Vertical profile of peroxyacetyl nitrate ( PAN ) from MIPAS-STR measurements over Brazil in February 2005 and its contribution to tropical UT NO y partitioning

We report on the retrieval of PAN (CH3C(O)OONO2) in the upper tropical troposphere from limb measurements by the remote-sensor MIPASSTR on board the Russian high altitude research aircraft M55-Geophysica. The measurements were performed close to Araçatuba, Brazil, on 17 February 2005. The retrieval was made in the spectral range 775–820 cm −1 where PAN exhibits its strongest feature but also more than 10 species interfere. Especially trace gases such as CH 3 Cl3, CFC113, CFC-11, and CFC-22, emitting also in spectrally broad not-resolved branches, make the processing of PAN prone to errors. Therefore, the selection of appropriate spectral windows, the separate retrieval of several interfering species and the careful handling of the water vapour profile are part of the study presented. The retrieved profile of PAN has a maximum of about 0.14 ppbv at 10 km altitude, slightly larger than the lowest reported values ( <0.1 ppbv) and much lower than the highest reported in the literature (0 .65 ppbv). Besides the NO y constituents measured by MIPAS-STR (HNO 3, ClONO2, HO2NO2, PAN), the in situ instruments aboard the Geophysica provide simultaneous measurements of NO, NO 2, and the sum NOy. Comparing the sum of in-situ and remotely derived NO+NO2+HNO3+ClONO2+HO2NO2+PAN with total Correspondence to: M. Höpfner (michael.hoepfner@imk.fzk.de) NOy a deficit of 30–40% (0.2–0.3 ppbv) in the troposphere remains unexplained whereas the values fit well in the stratosphere.


Introduction
PAN (CH 3 C(O)OONO 2 ) is the most common member of peroxyacyl nitrates playing an important role in tropospheric chemistry.In high concentrations -higher, than up to now measured in the atmosphere -it is known to be eye irritant and phytotoxic to plants.PAN was first discovered in a Los Angeles photochemical smog episode (Stephens et al., 1956).Biomass burning was also suggested to be a significant source of PAN (Holzinger et al., 2005).The formation of PAN in the atmosphere involves hydrocarbons (paraffins, olefins, aromatics) and oxides of nitrogen.It is initiated by the reaction of OH with hydrocarbons.After intermediate reactions involving acetaldehyde, the acetyl radical and molecular oxygen, the peroxyacetyl radical (CH 3 CO 3 ) is formed which further reacts with NO 2 to PAN (Singh, 1987).
The lifetime of PAN in the lower troposphere is in the order of hours and is dominated by thermolysis.In the upper troposphere, the lifetime, dominated by photolysis, is of the order of months, or even longer in dark Arctic regions (Talukdar et al., 1995;Kirchener et al., 1999).Median PAN/NO y ratios of more than 0.6 at altitudes from 4 km Published by Copernicus Publications on behalf of the European Geosciences Union.
C. Keim et al.: Tropical vertical profile of peroxyacetyl nitrate to 8 km have been observed in long-range transported Asian pollution plumes (Roberts et al., 2004).For details on the formation and distribution of PAN see Warneck (1999) and Finlayson-Pitts and Pitts (2000).
Remote sensing in the infrared provides an alternative and independent method for the measurement of PAN.From occultation measurements of ACE-FTS onboard SCISAT-1 it was possible to retrieve PAN in a young biomass burning plume (Coheur et al., 2007).Recently, Remedios et al. (2007a) have shown the clear presence of the signatures of PAN in the emission spectra obtained by the balloon born MIPAS.Global upper tropospheric PAN distributions were derived from MIPAS/Envisat spectra by Glatthor et al. (2007).
In this paper we report the retrieval of PAN from measurements of MIPAS-STR (Michelson Interferometer for Passive Atmosphere Sounding-STRatospheric aircraft, Piesch et al., 1996) an instrument operated on board the high-altitude aircraft Geophysica.The work was initiated by the observation that large differences exist in the upper troposphere (above 10 km) between MIPAS-STR measurements of HNO 3 and coincident in situ measurements of NO y -NO by SIOUX (StratospherIc Observation Unit for nitrogen oXides, Schmitt, 2003) also aboard the Geophysica.
In the following we give a short description of the MIPAS-STR instrument and its measurement strategy, an overview of the flight of 17 February 2005 and compare HNO 3 data from MIPAS-STR with coincident in-situ measurements of (NO y -NO-NO 2 ) to obtain an upper limit PAN profile (Sect.2).Further we give a simulation on the feasibility of detecting PAN from the MIPAS-STR observations (Sect.3), relevant general details on the data processing (Sect.4) and finally the retrieval of PAN from the measured spectra, including the er-ror estimation (Sect.5).The last section gives a summary and a discussion of the results.

