The CI is the standard method to detect clouds and aerosol with MIPAS . It is defined as the ratio between the mean radiances around the 792 cm-1 band with strong CO2 emissions and the atmospheric window region around 833 cm-1: CI=I¯1([788.25,796.25cm-1])I¯2([832.31,834.37cm-1]), where I¯1,2 is the mean radiance of each window. The CI is a continuous value, where large values indicate clear air conditions and small values indicate the presence of clouds or aerosol. In previous studies CI values below 1.8–6 have been used to indicate cloudy air and CI values above 6 indicate clear air . The CI detection threshold depends on altitude, latitude, and season, mainly because of the water vapour continuum contributing to the 833 cm-1 window radiance . The effect of the water vapour continuum is particularly pronounced at lower tropospheric altitudes.

Because the water vapour continuum absorption decreases with higher wavenumber e.g., we looked for additional windows at higher wavenumbers in MIPAS band A in order to achieve an altitude-, latitude-, and season-independent aerosol detection. For the selection of an appropriate window, which cannot be directly adopted from IR nadir measurements due to the strong trace gas emission lines measured in the IR limb geometry, we considered MIPAS clear air radiance profiles between about 7 and 25 km altitude. In the clear air profile in Fig.  bright colours indicate high radiances due to trace gas emissions and dark colours indicate atmospheric window regions with low radiances. These atmospheric windows are especially suited for aerosol detection, because trace gases have little impact here. The broad window around 830 cm-1 with low radiances at all altitudes is already used for the CI. Between about 940 and 970 cm-1 there are many narrow windows with low radiances between CO2 lines. The broadest of these windows is located between 960 and 961 cm-1. Therefore, we average over the 17 spectral points measured by MIPAS in this window and define the aerosol index (AI) as AI=I¯1([788.25,796.25cm-1])I¯2([960.00,961.00cm-1]), where I¯1,2 is the mean radiance of each window.

In the stratosphere at altitudes from about 50 km down to about 22 km we found a seasonal and diurnal cycle in the radiances between 960 and 961 cm-1 and hence in the AI (Fig. ). This diurnal cycle and the differences between the summer and the winter hemisphere are most likely caused by non-local thermodynamic equilibrium (non-LTE) effects of the CO2 laser bands between 950 and 970 cm-1 at altitudes of 50 km and above e.g.. In order to filter out this non-LTE feature we combined the AI with the CI, which is not affected by non-LTE effects, and defined the aerosol cloud index (ACI) as the maximum value of both: ACI=max(CI,AI).

Below about 25 km the ACI generally corresponds to the AI and above about 25 km it corresponds to the CI (Fig. ).

In order to demonstrate the benefits of the ACI and to derive a fixed threshold value that is applicable to MIPAS measurements, we investigated the behaviour of simulated and measured ACI profiles and compared them with the corresponding CI profiles.

In the radiative transfer simulations we focused on clear air and cloud simulations at altitudes between 5.5 and 19.5 km with a 0.5 km vertical sampling. We considered polar winter, polar summer, midlatitude, and tropical atmospheric conditions without any background aerosol. The simulation results for clear air conditions (Fig. ) show that the AI is significantly larger than the CI. They also show an altitude dependence for the CI but also for the AI/ACI especially at altitudes below 10 km. At altitudes above about 15 km the CI and AI converge towards each other. In the polar summer atmosphere they intersect at 17 km, which means that the ACI corresponds to the CI at altitudes above.

For the cloud simulations we placed 1 km thick ice cloud, sulfate aerosol, and volcanic ash layers at 6–7, 9–10, 13–14, and 17–18 km altitude in polar winter, polar summer, midlatitude, and tropical atmospheric conditions. For the ice cloud, volcanic ash, and sulfate aerosol simulations we assumed various realistic combinations of particle sizes, concentrations, and extinctions given at 948.5 cm-1 (see Tables 3–5 in ). The simulated extinction range was 1×10-3–1, 1×10-3–5×10-1, and 1×10-4–1×10-2 km-1 for ice, volcanic ash, and sulfate aerosol, respectively. The considered mode radii were 0.3–96, 0.1–5, and 0.01–1.5 µm, respectively. Examples of simulated profiles for ice, volcanic ash, and sulfate aerosol are shown in Figs.  to .

