Ash and sulfur dioxide in the 2008 eruptions of Okmok and Kasatochi: Insights from high spectral resolution satellite measurements

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jd013556, 2010 Ash and sulfur dioxide in the 2008 eruptions of Okmok and Kasatochi: Insights from high spectral resolution satellite measurements A. J. Prata, 1 G. Gangale, 2 L. Clarisse, 3 and F. Karagulian 3 Received 13 November 2009; revised 16 June 2010; accepted 24 June 2010; published 18 November [1] Ash particles and sulfur dioxide gas are two significant components of volcanic clouds that are important because of their effects on the atmosphere. Several different satellite instruments are capable of delivering quantitative measurements of ash and SO 2, but few can provide simultaneous assessments. High spectral resolution (n/dn 1200) infrared satellite data from the Atmospheric Infrared Sounder (AIRS) are utilized to detect volcanic ash within the 8 12 mm window region, and at the same time exploit the 4.0 mm and 7.3 mm bands of SO 2 to detect SO 2 at two different heights. The purpose is to study the interaction between gas and particles in dispersing volcanic clouds, and investigate the circumstances when the gas rich and ash rich parts of the plume are collocated and when they separate. Simultaneous retrievals of ash and SO 2 in the eruption clouds from Okmok and Kasatochi suggest that the two components were transported together for at least the first 3 days after the initial injection. Later (several days) transport is difficult to infer because of the lack of sensitivity of the ash algorithm to thin, dispersing ash clouds. For Kasatochi and Okmok, AIRS measured maximum masses of approximately 1.21 ± 0.01 Tg and 0.29 ± 0.01 Tg of SO 2, and 0.31 ± 0.03 Tg and 0.07 ± 0.03 Tg of fine ash (1 mm < radii < 10 mm), respectively. The retrieval schemes described here are capable of detecting the distribution of SO 2 simultaneously with estimates of ash concentrations from the same satellite instrument and represent an important improvement for observations of multispecies dispersing volcanic clouds. Analyses of other volcanic eruptions show that SO 2 and ash do not always travel together. Consequently, it is concluded that for dispersing volcanic clouds it is vital to be able to detect both SO 2 rich and ash rich clouds simultaneously in order to diagnose their effect on the atmosphere and the aviation hazard. Citation: Prata, A. J., G. Gangale, L. Clarisse, and F. Karagulian (2010), Ash and sulfur dioxide in the 2008 eruptions of Okmok and Kasatochi: Insights from high spectral resolution satellite measurements, J. Geophys. Res., 115,, doi: /2009jd Introduction [2] Sulfur dioxide gas and ash particles are often emitted together during volcanic eruptions and can reach different heights in the atmosphere to be transported in different directions. Both SO 2 and ash are important to study when in the atmosphere because of their effects on the radiation balance and on aviation. SO 2 in the stratosphere is oxidized and hydrated to form aqueous sulfuric acid (often referred to as sulfate) with a timescale of 3 4 weeks depending on temperature, and water molecule availability. Sulfate aerosols 1 Climate and Atmosphere Department, Norwegian Institute for Air Research, Kjeller, Norway. 2 Dipartimento di Ingegneria dei Materiali e dell Ambiente, Università di Modena e Reggio Emilia, Modena, Italy. 3 Spectroscopie de l Atmosphère, Service de Chimie Quantique Photophysique, Université Libre de Bruxelles, Brussels, Belgium. Copyright 2010 by the American Geophysical Union /10/2009JD absorb infrared radiation, warming the stratosphere, and reflect incoming shortwave solar radiation which cools the Earth s surface. [3] Ash too can affect the radiation balance, but because the particles are large, typically >0.5 mm (radius) they tend to fall out more quickly and spend less time high in the stable stratosphere. Thus the effects of ash tend to be local with most of the fine particles falling out of the atmosphere within the first few days [Rose and Durant, 2009]. [4] Passive infrared and ultraviolet satellite measurements from multispectral scanners (e.g., the Moderate Resolution Imaging Spectroradiometer (MODIS)) or spectrometers (e.g., the Ozone Monitoring Instrument (OMI)) provide total or partial column estimates of SO 2 [Corradini et al., 2010; Krotkov et al., 2006] but so far have not been used to determine vertical structure. Yang et al. [2009] have suggested a scheme to determine the altitude of volcanic emissions from OMI measurements by exploiting the spectral information present in these hyperspectral data. High resolution infrared measurements from AIRS (Atmospheric Infrared Sounder) 1of18