MIPAS-STR measurements on 17 February 2005
2.1 The MIPAS-STR instrument MIPAS-STR is a cryogenic Fourier transform emission sounder operating in the middle infrared (Fischer and Oelhaf, 1996;Keim et al., 2004).The emission method allows limb and upward viewing, yielding about 2 km vertical resolution below the flight level (up to 20 km).Reduced vertical information above the flight level is obtained by upward measurements with several elevation angles.The final results are 2-dimensional distributions of the trace gases along the flight track in an altitude range covering the lowest stratosphere and the upper troposphere.
The first deployment of MIPAS-STR was made during the Antarctic campaign APE-GAIA in 1999 (Höpfner et al., 2000).The performance of the instrument has been considerably improved in recent years.The pointing of the limb measurements has been operated at fixed tangent heights between 6 km and the flight altitude with a spacing of 1 km.Considering the instrumental field of view of 0.44 degrees (FWHM) over-sampling by a factor 2-3 was applied at the lower tangent heights.In addition upward measurements at elevation angles of 0, 1, 3 and 10 degrees, as well as zenith and cold blackbody (210 K) measurements were performed.Twosided interferograms were obtained with a maximum optical path difference L of 14.4 cm, resulting in an unapodised spectral resolution (1/2 L) of 0.035 cm −1 .For a flight altitude of 19 km the complete sequence, including calibration, takes 200 s.This results in a horizontal resolution in flight direction of about 36 km.The data shown in this paper are obtained from channel 1, which covers the wavenumber range of 770-970 cm −1 .

Flight scenario
The flight track of the Geophysica with the location of the tangent points of MIPAS-STR limb sequences is given in Fig. 1.From Arac ¸atuba (21.2 • S, 50.4 • W) the flight was conducted northbound and returned south on a straight track from 14 • S to 23 • S. Optically thick clouds were observed in the northern part of this leg which prevented trace gas retrieval for that region.However, cloudless condition were found in the southern part, just before the descent.The red rectangle in Fig. 1 indicates the tangent points of six cloud free limb sequences measured between 13:05 and 13:20 UTC (about 10 a.m.local time).These six limb sequences cover the track of the aircraft on the descent, which started at the southernmost point of the path.The flight thus gives an excellent opportunity to compare the MIPAS-STR profile with in situ data measured during descent.