While for clear air the AI/ACI values are systematically larger than the CI values both have similar values in case of sulfate aerosol and volcanic ash at cloud altitude. In some cases the AI becomes smaller than the CI, which means that it is even more sensitive towards the aerosol. Also, the deviation from the clear air profile is stronger for the AI than for the CI. For ice clouds, however, the CI is systematically smaller than the AI/ACI, which means that the CI is more sensitive towards ice. Since we focus on aerosol detection, this is not an issue for our study. More details on the simulated CI, AI, and ACI profiles and its sensitivity are discussed in the Appendix Sect. .

To derive an ACI aerosol/cloud detection threshold value we considered the simulated clear air profiles and the MIPAS measurement geometry after the modification in 2005 that reached down to about 10 km in the tropics and about 7 km in the polar regions. In the simulations the ACI is always larger than 7 in the polar winter atmosphere. In the polar summer and midlatitude atmosphere the ACI is larger than 7 at 7 km and above and in the equatorial atmosphere it is larger than 7 at 9 km and above. This means that a fixed ACI threshold value of 7 will be applicable to MIPAS measurements between 2005 and 2012 for the detection of aerosol.

Examples of the CI (Fig. b), the AI (Fig. c), and the ACI (Fig. d) are shown as a function of altitude along a single MIPAS orbit (Fig. a) measured on 18 August 2011. In mid-August most of the northern hemispheric stratosphere was affected by a sulphate aerosol layer caused by the Nabro eruption in June 2011 . Two months after the eruption of the Nabro volcano this layer was diluted, so that it was invisible to IR nadir measurements (Fig. f) and to the CALIPSO operational product but visible to OSIRIS solar scattering measurements and to MIPAS (Fig. d, e) as discussed below.

For the detection of aerosol and clouds along a MIPAS orbit we mainly rely on the established CI. Here we briefly discuss the purposes and shortcomings of the different CI thresholds for the example of a particular MIPAS orbit (Fig. b). A fixed CI threshold of 1.8 (CI below 2 shown in yellow) is used for cloud clearing for trace gas profile retrievals by ESA . This threshold captures tropospheric clouds only and PSCs in the Antarctic but not a volcanic aerosol layer in the northern hemispheric UTLS like the one caused by Nabro. Also, a fixed CI threshold of 4.5 (yellow, orange, red), which is used for more conservative cloud filtering, mainly captures PSCs , subvisible cirrus clouds (SVCs) , and tropospheric clouds but not the volcanic aerosol layer. A fixed CI threshold of 6 (yellow, orange, red, dark red) captures the UTLS aerosol layer but mistakes cloud-free regions (profiles 0–3, 56–60, and 92–95; see comparison with IR nadir data below) as cloudy. This feature of the CI of becoming smaller at lower altitudes in cloud-free conditions is addressed by , providing a variable CI threshold definition at altitudes above 10 km. Here we used a simplified variable CI threshold based on . The details are given in Appendix . This most advanced CI threshold definition (black crosses in Fig. b and d) allows cloudy and cloud-free tropospheric regions as seen by IR nadir instruments (Fig. f) to be discriminated and captures the UTLS aerosol layer as observed by OSIRIS.

Figure  suggests that a fixed ACI with values smaller than 7 allows tropospheric clouds as well as the stratospheric aerosol layer caused by the Nabro eruption to be detected. Hence, we assessed the performance of the ACI with a fixed threshold value of 7 for the detection of aerosol and clouds by comparing the ACI aerosol/cloud detections with CI cloud detections relying on the thresholds presented by for altitudes above 10 km combined with a fixed CI threshold of 2 at 10 km and below (black crosses in Fig. b and d). In Fig. b and d the profiles 0–3, 56–60, and 92–95 are identified as clear air by both the CI and the ACI. The UTLS aerosol layer is also captured by both. However, the CI and the ACI cloud/aerosol detections are not completely identical. Below 10 km the CI with a threshold of 2 identifies slightly less clouds than the ACI with a threshold of 7. Examples can be found in profiles 16, 18, and 19 around 10 km and between profiles 30 and 40, where the CI detects clear air down to the lowest tangent altitude, whereas the ACI indicates clouds at the lowest tangent altitude. Above 10 km there are a few profiles where the CI identifies clouds/aerosol but not the ACI and vice versa (e.g. profiles 5, 54, 41–44). However, the main differences can be seen in profiles 16–18, where the CI indicates clouds/aerosol down to the tropopause whereas the ACI indicates a stratospheric aerosol layer with clear air below, and between profiles 23 and 35, where the CI indicates a thin stratospheric aerosol layer whereas the ACI indicates aerosol down to the tropopause.