2 and IASI (Infrared Atmospheric Sounding Interferometer), in principle, can be used to infer vertical structure of gases in the atmosphere, essentially because different channels have different vertical sensitivities to emission and absorption of infrared radiation. Coheur et al. [2005] and Clerbaux et al. [2009] have presented vertical retrievals of O 3 and CO using IASI and ACE/FTS (Atmospheric Chemistry Experiment/ Fourier Transform Spectrometer) measurements, respectively. Divakarla et al. [2008] have used AIRS data to determine vertical O 3 profiles, while Eckhardt et al. [2008] used satellite data in combination with a Lagrangian dispersion model to determine the vertical injection height profile of SO 2 from the volcanic eruption of Jebel at Tair. Kristiansen et al. [2010] explore this technique further for the 2008 eruptions of Kasatochi. [5] Carn et al. [2005] and Prata and Bernardo [2007] have shown that it is possible to retrieve the partial column of SO 2 from AIRS measurements and that most of the information comes from the middle to upper troposphere or the upper troposphere/lower stratosphere (UTLS). Boundary layer SO 2 emissions are effectively blocked out by water vapor absorption, which is appreciable in the 7 8 mm region. Clarisse et al. [2008] have used high spectral resolution IASI measurements to determine middle to upper troposphere volcanic SO 2 and Karagulian et al. [2010] have shown that volcanic H 2 SO 4 and ash can also be detected in IASI data. [6] The ultraviolet spectrometers, OMI and GOME 2 (the second Global Ozone Monitoring Experiment) are both capable of detecting SO 2 emissions close to the surface, but so far it has not been demonstrated that the infrared sensors have this sensitivity, although Prata and Bernardo [2007] and Karagulian et al. [2010] have suggested that this may be possible. Gangaleetal.[2010] have shown that fine ash can be detected in AIRS data by exploiting the spectral behavior of silicate particles across the region cm 1. [7] The main aim of this paper is to show for the first time that SO 2 can be detected and quantified in the weak n 1 + n 3 band of SO 2 situated near 2500 cm 1. We also utilize the n 3 (1362 cm 1 )SO 2 retrieval scheme of Prata and Bernardo [2007] and the ash scheme of Gangale et al. [2010] to investigate the collocation of SO 2 and ash in the atmosphere. The analyses rely on measurements from AIRS for the recent eruptions of Kasatochi and Okmok. Although not used here, IASI data are also capable of providing boundary layer, middle to upper troposphere, and UTLS SO 2 measurements and fine ash loadings. [8] The paper is intended to be brief and is organized as follows. We investigate the possibility that boundary layer SO 2 can be determined in a quantitative manner using AIRS spectral radiances across the weak n 1 + n 3 SO 2 band. Further we suggest that by using AIRS radiance retrievals of boundary layer SO 2, upper tropospheric/lower stratosphere SO 2 from the strong n 3 band of SO 2 and ash retrievals, it is possible to determine whether volcanic SO 2 and ash are collocated and moving together. We test our hypotheses using AIRS measurements from the July/August 2008 eruptions of Okmok and Kasatochi. [9] The Kasatochi eruption produced a large SO 2 atmospheric loading (>1 Tg, see later) and is an excellent candidate to test the use of the weak n 1 + n 3 band of SO 2. Okmok produced much less SO 2 and so provides a test of a lower limit for SO 2 detection in this band. Both eruptions produced fine ash. The salient details of the methodology and the satellite data used in this investigation are provided next, followed by brief descriptions of the algorithms used to determine SO 2 and fine ash. The retrieval results for the eruptions of Kasatochi and Okmok are presented, and finally the main results are discussed with particular reference to the collocation of SO 2 and ash and their vertical structure. 2. Satellite Measurements and Data Analysis [10] High spectral resolution measurements from AIRS are utilized in this study. AIRS is an echelle grating spectrometer operating at infrared wave numbers between 650 and 2665 cm 1 [Chahine et al., 2006]. There are a few gaps in the spectral coverage, notably between 1135 cm 1 and 1215 cm 1 which precludes using the SO 2 n 1 band. The n 1 band is sensitive to SO 2 emissions at low levels in the atmosphere and has been successfully used to determine boundary layer SO 2 from other instruments, notably ASTER [Realmuto et al., 1997]. The AIRS instrument is on board the NASA Aqua polar orbiting satellite at an altitude of about 700 km above the Earth s surface in a Sun synchronous orbit with local equator crossing times of 0130 local time (LT) and 1330 LT. The primary goal is to provide global atmospheric retrievals of temperature, moisture, and other gases for numerical weather prediction and climate. AIRS scans a swath of ±49 from nadir with an instantaneous field of view of 1.1 providing nadir pixels with dimensions km 2, increasing to km 2 at the swath edge. There are 90 pixels ( 1800 km) along each scan line and each image granule is divided into 135 along track lines ( 2700 km) for convenience in processing and product delivery. [11] Level 1b data (V5 AIRS Cal Rad L1B) were obtained from the NASA GES DISC data archive using the Mirador data access tool. These data are calibrated, geolocated and quality controlled and delivered as radiance spectra at 2378 wave numbers at each of the pixels within a granule. Data within the range cm 1 are primarily used here, but the SO 2 absorption feature situated near 2500 cm 1 is also analyzed to investigate the possibility of determining low level SO SO 2 Algorithms mm Retrievals [12] SO 2 partial column abundance is determined from the AIRS data using the strong n 3 SO 2 absorption band centered near to 1363 cm 1 (7.3 mm), based on the scheme proposed by Prata and Bernardo [2007]. This band lies in a region of strong water vapor absorption and it becomes difficult to unambiguously determine SO 2 absorption whenever the water vapor concentration is high. The retrievals are only reliable for SO 2 clouds that lie above about 3 km or whenever the atmosphere is dry, but some allowance is made in the retrieval scheme for water vapor effects. The majority of the SO 2 clouds from Okmok and Kasatochi were detected well above 3 km, especially after a few days and Kristiansen et al. [2010] found that most of the SO 2 from Kasatochi was above the tropopause. Thus the AIRS 7.3 mm retrievals are expected to be only minimally affected by water vapor. Furthermore, the retrieval scheme provides a measure of goodness of fit (the r 2 correlation) (see Prata and Bernardo 2of18

