The EOS MLS instrument on NASA's Aura satellite has made the first continuous global measurements of HCN in the middle atmosphere. The measurements are in broad agreement with most earlier measurements, showing HCN to be a long-lived tracer with sources in the troposphere and sinks in the middle atmosphere. There is no evidence in the data for a proposed mesospheric source of HCN

Jimenez, C. J.

Pumphrey, H. C.

Waters, J. W.

English

Edinburgh Research Explorer

     Edinburgh Research Explorer                                      Measurement of HCN in the middle atmosphere by EOS MLSCitation for published version:Pumphrey, HC, Jimenez, CJ & Waters, JW 2006, 'Measurement of HCN in the middle atmosphere by EOSMLS' Geophysical Research Letters, vol 33, no. 8, L08804, pp. 1-4., 10.1029/2005GL025656Digital Object Identifier (DOI):10.1029/2005GL025656Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Publisher final version (usually the publisher pdf)Published In:Geophysical Research LettersPublisher Rights Statement:Published in Geophysical Research Letters by the American Geophysical Union (2006)General rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 20. Feb. 2015Measurement of HCN in the middle atmosphere by EOS MLSH. C. Pumphrey,1 Carlos J. Jimenez,1 and J. W. Waters2Received 30 December 2005; revised 20 February 2006; accepted 21 March 2006; published 22 April 2006.[1] The EOS MLS instrument on NASA’s Aura satellitehas made the first continuous global measurements of HCNin the middle atmosphere. The measurements are in broadagreement with most earlier measurements, showing HCNto be a long-lived tracer with sources in the troposphere andsinks in the middle atmosphere. There is no evidence in thedata for a proposed mesospheric source of HCN.Citation: Pumphrey, H. C., C. J. Jimenez, and J. W. Waters(2006), Measurement of HCN in the middle atmosphere by EOSMLS, Geophys. Res. Lett., 33, L08804, doi:10.1029/2005GL025656.1. Introduction[2] It has been known for many years that hydrogencyanide (HCN) is a minor constituent of the atmosphere. Acompilation of early measurements and modelling work[Cicerone and Zellner, 1983] suggests that the gas has itssources at the Earth’s surface, its sinks in the middleatmosphere and is well-mixed in the troposphere. A typicaltropospheric mixing ratio is about 0.18 ppbv. More recentstudies [Li et al., 2000; Singh et al., 2003] suggest a morecomplex picture in the troposphere, with stronger sourcesand the ocean as a stronger sink.[3] In the middle atmosphere, HCN is believed to be lostslowly due to reactions with OH and O1D. As a result, themixing ratio decreases with both altitude and latitude. Thispicture has, to date, been based on a small number ofmeasurements using various techniques.[4] The MLS instrument, one of four experiments onNASA’s Aura satellite, has made global measurements ofHCN continuously since August 2004. The signal from theHCN molecule is not large compared to the instrumentnoise, so that the measurements are best used to constructweekly zonal means. Despite their limitations, these dataallow us, for the first time, to observe the seasonal cycle ofmiddle-atmosphere HCN.2. Measurements[5] The Microwave Limb Sounder [Waters, 2006, 1999]is essentially a small radio telescope. The 1.6 m dishantenna is moved mechanically so that its field of viewscans across the Earth’s limb every 24.7 s. The radiation itreceives is converted to a lower frequency in severalheterodyne mixers (radiometers) and analyzed by 32 spec-trometers (28 filter banks and 4 digital autocorrelators).Most of the filter banks are located so that they cover afrequency range centered on a single spectral line of a targetmolecule. Microwave spectral lines show strong pressurebroadening: a line which is 1 MHz wide at 0.3 hPa (56 km)will be 300 MHz wide at 100 hPa (16 km). For this reasonthe filter banks are designed with narrower filters at theband center. The heterodyne nature of the radiometersmeans that each filter bank is affected by two quite separatespectral regions or sidebands, one each side of the localoscillator (LO) frequency. The sideband not containing thetarget line is not filtered out. Instead, the LO frequency ischosen so that, for the most important measurements, thenon-target sideband is in a spectral region with no stronglines.