The tropospheric aerosol/cloud detections of the CI and ACI are confirmed by geostationary IR nadir measurements by MTSAT (15:00 UTC), IODC (18:00 UTC), and GOES East (15:00 UTC) (the IR nadir images were obtained from NERC Satellite Receiving Station, Dundee University, Scotland, http://www.sat.dundee.ac.uk/) along the orbit track (Fig. f) and closest to the MIPAS measurement time (15:05–16:45 UTC). In the IR nadir images clear air is indicated by dark/black colours, high-altitude clouds are bright white, and low-altitude clouds are indicated by greyish colours. For the clear air profiles 0–2 north west of Australia (22–20∘ S), 53–60 west of South America (2∘ N–23∘ S), and 92–95 over the Indian Ocean the IR nadir images show only low-altitude clouds, which are below the lowest tangent altitude of MIPAS. Over Asia (7∘ S–15∘ N) many high-altitude clouds are present (profiles 4–15). Over northern China and Mongolia there is a gap in the high-altitude clouds (profiles 17–18). Over North and Central America (10–60∘ N) there are patchy cloud patterns, which is also reflected in the alternating cloudy and clear air profiles (profiles 38–51) measured by MIPAS. From 26 to 60∘ S (profiles 61–70) there is a large field of high-altitude clouds. At latitudes higher than 60∘  the results from geostationary images become uncertain, hence they are not discussed here. A more detailed assessment of the detection sensitivity and altitude information accuracy of the MIPAS measurements of the UTLS aerosol layer will be presented in a future study.

The simulations as well as the comparison with the most sensitive variable CI threshold values indicated that a fixed ACI value of 7 is an appropriate threshold value. Hence, in the following we use the ACI with a fixed threshold value of 7 down to 7–9 km as an alternative to the CI with variable thresholds above 10 km and a fixed threshold of 2 below 10 km. The example (Fig. ) shows the advantage of the ACI, namely to have a fixed threshold that is sensitive to thin aerosol layers and thick clouds, while not mistaking tropospheric clear air as cloudy.

For the measurements before 2005 we also analysed altitudes below 7–9 km. For these altitudes the simulations in Sect.  indicated that the ACI for clear air falls below 7 at altitudes below 7–9 km. However, we often found ACI values significantly larger than 7 down to the lowest tangent altitudes (Figs. , ) in the MIPAS measurements before 2005. As the most likely reason for the discrepancy between the measurements and simulations below 9 km we identified the water vapour continuum assumed in the simulations. On the one hand, we used climatological water vapour profiles that inherently do not cover the complete variability in the atmosphere; on the other hand, in JURASSIC the Mlawer–Tobin–Clough–Kneizys–Davies 1.10 scheme (MT_CKD) is used for the water vapour continuum representation. This scheme was found to represent real conditions with insufficient accuracy at lower altitudes . Hence, the ACI has the potential to be also applied to the MIPAS measurements before 2005, where the lowest tangent altitude reached down to nearly 5 km at all latitudes.