3 [2007] for more details) between the AIRS measurements and a library SO 2 spectrum. Whenever this value falls below a threshold the retrievals are rejected; typically values of r are used. Typical errors on individual partial column amounts are ±3 to ±6 DU (1 Dobson unit, DU = molecules cm 2 at S.T.P.). This results in an SO 2 mass error in a typical scene covering 300 AIRS pixels of about ±0.01 Tg, assuming that the retrievals are uncorrelated. In practice the errors are computed by repeating retrievals with values of r 2 that are ±5% different to the nominal value of r 2 = Full details of the retrieval scheme, with several examples, can be found in the paper by Prata and Bernardo [2007]. [13] The narrow swath width of the AIRS sensor and the large horizontal extent of the Okmok and Kasatochi SO 2 clouds inevitably mean that parts of the clouds are not measured. The SO 2 abundances are often underestimated by AIRS. For convenience we will refer to the partial column SO 2 derived from the 7.3 mm scheme as tropospheric or upper troposphere lower stratosphere (UTLS) SO 2, acknowledging that some of this SO 2 will reside in the lower troposphere and that any SO 2 below 3 km is unlikely to be detected mm Retrievals [14] The n 1 + n mm (2500 cm 1 )SO 2 absorption feature has previously been used to detect SO 2 in the atmosphere of Jupiter s moon Io [e.g., Hapke, 1979] and in the volcanic plume of Pelé on Io [Spencer et al., 2000], but as far as we know it has not been exploited for retrieving SO 2 in the Earth s atmosphere from Earth orbiting satellites. The band is much weaker than either the 7.3 mm (n 3 ) or the 8.6 mm (n 1 ) bands (the HITRAN 2000 integrated band strengths for the SO 2 n 1 + n 3, n 1 and n 3 bands are cm 1 /(molecules cm 2 ), cm 1 /(molecules cm 2 ), and cm 1 /(molecules cm 2 ), respectively), but because there is less interference from other absorbers, notably water vapor and also because the weighting function for this channel peaks close to the surface (see Appendix A), the potential for measuring SO 2 emissions in the boundary layer is enhanced. The retrieval scheme adopted here is similar to that used for the n 3 band. In order to unambiguously detect SO 2 in this band, a normalized spectral library line shape (taken from the HITRAN 2000 database [Rothman et al., 2003]) is compared to the normalized absorbance spectrum determined from the AIRS measurements on a pixel by pixel basis, in a similar manner to the scheme used for the n 3 band. [15] The following is a brief description of the basis for the retrieval scheme adopted. The Beer Bougier Lambert law for radiance exiting a layer of gas of concentration, q(z) with absorption coefficient k n and thickness dh may be written, I 0 ¼ I ;0 exp Z dh o k ðzþqðzþdz ; ð1þ where I n is the radiance at wave number n leaving the SO 2 layer measured at the satellite, z is height, and I n,0 is the radiance entering the SO 2 layer from below. Radiation reflected off the gas layer, radiation emitted by the atmosphere above the gas layer and radiation emitted by the gas layer itself are ignored. The absorbance spectrum is determined from A ¼ Ln I p t;l t ; ð2þ I pr;l r where I pt,l t and I pr,l r are measured AIRS radiance spectra (functions of n omitted for notational convenience) for the target pixel [p t, l t ] and a reference pixel, [p r, l r ], respectively, and p and l represent pixel and line number. The method for determining the optimal reference pixel relies solely on the degree of correlation between the absorbance spectrum computed above and the library (synthetic) spectrum, computed from S ¼ Ln I s ; ð3þ where I s is the synthetic radiance spectrum with 100 DU of SO 2 (an arbitrary amount, spread uniformly from the surface to 3 km) and I 0 is the radiance spectrum with background SO 2. The reference spectrum, I pr,l r is deemed to be the pixel that produces the highest r 2 correlation between A n and S n. Thus for each target pixel, the correlation with every other pixel is calculated. Only radiances within the region between cm 1 are used in the correlation. Although the band is unaffected by water vapor, the band strength is low and the model makes many simplifying assumptions, thus unless stated otherwise we only consider retrievals for which r [16] Once a pixel is determined to contain SO 2 the absorbance is integrated across the band to give the path concentration, u. This follows if it is assumed that the absorption coefficient is independent of the path, then Z 1 Z Z qðzþdz k d ¼ A d; ð4þ D D 0 u ¼ u ¼ Z 1 0 I 0 qðzþdz; R R D A d D k d : Thus the SO 2 path concentration is determined by integrating the measured absorbance spectrum over the band (Dn) and dividing by the band integrated absorption coefficient, obtained from the HITRAN 2000 database. The band used extends from 2470 cm 1 to 2525 cm 1, or 54 AIRS channels. The absorbance, based on interpolating the HITRAN 2000 database to the AIRS channel wave numbers and then integrating from 2470 to 2525 cm 1, with no spectral weights (i.e., assuming boxcar filter functions for all channels) is used to calculate R Dnk n dn. Some updates to the SO 2 line strengths have been made in the latest HITRAN database (HITRAN 2008) [Rothman et al., 2009], suggesting accuracies of 2 3% for the line intensities of the SO 2 bands used here. The integrated band intensity based on HITRAN 2000 is cm 1 /(molecule cm 2 ), compared to the experimentally determined values of cm 1 / (molecule cm 2 )ofpine et al. [1977], cm 1 / (molecule cm 2 ) of Lafferty et al. [1996] and cm 1 /(molecule cm 2 )ofsumpf [2001]. These values ð5þ ð6þ 3of18

4 Figure 1. Line strengths ( 10 22,incm 1 /(molecules cm 2 )) for several gases that have absorption bands within the spectral region of interest. The line strengths for each molecule have been scaled appropriately for illustration purposes only. Data from the HITRAN 2000 database [Rothman et al., 2003]. suggest a variation of around 5% in knowledge of the band intensities. [17] In this initial study we ignore (possibly important) spectral effects, such as line broadening, temperature dependence, and interference from other absorbers and isotopes of SO 2. Other absorbers (all weak) that may affect the SO 2 band are H 2 S lines extending from 2300 cm 1 to about 2800 cm 1, the combination bands of N 2 O( 2462 and 2563 cm 1 ), the edge of the 4.3 mm bandofco 2 that extends to 2349 cm 1, and water vapor. The overtone and combination bands of CH 4 ( 2600 cm cm 1 ) and O 3 ( cm 1 ) are weak and do not interfere. Figure 1 shows the line strengths and locations of these bands in relation to the n 1 + n 3 SO 2 band. The line intensities have been suitably scaled for illustration; CO 2 is much stronger than the other absorbers but is sufficiently far away from the SO 2 band that interference is unlikely. Water vapor lines are also weak through the interval. N 2 O is a possible interference but it is an uncommon constituent in volcanic plumes. The only gas that may interfere is H 2 S as this has overlapping lines and is a common constituent in volcanic plumes. Interference by gases is easily seen in the absorbance spectra and reduces the correlation coefficient. [18] Figure 2 shows the brightness temperature spectrum of a pixel containing SO 2 (black colored line), the reference spectrum (red colored line, offset by 4 K for ease of com- Figure 2. Brightness temperature spectra from two AIRS pixels: red colored line is a reference spectrum with no SO 2 present; black colored line is a spectrum with SO 2. The spectral region covering the n 1 + n 3 SO 2 combination band is indicated and the normalized filter functions for two MODIS channels are also shown. The reference spectrum has been offset by 4 K for ease of comparison. 4of18

