Airborne Multi-AXis Differential Optical Absorption Spectroscopy (AMAX-DOAS) fills an existing gap in observing capabilities from ground-based monitoring networks and satellite measurements (local to global scale) due to several advantages:

(1) Better sensitivity in the free troposphere by maximizing overlap between photon paths and trace gases (exploiting the mobility of the aircraft)

(2) Better vertical profiling capabilities: at constant flight altitude the vertical resolution is best near instrument altitude (few 100m)

(3) The remote sensing capability enables the measurement of trace gases above and below the aircraft, thus making AMAX-DOAS measurements sensitive to whole tropospheric and stratospheric columns.


Our AMAX-DOAS instruments typically consist of two synchronized spectrograph/CCD detectors that receive scattered light sequentially from any telescope, and simultaneously observe the spectral ranges from ~315 – 465nm (reactive trace gases) and ~390 – 525nm (aerosol properties). Telescopes are housed in different pylons, adapted to the specific aircraft. The forward viewing directions are actively stabilized by a real time motion compensation system to ensure constant lines of sight, which is crucial to optimize vertical profile retrievals. 


AMAX-DOAS measurements of trace gases and aerosols have been conducted during several field campaigns: Twin Otter flights in Colorado and California (2008 & 2009); CALNEX/CARES in California (2010); GV flights during HEFT-10 (2010); TORERO over the tropical Pacific Ocean (2012); TI3GER in Colorado and Hawaii (2022); and CUPiDS in New York (2023).

Upcoming AMAX-DOAS flight campaigns include CDPHE Front Range in Colorado (2024); USOS in Utah (2024); and TI3GER 2 in Hawaii (2024).


Twin Otter
First generation AMAX-DOAS pylon.


AMAX-DOAS pylon wing mounted to the NSF/NCAR GV. The (heated) forward and zenith telescope windows can be seen.


amax pylon
NSF/NCAR GV wing mounted AMAX-DOAS pylon.


Relevant publications:

Koenig et al.: Ozone depletion due to dust release of iodine in the free troposphere, Science Advances, 7, 52, doi:10.1126/sciadv.abj6544, 2021.

Koenig et al.: Quantitative detection of iodine in the stratosphere, Proc. Nat. Acad. Sci., 117, 4, 1860-1866, doi:10.1073/pnas.1916828117, 2020.

Wales et al.: Stratospheric injection of Brominated Very Short-Lived Substances: Aircraft Observations in the Western Pacific and Representation in Global Models, J. Geophys. Res. Atmos., 123, 5690-5719, doi:10.1029/2017JD027978, 2018.

Koenig et al.: BrO and Bry profiles over the Western Pacific: Relevance of Inorganic Bromine Sources and a Bry Minimum in the Aged Tropical Tropopause Layer, Atmos. Chem. Phys., 17, 15245-15270, doi:10.5194/acp-17-15245-2017, 2017.

Dix et al.: Parameterization Retrieval of Trace Gas Volume Mixing Ratios from Airborne MAX-DOAS, Atmos. Meas. Tech., 9, 5655-5675, doi:10.5194/amt-9-5655-2016, 2016.

Baidar et al.: Weakening of the weekend ozone effect over California's South Coast Air Basin, Geophys. Res. Lett., 42 (21), 9457-9464, doi:10.1002/2015GL066419, 2015.

Saiz-Lopez et al.: Injection of iodine to the stratosphere, Geophys. Res. Lett., 42 (16), 6852-6859, doi:10.1002/2015GL064796, 2015.

Wang et al.: Active and widespread halogen chemistry in the tropical and subtropical free troposphere, Proc. Natl. Acad. Sci., 112 (30), 9281-9286, doi:10.1073/pnas.1505142112, 2015.

Volkamer et al.: Aircraft measurements of BrO, IO, glyoxal, NO2, H2O, O2-O2 and aerosol extinction profiles in the tropics: Comparison with aircraft-/ship-based in situ and lidar measurements, Atmos. Meas. Tech., 8, 2121-2148, doi:10.5194/amt-8-2121-2015, 2015.

Spinei et al.: Ground-based direct-sun DOAS and airborne MAX-DOAS measurements of the collision-induced oxygen complex, O2O2, absorption with significant pressure and temperature differences, Atmos. Meas. Tech., 8, 793-809, doi:10.5194/amt-8-793-2015, 2015.

Baidar et al.: Combining Active and Passive Airborne Remote Sensing to Quantify NO2 and Ox production near Bakersfield, CA, Br. J. Environ. Clim. Change, 3 (4), 566-586, doi:10.9734/BJECC/2013/5740, 2013

Oetjen et al.: Airborne MAX-DOAS Measurements over California: Testing the NASA OMI Tropospheric NO2 product, J. Geophys. Res. - Atmos., 118 (13), 7400-7413, doi:10.1002/jgrd.50550, 2013.

Baidar et al.: The CU Airborne MAX-DOAS Instrument: Vertical Profiling of Aerosol Extinction and Trace Gases, Atmos. Meas. Tech., 6 (3), 719-739, doi:10.5194/amt-6-719-2013, 2013.