Mountaintop MAX-DOAS

The Idea 

A particular application of the MAX-DOAS technique involves deploying instruments at high-altitude mountain sites in remote locations. Typical ground-based remote sensing instruments are “stuck” within the boundary layer, which is heavily impacted by local traffic, industrial emissions and interactions with the surface. Mountain-top platforms allow us to rise above, thereby increasing sensitivity to the higher atmosphere, particularly the free troposphere. Because the air is thinner and cleaner at high altitudes, the instruments can “see” much farther (horizontal light paths >30 km). This increases their sensitivity, enabling the detection of free radicals and other trace gases at their subtle background concentrations. In contrast to airborne campaigns, measurements from mountain-tops can be performed continuously over several years with relatively minor effort on data acquisition and instrument maintenance. This allows us to investigate long-term chemistry trends and transport phenomena. Our mountain top instruments have been deployed in three locations (see below). To date (2026) they have acquired in total almost 15 years of data and will continue to do so. 

 

Typical mountain-top MAX-DOAS viewing geometry. 

 

 

Locations of the Volkamer-Group mountain-top MAX-DOAS instruments. 

The Science 

The free troposphere acts as a critical reservoir and transport pathway for reactive trace gases, yet it remains undersampled by traditional ground-based networks, and aircraft. Our mountain-top MAX-DOAS instruments (MT MAX-DOAS) address this data gap, characterizing the vertical distribution and chemical evolution of free radicals and trace gases, such as ozone precursors, halogen oxide radicals and sulfur dioxide. 

This data has already contributed to various fields of atmospheric research including:  

  • Halogen Radicals and Mercury Oxidation: MT MAX-DOAS provides empirical constraints on iodine and bromine. Observations at Storm Peak Laboratory reported the first ground-based evidence of elevated iodine monoxide (IO) over the central continental United States, with columns up to three times higher than predicted by global chemical transport models (Lee et al., 2024). Our findings suggest that iodine may be a competitive oxidant for elemental mercury. To date, this iodine-induced mercury oxidation is missing in atmospheric models.
  • Chemistry of volcanic emissions: MT MAX-DOAS observations allow for monitoring of long-term transport and chemistry of volcanic emissions from major eruptions. Our measurements at Maido observatory contributed to:
  1. Understanding of a stratospheric ozone depletion following the Hunga Tonga eruption (Evan et al., 2023), which was driven by intricate chemistry involving water vapor and chlorine species.
  2. Revealing a near-complete gaseous elemental mercury depletion within volcanic plumes, suggesting significant mercury scavenging by volcanic aerosols (Koenig et al., 2023)

 

Figure from Lee et al., 2024: Detection of iodine monoxide (IO) radicals above Storm Peak Laboratory on two example days. (a) and (b) show IO absorption signatures in the skylight spectra of our mountain-top MAX-DOAS for a “low IO” case (a) and a “high IO” case. (c) Back trajectories for both days show the origin of the observed air-masses. 

 

Figure from Koenig et al., 2024: Principles and components of an advanced retrieval algorithm for Bromine monoxide (BrO) vertical profiles from mountain-top measurements at Maido Observatory (Reunion Island). The algorithm maximizes information on the vertical distribution by combining measurements at multiple solar zenith angles and instrument viewing directions with chemical modeling. Using such approaches, independent concentration values at up to six altitudes can be obtained without any airborne equipment. 

 

Figure from Evan et al., 2023: After the Hunga Tonga volcanic eruption, mountain-top MAX-DOAS measurements complemented balloon-borne observations at Réunion Island, to understand how volcanic injection of H2O vapor, sulfur dioxide (SO2), and HCl cause rapid chlorine activation on hydrated volcanic aerosol and O3 depletion in the stratosphere. 

 

Related publications 

Koenig, T. K., Hendrick, F., Kinnison, D., Lee, C. F., Van Roozendael, M., and Volkamer, R.: Troposphere–stratosphere-integrated bromine monoxide (BrO) profile retrieval over the central Pacific Ocean, Atmos. Meas. Tech., 17, 5911–5934, https://doi.org/10.5194/amt-17-5911-2024, 2024. 