5 Figure 3. Library (synthetic) normalized spectrum (solid line) based on HITRAN 2000 line strengths and re sampled to the AIRS channels, and AIRS measurements (black dots) for a single AIRS pixel where boundary layer SO 2 is detected. In this case the correlation coefficient (r 2 ) between the synthetic spectrum and the measured spectrum is parison), and the filter functions of MODIS channels 22 (centered near 3.98 mm) and 23 (centred near 4.06 mm), which cover the SO 2 band. The SO 2 n 1 + n 3 feature is easily seen in the spectrum and not in the reference spectrum. Since MODIS channels 22 and 23 cover this region, it may be possible to use these bands to detect boundary layer SO 2 ; however, the problems discussed above are much more difficult to overcome for broadband channels than for the high spectral resolution AIRS measurements and it is likely that there may be many ambiguous situations which prohibit SO 2 detection from the MODIS 3 4 mm channels. [19] Figure 3 shows an example of the retrieval of SO 2 from the n 1 + n 3 band from a single AIRS pixel. The solid line is a normalized library spectrum for SO 2 resampled to the AIRS resolution. The normalization (for illustration only) is done by dividing by the maximum line strength and multiplying by the maximum of the measured absorbance spectrum. The measurements are shown by black dots. It is the degree of correlation between the normalized spectrum and the measured absorbance spectrum that provides an assessment of the presence of SO 2. Only when this correlation is high (a value of r is used, unless otherwise indicated) is the absorption calculated according to (6). Henceforth we will refer to the n 1 + n 3 band retrieval as boundary layer SO 2 without being specific as to the vertical extent of the SO 2 column, which is discussed in the next subsection and in Appendix A. [20] There are two additional effects that may cause difficulties for retrieving SO 2 in the 4 mm region that are negligible at 7.3 mm. First, during the day, there can be a substantial radiance contribution from reflection of solar energy. The size of this contribution depends on the time of day, the viewing geometry and the nature of the surface. Since this contribution is an additive term in the radiative transfer, it is not removed when deriving the absorbance spectrum. To alleviate this effect, a portion of the radiance at wave numbers outside the n 1 + n 3 band, just below and just above, is calculated and subtracted from the spectra before the absorbance is calculated. This has the effect of creating a zero baseline for the spectra and may introduce a bias into the retrievals. More detailed radiative transfer calculations are being done to investigate the impact of this and will be reported in a future paper. Note that in many cases the SO 2 clouds are above ocean for which the bidirectional reflectance function at 4 mm is small and the solar reflected contribution is therefore small. [21] The second effect is due to the extra radiance generated by the volcano, in particular from the hightemperature lava within the crater or in the flowing lava field. The lava, or hot spot may reach temperatures in the range K causing an increase in radiance of the order 10 3 at 4 mm, compared to the radiance from a pixel at 300 K. Because the AIRS pixels are generally much larger than the hot spot the effect is somewhat diminished but may still be a source of error. To alleviate this effect, the magnitude of the radiance of the target pixel and the reference pixel are compared and rejected if the absolute difference in brightness temperature averaged over the band exceeds a threshold (10 K is used currently). [22] A proper error analysis has not been performed for the 4 mm retrieval as this requires a more careful study of the radiative transfer and a better characterization of the effects noted above. Here the 1 s error on the total SO 2 mass retrieved is estimated by repeating retrievals using different values of r Boundary Layer SO 2 From 4 mm Band [23] The detection of SO 2 in the AIRS 4 mm radiances is a new observation and it is pertinent to investigate how much of the SO 2 column is observed using this band. The band is much weaker than either the 7.3 or 8.6 mm bands and the radiative transfer is more complicated because it is possible to have solar reflected radiation as well as emitted thermal radiation. A full treatment of the problem requires more 5of18

6 Figure 4. (a) Skew T Log(P) plot of two radiosonde ascents at Cold Bay, Alaska ( W, N). The black colored line is at 1200 UTC (0400 LT) and the red colored line is at 0000 UTC (1600 LT). The dashed lines are dew point temperatures. (b) Signal to noise ratio (SNR) corresponding to the two profiles shown. SNR exceeds 1 below 1 km for both profiles and between 3 and 5 km for the 0000 UTC profile. detailed analysis and is beyond the remit of this paper, but here we provide a simplified analysis which demonstrates that information in the 4 mm band is primarily concentrated in the boundary layer and near to the surface. [24] Following Prata and Bernardo [2007] we use the signal to noise ration (SNR) as a measure of the vertical extent of the information content of this band. The SNR is defined as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R 1 D D SNR ¼ DS2 d : NEDI [25] The SNR is calculated from radiative transfer (RT) simulations using temperature and dew point profiles from nearby radiosonde ascents, in this case from two profiles made at Cold Bay, Alaska ( W, N) at 12 and 00 UTC on 8 August Thirty one RT calculations are performed by inserting 1 ppmv SO 2 at each level, one at a time, of the 31 levels in the profile and computing the radiance leaving the top of the atmosphere after integrating across the band from 2450 to 2540 cm 1 and subtracting this integrated radiance from that corresponding to the case with no SO 2 (the reference radiance). [26] The noise equivalent radiance is assumed to be that due to a ±0.1 K temperate change. The SNR profiles for the 1200 and 0000 UTC cases are shown together in Figure 4b, and the temperature and dew point profiles are plotted in Figure 4a on a skew T Log(p) plot. The temperature and dew point profiles show a boundary layer up to about 800 hpa and a significant surface inversion in the earlier profile, with middle level inversions around 450 hpa and 320 hpa. Signal to noise levels are low, but exceed unity below about 870 hpa and between 720 and 520 hpa. It is concluded from this that the penetration depth for the n 1 + n 3 band is deeper than that for the 7.3 mm band [see Prata and Bernardo, 2007, Figure 4 and Figure 13], and that there is sensitivity to SO 2 layers within the boundary layer. In Appendix A we use a band model to show that the contribution of the radiance from the n 1 + n 3 band to the total radiance is mostly concentrated below 6 km for SO 2 plumes up to 8 km high. 6of18

