of 2011, 2014 and 2016.

The HNO3 time series were successfully fitted with multivariate regressions in order to identify the various factors responsible for the variability in the observations. To the best of our knowledge, this is the first time that such regression models have been applied to HNO3 time evolution. A specific set of explanatory variables was retained for each equivalent latitude band following an iterative procedure, according to the influence of each of these variables in the regression. The regression model allowed good representation of the IASI observations in most cases (correlation coefficients between 0.81 and 0.94). However, the variability recorded in the tropics could not be reproduced that well, with only about 20 to 40 % correctly accounted for. The regression for other parts of the globe yielded better results, especially in the southern polar regions, where a high percentage (60–80 %) of the observed variability is reproduced by the regression. Generally, it was found that the annual cycle is the factor responsible for the largest part of the variability, showing a hemispheric pattern. The Brewer–Dobson circulation, and also the solar insolation seasonality, which are embedded in the harmonic terms, seem to be the main drivers of variability; the Brewer–Dobson circulation carries NOy towards the poles and both processes bring HNO3 concentrations to their maxima during the local winter when production is enhanced and destruction inhibited. We also interestingly show that polar stratospheric clouds are the second most important driver of the variability of HNO3 in the southern polar latitudes (65–90∘ S). The influence of PSCs is, as expected, less marked in the Northern Hemisphere, but accounting for PSCs still significantly improves the model-to-observation agreement especially during the colder northern winters (R from 0.83 to 0.86). While we feel that the VPSC proxy used here for the PSCs (including only the NAT) is generally good, it is not excluded that adding other forms of PSCs would further improve the model. In any case, the present work shows the potential of using IASI measurements to study the polar denitrification processes in depth.

In the mid- and tropical latitudes, the annual cycle is still prominent, but the relative influence of the QBO increases. Most of the weak seasonality revealed by IASI in the tropical regions is explained by the annual cycle (as well as a potential contribution of African fires and lightning as additional NOx sources), the QBO and the MEI.

More generally, this study shows that the IASI data allow a good analysis and understanding of the HNO3 variability in the atmosphere. The measurements are made with exceptional spatial and temporal sampling, which allows a detailed analysis of the polar regions throughout the entire year. The amount of data allows for a thorough monitoring of the processes regulating the HNO3 variability, such as denitrification processes in the southern polar regions, or seasonal variability in tropical regions. The IASI HNO3 time series will soon be extended with the launch of Metop C in September 2018, which should further improve the regression model. As shown here by the still significant residuals at some periods and locations, other factors could also probably be included to acquire a full and coherent representation of the HNO3 total columns variability.