For the separation between aerosol and ice clouds by IR nadir measurements spectral windows around 8.5, 11, and 12 µm (1176, 909, 833 cm-1, respectively) are employed . For these windows the optical properties of ice differ most strongly from aerosol such as volcanic ash, soil-derived aerosol, and sulfate aerosol . For the IR limb emission measurements of MIPAS we identified three narrow windows that have very little interference with trace gases and exploit the spectral differences between the optical properties of ice and, in our case, volcanic aerosol:

830.6–831.1 cm-1

The optical properties, i.e. extinction coefficient (βe) and single scattering albedo, are determined by the microphysical properties of the particles, i.e. complex refractive index, particle size, and particle shape. The imaginary and real parts of the complex refractive indices of ice , sulfate aerosol , and two representatives of volcanic ash (, volcanic ash; , basalt) are shown in Fig. a and b, where our windows are indicated by grey bars. The imaginary part of the refractive index (Fig. a) has a positive spectral gradient for ash and sulfate between 830 and 960 cm-1, whereas the spectral gradient of ice is negative. Also, between 830 and 1224 cm-1 the spectral gradient is positive for sulfate but negative for ice. For the real part of the refractive indices (Fig. b), the spectral gradient between 830 and 960 cm-1 is positive for ash and sulfate but negative for ice. The spectral gradient between 960 and 1224 cm-1 is negative for ash and sulfate but positive for ice.

As the imaginary part of the complex refractive index describes the absorption and the real part describes the scattering, these differences between ice clouds and aerosol (volcanic ash and sulfate aerosol) propagate to the optical properties (Fig. c and d). To calculate the extinction coefficient and single scattering albedo we used the same log-normal size distributions as for sulfate aerosol, volcanic ash, subvisible cirrus (small ice particles), and tropical cirrus (large ice particles). The extinction coefficient spectra (Fig. c) were normalised to 1 at 960 cm-1. For sulfate aerosol and ash they exhibit similar spectral gradients as the imaginary part of the refractive index. However, in addition to the refractive indices, the particle size has an impact on the optical properties. For large ice particles the extinction coefficient spectrum is flat and for small ice particles it exhibits a pronounced minimum around 960 cm-1. The single scattering albedo depends on particle size and wavenumber (Fig. d). The scattering contributions range from 30 to 80 % for small ice particles and are nearly constant around 55 % for large ice particles. The scattering contribution of the sulfate aerosol is generally below 10 % and for volcanic ash it ranges from 15 to 80 %. Hence, scattering effects can not be neglected for any particle type discussed here.

Having identified three window regions with small radiance contributions by atmospheric trace gases and with significant differences in the optical properties for ice and volcanic aerosol, we expected that these differences could also be found in the radiance spectra measured by MIPAS. For IR nadir measurements it is common practice to use brightness temperature differences (BTDs) for the discrimination between volcanic or soil-derived aerosol and ice clouds e.g.. In order to identify characteristic patterns in BTD correlations for IR limb emission measurements, we evaluated the radiative transfer simulations for clear air, ice, volcanic ash, and sulfate aerosol under various atmospheric conditions (). In contrast to IR nadir BTD analyses that often correlate 11–12 µm with 8–11 µm e.g., we found for MIPAS IR limb spectra the clearest correlation patterns for the BTDs between the 830 and 1224 cm-1 (12.0–8.2 µm) windows and the 960 and 1224 cm-1 (10.4–8.2 µm) windows.

The simulation results are shown for all realistic ice cloud (Fig. a), sulfate aerosol (Fig. b), volcanic ash (Fig. c), and “clear air” (Fig. d) scenarios. The tangent altitudes of the simulations range from 6 to 19.5 km, which is from above to below the simulated clouds. Except for the clear air simulations only the scenarios for ACI < 7 are shown. In the simulations, the individual results for the BTDs are colour-coded by the ACI because we found a clear sensitivity to the ACI in the simulations. The solid black lines are nearly identical to the diagonal and split the plot into two parts (upper and lower part). Here, we use them to guide the reader through each panel of Fig. . In fact, these are our threshold functions that will be derived in Sect. .