[6] Hydrogen cyanide is a linear molecule with a largedipole moment. Linear molecules have a simple microwavespectrum, with lines at integer multiples of a fundamentalfrequency: 88.6 GHz in the case of HCN. (Each HCN line issplit into several hyperfine components but these are tooclose together concern us here.) MLS has two filter banks(Band 6F and Band 27M) which are affected by the secondof these lines, which lies at 177.26 GHz. Band 27M wasincluded in the instrument specifically to measure HCN.However, as Figure 1 shows, although HCN is the largestsignal in the lower sideband it is small compared to thesignals from ozone and nitric acid in the upper sideband.Figure 2 shows some radiances as measured by the instru-ment. The HCN signal is not large compared to the randommeasurement noise for a single scan. By averaging theradiances for a week, over a 10 latitude band, the noiseis reduced to a level which allows the HCN signal to beobserved throughout the middle atmosphere.3. Retrieval of Mixing Ratios[7] HCN is one of the products retrieved by thestandard MLS retrieval software [Livesey and Wu, 1999;Livesey et al., 2006]. However, the smallness of thesignal means that the retrieved product is usable overonly a very small vertical range. Above 48 km the signalis lost in the noise. Below 32 km the signal is super-imposed on stronger emissions from HNO3 and O3. Thecurrent operational retrieval software is unable to obtainusable HCN mixing ratios in these conditions, returningvalues which have obvious systematic errors, being fre-quently large and negative.[8] In order to obtain HCN mixing ratios over a largervertical range, we form a weekly zonal mean of the radiancesand from these retrieve weekly zonal means of temperatureand mixing ratio. The calibrated radiance (level 1B) dataare provided with tangent altitude ht as a vertical co-ordinate;ht varies somewhat from one scan to the next. The weeklyzonal mean is formed by interpolating the radiances onto afixed set of tangent height values and taking the mean ofthese interpolated radiances for all scans within a given weekand latitude bin. Operational issues mean that on someGEOPHYSICAL RESEARCH LETTERS, VOL. 33, L08804, doi:10.1029/2005GL025656, 20061School of GeoSciences, University of Edinburgh, Edinburgh, UK.2NASA Jet Propulsion Laboratory, Pasadena, California, USA.Copyright 2006 by the American Geophysical Union.0094-8276/06/2005GL025656L08804 1 of 4occasions, the 7 days may not be consecutive, but they arespread over no more than 17 days.[9] The retrieval uses the maximum a postiori probability(MAP) formula as described by Rodgers [2000]. The apriori mixing ratio for HCN is 0.18 ppbv in the troposphereand decreases monotonically with height throughout themiddle atmosphere to a value of 1 pptv at 95 km. As theproblem is somewhat nonlinear, the MAP formula is usediteratively, a Marquadt-Levenberg scheme ensures conver-gence. The retrieval is performed in two phases. A firstphase estimates the zonal mean temperature T, O3 and thegeopotential height hr at a reference pressure, usingradiances from Band 1F and a few channels from band6F only. Band 1F is centered on the 118 GHz O2 line andis the main source of temperature information for MLS.Ozone is retrieved at the same time as there are interferingO3 lines in band 1F. A second phase adds bands 27M andmost of band 6F to the measurement vector, while HCNand HNO3 are added to the list of quantities estimated.The results of the first phase are used as the starting pointfor the second phase. The two-phase strategy is usedbecause the temperature retrieval is highly nonlinear andrequires more iterations.4. Results[10] Figure 3 shows a 7-day zonal mean of HCN mixingratio. The mixing ratios of 0.25 ppmv which occur between24 and 30 km are somewhat larger than the accepted valuein the troposphere. This may be due to spectroscopicuncertainties and should not necessarily be interpreted asa source of HCN in the lower stratosphere. Qualitatively, thevalues above 24 km are much as one would expect for aFigure 1. Calculated limb radiances in the (right) upper and (left) lower sidebands of MLS. The solid, dashed and dot-dashed lines are for tangent heights of 45, 31 and 17 km respectively. The molecules responsible for some of the moreprominent emission lines are marked at the top of the figure. Note that HCN causes the largest signal in the lower sideband,but that a line of vibrationally excited O3 appears in almost