7 PRATA ET AL.: AIRS ASH AND SO2 RETRIEVAL Figure 5. Images of (a) linear coefficient L2, (b) linear coefficient L3, and (c) concavity coefficient for AIRS granule 232 obtained on 8 August 2008 at 2311 UTC. The coefficients have been multiplied by factors of 104, 104, and 106, respectively Ash Algorithm [27] Gangale et al. [2010] have described a new way to detect dispersing ash using AIRS measurements. The technique is essentially an extension of the reverse absorption method of Prata [1989] that uses two bands in the infrared region between 8 and 12 mm. Brightness temperatures in the shorter wavelength channel (usually centered near 11 mm) are greater than those in the longer wavelength channel (usually centered near 12 mm) for clear and partially cloudy pixels. But the reverse is found when the pixels contain ash. Exceptions happen when the assumptions break down and ambiguous results are obtained [see Prata et al., 2001]. Since AIRS has several hundred channels within the 8 12 mm region it is possible to alleviate these ambiguities and also utilize the extra information to determine ash properties. [28] Gangale et al. [2010] analyzed the information available in the high spectral resolution AIRS spectra using three subintervals to detect ash: (1) cm 1 (region R1), (2) cm 1 (R2), and (3) cm 1 (R3). The spectral signature of brightness temperature in R1 was found to have a markedly concave shape (quadratic variation with wave number), especially for ash with a high silicate content. The ash signature in R2 was found to be linear having a negative slope with wave number, while in R3 the variation was also linear but with a positive slope with wave number. The overall shape across the region cm 1 resembles a V as first noted by DeSouza Mechado et al. [2006] for windblown (silicate bearing) sand and dust. AIRS spectra can be automatically analyzed to determine the spectral fits, the Concavity (C) in R1, and the linear fits L2, L3 in R2 and R3, respectively. The parameters C, L2 and L3 are the coefficients determined from 2nd and 1st order fits to the specified regions, respectively. [29] To illustrate this, Figure 5 shows three images of L2, L3, and C for an AIRS granule containing the eruption cloud from Kasatochi on 8 August 2008 at 1341 UTC. All three spectral fits suggest anomalous features in the data within the region W, N, although C is weak. The strongly negative values (red colored) of L2 suggest ash; 7 of 18

8 Figure 6. Temporal evolution of boundary layer SO 2, tropospheric (UTLS) SO 2, and fine ash mass determined from AIRS high spectral resolution data for the 8 August 2008 eruption of Kasatochi. The shaded region (in grey) around the BL SO 2 indicates the estimated 1 s error on the retrieval. while the strongly negative values of L 3 suggest SO 2. The spatial coincidence of C and L 2 confirms an ash signature; the strength of C can be used to infer the composition of the ash [Gangale et al., 2010]; a stronger signal suggests higher silicate content in the ash. Positive L 3 also suggests ash, but because SO 2 interferes in this region and has a negative L 3, it is more difficult to use this region to perform ash retrievals. Retrieving ash using this technique requires an ash microphysical model to perform a minimization between the modelled spectral behavior of ash spectra and the AIRS measurements within the three spectral regions. The modelled fits to the AIRS measurements provide estimates of mean effective particle radii, optical thickness and from these estimates, the mass of ash within an AIRS pixel may be evaluated. More details of the retrieval scheme are given by Gangale et al. [2010]. [30] Having briefly introduced the tools used in this study to quantify SO 2 and ash, we now turn to a few examples from the July and August 2008 eruptions of Okmok and Kasatochi. 3. Kasatochi [31] Detailed analyses of the events of the August 2008 eruption of Kasatochi are discussed elsewhere in this Special Issue. Here we concentrate on the SO 2 and the fine ash emissions as observed by AIRS, with the aim of investigating the collocation of these substances in the horizontal and vertical. [32] Three large eruptions were reported: the first large eruption occurred at 2200 UTC on 7 August and a third eruption occurred at 0435 UTC on 8 August, about 9 h before the first AIRS overpass with detectable SO 2. The AIRS pass at 0005 UTC on 8 August detected no SO 2. Following the three large eruptions of 7 August, a period of continuous degassing and ash emissions occurred (BGVN, 3307, 2008), thus the AIRS retrievals at 1341 UTC on 8 August represent only SO 2 emitted up to that time. Ash was reported to have reached altitudes of 13.7 km on August 7, and >10.7 km on August 8 (BGVN, 3307, 2008). Kristiansen et al. [2010], determined the height (a.s.l.) of the Kasatochi SO 2 emissions to be 7 km and 12 km with smaller emissions to 20 km. Between 0005 UTC 8 August and 1223 UTC 13 August, SO 2 was detected in 68 AIRS granules using the 7.3 mm retrieval scheme. [33] Figure 6 shows the variation of UTLS SO 2 mass with time for the first 7 days after the initial eruptions. The steep rise in SO 2 mass (Figure 6), followed by a plateau and then gradual decline is typical of what is normally found in satellite retrievals of SO 2 mass. The reason why the maximum mass is not observed immediately is not clear; Rose et al. [2000] have suggested this may be due to conversion of H 2 StoSO 2 which causes a peak in the SO 2 in the second day after the eruption, but in the case of infrared measurements it is possibly due to saturation of the bands, and for the n 3 band lack of sensitivity to low level SO 2. The temporal variation may be explained as follows: after some time as the SO 2 rises, disperses and dilutes, the infrared absorption decreases and the mass, which is the product of the partial column amount and the area, can be better delineated. Later, as the SO 2 cloud continues to dilute, conversion to sulfate occurs, rendering detection by infrared radiation difficult. Masking by clouds and water vapor may also play a role in causing temporal variation in the estimated mass. Another effect that must be considered is the incomplete coverage of the narrow AIRS swath with respect to the horizontal spatial extent of the SO 2 cloud, which causes fluctuations in the sensed mass, not necessarily related to the SO 2 cloud mass. [34] The evolution of the boundary layer SO 2 determined from the n 1 + n 3 band for the first 3 days after the eruption is also plotted on Figure 6. There is an initial steep rise in SO 2 and then a rapid decay to values below the detection limit. It is tempting to suggest that summing the boundary layer and UTLS SO 2 amounts would provide a better estimate of the 8of18