Lee, C. F., T. Elgiar, L. M. David, T. Y. Wilmot, M. Reza, N. Hirshorn, I. B. McCubbin, V. Shah, J. C. Lin, S. N. Lyman, A. G. Hallar, L. E. Gratz, R. Volkamer: Elevated Tropospheric Iodine Over the Central Continental United States: Is Iodine a Major Oxidant of Atmospheric Mercury?, Geophys. Res. Lett., 51, e2024GL109247. https://doi.org/10.1029/2024GL109247, 2024. 

Derry, E. J., T. R. Elgiar, T. Y. Wilmot, N. W. Hoch, N. S. Hirshorn, P. Weiss-Penzias, C. F. Lee, J. C. Lin, A. G. Hallar, R. Volkamer, S. N. Lyman, and L. E. Gratz: Elevated oxidized mercury in the free troposphere: analytical advances and application at a remote continental mountaintop site, Atmos. Chem. Phys., 24, 9615–9643. https://doi.org/10.5194/acp-24-9615-2024, 2024. 

Evan, S., J. Brioude, K. H. Rosenlof, R.-S. Gao, R. W. Portmann, R. Volkamer, C. F. Lee, J.-M. Metzger, K. Lamy, P. Walter, S. L. Alvarez, J. H. Flynn, E. Asher, M. Todt, S. M. Davis, T. Thornberry, H. Vomel, F. G. Wienhold, R. M. Stauffer, L. Millan, M. L. Santee, L. Froidevaux, and W. G. Read: Rapid ozone loss following humidification of the stratosphere by the Hunga Tonga Eruption, Science, 382, eadg2551(2023). https://doi.org/10.1126/science.adg2551, 2023. 

A. Koenig, O. Magand, C. Rose, A. Di Muro, Y. Miyazaki, A. Colomb, M. Rissanen, C. F. Lee, T. K. Koenig, R. Volkamer, J. Brioude, B. Verreyken, T. Roberts, B. A. Edwards, K. Sellegri, S. Arellano, P. Kowalski, A. Aiuppa, J. Sonke, A. Dommergue. Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols. Environ. Sci.: Atmos., https://doi.org/10.1039/D3EA00063J, 2023. 

Verreyken, B., C. Amelynck, J. Brioude, J.-F. Müller, N. Schoon, N. Kumps, A. Colomb, J.-M. Metzger, C. F. Lee, T. K. Koenig, R. Volkamer, and T. Stavrakou: Characterization of African biomass burning plumes and impacts on the atmospheric composition over the south-west Indian Ocean, Atmos. Chem. Phys., 20, 14821-14845, https://doi.org/10.5194/acp-20-14821-2020, 2020. 

Zhu, L., D. J. Jacob, S. D. Eastham, M. P. Sulprizio, X. Wang, T. Sherwen, M. J. Evans, Q. Chen, B. Alexander, T. K. Koenig, R. Volkamer, L. G. Huey, M. Le Breton, T. J. Bannan, and C. J. Percival: Effect of sea-salt aerosol on tropospheric bromine chemistry, Atmos. Chem. Phys., 19, 6497-6507, doi:10.5194/acp-19-6497-2019, 2019. 

Coburn, S., B. Dix, E. Edgerton, C. D. Holmes, D. Kinnison, Q. Liang, A. ter Schure, S. Y. Wang, and R. Volkamer: Mercury oxidation from bromine chemistry in the free troposphere over the Southeastern US, Atmos. Chem. Phys., 16, 3743-3760, doi:10.5194/acp-16-3743-2016, 2016. 

Sherwen, T., M. J. Evans, L. J. Carpenter, S. J. Andrews, R. T. Lidster, B. Dix, T. K. Koenig, R. Volkamer, A. Saiz-Lopez, C. Prados-Roman, A. S. Mahajan, and C. Ordóñez: Iodine's impact on tropospheric oxidants: a global model study in GEOS-Chem, Atmos. Chem. Phys., 16, 1161-1186, https://doi.org/10.5194/acp-16-1161-2016, 2016. 

Saiz-Lopez, A., S. Baidar, C. A. Cuevas, T. K. Koenig, R. P. Fernandez, B. Dix, D.E. Kinnison, J.-F. Lamarque, X. Rodriguez-Lloveras, T. L. Campos, and R. Volkamer: Injection of iodine to the stratosphere, Geophys. Res. Lett., 42 (16), 6852–6859, doi:10.1002/2015GL064796, 2015.