the same place in the upper sideband.Figure 2. Measured radiances in bands 6 and 27 of MLS.The measurements are shown as horizontal bars, the widthof the bar indicates the width of the spectrometer channel.Results are shown for three tangent heights: 17, 31 and45 km. The data are averaged over a 10 latitude bandcentered on the equator; this reduces the measurement noisefrom the single-scan values of 0.2–1 K down to 0.007–0.04 K. The lower noise levels correspond to the broaderchannels. The curves are calculated radiances for tangentheights 17 km (thick), 31 km (medium) and 45 km (thin).Dashed lines are for the upper sideband, solid lines for thelower sideband. Note how the HCN line is clearly separatedfrom the vibrationally excited Ozone line in the uppersideband at 45 and 31 km, but that the two lines merge intoone broad feature at 17 km.Figure 3. Weekly zonal mean HCN mixing ratio (in ppbv)for the days centered on 1 September 2004.L08804 PUMPHREY ET AL.: HCN FROM EOS MLS L088042 of 4tropospheric source gas; they decrease with both altitudeand latitude.[11] Figure 4 shows a time series of mixing ratio at the10 hPa pressure level. High values occur in low latitudes atall seasons, and extremely low values are seen in southernhigh latitudes during the winter; these are caused by thedescent of air from the mesosphere in the polar vortex. Thedecrease in the equatorial values over the 12 months of datashown remains unexplained. However, we note that CH3CNhas been observed to show variability on timescales longerthan a year [Livesey et al., 2001]. Figure 5 shows a timeseries at 80S. The pattern of decreasing values (and hencedownward-sloping contours) for most of the year, with asharp change as the polar vortex breaks up is also a featurethat has been observed in CH3CN [see Livesey et al., 2001,Figure 5].5. Comparisons With Other Measurements[12] Figure 6 shows the ranges of various HCN measure-ments, with the MLS data superimposed. The MLS dataappear to show a positive bias, with the low-latitude valuesbeing greater than those returned by other measurementtechniques. The high latitude values are also greater than theone other high-latitude measurement shown. Comparisonswith ATMOS data (not shown) also suggest that the MLSdata have a positive bias.Figure 5. Time series of weekly zonal mean HCN mixingratio (in ppbv) at 80S. Day 1 is 1 Jan 2004.Figure 4. Time series of weekly zonal mean HCN mixingratio (in ppbv) at 10 hPa (approx 32 km). Day 1 is 1 Jan2004.Figure 6. MLS zonal means (thick lines) at 20N (dashed)and 80S (solid). Profiles from the MkIV balloon instru-ment (A. Kleinbohl et al., On the stratospheric chemistry ofhydrogen cyanide, submitted to Geophysical ResearchLetters, 2006) taken at 35N in September 2004 and 2005are shown black diamonds. The ranges of various historic insitu and ground-based measurements are also shown. Notethat recent changes to the HITRAN database have increasedthe MkIV mixing ratios by 5–9% and would have a similareffect on some of the historical data. (Figure based onSchneider et al. [1997, and references therein].)Figure 7. (top) Two HCN profiles (solid and dashed lines)used as input to the forward model. Points with error barsare Kopp’s measurements. The dotted line is a retrievedHCN profile. (bottom) Radiance profiles (solid and dashedlines) corresponding to the profiles Figure 7 (top). The thickdotted line is a mean of the 7-day means, taken over all7-day periods and all latitude bands. The thin dotted lines arethe extrema of the 7-day means. For this channel, the noise is1 K on a single scan and 0.04 K on a 7-day mean.L08804 PUMPHREY ET AL.: HCN FROM EOS MLS L088043 of 4[13] Above 45 km the mixing ratio decreases with height.No correlative data are available in the lower mesospherebut the measurements of Kopp [1990] show values of 0.2–1.0 ppbv between 70 and 90 km. For these values to bereasonable, there would need to be a reaction producingHCN. Kopp [1990] proposes two possibilities: CH3 + N!HCN + H2 and3CH2 + N! HCN + H. In both reactions theatomic nitrogen is assumed to be produced in auroralconditions. The MLS signal from these altitudes is small,so that the retrieved zonal mean is strongly affected by the apriori. We therefore ask: what would the radiances be like, ifthe mixing ratios were similar to those inferred by Kopp?Figure 7 shows two profiles: one being the a priori used forthe retrieval, the