The SIOUX instrument
Nitric oxide (NO) and total reactive nitrogen (NO y ) were measured with the SIOUX (Stratospheric Observation Unit for nitrogen oXides) instrument.
SIOUX includes a two-channel high sensitivity NO detector using gas-phase O 3 /NO-chemiluminescence technique.For detection of total reactive nitrogen (defined as NO y = NO + NO 2 + NO 3 + HNO 3 + HNO 2 + HNO 4 + PAN + RONO 2 + 2 • N 2 O 5 + halogen nitrates + organic nitrates + aerosol nitrates), higher oxidised nitrogen compounds are reduced to NO using a heated (300 • C) catalytic gold converter operated with addition of CO (e.g., Fahey et al., 1985).The conversion efficiency of the NO y converter to HCN was investigated in laboratory tests and amounts to 2-15% of the HCN mixing ratio increasing with increasing ozone concentration.In the troposphere this may result in an artifact NO y signal of 4 pptv (<1% of the NO y signal) for a HCN mixing ratio of 200 pptv.Further details of the measurement technique are given in Ziereis et al. (2000).The SIOUX instrument has already been used during several field campaigns on board the Geophysica (Grooß et al., 2005;Voigt et al., 2005Voigt et al., , 2006)).It is housed in a container under the right wing of the Geophysica.Ambient sample air is passed through a rearward fac- ing inlet to avoid sampling of larger aerosol particles.Small aerosols (<1 µm) may enter the inlet, however, the contribution of nitrate contained in these particles to the NO y signal is considered negligible.The overall accuracy of the NO and NO y measurements is 7% and 12% (1σ ), respectively, with a time resolution of 1 s.
The concentration of NO 2 has been be calculated assuming a photochemical steady state between daytime NO 2 and NO according to Eq. ( 1) (e.g., Schlager et al., 1997).
where square brackets indicate concentrations, k(T) denotes the temperature-dependent rate coefficient of the reaction of O 3 with NO, and J NO 2 is the NO 2 photolysis frequency.The J NO 2 values are calculated with the radiative transfer model of Ruggaber et al. (1994).The O 3 concentrations are taken from measurements of FOZAN (Fast OZone ANalyzer, Ulanovsky et al., 2001), an other in situ instrument aboard the Geophysica, and the temperatures from a high-precision TDC (thermodynamic complex) sensor (Rosemount sensor customized at CAO, Central Aerological Observatory, Dolgoprudny, Russia) also aboard the same aircraft.Individual uncertainties for O 3 , k, and J NO 2 are 5%, 35%, and 25%, respectively.The overall accuracy of the NO 2 calculation is 25% (1 σ ).In Fig. 2 the mean HNO 3 volume mixing ratio (vmr) from the six southernmost limb sequences (see Fig. 1) is compared to the in situ observation of NO y -NO-NO 2 during descent.HNO 3 was retrieved in its ν5 and 2ν9 bands as described in Wang et al. (2007) using the MIPAS spectroscopic database version PF3.1 (Flaud et al., 2003).NO y and NO are measured directly by SIOUX, while NO 2 is calculated as described above.The vmr profiles of ClONO 2 and HO 2 NO 2 , also included in NO y , were retrieved from the MIPAS-STR measurements (see Höpfner et al. (2007) and Stiller et al. (2007) for the method), but due to their very low vmr (see Figs. 5 and 13) neglected in the comparison.
The altitude of the cold point tropopause (see Fig. 12) is about 18 km.Below this altitude, NO y -NO-NO 2 is always higher than HNO 3 by up to 0.32 ppbv.In the following we investigate how much of this difference can be attributed to PAN.

Spectral simulations for PAN
A well suited band for mid-IR PAN analysis is located between 775 and 820 cm −1 (Glatthor et al., 2007;Remedios et al., 2007a).To indicate the contribution of different atmospheric trace species in this spectral region we show simulations performed with KOPRA (Karlsruhe Optimised and Precise Radiative transfer Algorithm, Stiller et al., 2000) for a tangent height of 8 km located at the southern part of the flight.In total 38 different trace gases are considered, the 14 strongest radiances are shown in Fig. 3.The band of PAN is mainly interfered by CO 2 , H 2 O, O 3 , CCl 4 , CFC-22, CFC-113, CH 3 CCl 3 , and ClONO 2 .The infrared cross-section data for PAN at 295 K (Allen et al., 2005a) and the newer data at lower temperatures of 273 K, and 250 K (Allen et al.,Fig. 4. Simulated difference spectra (with-without PAN) in the broad-band range of PAN for all tangent heights from 8 to 18.6 km.The noise level of MIPAS-STR is about 15 nW/(cm 2 sr cm −1 ) 2005b) are adopted in this paper.For the simulation, the temperature profile is taken from the ECMWF model (Fig. 12 supports this choice as reasonable) and the vmr profiles for all gases except for H 2 O and PAN are taken from a tropical climatology (Remedios et al., 2007b).For PAN a midlatitude profile of the MOZART Model (Model for OZone And Related chemical Tracers Horowitz et al., 2003) is used.The water profile is estimated from in situ measurements of FLASH (FLuorescence Airborne Stratospheric Hygrometer, Sitnikov et al., 2007) and FISH (Fast In situ Stratospheric Hygrometer, Zöger et al., 1999) aboard the Geophysica (see Fig. 6).
The sensitivity of the MIPAS-STR observation to PAN is demonstrated by plotting simulated difference spectra (withwithout PAN) for various tangent heights between 8 and 18.6 km (see Fig. 4).
In small spectral regions, the information on PAN is reduced due to saturation caused by interfering trace gases.This is the case around the CO 2 Q-branch (792 cm −1 ) and at the position of strong CO 2 and H 2 O lines.Apart from these regions the radiance abates rather quickly with increasing tangent height.This quick decrease is not surprising, the emitted radiance is (mainly) proportional to the concentration of the trace gas and via Planck's law also dependent on its temperature.As the temperature and the pressure (drives the concentration, if not compensated by a strong vmr increase) decrease with increasing altitude, the radiance decreases.At 13, 14, and 15 km, the maximum radiance is only 50, 25, and 12.5 nW/(cm 2 sr cm −1 ), respectively, comparable with the spectral noise 15 nW/ cm 2 sr cm −1 in the single MIPAS-STR spectra.
However, with the high resolution spectra the broadly emitting PAN can be retrieved by multi-line retrieval from much lower radiances.In the present work 1171 independent spectral points were used to obtain a PAN profile.