The ice simulations (Fig. a) comprise extinctions ranging from 0.1 to 1 km-1 and mode radii ranging from 0.3 to 96 µm. They fall in the middle of the BTD correlation plot, forming a longish shape and reaching from negative BTDs to positive BTDs (about -45 to 5 K) on both axes. Nearly all simulations fall into the lower part (below the nearly diagonal separation lines). In the little inset of Fig. a we filtered the comprehensive simulations for realistic combinations of particle sizes and extinctions (number concentrations). Hence, we removed all simulations with mode radii of 6 µm and below and extinctions of 5×10-2 km-1 and larger from the polar summer, midlatitude, and tropical atmosphere because we consider such small ice particle sizes and high extinction (number concentrations) only likely in PSCs . SVCs may have such small ice particle sizes, but they inherently have lower extinctions . The ice simulations show a dependence on the ACI, where optically thick scenarios (ACI < 2) form a narrow cluster below the black line at BTDs between 0 and -20 K. Optically less dense ice scenarios (ACI > 2) usually have a lower BTD on both or only one of both axes.

The sulfate simulations (Fig. b) performed for extinctions between 1×10-4 and 1×10-2 km-1 and mode radii between 0.01 and 1.5 µm have ACI values ranging from larger than 2 to larger than 7. None of the scenarios that include post-Pinatubo particle sizes and concentrations gets optically thick. Scenarios for the 1 km thick sulfate aerosol layer with an extinction coefficient of 1×10-4 km-1 at 947 cm-1 (10.5 µm) have ACIs larger than 7 and hence would be identified as clear air. Scenarios with extinction coefficients of 5×10-4 km-1 and higher have ACIs below 7. They cluster above the diagonal at low BTDs (smaller than -20 K on the x axis). The scenarios that fall in the lower part of the plot are all for tangent altitudes below 8 km. Comparing the sulfate aerosol simulations with the ice cloud simulations, there is nearly no overlap except for very few scenarios below 8 km.

For the simulations of volcanic ash we used the “basalt” refractive indices (Fig. c) and the “volcanic ash” refractive indices (not shown). The simulations that were performed for extinctions between 1×10-3 and 5×10-1 km-1 and mode radii between 0.1 and 5 µm cover optically thick (ACI < 2) to thin conditions. The simulated scenarios form a diagonal shape in the middle of the BTD correlation plot and extend from the lower left to the upper right of the plot. Also, a dependence on the ACI, similar to the ice cloud simulations, with high values (optically thin) at bottom left and small values (optically thick) at top right can be seen. Several ash simulations fall in the lower part of the plot and are congruent with the ice simulations. Hence, only the ash scenarios that fall in the upper part of the plot are separable from ice clouds. In the simulations we found a particle size dependency (not shown), where smaller particles fall in the upper part and larger particles fall in the lower part (ice region) of the plot. A size dependency for detecting volcanic ash with IR limb emission measurements has also been reported by with a maximum detectable mode radius of 1.5 µm for volcanic ash . Here, for volcanic ash we found scenarios with a mode radius of 5 µm that do not overlap with ice. From the simulations presented here, we derived that volcanic ash particles with mode radii between 0.3 and 5 µm and extinction coefficients between 1×10-3 and 5×10-1 km-1 can be discriminated from ice clouds at altitudes down to 6 km. Comparing the ash simulations with the sulfate simulations, there is substantial overlap. Hence, further criteria are required for the discrimination between sulfate aerosol and volcanic ash. As the focus here is on filtering out ice spectra from all cloudy spectra, we do not pursue an aerosol classification at this point.

Comparing the clear air simulations for the four atmospheres (polar winter, polar summer, midlatitudes, equatorial) at altitudes between 6 and 19.5 km (Fig. d) with the ice and aerosol simulations shows that there is an overlap. As already described in Sect. , there are a few simulated clear air scenarios with ACI < 7 below 9 km. However, all clear air scenarios at altitudes below 14 km fall in the lower part (ice region) of the BTD correlation plot and hence will be filtered out together with the ice scenarios. Some scenarios above 14 km fall in the upper part of the BTD correlation plot, but they all have an ACI > 10. Using an ACI < 7 as a pre-condition, these clear air scenarios will be filtered out also.

Having expectations from the simulations on what the BTD correlations look like for ice clouds, sulfate aerosol, and volcanic ash, we show four selected cases of UTLS aerosol measurements in order to verify the simulations (Fig. ). For each case we used all measurements of an entire day, which is about 14 orbits, in the latitude range given below and at altitudes between MIPAS lowest tangent altitude (about 7 km) and 25 km. For the longitude range we did not introduce a limitation in order to include also clear air and ice clouds. In Fig.  all spectra with cloud/aerosol detections (ACI < 7) are shown. Targeting at successfully filtering out ice clouds, we expect that the aerosol clearly stands out from the ice region as indicated by the simulations. For better comparison we also added the nearly diagonal separation lines introduced in Fig. .