9 Figure 7. (a) Boundary layer SO 2 determined from AIRS measurements of the n 1 + n 3 band and (b) tropospheric (UTLS) SO 2 determined from AIRS measurements of the n 3 band. (c) Ash mass loadings (gm 2 )determinedfrom AIRS measurements in the mm region. AIRS granule: 137 acquired on 8 August 2008 at 1341 UTC. total column, but as the vertical extent of both partial columns are not known this is problematic. Certainly it seems plausible that in the early stages of the eruption more SO 2 is residing in the lowest layers, consistent with the AIRS retrievals. With time, the boundary layer SO 2 is removed by wet and dry deposition and also transformed to sulfate and the upper level SO 2 is transported and dispersed in the high level winds [Kristiansen et al., 2010]. [35] Fine ash retrievals using AIRS radiances in the 8 12 mm region are also shown in Figure 6. The ash mass is quite low and fluctuating, but within experimental errors. After 3 days the ash mass has fallen below the detection limit, but it is also quite reasonable to assume that a large fraction of the mass has fallen out rather than dispersed [Rose et al., 2001]. Ash mass seems to peak at around the same time as the UTLS SO 2 and this raises the question about the vertical location of the ash. Is it collocated with the boundary layer SO 2, with the UTLS SO 2 or at some other place? To try to answer this question, the spatial patterns for boundary layer (Figure 6, top) and UTLS (Figure 6, middle) SO 2 for the earliest AIRS overpass (1341 UT 8 August) after the latest eruption are shown in Figure 7. There is a striking resemblance between the two patterns, strongly suggesting that the boundary layer and UTLS parts of the SO 2 column are collocated. Figure 6 (bottom) shows the locations of AIRS ash retrievals (colored dots) overlayed by contours of the UTLS SO 2. The correspondence is again striking, suggesting that the ash was collocated with the SO 2. There is a central portion within the plume where it has not been possible to determine SO 2 from the n 3 band, which also corresponds to an ash rich region. It is possible that the n 3 band is saturated in this region making retrievals impossible. Figure 8 shows a compilation of retrievals for the AIRS overpass at 0050 UT on 9 August Figure 8a shows the BL SO 2, Figure 8b shows the tropospheric or UTLS SO 2, Figure 8c shows MODIS fine ash mass retrievals [see Corradini et al., 2010], with contours of UTLS SO 2 overlayed, Figure 8d shows AIRS ash retrievals, Figure 8e shows OMI SO 2 (15 km) retrievals (courtesy S. Carn) measured about 15 min later, Figure 8f shows OMI aerosol index (courtesy S. Carn), and Figure 8g shows the MODIS true color image taken at the same time as the AIRS overpass. The spatial patterns show a high degree of correlation, but the differences are worth examining. The AIRS BL SO 2 retrievals are more spatially confined than the AIRS UTLS SO 2 but have maxima in the same locations. The OMI retrievals have a broad maximum in a different location to the AIRS retrievals. The fine ash patterns from MODIS are similar to the AIRS retrievals but have better spatial resolution and so show more detail and are broadly similar to the UTLS SO 2 but there is little ash detected in the northern part of the pattern (between 172 W 166 W, north of 51 N) and only part of the southern ash stream is detected (see Figure 8g). The AIRS ash retrievals are qualitatively similar to those from MODIS, and within the errors expected for ash retrievals, which can be as large as 50%. Intriguingly, AIRS ash retrievals show a portion of ash to the north and east of Kasatochi, that is not detected in the MODIS retrievals. This may be a thin veil of ash that is better delineated in high spectral data than in broadband measurements [Gangale et al., 2010]. Given the difference in spatial and spectral resolution of the sensors and the 9of18

10 PRATA ET AL.: AIRS ASH AND SO2 RETRIEVAL Figure 8. The evolving SO2 and ash cloud from Kasatochi on 9 August 2008, 0110 UTC. (a) Boundary layer SO2 determined from the AIRS n 1 + n 3 band, (b) tropospheric (UTLS) SO2 derived from the AIRS n 3 band, (c) ash mass loading (gm 2 determined from MODIS in the m region, (d) AIRS ash mass loadings, (e) OMI 15 km column SO2 (courtesy Simon Carn), (f) OMI aerosol index (courtesy Simon Carn). Large positive values of the index suggest absorbing particles, in this case ash, and (g) MODIS (1 km) true color image acquired at the same time as the AIRS data. 10 of 18