other being identical except in the region ofKopp’s data, where it matches those measurements.Corresponding calculated radiance profiles are also shownin Figure 7 along with the mean and limits of all the 107-day means of the MLS data. It is clear that a layer ofupper mesosphere HCN of about 0.6 ppbv would cause asignal of about 0.4 K in the center channel of band 27M,and that this signal is not present in the measured data. Wecan tentatively suggest that the mixing ratio in the meso-pause layer is not greater than 0.1 ppbv.6. Conclusions[14] Qualitatively, HCN as observed by MLS agrees wellwith that observed in the stratosphere by a variety oftechniques and predicted theoretically. Quantitatively, theHCN estimates reported here appear to have a positive biaswhen compared to earlier measurements. These resultsmake it clear that the MLS technique is a viable way tomeasure HCN mixing ratio in the middle atmosphere, butthat to achieve good results, a system with a lower mea-surement noise would be required. Figures 1 and 2 suggestthat an instrument designed specifically to measure HCNwould have to be a single-sideband instrument or have avery different LO frequency from that used by EOS MLS.[15] Recent work on HCN in the troposphere [Li et al.,2000; Singh et al., 2003] suggests that satellite observationsof this molecule in the UTLS region would be valuable fortracking the fate of biomass-burning products. A microwavelimb sounding instrument would be one way to achieve thisresult. The double-sideband nature of EOS MLS means thatroutine observations of HCN in the UTLS are not practical.However, a case has been observed [Livesey et al., 2004] inwhich an unusually large amount of CH3CN (anotherbiomass-burning product) was injected into the lowerstratosphere. It is possible that events of this type wouldbe observable with EOS MLS.[16] Acknowledgments. We thank Armin Kleinboehl and GeoffreyToon for providing preliminary HCN profiles from the JPL MkIV ballooninterferometer. MLS work in the UK is funded by NERC.ReferencesCicerone, R. J., and R. Zellner (1983), The atmospheric chemistry of hy-drogen cyanide (HCN), J. Geophys. Res., 88, 10,689–10,696.Kopp, E. (1990), Hydrogen constituents of the mesosphere inferred frompositive ions: H2O, CH4, H2CO, H2O2, and HCN, J. Geophys. Res., 95,5613–5630.Li, Q., D. J. Jacob, I. Bey, R. M. Yantosca, Y. Zhao, Y. Kondo, andJ. Notholt (2000), Atmospheric hydrogen cyanide (HCN): Biomass Burn-ing source, ocean sink?, Geophys. Res. Lett., 27, 357–360.Livesey, N. J., and D. L. Wu (1999), EOS MLS retrieval processes algo-rithm theoretical basis, Tech. Rep. JPL D-16159, Jet Propul. Lab., Pasa-dena, Calif.Livesey, N. J., J. W. Waters, R. Khosravi, G. P. Brasseur, G. S. Tyndall, andW. G. Read (2001), Stratospheric CH3CN from the UARS microwavelimb sounder, Geophys. Res. Lett., 28, 779–782.Livesey, N. J., M. D. Fromm, J. W. Waters, G. L. Manney, M. L. Santee,and W. G. Read (2004), Enhancements in lower stratospheric CH3CNobserved by UARS MLS following boreal forest fires, J. Geophys. Res.,109(D6), D06308, doi:10.1029/2003JD004055.Livesey, N. J., W. V. Snyder, W. G. Read, and P. A. Wagner (2006),Retrieval algorithms for the EOS Microwave Limb Sounder (MLS) in-strument, IEEE Trans. Geosci. Remote Sens, in press.Rodgers, C. D. (2000), Inverse Methods for Atmospheric Sounding: Theoryand Practise, World Sci., Hackensack, N. J.Schneider, J., V. Bu¨rger, and F. Arnold (1997), Methyl cyanide and hydro-gen cyanide measurements in the lower stratosphere: Implications formethyl cyanide sources and sinks., J. Geophys. Res., 102, 25,501–25,506.Singh, H. B., et al. (2003), In situ measurements of HCN and CH3CN overthe Pacific Ocean: Sources, sinks, and budgets, J. Geophys. Res.,108(D20), 8795, doi:10.1029/2002JD003006.Waters, J. W. (1999), An overview of the EOS MLS experiment, Tech. Rep.JPL D-15745, Jet Propul. Lab., Pasadena, Calif.Waters, J. W. (2006), The Earth Observing System Microwave Limb Soun-der (EOS MLS) on the Aura satellite, IEEE Trans. Geosci. Remote Sens.,in press.C. J. Jimenez and H. C. Pumphrey, Institute for Atmospheric andEnvironmental Sciences, School of GeoSciences, University of Edinburgh,Edinburgh EH9 3JN, UK. (h.c.pumphrey@ed.ac.uk)J. W. Waters, NASA Jet Propulsion Laboratory, Mail Stop 183-701, 4800Oak Grove Drive, Pasadena, CA 91109–8099, USA.L08804 PUMPHREY ET AL.: HCN FROM EOS MLS L088044 of 4