For 17 May 2011 (Fig. a) we expected to find only ice clouds after filtering out the clear air spectra in the entire Southern Hemisphere at latitudes between 0 and 60∘ S (since there are no reports of volcanic eruptions; ). As in the ice cloud simulations (Fig. ), the measurements with ACI < 7 form a relatively narrow diagonal group with BTDs ranging from 0 to -40 K on both axes.

For 29 July 2011 (Fig. b) we expected to find sulfate aerosol from the Nabro eruption covering the entire northern hemispheric UTLS (0–90∘ N) and ice clouds as well. In the measurements with ACI < 7 we again observe a relatively narrow diagonal group just below the separations lines as in the ice simulations. In addition there is a second cluster with ACI values between 5 and 7 and at BTDs between -30 and -50 K on the abscissa and between -20 and -40 K on the ordinate, just as expected from the simulations for sulfate aerosol (Fig. b).

For 16 June 2011 (Fig. c) we expected to find volcanic ash at latitudes between 0 and 60∘ S originating from the eruption of the Puyehue–Cordón Caulle and ice clouds as well. As seen in the two cases before, in the measurements with ACI < 7 there is again a relatively narrow diagonal group just below the separation lines as predicted by the ice simulations. Furthermore, there are many spectra above the separation lines forming an arc-shaped structure and with ACI values ranging from below 2 to 7. These measurements fall in the region covered by the volcanic ash simulations (Fig. a).

For 29 January 2011 (Fig. d) we again expected to find tropospheric ice clouds after filtering out clear air spectra and polar stratospheric clouds in the Northern Hemisphere (0–90∘ N). As in the cases before, there is the narrow diagonal ice group with ACI values ranging from below 2 to 7, but there is also a second cluster just above the separation lines with ACI values ranging from below 2 to 7. If only measurements between 0 and 60∘ N are considered, the second cluster disappears (Fig. ). This example demonstrates that non-ice PSCs can also be separated from ice PSCs using this BTD correlation. To determine the non-ice PSC types, particle sizes and concentrations that can be separated from ice PSCs with this method require detailed simulations which are not within the scope of this study. The classification of PSCs measured by IR limb sounders is the subject of multiple studies and references therein.

As the simulations and measurements showed, the ice scenarios fall in the lower half of the BTD correlation plot and a substantial number of the aerosol simulations (volcanic ash and sulfate aerosol) falls in the upper half (Figs.  and ). In order to deduce an ice filtering threshold for MIPAS measurements, we analysed the 2011 MIPAS data on a day-by-day basis and analysed monthly and annual count statistics. From these we found that filtering out the ice clouds is best done with two threshold functions. For ACI values below 4 the upper edge of the ice cluster (BTDs larger than 30.4 K on the x axis) is very sharp in the simulations (Fig. a) as well as in the measurements (Fig. ). The corresponding threshold function is BTD960-1224=0.87×BTD830-1224+6K. For ACI values larger than 4 the BTD scatter plot can become quite diffuse for 2011 because there were three volcanic eruptions (Grímsvötn, Puyehue–Cordón Caulle, Nabro) that injected a substantial amount of SO2 and volcanic ash into the atmosphere. We also analysed the measurements from 2003, a year with very little volcanic emissions during the MIPAS measurement period. For BTDs below -30.4 K we obtained the following threshold function: BTD960-1224=1.33×BTD830-1224+20K.

To identify aerosol in the MIPAS data we used the condition that the BTDs must exceed at least one of the two threshold functions given in Eqs. () and () (in Fig.  the black solid lines must be exceeded).