11 Figure 9. Spectra for a single AIRS pixel showing the correlation between a reference spectrum (redcolored lines) and AIRS measurements (black colored lines). (a) The 7.3 mm, n 3 band, (b) the 4 mm, n 1 + n 3 combination band, and (c) the spectrum of fine ash. In the case of ash the reference line is a black body with a brightness temperature equivalent to the measured value at 10 mm. The correlation between the reference and the measured spectra is much better for the 4 mm bandthanthe7.3mm band, suggesting that the SO 2 is lower down in the atmosphere, while the ash spectrum shows negative slope in R2 consistent with ash and negative slope in R3consistentwithlowertroposphereSO 2.(R2= cm 1 and R3 = cm 1 ). assumptions needed to perform the ash retrievals, agreement of 10 30% in mass loading is regarded as good. [36] The MODIS true color and OMI aerosol index detect the stream of ash quite well. The poor detection of fine ash in the stream may be due in part to the high optical depth of the ash there, which limits its detection from infrared measurements. The hole in the pattern, centered near 170 W, 51 N, where little ash and SO 2 are detected has a strong positive aerosol index signal. The correlation with the MODIS true color image is poor, so it remains unclear whether or not there is ash in this region. [37] Another way to use the AIRS data to verify whether a particular pixel contains mostly BL SO 2 or mostly UTLS SO 2 is to look directly at the spectra. Figure 9 shows AIRS spectra for a pixel in AIRS granule 008 obtained at 1341 UT on 8 August Figure 9 (top) shows the 1363 cm 1 AIRS measurements (dots) together with a library spectrum (red colored line), where the correlation between the two is 11 of 18

12 Figure 10. Spectra for a single AIRS pixel showing the correlation between a reference spectrum (redcolored lines) and AIRS measurements (black colored lines). (a) The 7.3 mm, n 3 band, (b) the 4 mm, n 1 + n 3 combination band, and (c) the spectrum of fine ash. In this case the correlation between the reference and measured spectra is much better for the 7.3 mm band, and the ash spectrum has a marked concavity and a positive slope in region R3, consistent with high level ash and SO 2. not good. Figure 9 (middle) shows the 2500 cm 1 AIRS measurements (dots) with a library spectrum (red colored line), where the correlation is good. Figure 9 (bottom) shows the ash spectrum; in this case the reference is a blackbody with no spectral variation. The spectra suggest that in this case, most of the SO 2 is in the lower part of the troposphere and that there is an ash signal; the slope of the ash spectrum between cm 1 is negative, with some concavity. The negative slope of the spectrum between 1070 cm 1 and 1130 cm 1 is likely due to absorption by the n 1 band of SO 2 and this is also consistent with lower troposphere SO 2. There are other examples of spectra (Figure 10) where the correlation for the UTLS SO 2 is much better than that for the BL SO 2 and we may conclude that in those cases, most of the SO 2 is in the UTLS. If there is also an ash signature, as there is in Figure 10, then by further examining the spatial patterns, it is possible to diagnose whether the ash rich portion is low (BL) or high (UTLS), or perhaps somewhere different to the SO 2 rich portion. [38] Later AIRS observations of the evolving Kasatochi cloud on 9 and 10 August (Figure 11) show that the ash remained collocated with the UTLS SO 2 and so we may conclude that at least some ash has travelled with the upper level SO 2. Some of the spatial differences in Figure of 18

13 Figure 11. AIRS n 3 (7.3 mm), tropospheric (UTLS) SO 2 retrievals for the Kasatochi eruption on 9 and 10 August 2008 as contours in units of DU, overlayed with simultaneous AIRS retrievals of fine ash mass in units gm 2 (colored dots). between the SO 2 contours and the ash concentrations may be due to transport effects but in general the correspondence is strong, suggesting collocation between the SO 2 and ash. This is an important observation because it demonstrates that for the Kasatochi eruption measurements, observations of the location of SO 2 could be used as a surrogate for the location of ash, which is hazardous to jet aircraft. The UTLS SO 2 spread over great distances and was observed over Svalbard [Kristiansen et al., 2010] on 15 August. Figure 12 shows daily composites of AIRS UTLS SO 2 from 8 to 13 August 2008, illustrating the spread and detectability of the SO 2 from the n 3 band. By 11 August the BL SO 2 is no longer detected and it is very difficult to detect ash with any certainty, thus being able to use the UTLS SO 2 as a marker for ash would be very useful, however some in situ validation of the constituents in dispersed volcanic clouds is required. 4. Okmok [39] Okmok volcano (53.43 N, W, 1073 m) erupted at around 1943 UT on 12 July (BVGN 3307, 2008) and the first AIRS overpass was at 2329 UT, about 4 h later. Further eruptions, some ash rich, occurred throughout the latter half of July and into mid August. Ash columns were reported to reach altitudes of between 4 kmto 11 km a.s.l. (BGVN 3307, 2008). The ash rich eruption of 3 August is noticeable in MODIS (250 m and 1 km data) but at the spatial resolution of AIRS ( 14 km) no ash could be detected. Thus we concentrate on the early and largest eruptions of 12 July. The same AIRS analyses that were conducted for Kasatochi are repeated for Okmok, giving retrievals of BL SO 2, UTLS SO 2 and fine ash. The summary results are provided in Figure 13. Masses for all three emissions are much lower for Okmok compared to Kasatochi: maximum UTLS SO 2 of 0.3 Tg(SO 2 ) occurs about 24 h after the initial injection and then decays, remaining at a near constant value of 0.2 Tg for the next 4 days. The spread of the Okmok UTLS SO 2 across the North Pacific and onto Canada and northern United States may be seen in the composite of Figure 14. No such spread can be observed in the BL SO 2 mass which is both much lower, peaking at under 0.2 Tg and rapidly declining to less that 0.05 Tg thereafter. Fine ash mass is also lower than Kasatochi, reaching maximum amounts of 0.08 Tg on 14 July. It is important to point out that because of the large footprint (field of view) of the AIRS pixels, many smaller ash rich plumes are not detected. We did consider several AIRS granules where ash was observed in contemperaneous MODIS/Aqua 1 km data but not detected in the AIRS spectra. There is also a strong possibility that even though the new ash retrieval has better sensitivity to thin ash veils, it is still not sensitive enough to detect all potentially hazardous ash clouds. Thus it remains a continuing problem to be able to detect thin, dispersing ash clouds from infrared satellite measurements. [40] The horizontal and vertical collocation of the Okmok SO 2 and ash were also studied for the 12 July event. Figure 15 shows the BL (top) and UTLS (bottom) SO 2 derived from 13 of 18