Level-1 processing
Level-1 processing of the MIPAS-STR data provides the input data for the subsequent profile retrieval.Basically, it converts raw interferograms of the atmospheric measurements stored during the flight into radiometrically calibrated atmospheric spectra for each tangent height or elevation angle.The spectral gain and offset of the instrument were obtained from the zenith and cold blackbody measurements of each individual sequence.The zenith spectra were corrected for the contained atmospheric features.Level-1 processing also provides the auxiliary data which are derived from the stored housekeeping information as well as from the line of sight calibration and the field of view measurements made before and after the flight.The auxiliary data include information on the corrected flight altitudes, elevation-and azimuth angles, and relevant instrument parameters.

Level-2 processing
Vertical profiles of the atmospheric parameters (vmr of gases, temperature, pressure and absorption/emission of aerosols) are retrieved by use of the atmospheric radiative transfer model KOPRA and its inversion algorithm KOPRAFIT.The profiles are iteratively changed to minimise the residuum between measured spectra and forward calculated spectra of a complete sequence.Regularisation of the profile shape against an a priori profile is necessary for each retrieved atmospheric parameter because the chosen retrieval grid (0.5 km) is finer than the achievable vertical resolution.In KOPRAFIT the Tikhonov-Philips regularisation method (Tikhonov, 1963;Phillips, 2003) was adopted: where i denotes the iteration index; x the vector with the unknowns; x a the a priori values; y the measurement vector; S y the measurement covariance matrix of y; f the forward model; K the spectral derivatives matrix; γ the regularisation parameter and L the first derivative regularisation operator.
The regularisation strength is chosen as small as possible, just large enough to avoid oscillations in the resulting profile.The achieved vertical resolution of the retrieved profile is the FWHM (full width at half maximum) of the columns of the averaging kernel matrix, given by:

PAN retrieval method
Here we describe the strategy used for the retrieval of PAN.To minimise the error contribution from spectral noise, we have averaged all spectra of the same tangent height/elevation angle within the six southernmost limb sequences (see Fig. 1), which reduces the noise from 15 to 6 nW/(cm 2 sr cm −1 ).The MIPAS-STR spectra are averaged over the same region where the in situ measurements while the descent took place.The small variability of the individual spectra and also of the 6 individual profiles (e.g. of temperature and CFC-11) permits us to do the average and the comparison with the in situ profiles.Furthermore, we have used all spectral points between 775 and 820 cm −1 , with the exception of the region 790-794 cm −1 .We excluded this interval to avoid any error on the retrieval from line-mixing of the CO 2 Q-branch located there.A summary of atmospheric parameters (12 species and temperature) that have been considered in the retrieval scheme is given in Table 1.Among those parameters, five species (CH 3 CCl 3 , CFC-113, CFC-22, CFC-11, and ClONO 2 ) have been determined in steps previous to the PAN retrieval and are kept constant.The remaining profiles are fitted simultaneously with PAN.
ClONO 2 is fixed to the profile derived from the nearby ν 4 Q-branch in the interval 779.5-781 cm −1 (Höpfner et al., 2007).CFC-11 has been determined on the basis of the major band in the interval 838-856 cm −1 (von Clarmann et al., 2007a) and CFC-22 has been obtained from its signature at 828.7-829.4cm −1 (Moore and Remedios, 2008).CH 3 CCl 3 and CFC-113 profiles are firstly estimated from the tropic climatology and then scaled to remove their spectral signatures from the residuum.The scaling factor corrects the profiles for the annual decrease.
The vmr profiles of all five pre-determined species are plotted in Fig. 5.As a priori vmr profile for PAN (PAN a priori ) a mid-latitude profile of the MOZART model is used (see Fig. 7).Beside trace gases and temperature we determine a continuum extinction profile for aerosols and a tangent height constant radiation offset for minor calibration errors.
Water continuum emission and possible aerosol broadband emission increase with decreasing tangent altitude.Both affect the baseline of the spectra.As the retrieval of PAN is sensitive on the quality of the baseline, we did not use spectra below 8 km altitude.The continuum contribution in these spectra exceed our threshold of 400 nW/(cm 2 sr cm −1 ) in the laser band region (960 cm −1 ).

Determination of the H 2 O a priori profile
Although H 2 O is simultaneously fitted with PAN, an impact of the applied a priori profile for water vapour on the PAN Also shown are the a priori profile and the upper limit corresponding to the NO y measurements.result has been observed.The use of a climatological H 2 O a priori profile resulted in instabilities in the PAN profile retrieval.This was caused by the incorrect vertical position of the hygropause mapped into the resulting water vapour profile through the Tikhonov-Phillips regularisation constraint.
To solve this problem we adopted a 2-step approach.In the first step we use a zero a priori H 2 O profile and a relatively strong regularisation.This leads to a H 2 O profile (H 2 O first ) with reasonable position of the hygropause but relatively low vertical resolution.Its values are found to be higher than the in situ data between 10 and 12 km.
In the next step with weakened constraint, H 2 O first is used as the a priori to get the next H 2 O profile.This profile has been used as the "selected" H 2 O a priori in the PAN retrieval.As shown in Fig. 6 the fitted H 2 O vmr profile is very similar to the selected a priori profile above 11 km but larger at lower altitudes.Both the selected a priori and the fitted profile tend to have some zigzag structure around 12-13 km.Such kind of feature is also present in the in situ data observed by the instruments FLASH and FISH (Fig. 6) just above the hygropause and, thus, seems to be real.

PAN cross sections
Cross sections for PAN have been measured at 295 K, 273 K and 250 K (Allen et al., 2005a,b) whereas the relevant temperature for our measurements is between 197 K and 250 K.The cross sections increase from 273 K to 250 K by 8%, from 295 K to 273 K by 12%.For the PAN profile we extrapolated linearly, using the cross section measured at 273 K and 250 K, according to the atmospheric temperature profile.

The resulting PAN-profile
Figure 7 shows the retrieved profile of PAN from MIPAS-STR (in the following referred to as PAN MIPAS−STR ).Error bars indicating the total error (see Sect. 4.7 and Fig. 11) and the noise error are added in the profile.The vmr profile peaks at 10 km altitude with a value of about 0.14 ppbv and an error of 15%.At 14 km, still 0.04 ppbv of PAN are observed with an error of proximately 22%.
The averaging kernel matrix for PAN MIPAS−STR is used to determine the sensitivity of the retrieval at different altitudes (see Fig. 8).The columns of the matrix are the answers of the retrieval to a delta function in the associated altitude.The diagonal structures in the altitude range of the limb sequences between 8 and 18.6 km is clearly visible in Fig. 8. Below this range no measurements are available.The vertical resolution, determined as FWHM of each column of the averaging kernel matrix is given in Fig. 9. Above the flight level of 19 km the diagonal structure broadens strongly showing that there the vertical information is strongly reduced compared to the limb-range where a vertical resolution of km has been achieved (see Fig. 9).

