# Differential absorption lidar (DIAL / IPDA) for trace gas
*methodologies*

Differential absorption lidar transmits two closely-spaced wavelengths (online, on a gas absorption line; offline, off it) and retrieves trace-gas column or profile from the differential return attenuation. Acquisition: dual-wavelength on/off-line pulses (integrated-path IPDA against the surface return, or range-resolved DIAL). Retrieval: column-integrated dry-air mixing ratio of the target gas. Example instrument: MERLIN (methane, IPDA, ~1645 nm, CNES/DLR, phase D, launch ~2027).

## Specifications
- **family**: LiDAR
- **entity type**: methodology
- **last verified date**: 2026-06-05
- **verified by**: agency-doc
- **claim status**: unclaimed
- **attributes**: {"family":"LiDAR","summary":"Differential absorption lidar transmits two closely-spaced wavelengths (online, on a gas absorption line; offline, off it) and retrieves trace-gas column or profile from the differential return attenuation. Acquisition: dual-wavelength on/off-line pulses (integrated-path IPDA against the surface return, or range-resolved DIAL). Retrieval: column-integrated dry-air mixing ratio of the target gas. Example instrument: MERLIN (methane, IPDA, ~1645 nm, CNES/DLR, phase D, launch ~2027)."}
- **technology**: lidar

## Editorial
Differential absorption lidar (DIAL) and its integrated-path variant (IPDA) retrieve atmospheric trace-gas concentrations by transmitting two closely spaced laser wavelengths: an online wavelength tuned to a strong absorption feature of the target gas and an offline reference wavelength positioned just outside that feature. The ratio of the backscattered signals at the two wavelengths carries the round-trip absorption, from which column-integrated or range-resolved dry-air mixing ratios are derived.

IPDA targets the surface return or a hard reflector to accumulate the full column signal, making it particularly suited to global greenhouse gas monitoring from orbit. MERLIN, a joint CNES-DLR mission, uses an IPDA lidar at approximately 1645.552 nm, placed on a methane R6 absorption line, with a closely offset offline pulse. Its target retrieval precision for column-average methane (XCH4) is 1% (approximately 18 ppb), sufficient to detect regional flux signals against the global background.[^s-mdpi-merlin-remote-sensing][^s-eoportal-merlin][^s-researchgate-merlin-overview] As of available mission documentation, MERLIN is in phase D, with a planned launch on a MYRIADE Evolutions platform; schedule details may have evolved after the research date.[^s-eoportal-merlin]

China's ACDL instrument, operating at 1572 nm via an IPDA-DIAL channel for column CO2 (XCO2), flew on DQ-1 (Daqi-1) and demonstrated global XCO2 column retrieval from spaceborne IPDA lidar, confirming the technique's maturity for carbon cycle monitoring.[^s-amt-acdl-xco2][^s-amt-acdl-calibration] The same ACDL instrument is also carried on DQ-2 (Daqi-2), which launched in April 2026 and continues CO2 column measurements via IPDA.[^s-wmo-oscar-acdl-dq2]

Key data products are column-average dry-air mixing ratios (XCH4, XCO2), supporting greenhouse gas flux estimation and carbon cycle monitoring from orbit.

## Sources
- [s-mdpi-merlin-remote-sensing] | MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane - Remote Sensing 2017 | https://www.mdpi.com/2072-4292/9/10/1052 | tier=peer-reviewed | accessed=2026-06-05
  Peer-reviewed mission overview; IPDA DIAL principle, online wavelength ~1645.552 nm on CH4 R6 absorption line, offline reference, XCH4 column retrieval, 1% (~18 ppb) precision target; CNES platform + DLR payload
- [s-eoportal-merlin] | MERLIN - eoPortal | https://www.eoportal.org/satellite-missions/merlin | tier=community | accessed=2026-06-05
  Mission profile: phase D, launch ~2027, sun-synchronous orbit, nadir-pointing IPDA lidar; MYRIADE Evolutions platform
- [s-amt-acdl-xco2] | First global XCO2 observations from spaceborne lidar: methodology and initial result - ScienceDirect 2025 | https://www.sciencedirect.com/science/article/abs/pii/S003442572500358X | tier=peer-reviewed | accessed=2026-06-05
  Peer-reviewed; confirms ACDL 1572 nm IPDA-DIAL technique demonstrated operationally for CO2 column retrieval on DQ-1
- [s-amt-acdl-calibration] | ACDL/DQ-1 calibration algorithms Part 1 - AMT 2025 | https://amt.copernicus.org/articles/18/2021/2025/ | tier=peer-reviewed | accessed=2026-06-05
  Confirms ACDL 1572 nm IPDA channel for XCO2 and 532 nm HSRL for aerosol on DQ-1 launched April 2022
- [s-researchgate-merlin-overview] | MERLIN (Methane Remote Sensing Lidar Mission): an overview - ResearchGate | https://www.researchgate.net/publication/280090102_MERLIN_Methane_Remote_Sensing_Lidar_MIssion_an_overview | tier=third-party | accessed=2026-06-05
  Conference paper confirming DIAL/IPDA dual-wavelength measurement principle and methane science case
- [s-wmo-oscar-acdl-dq2] | WMO OSCAR - ACDL instrument record (DQ-1 and DQ-2) | https://space.oscar.wmo.int/instruments/view/acdl | tier=agency-doc | accessed=2026-06-06
  Agency-doc: ACDL listed on both DQ-1 (Jul 2022) and DQ-2 (Apr 2026); CO2 mole fraction listed as primary measurement variable for both missions; 1572 nm dual-wavelength IPDA channel described. Confirms ACDL on Daqi-2 measures CO2 via IPDA, backing the daqi-2 flies_on relation.

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Source: https://eo-atlas.org/methodologies/lidar-dial
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