Atmospheric hydrogen cyanide (HCN): Biomass Burning source, ocean sink?,

Enhancements in lower stratospheric CH3CN observed by UARS MLS following boreal forest fires,

Hydrogen constituents of the mesosphere inferred from positive ions: H2O, CH4,H 2CO, H2O2, and HCN,

In situ measurements of HCN and CH3CN over the Pacific Ocean: Sources, sinks, and budgets,

Inverse Methods for Atmospheric Sounding: Theory and Practise, World Sci.,

Methyl cyanide and hydrogen cyanide measurements in the lower stratosphere: Implications for methyl cyanide sources and sinks.,

Retrieval algorithms for the EOS Microwave Limb Sounder (MLS) instrument,

Stratospheric CH3CN from the UARS microwave limb sounder,

The atmospheric chemistry of hydrogen cyanide

The Earth Observing System Microwave Limb Sounder (EOS MLS) on the Aura satellite,

Measurement of HCN in the middle atmosphere by EOS MLS

     Edinburgh Research Explorer                                      Measurement of HCN in the middle atmosphere by EOS MLSCitation for published version:Pumphrey, HC, Jimenez, CJ & Waters, JW 2006, 'Measurement of HCN in the middle atmosphere by EOSMLS', Geophysical Research Letters, vol. 33, no. 8, L08804, pp. 1-4. https://doi.org/10.1029/2005GL025656Digital Object Identifier (DOI):10.1029/2005GL025656Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Publisher's PDF, also known as Version of recordPublished In:Geophysical Research LettersPublisher Rights Statement:Published in Geophysical Research Letters by the American Geophysical Union (2006)General rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 30. Nov. 2025Measurement of HCN in the middle atmosphere by EOS MLSH. C. Pumphrey,1 Carlos J. Jimenez,1 and J. W. Waters2Received 30 December 2005; revised 20 February 2006; accepted 21 March 2006; published 22 April 2006.[1] The EOS MLS instrument on NASA’s Aura satellitehas made the first continuous global measurements of HCNin the middle atmosphere. The measurements are in broadagreement with most earlier measurements, showing HCNto be a long-lived tracer with sources in the troposphere andsinks in the middle atmosphere. There is no evidence in thedata for a proposed mesospheric source of HCN.Citation: Pumphrey, H. C., C. J. Jimenez, and J. W. Waters(2006), Measurement of HCN in the middle atmosphere by EOSMLS, Geophys. Res. Lett., 33, L08804, doi:10.1029/2005GL025656.1. Introduction[2] It has been known for many years that hydrogencyanide (HCN) is a minor constituent of the atmosphere. Acompilation of early measurements and modelling work[Cicerone and Zellner, 1983] suggests that the gas has itssources at the Earth’s surface, its sinks in the middleatmosphere and is well-mixed in the troposphere. A typicaltropospheric mixing ratio is about 0.18 ppbv. More recentstudies [Li et al., 2000; Singh et al., 2003] suggest a morecomplex picture in the troposphere, with stronger sourcesand the ocean as a stronger sink.[3] In the middle atmosphere, HCN is believed to be lostslowly due to reactions with OH and O1D. As a result, themixing ratio decreases with both altitude and latitude. Thispicture has, to date, been based on a small number ofmeasurements using various techniques.[4] The MLS instrument, one of four experiments onNASA’s Aura satellite, has made global measurements ofHCN continuously since August 2004. The signal from theHCN molecule is not large compared to the instrumentnoise, so that the measurements are best used to constructweekly zonal means. Despite their limitations, these dataallow us, for the first time, to observe the seasonal cycle ofmiddle-atmosphere HCN.2. Measurements[5] The Microwave Limb Sounder [Waters, 2006, 1999]is essentially a small radio telescope. The 1.6 m dishantenna is moved mechanically so that its field of viewscans across the Earth’s limb every 24.7 s. The radiation itreceives is converted to a lower frequency in severalheterodyne mixers (radiometers) and analyzed by 32 spec-trometers (28 filter banks and 4 digital autocorrelators).Most of the filter banks are located so that they cover afrequency range centered on a single spectral line of a targetmolecule. Microwave spectral lines show strong pressurebroadening: a line which is 1 MHz wide at 0.3 hPa (56 km)will be 300 MHz wide at 100 hPa (16 km). For this reasonthe filter banks are designed with narrower filters at theband center. The heterodyne nature of the radiometersmeans that each filter bank is affected by two quite separatespectral regions or sidebands, one each side of the localoscillator (LO) frequency. The sideband not containing thetarget line is not filtered out. Instead, the LO frequency ischosen so that, for the most important measurements, thenon-target sideband is in a spectral region with no stronglines.