Residual spectra
We investigate the quality in the spectral domain of the PAN retrieval described in Sect.4.3 (RUN fit ) in comparison with that resulting from two further approaches (see Table 2).For the test "RUN limit " we fixed the PAN profile to PAN limit (see Fig. 2) and retrieved all other parameters as described in Sect.4.3.The test RUN zero has been handled similarly but all PAN vmrs are fixed to zero.For all three runs, the residual spectra are shown in the lower panels of Fig. 10 for two selected tangent heights, 11 km (left part) and 13 km (right part).The top panels show the corresponding measured spectra.The rms (root mean square) of the residuum is considerately lower 14.8 nW/ cm 2 sr cm −1 for the run RUN fit , than for RUN limit 32.1 nW/ cm 2 sr cm −1 and RUN zero 27.3 nW/ cm 2 sr cm −1 .The rms of RUN fit is higher than the spectral noise 6 nW/ cm 2 sr cm −1 , because the residuum still contains residuals of lines, especially for low tangent heights.The broadband structure similar to the PAN contribution (see Figs. 3 and 4), present in the residua of RUN limit and RUN zero , is, however, removed in RUN fit .

Error estimation
In this section we analyse the effects of various error sources on the retrieved PAN vertical profile.We distinguish instrument-related error sources such as calibration and spectral noise and retrieval-related error sources such as spectroscopy and the errors in the used profiles.Here we consider temperature, water vapour, CCl 4 and the five interfering species (CH 3 CCl 3 , CFC-113, CFC-22, CFC-11, and ClONO 2 ) whose profiles have been kept constant during the PAN retrieval.Figure 11 presents the total error together with the individual errors described in the following paragraphs.
1. Temperature: A comparison of the retrieved vertical temperature profile from MIPAS-STR with that of ECMWF and in situ observations by the Rosemount TDC is shown in Fig. 12.In general, good agreement is found between all profiles, providing us the confidence in the level-1 processing for the spectral band in which PAN is also retrieved.Since the MIPAS-STR temperature is still slightly lower in the comparison, especially in the lower part, the contribution from a 2 K shift of the temperature profile is considered in the PAN error estimation.
2. Water vapour: Two different H 2 O a priori profiles (test 1 and test 2 in Fig. 6), are used to estimate the contribution of the H 2 O a priori profile on the PAN error budget.In both a priori test profiles the zigzag at 13 km is removed.Additionally, the a priori values in test 2 have been increased for altitudes below 11 km, adapting the FLASH measurement.Test 1 only weakly influences PAN MIPAS−STR , whereas test 2 leads to differences in the order of about 5%.
3. The five pre-determined species: An uncertainty of in each of the vmr profiles (CH 3 CCl 3 , CFC-113, CFC-22, CFC-11, and ClONO 2 ), which have been deterin previous steps kept constant during the PAN retrieval, is assumed.The real profiles of these species are not important, but how good spectral natures may be removed from the measured spectra.So the spectroscopic error, normally dominating the error budged, can be omitted.The remaining error shrinks to about 5%.

PAN cross sections:
To consider atmospheric temperatures lower than 250 K we linearly extrapolated the cross sections measured at 273 K and 250 K.For the error from the PAN cross section, we added the temperature dependent term (T−250 K)×0.16% to the error of 3% given by Allen et al. (2005b) for 250 K.The first term, roughly 4% for 25 K difference is the dominant term at temperatures close to 200 K.
Figure 11 presents each individual error contribution together with the total error calculated from these by the root square sum of all individual errors for each altitude.The high relative errors are at altitudes with low vmr values (see Atmos. Chem. Phys., 8, 4891-4902, 2008 www.atmos-chem-phys.net/8/4891/2008/Fig. 7).In the altitude range spanned by the tangent points from 9 km to 18 km, the total relative error is between 15% and 20%.In the lower part (up to about 14 km), errors in the temperature and PAN cross section dominate, whereas above spectral noise and PAN cross section are the major error sources.Error bars for the total error are given with the PAN MIPAS−STR profile in Fig. 7.