For the MIPAS orbit discussed in Sect.  the result using our ice filtering thresholds is shown in Fig. e. Figure e shows the ACI as in Fig. d but indicating in grey all spectra that fall in the ice region. As expected (see Sect. ), nearly all aerosol/cloud detections in the northern hemispheric troposphere are identified as ice clouds. On top of these ice clouds there is a layer in the UTLS with ACI values between 4 and 7, which is the Nabro sulfate aerosol. In the Antarctic region a large fraction of the stratospheric clouds falls in the ice region, which can be expected in southern hemispheric winter, but around profiles 83–85 non-ice PSCs are also identified, consistent with the report of a NAT belt in the region downstream of the Antarctic peninsula .

Considering Eqs. () and () derived from 2 years of MIPAS measurements (2003, 2011) and the ice simulations (Fig. a), it becomes obvious that the line corresponding to Eq. () is very close to the simulations and that there is a gap between the line corresponding to Eq. () and the simulation results. Comparing the measurements (Fig. ) and the simulations in more detail, we found that the narrow ice pattern below the threshold functions in the measurements is mostly reproduced by the ice cloud simulations. Especially for ACI values below 2 simulations and measurements agree well. However, for higher ACI values there are many scenarios that fall in a range that was not covered by the measurements (about 0 to -40 K on the abscissa and -20 to -50 K on the ordinate). These scenarios have in common that the assumed mode radius of the particles is 6 µm or smaller and the extinction coefficient is smaller than or equal to 1×10-2 km-1. We considered such small ice particles to be likely for ice PSCs and possibly SVCs . However, in the tropics and at midlatitudes we could not find MIPAS measurements that fall into this range. Hence, according to the MIPAS measurements 1 km thick ice clouds with mode radii smaller than 6 µm seem very unlikely in the tropics and at midlatitudes. Only in the polar regions, which are excluded in Fig. a, b, c, did we find MIPAS measurements that show the same BTDs as the ice cloud simulations for mode radii down to 3 µm.

As already discussed in Sect. , there are a few simulated scenarios in the tropical atmosphere for the 18 km ice cloud that slightly exceed the upper threshold function. For these particular scenarios we assumed mode radii of 6 µm or less and the extinction coefficients are between 5×10-2 and 1 km-1. In the tropics mode radii of 6 µm or less can only be found in SVCs. However, SVCs do not have such high particle concentrations and extinctions . Hence, in the inset of Fig. a the simulation results are shown only for ice particle size distributions with mode radii larger than 6 µm. The pattern of these ice cloud simulations is in very good agreement with the measurements shown in Fig. . Further all, but 7 out of 3333, simulated scenarios fall below the threshold functions derived from the measurements. These seven scenarios occur only for the ice cloud at 18 km altitude in the tropics and they all have a BTD larger than -25 K on the abscissa. We checked the MIPAS measurements between 20∘ (30∘) N and 20∘ (30∘) S and found that in 48 (62) out of 58 945 (78 563) cloudy profiles in 2011 the upper threshold was exceeded (both 0.08 %). In 2003, a year with less volcanic aerosol, only 15 (18) out of 56 375 (74 767) (0.02–0.03 %) cloudy profiles exceeded the upper threshold. Although we do not consider 0.02–0.08 % to be a strong evidence in the measurements that tropical high-altitude clouds consisting of small ice particles could exceed the ice separation threshold, we would like to point out that there is at least the theoretical possibility.

The sulfate aerosol simulations (Fig. b) form a group just above the lower threshold function. This group matches very well the sulfate aerosol observations after the Nabro eruption (Fig. b). There are also a few scenarios for which the simulated BTD does not exceed the aerosol detection threshold. These scenarios occur all at tangent altitudes below 8 km. However, in the MIPAS measurements we found numerous cases of aerosol detection below 8 km altitude. This is most likely due to the fact that the aerosol layers in reality have a larger vertical extent than the 1 km assumed in the simulations. This effect and an analysis of the altitude information including comparisons with lidar measurements will be discussed in detail in a separate study.

The volcanic ash simulations using “basalt” refractive indices reported by (Fig. c) represent the volcanic ash measurements after the Puyehue–Cordón Caulle eruption (Fig. c) better than simulations using the “volcanic ash” refractive indices reported by . Thus, the volcanic ash simulations are very sensitive to the refractive index data and therefore to the type of volcanic ash present.