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The MLS HO2 Product

Contact: Luis Millan

Basic Information

HO2 is an important reactive species in atmospheric chemistry. It is closely linked with OH and they are often known as the HOx family that plays an important role in controlling middle atmospheric ozone through catalytic HOx reaction cycles.
The HO2 Molecule
HO2 molecule visualization

How it is part of MLS Science Objectives

MLS makes the first global measurements of HO2 that, along with its measurements of OH, provide needed new information on stratospheric hydrogen chemistry that affects ozone. Lack of global OH and HO2 measurements has previously been a major limitation in constraining models of stratospheric chemistry. MLS HO2 measurements, together with OH measurements, provide unique observational evidences that make it possible to study the global distribution and variability of HOx, their partitioning, and their impact on ozone.

How EOS MLS measures HO2

The standard product for HO2 is taken from the 640 GHz retrieval. Significant averaging (e.g., weekly zonal means) is required to obtain useful HO2 data from MLS observations. Averages of nighttime observations (from the descending part of the orbit), when HO2 abundances are expected to be essentially zero show significant non-zero values, indicative of biases. These need to be removed before any scientific study by taking day/night (or ascending/descending) differences. We believe that these differences will be reliable at levels at and above the 22 hPa pressure surfaces. At pressures greater than 22 hPa the bias is larger than we can confidently expect simple day/night differences to remove.

HO2 has been validated using the balloon-borne FIRS-2 instrument during field campaigns [Pickett et al., 2006, 2008].

HO2 Information from the Spectroscopy Database

Quick Product Information for data version v4.2

**Day/night differences should be used in analyzing this product**
  • Swath Name: HO2
  • Status Flag: Only use profiles for which the Status field is an even number.
  • Useful Range: 22 - 0.046 hPa
  • DAAC Short Name: ML2HO2
  • Precision: Only use values for which the estimated precision is a positive number.
  • Quality Threshold: MLS v4.2x HO2 data can be used irrespective of the value of the Quality field.
  • Convergence Threshold: <1.1
Download EOS Aura MLS HO2 v4.2 data

Publications related to the MLS HO2 data product


  1. Kulikov, M., A. Nechaev, M. Belikovich, T. Ermakova, A. Feigin, "Technical note: Evaluation of the simultaneous measurements of mesospheric OH, HO<sub>2</sub>, and O<sub>3</sub> under a photochemical equilibrium assumption – a statistical approach", Atmospheric Chemistry and Physics 18, 10, 7453-7471, doi:10.5194/acp-18-7453-2018, 2018. reprint
  2. Zou, Z., X. Xue, C. Shen, W. Yi, J. Wu, T. Chen, X. Dou, "Response of Mesospheric HO2 and O3 to Large Solar Proton Events", Journal of Geophysical Research: Space Physics, doi:10.1029/2018ja025481, 2018. reprint


  1. Hegglin, M., S. Tegtmeier, J. Anderson, A. Bourassa, S. Brohede, D. Degenstein, L. Froidevaux, B. Funke, J. Gille, A. Jones, Y. Kasai, E. Kyrola, J. Lumpe, J. Neu, E. Remsberg, A. Rozanov, M. Toohey, J. Urban, T. von Clarmann, K.A. Walker, R. Wang, "The SPARC Data Initiative: Assessment of stratospheric trace gas and aerosol climatologies from satellite limb sounders", SPARC Report 8,, 2017. reprint
  2. James, A., D. Moon, W. Feng, P. Lakey, V. Frankland, D. Heard, J. Plane, "The uptake of HO2 on meteoric smoke analogues", Journal of Geophysical Research: Atmospheres 122, 1, 554-565, doi:10.1002/2016jd025882, 2017. reprint
  3. Li, K., Q. Zhang, S. Wang, S. Sander, Y. Yung, "Resolving the Model-Observation Discrepancy in the Mesospheric and Stratospheric HOx Chemistry", Earth and Space Science, doi:10.1002/2017ea000283, 2017. reprint


  1. Millan, L., S. Wang, N. Livesey, D. Kinnison, H. Sagawa, Y. Kasai, "Stratospheric and mesospheric HO2 observations from the Aura Microwave Limb Sounder ", Atmospheric Chemistry and Physics 15, 2889-2902, doi:10.5194/acp-15-2889-2015, 2015. reprint
  2. Wang, S., Q. Zhang, L. Millan, K. Li, Y.L. Yung, S.P. Sander, N.J. Livesey, M.L. Santee, " First evidence of middle atmospheric HO2 response to 27 day solar cycles from satellite observations ", Geophysical Research Letters 42, 22, 10,004-10,009, doi:10.1002/2015gl065237, 2015. reprint