Discussion
This work was initiated by the comparison of the MIPAS-STR HNO 3 profile with the difference profile NO y -NO, measured by the in-situ instrument SIOUX.The disagreement between the two profiles posed the question, which of the constituents of NO y have to be considered additionally.The profiles of ClONO 2 and HO 2 NO 2 were retrieved from the MIPAS-STR spectra, and NO 2 was calculated from O 3 and NO.However, the consideration of those gases did not change the situation, as their vmrs are very small.So we tried successfully to retrieve PAN vmrs from the MIPAS-STR spectra.But the retrieved PAN profile only accounts for a sixth to a half (depending on the height) of the deficit NO y -NO-NO 2 -HNO 3 -ClONO 2 -HO 2 NO 2 .In Fig. 13 we show all derived profiles of the individual constituents of NO y , their sum and the measured NO y profile.In this figure, the MIPAS-STR profiles are interpolated to the in situ measurement altitudes, the data gaps result from calibration cycles of the SIOUX instrument.
We also degraded all in situ profiles to a resolution, as if they had been measured by MIPAS-STR.This was done according to Eq. ( 4) (e.g., von Clarmann et al., 2007b).
where x is the in situ profile (replacing the true atmospheric state in the original Equation), x a is the a priori profile, and A is the averaging kernel as given in Eq. ( 3).The left side x is the profile virtually measured by MIPAS-STR.As a priori profiles for NO and NO 2 we used their profiles given in the climatology (Remedios et al., 2007b).For NO y we used the sum of HNO 3 and ClONO 2 .As the averaging kernels result from a real retrieval they are not available.But as already described near Eq. ( 3), the averaging kernel reflects the vertical spacing of the tangent points.This results in a similarity of the averaging kernels of the different trace gases.We therefore can use the averaging kernel of the PAN retrieval to simulate the NO y , NO, and NO 2 measurements.Figure 14 shows the profile of NO y together with the sum of NO, NO 2 , HNO 3 , ClONO 2 , HO 2 NO 2 , and PAN in the left panel.We give both, the degraded and the not degraded profiles.1-Sigma error bares are added to the degraded profiles.The error of the sum is the root square sum of the errors of the individual gases.In the left panel we show the (degraded) residual NO y -NO-NO 2 -HNO 3 -ClONO 2 -HO 2 NO 2 -PAN.
The error bares also give the 1-σ error, calculated as the root square sum of the errors of the individual gases.At altitudes above the tropopause, where the tropospheric constituents of NO y can be neglected, the profile of NO y agrees well with the calculated sum.This gives us confidence in the accuracy of the measurements.Below, the discrepancy reaches 0.29±0.04ppbv at 11.5 km (see Fig. 14).We therefore conclude that not all contributing reactive nitrogens have been accounted for in the sum.Murphy et al. (2004) report on two compounds (HO 2 NO 2 and CH 3 O 2 NO 2 ) which become important at low temperatures in the upper troposphere.They determined their contribution to NO y to be 30% and more at temperatures below 230 K.This could explain the discrepancy, as these compounds are measured in the sum NO y , but only HO 2 NO 2 could be considered in the calculated sum.In Fig. 15 we show the relative contribution of PAN to NO y .In the left panel we show the (degraded) profiles, and in the right panel we show there ratio.The ratio peaks 30% at 9.5 km.Roberts et al. (2004) measured ratios of more than 60% in long-range transported Asian pollution plumes.Singh (1996) report 23% in the western subtropical south Atlantic for the altitude range 7-12 km.The later is in accordance with our measurements in the same region.Singh (1996) sorted their measurements according to the CO vmr in "influenced by anthropogenic pollution" and "relatively clean background".The corresponding median PAN vmrs show no significant difference above 8 km (65 pptv and 45 pptv), and are somehow in agreement with our measurements.The "polluted" median NO y vmr (340 pptv -100 pptv for "clean") is increased by a factor of three due to the pollution, but is still lower than our measurement of about 500 pptv (9-12 km).The PAN to NO y ratio of the "polluted" measurements (20%) is only little smaller than for the whole data set and thus still in accordance with our value.However, compared to Glatthor et al. (2007, 0.33 ppbv @ 8 km and 0.23 ppbv @ 11 km) who use the same method, our PAN vmrs are smaller.