[6] Hydrogen cyanide is a linear molecule with a largedipole moment. Linear molecules have a simple microwavespectrum, with lines at integer multiples of a fundamentalfrequency: 88.6 GHz in the case of HCN. (Each HCN line issplit into several hyperfine components but these are tooclose together concern us here.) MLS has two filter banks(Band 6F and Band 27M) which are affected by the secondof these lines, which lies at 177.26 GHz. Band 27M wasincluded in the instrument specifically to measure HCN.However, as Figure 1 shows, although HCN is the largestsignal in the lower sideband it is small compared to thesignals from ozone and nitric acid in the upper sideband.Figure 2 shows some radiances as measured by the instru-ment. The HCN signal is not large compared to the randommeasurement noise for a single scan. By averaging theradiances for a week, over a 10 latitude band, the noiseis reduced to a level which allows the HCN signal to beobserved throughout the middle atmosphere.3. Retrieval of Mixing Ratios[7] HCN is one of the products retrieved by thestandard MLS retrieval software [Livesey and Wu, 1999;Livesey et al., 2006]. However, the smallness of thesignal means that the retrieved product is usable overonly a very small vertical range. Above 48 km the signalis lost in the noise. Below 32 km the signal is super-imposed on stronger emissions from HNO3 and O3. Thecurrent operational retrieval software is unable to obtainusable HCN mixing ratios in these conditions, returningvalues which have obvious systematic errors, being fre-quently large and negative.[8] In order to obtain HCN mixing ratios over a largervertical range, we form a weekly zonal mean of the radiancesand from these retrieve weekly zonal means of temperatureand mixing ratio. The calibrated radiance (level 1B) dataare provided with tangent altitude ht as a vertical co-ordinate;ht varies somewhat from one scan to the next. The weeklyzonal mean is formed by interpolating the radiances onto afixed set of tangent height values and taking the mean ofthese interpolated radiances for all scans within a given weekand latitude bin. Operational issues mean that on someGEOPHYSICAL RESEARCH LETTERS, VOL. 33, L08804, doi:10.1029/2005GL025656, 20061School of GeoSciences, University of Edinburgh, Edinburgh, UK.2NASA Jet Propulsion Laboratory, Pasadena, California, USA.Copyright 2006 by the American Geophysical Union.0094-8276/06/2005GL025656L08804 1 of 4occasions, the 7 days may not be consecutive, but they arespread over no more than 17 days.[9] The retrieval uses the maximum a postiori probability(MAP) formula as described by Rodgers [2000]. The apriori mixing ratio for HCN is 0.18 ppbv in the troposphereand decreases monotonically with height throughout themiddle atmosphere to a value of 1 pptv at 95 km. As theproblem is somewhat nonlinear, the MAP formula is usediteratively, a Marquadt-Levenberg scheme ensures conver-gence. The retrieval is performed in two phases. A firstphase estimates the zonal mean temperature T, O3 and thegeopotential height hr at a reference pressure, usingradiances from Band 1F and a few channels from band6F only. Band 1F is centered on the 118 GHz O2 line andis the main source of temperature information for MLS.Ozone is retrieved at the same time as there are interferingO3 lines in band 1F. A second phase adds bands 27M andmost of band 6F to the measurement vector, while HCNand HNO3 are added to the list of quantities estimated.The results of the first phase are used as the starting pointfor the second phase. The two-phase strategy is usedbecause the temperature retrieval is highly nonlinear andrequires more iterations.4. Results[10] Figure 3 shows a 7-day zonal mean of HCN mixingratio. The mixing ratios of 0.25 ppmv which occur between24 and 30 km are somewhat larger than the accepted valuein the troposphere. This may be due to spectroscopicuncertainties and should not necessarily be interpreted asa source of HCN in the lower stratosphere. Qualitatively, thevalues above 24 km are much as one would expect for aFigure 1. Calculated limb radiances in the (right) upper and (left) lower sidebands of MLS. The solid, dashed and dot-dashed lines are for tangent heights of 45, 31 and 17 km respectively. The molecules responsible for some of the moreprominent emission lines are marked at the top of