  1. Khosravi, M., P. Baron, J. Urban, L. Froidevaux, A.I. Jonsson, Y. Kasai, K. Kuribayashi, C. Mitsuda, D.P. Murtagh, H. Sagawa, M.L. Santee, T.O. Sato, M. Shiotani, M. Suzuki, T. von Clarmann, K.A. Walker, S. Wang, "Diurnal variation of stratospheric and lower mesospheric HOCl, ClO and HO2 at the equator: comparison of 1-D model calculations with measurements by satellite instruments", Atmospheric Chemistry and Physics 13, 7587-7606, doi:10.5194/acp-13-7587-2013, 2013. reprint


  1. Jackman, C.H., D.R. Marsh, F.M. Vitt, R.G. Roble, C.E. Randall, P.F. Bernath, B. Funke, M. Lopez-Puertas, S. Versick, G.P. Stiller, A.J. Tylka, E.L. Flemming, "Northern Hemisphere atmospheric influence of the solar proton events and ground level enhancement in January 2005", Atmospheric Chemistry and Physics 11, 6153-6166, doi:10.5194/acp-11-6153-2011, 2011. reprint


  1. Pickett, H.M., B.J. Drouin, T. Canty, R.J. Salawitch, R.A. Fuller, V.S. Perun, N.J. Livesey, J.W. Waters, R.A. Stachnik, S.P. Sander, W.A. Traub, K.W. Jucks, K. Minschwaner, "Validation of Aura Microwave Limb Sounder OH and HO2 measurements", Journal of Geophysical Research 113, D16S30, doi:10.1029/2007JD008775, 2008. reprint


  1. Kovalenko, L.J., K.W. Jucks, R.J. Salawitch, G.C. Toon, J-F. Blavier, D.G. Johnson, A. Kleinbohl, N.J. Livesey, J.J. Margitan, H.M. Pickett, M.L. Santee, B. Sen, R.A. Stachnik, J.W. Waters, "Observed and Modeled HOCl Profiles in the Midlatitude Stratosphere: Implication for Ozone Loss", Geophysical Research Letters 34, L19801, doi:10.1029/2007GL031100, 2007. reprint
  2. Mueller, E.R., R. Henschke, W.E. Robotham, L.A. Newman, L.M. Laughman, R.A. Hart, J. Kennedy, H.M. Pickett, "Terahertz local oscillator for the Microwave Limb Sounder on the Aura Satellite", Applied Optics 46, no. 22, 4907-4915, doi:10.1364/AO.46.004907, 2007. reprint
  3. Verronen, P.T., C.J. Rodger, M.A. Clilverd, H.M. Pickett, E. Turunen, "Latitudinal extent of the January 2005 solar proton event in the Northern Hemisphere from satellite observations of hydroxyl", Annales Geophysicae 25, 2203-2215, doi:10.5194/angeo-25-2203-2007, 2007. reprint


  1. Canty, T., H.M. Pickett, R.J. Salawitch, K.W. Jucks, W.A. Traub, J.W. Waters, "Stratospheric and Mesospheric HOx: Results from Aura MLS and FIRS-2", Geophysical Research Letters 33, L12802, doi:10.1029/2006GL025964, 2006. reprint
  2. Cofield, R.E., P.C. Stek, "Design and field-of-view calibration of 114-660 GHz optics of the Earth Observing System Microwave Limb Sounder", IEEE Transactions on Geoscience and Remote Sensing 44, no. 5, 1166-1181, doi:10.1109/TGRS.2006.873234, 2006. reprint
  3. Pickett, H.M., W.G. Read, K.K. Lee, Y.L. Yung, "Observation of Night OH in the Mesosphere", Geophysical Research Letters 33, L19808, doi:10.1029/2006GL026910, 2006. reprint
  4. Pickett, H.M., B.J. Drouin, T. Canty, L.J. Kovalenko, R.J. Salawitch, N.J. Livesey, W.G. Read, J.W. Waters, K.W. Jucks, W.A. Traub, "Validation of Aura MLS HOx measurements with remote-sensing balloon instruments", Geophysical Research Letters 33, L01808, doi:10.1029/2005GL024048, 2006. reprint


  1. Osterman, G.B., R.J. Salawitch, B. Sen, G.C. Toon, R.A. Stachnik, H.M. Pickett, J.J. Margitan, J-F. Blavier, D.B. Peterson, "Balloon-borne measurements of stratospheric radicals and their precursors: Implications for the production and loss of ozone", Geophysical Research Letters 24, 1107-1110, doi:10.1029/97GL00921, 1997. reprint


  1. Stachnik, R.A., J.C. Hardy, J.A. Tarsala, J.W. Waters, N.R. Erickson, "Submillimeterwave heterodyne measurements of stratospheric ClO, HCl, O3, and HO2: First results", Geophysical Research Letters 19, 1931-1934, doi:10.1029/92GL01884, 1992. reprint

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