14 Figure 12. Daily composited AIRS n 3 (7.3 mm), tropospheric (UTLS) SO 2 retrievals for the Kasatochi eruption starting on 8 August The large spread of the UTLS SO 2 from Kasatochi suggests the SO 2 was high in the atmosphere; no such spread could be observed for the boundary layer SO 2 or ash. Shown are (a) 8 August, (b) 9 August, (c) 10 August, (d) 11 August, (e) 12 August, (f) 13 August. Figure 13. Temporal evolution of boundary layer SO 2, tropospheric (UTLS) SO 2,andfineashmass determined from AIRS high spectral resolution data for the 12 July 2008 eruption of Okmok. The shaded region (in grey) around the BL SO 2 indicates the estimated 1 s error on the retrieval. 14 of 18

15 Figure 14. Daily composited AIRS 7.3 mm SO 2 retrievals for the Okmok eruption. The time range shown is from 12 July to 20 July AIRS. The SO 2 clouds are confined to the same spatial area, but the locations of the maxima are slightly different and the western edge of the BL cloud is connected whereas it is not for the UTLS cloud. The whole region is contaminated with meteorological clouds and these may be masking the retrieval of BL SO 2. This may explain why only a few SO 2 affected pixels can be retrieved. The inset plots on Figure 15 show the spectral fits between the AIRS measurements and the library spectra for both the 7.3 and 4 mm bands for two pixels in each image. The r 2 correlations between the measured and library spectra are high (r ) for the 7.3 m retrievals, but a little lower (r 2 > 0.80) for the 4 mm, suggesting that clouds are interfering with these SO 2 retrievals. Since retrievals for pixels with a low r 2 correlation are not performed, the SO 2 4 mm retrievals are underestimating the mass in cloudy regions. Inspection of the spectral correlation is necessary in order to have confidence in the 4 mm retrievals, especially in the presence of meteorological cloud. [41] Dispersion modeling suggests that the Okmok SO 2 was mostly confined to a region between 10 and 17 km (M. Fromm, private communication, 2008), and CALIOP data from a Calipso overpass on 16 July 2008 suggests aerosols at 5, 12 and 17 km. Since the ash amount was quite small (compared to other eruptions) and difficult to detect in satellite imagery [see also Corradini et al., 2010], it is difficult to be conclusive about the location of the ash at later times. However, AIRS data do show the presence of ash as indicated from the analyses of the C, L 2, and L 3 images. 5. Discussion and Conclusions [42] In this work we have set out to investigate whether high resolution infrared spectral radiances can be used to detect boundary layer emissions of volcanic SO 2 using the combination n 1 + n 3 band. AIRS spectra for the eruptions of Kasatochi with estimated SO 2 mass emissions exceeding 1 Tg and Okmok, with much less emitted mass ( 0.3 Tg), were used to demonstrate for the first time that the combination band of SO 2 centered near 2500 cm 1 does indeed have sufficient sensitivity to detect boundary layer volcanic emissions. We note some differences and similarities between the spatial patterns of BL SO 2, UTLS SO 2 and fine ash mass, and their temporal evolutions, to infer their vertical and horizontal collocation. An interesting result from this analysis is that the lower AIRS SO 2 mass estimates derived using the strong n 3 band compared to OMI measurements [e.g., Prata and Bernardo, 2007] can be explained to some extent to that bands lack of sensitivity to lower troposphere SO 2. When the contribution from the BL SO 2 is added to the UTLS SO 2 there is a large increase in the total column estimate of up to 100% in some cases. However, the retrieval scheme used does not offer precise information on the vertical extent of the SO 2 partial columns being sensed in the various wave bands and so a simple addition of the masses determined from the n 1 + n 3 and n 3 bands is likely to be wrong. [43] Another key finding from this study is that for both the Kasatochi (8 14 August 2008) and Okmok (12 15 July 2008) events, the ash and UTLS SO 2 travelled together and were collocated in all three spatial dimensions. That this was so is not necessarily obvious, and analysis of other volcanic eruptions where ash and SO 2 are present shows that the fine ash and SO 2 can travel in different directions and speeds. This was found for the November 2006 eruption of Karthala, Comoros [Prata and Kerkmann, 2007] and for the 2001 Reventador, Ecuador eruption where the fine ash was found to travel westward while the UTLS SO 2 travelled eastward [Tupper et al., 2006]. Knowledge of the collocation of ash and SO 2 is important for the ash/aviation hazard problem [Prata, 2009]. Recently, it has been proposed that detection of SO 2 may be used as an indication of the presence of ash for initiating aviation warnings [Carn et al., 2008], since detection of SO 2 from satellite measurements is considerably easier than detection of ash. Retrievals of multiple 15 of 18

16 Figure 15. (top) Boundary layer and (bottom) tropospheric SO 2 retrievals for an eruption of Okmok on 12 July UT, determined from AIRS 4 mm and 7.3 m retrieval schemes, respectively. The inset plots show the degree of correlation between the measured AIRS spectra and library spectra for both bands at two different pixels. chemical species [e.g., Karagulian et al., 2010] and particles, from a single instrument or from multiple instruments on the same satellite platform, or from nearly simultaneous satellite platforms (e.g., the A train) will aid in the understanding of the movement of dispersing volcanic clouds, as well as particle gas interactions and the impact these emissions have on the radiative budget of the atmosphere. Appendix A: Band Model and Weighting Functions [44] Here we use a band model, idealized SO 2 profiles and radiative transfer to investigate the sensitivity of the 4 mm band to SO 2 in the boundary layer and lower troposphere. Pierlussi et al. [1984] have described a double exponential band model for use with the n 1 + n 3 band of SO 2. The model has the form: ¼ exp 10 ax ; where, t is the transmittance and, X ¼ log C þ log W ; W ¼ p n T m 0 U; p 0 T U is the SO 2 absorber amount, W is a scaled absorber amount, p is pressure, T is temperature, and p 0, T 0 are values at STP. Values for the constants a, n, m, and the 16 of 18