the figure. Note that HCN causes the largest signal in the lower sideband,but that a line of vibrationally excited O3 appears in almost the same place in the upper sideband.Figure 2. Measured radiances in bands 6 and 27 of MLS.The measurements are shown as horizontal bars, the widthof the bar indicates the width of the spectrometer channel.Results are shown for three tangent heights: 17, 31 and45 km. The data are averaged over a 10 latitude bandcentered on the equator; this reduces the measurement noisefrom the single-scan values of 0.2–1 K down to 0.007–0.04 K. The lower noise levels correspond to the broaderchannels. The curves are calculated radiances for tangentheights 17 km (thick), 31 km (medium) and 45 km (thin).Dashed lines are for the upper sideband, solid lines for thelower sideband. Note how the HCN line is clearly separatedfrom the vibrationally excited Ozone line in the uppersideband at 45 and 31 km, but that the two lines merge intoone broad feature at 17 km.Figure 3. Weekly zonal mean HCN mixing ratio (in ppbv)for the days centered on 1 September 2004.L08804 PUMPHREY ET AL.: HCN FROM EOS MLS L088042 of 4tropospheric source gas; they decrease with both altitudeand latitude.[11] Figure 4 shows a time series of mixing ratio at the10 hPa pressure level. High values occur in low latitudes atall seasons, and extremely low values are seen in southernhigh latitudes during the winter; these are caused by thedescent of air from the mesosphere in the polar vortex. Thedecrease in the equatorial values over the 12 months of datashown remains unexplained. However, we note that CH3CNhas been observed to show variability on timescales longerthan a year [Livesey et al., 2001]. Figure 5 shows a timeseries at 80S. The pattern of decreasing values (and hencedownward-sloping contours) for most of the year, with asharp change as the polar vortex breaks up is also a featurethat has been observed in CH3CN [see Livesey et al., 2001,Figure 5].5. Comparisons With Other Measurements[12] Figure 6 shows the ranges of various HCN measure-ments, with the MLS data superimposed. The MLS dataappear to show a positive bias, with the low-latitude valuesbeing greater than those returned by other measurementtechniques. The high latitude values are also greater than theone other high-latitude measurement shown. Comparisonswith ATMOS data (not shown) also suggest that the MLSdata have a positive bias.Figure 5. Time series of weekly zonal mean HCN mixingratio (in ppbv) at 80S. Day 1 is 1 Jan 2004.Figure 4. Time series of weekly zonal mean HCN mixingratio (in ppbv) at 10 hPa (approx 32 km). Day 1 is 1 Jan2004.Figure 6. MLS zonal means (thick lines) at 20N (dashed)and 80S (solid). Profiles from the MkIV balloon instru-ment (A. Kleinbohl et al., On the stratospheric chemistry ofhydrogen cyanide, submitted to Geophysical ResearchLetters, 2006) taken at 35N in September 2004 and 2005are shown black diamonds. The ranges of various historic insitu and ground-based measurements are also shown. Notethat recent changes to the HITRAN database have increasedthe MkIV mixing ratios by 5–9% and would have a similareffect on some of the historical data. (Figure based onSchneider et al. [1997, and references therein].)Figure 7. (top) Two HCN profiles (solid and dashed lines)used as input to the forward model. Points with error barsare Kopp’s measurements. The dotted line is a retrievedHCN profile. (bottom) Radiance profiles (solid and dashedlines) corresponding to the profiles Figure 7 (top). The thickdotted line is a mean of the 7-day means, taken over all7-day periods and all latitude bands. The thin dotted lines arethe extrema of the 7-day means. For this channel, the noise is1 K on a single scan and 0.04 K on a 7-day mean.L08804 PUMPHREY ET AL.: HCN FROM EOS MLS L088043 of 4[13] Above 45 km the mixing ratio decreases with height.No correlative data are available in the lower mesospherebut the measurements of Kopp [1990] show values of 0.2–1.0 ppbv between 70 and 90 km. For these values to bereasonable, there would need to be a reaction producingHCN. Kopp [1990] proposes two possibilities: CH3 + N !HCN + H2 and3CH2 + N! HCN + H. In both reactions theatomic nitrogen is assumed to be produced in auroralconditions. The MLS signal from these altitudes is small,so that the retrieved zonal mean is strongly affected by the apriori. We therefore ask: what would the radiances be like, ifthe mixing ratios were similar to those inferred by Kopp?Figure 7 shows two profiles: one being the a priori used forthe retrieval, the other