Summary
We investigated the retrieval of the vertical profile (8-19 km) of PAN using MIPAS-STR emission spectra obtained in the tropics in February 2005.The largest peak in the retrieved PAN vmr profile is located at 10 km altitude with an amount of about 0.14 ppbv.Above 10 km PAN decreases with a secondary smaller maximum at 16 km (≈0.06 ppbv).The total relative error is estimated to be about 15-20% between 9 and 18 km.
We used simultaneous in situ measurements to derive the ratio PAN/NO y which peaks around 30% at 9.5 km.We calculated the sum of all measured (in-situ and remote) NO y constituents (NO, NO 2 , HNO 3 , ClONO 2 , HO 2 NO 2 , and PAN) and compared it to the measured total NO y .The deficit peaks at 0.29±0.04ppbv at 11.5 km.

Fig. 1 .
Fig. 1.Flight track of Geophysica and location of the tangent points of the MIPAS-STR instrument on 17 February 2005.The black arrow denotes the flight direction.The colour coding of the tangent points indicates their altitude, from blue for the lowest altitude at 6 km to orange at the aircraft flight level at 19 km.The red rectangle surrounds the six cloud-free sequences, used in this work and the aircrafts descent, where the in-situ profiles are located.

Fig. 2 .
Fig. 2. Comparison of HNO 3 from MIPAS-STR with in situ measured NO y -NO-NO 2 (left) and the difference of NO y -NO-NO 2 -HNO 3 (right).In the right panel we give also the smoothed difference used in Sect.4.6.

Fig. 3 .
Fig. 3. Simulated spectra in the broad-band range of PAN (black) for a tangent height of 8 km.The lower plot is a zoom of the upper one in y-direction.

Fig. 5 .
Fig. 5. Vertical profiles of five interfering species determined before the retrieval of PAN.

Fig. 7 .
Fig. 7. PAN vertical profile retrieved from MIPAS-STR (17 February 2005, 13:15 UTC; location: 22.0 • S and 47.7 • W).The error bars give the noise error (red) and the estimated total error (black).Also shown are the a priori profile and the upper limit corresponding to the NO y measurements.

Fig. 8 .
Fig. 8. Averaging kernel for the retrieval of PAN with MIPAS-STR.

Fig. 9 .
Fig. 9. Achieved vertical resolution of the retrieved PAN profile.The tangent heights are indicated by dotted lines.

Fig. 11 .
Fig. 11.Estimated total and individual relative errors in PAN MIPAS−STR analysis.

Fig. 12 .
Fig. 12.Comparison of the temperature profile retrieved simultaneously with PAN from MIPAS-STR spectra with that of ECMWF and in situ instrument TDC aboard the Geophysica.

Fig. 13 .
Fig. 13.Comparison of NO y with the profiles of the individual constituents and their sum.

Fig. 14 .
Fig. 14.Comparison of measured NO y with the sum of the individual constituents.All in situ profiles are degraded to the MIPAS-STR vertical resolution

Fig. 15 .
Fig. 15.Comparison of measured NO y with the PAN profile and the relative contribution of PAN to NO y .The profile of NO y is degraded to the vertical resolution of the PAN profile.

Table 1 .
Adjusted atmospheric parameter during the PAN retrieval.

Table 2 .
Three test cases for examination of the spectral fit quality.MIPAS-STR measured spectra in black (top panels) and the residual spectra (forward calculation -measurement) in blue, red and black of RUN fit and the tests RUN limit and RUN zero (lower panels) at tangent heights of 11 km (left panels) and 13 km (right panels).