being identical except in the region ofKopp’s data, where it matches those measurements.Corresponding calculated radiance profiles are also shownin Figure 7 along with the mean and limits of all the 107-day means of the MLS data. It is clear that a layer ofupper mesosphere HCN of about 0.6 ppbv would cause asignal of about 0.4 K in the center channel of band 27M,and that this signal is not present in the measured data. Wecan tentatively suggest that the mixing ratio in the meso-pause layer is not greater than 0.1 ppbv.6. Conclusions[14] Qualitatively, HCN as observed by MLS agrees wellwith that observed in the stratosphere by a variety oftechniques and predicted theoretically. Quantitatively, theHCN estimates reported here appear to have a positive biaswhen compared to earlier measurements. These resultsmake it clear that the MLS technique is a viable way tomeasure HCN mixing ratio in the middle atmosphere, butthat to achieve good results, a system with a lower mea-surement noise would be required. Figures 1 and 2 suggestthat an instrument designed specifically to measure HCNwould have to be a single-sideband instrument or have avery different LO frequency from that used by EOS MLS.[15] Recent work on HCN in the troposphere [Li et al.,2000; Singh et al., 2003] suggests that satellite observationsof this molecule in the UTLS region would be valuable fortracking the fate of biomass-burning products. A microwavelimb sounding instrument would be one way to achieve thisresult. The double-sideband nature of EOS MLS means thatroutine observations of HCN in the UTLS are not practical.However, a case has been observed [Livesey et al., 2004] inwhich an unusually large amount of CH3CN (anotherbiomass-burning product) was injected into the lowerstratosphere. It is possible that events of this type wouldbe observable with EOS MLS.[16] Acknowledgments. We thank Armin Kleinboehl and GeoffreyToon for providing preliminary HCN profiles from the JPL MkIV ballooninterferometer. MLS work in the UK is funded by NERC.ReferencesCicerone, R. J., and R. Zellner (1983), The atmospheric chemistry of hy-drogen cyanide (HCN), J. Geophys. Res., 88, 10,689–10,696.Kopp, E. (1990), Hydrogen constituents of the mesosphere inferred frompositive ions: H2O, CH4, H2CO, H2O2, and HCN, J. Geophys. Res., 95,5613–5630.Li, Q., D. J. Jacob, I. Bey, R. M. Yantosca, Y. Zhao, Y. Kondo, andJ. Notholt (2000), Atmospheric hydrogen cyanide (HCN): Biomass Burn-ing source, ocean sink?, Geophys. Res. Lett., 27, 357–360.Livesey, N. J., and D. L. Wu (1999), EOS MLS retrieval processes algo-rithm theoretical basis, Tech. Rep. JPL D-16159, Jet Propul. Lab., Pasa-dena, Calif.Livesey, N. J., J. W. Waters, R. Khosravi, G. P. Brasseur, G. S. Tyndall, andW. G. Read (2001), Stratospheric CH3CN from the UARS microwavelimb sounder, Geophys. Res. Lett., 28, 779–782.Livesey, N. J., M. D. Fromm, J. W. Waters, G. L. Manney, M. L. Santee,and W. G. Read (2004), Enhancements in lower stratospheric CH3CNobserved by UARS MLS following boreal forest fires, J. Geophys. Res.,109(D6), D06308, doi:10.1029/2003JD004055.Livesey, N. J., W. V. Snyder, W. G. Read, and P. A. Wagner (2006),Retrieval algorithms for the EOS Microwave Limb Sounder (MLS) in-strument, IEEE Trans. Geosci. Remote Sens, in press.Rodgers, C. D. (2000), Inverse Methods for Atmospheric Sounding: Theoryand Practise, World Sci., Hackensack, N. J.Schneider, J., V. Bürger, and F. Arnold (1997), Methyl cyanide and hydro-gen cyanide measurements in the lower stratosphere: Implications formethyl cyanide sources and sinks., J. Geophys. Res., 102, 25,501–25,506.Singh, H. B., et al. (2003), In situ measurements of HCN and CH3CN overthe Pacific Ocean: Sources, sinks, and budgets, J. Geophys. Res.,108(D20), 8795, doi:10.1029/2002JD003006.Waters, J. W. (1999), An overview of the EOS MLS experiment, Tech. Rep.JPL D-15745, Jet Propul. Lab., Pasadena, Calif.Waters, J. W. (2006), The Earth Observing System Microwave Limb Soun-der (EOS MLS) on the Aura satellite, IEEE Trans. Geosci. Remote Sens.,in press.C. J. Jimenez and H. C. Pumphrey, Institute for Atmospheric andEnvironmental Sciences, School of GeoSciences, University of Edinburgh,Edinburgh EH9 3JN, UK. (h.c.pumphrey@ed.ac.uk)J. W. Waters, NASA Jet Propulsion Laboratory, Mail Stop 183-701, 4800Oak Grove Drive, Pasadena, CA 91109–8099, USA.L08804 PUMPHREY ET AL.: HCN FROM EOS MLS L088044 of 4

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Measurement of HCN in the middle atmosphere by EOS MLS

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