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COMEX Science Overview

The final report for COMEX can be found here.

The COMEX (CO2 and Methane EXperiment) campaign is a collaborative NASA/ESA airborne campaign that is collecting data to determine greenhouse gas source emissions based on both in situ and remote sensing data. While in situ data only measures gas concentration at the instrument’s location (where the airplane flies or car drives), remote sensing data records the entire column below the airplane – thus it cannot miss the plume by flying above it.

The COMEX campaign combines two distinct remote sensing approaches, imaging spectroscopy with relatively broad spectral resolution but fine spatial resolution, and non-imaging spectroscopy at moderate spectral resolution. The AVIRIS Classic (AVIRIS-C) and AVIRIS-Next Generation (AVIRIS-NG) are imaging spectrometers that flew on the NASA ER2 and a Twin Otter airplane during COMEX. The University of Bremen, MAMAP (Methane Airborne MAPper) instrument is a non-imaging spectrometer that flew on the CIRPAS (Center for Interdisciplinary Remotely-Piloted Aircraft Studies) Twin Otter airplane.
COMEX data will test for natural synergies between these two types of remote sensing instruments in the airborne data for concurrent collection. Selection of the AVIRIS NG and MAMAP instruments was important–they are simulator instruments for the HyspIRI and CarbonSat satellite missions, of NASA and ESA, respectively. Through radiative transfer calculations, COMEX will demonstrate these synergies for space sensors.

COMEX will test the specific synergy that surface spectral albedo obtained at higher spatial resolution by AVIRIS-C and AVIRIS-NG (HyspIRI) can improve MAMAP measurements. Meanwhile, MAMAP’s higher spectral resolution improves atmospheric corrections for AVIRIS-C and AVIRIS-NG.

•COMEX will investigate the spatial and spectral resolution tradeoffs and synergies for methane and carbon dioxide anomaly (plume) detection and flux inversion to derive emissions.

The spectral features that are used to remote sense methane and carbon dioxide from reflected sunlight lie in the near infrared. Because water is very dark in the near infrared, remote sensing over the ocean and lakes are highly challenging. One solution is to tilt the sensor (a new MAMAP feature) or fly in a specific pattern with an imaging spectrometer like AVIRIS to use sunglint – the bright speckles of reflected sunlight from water surface wave facets. Sunglint greatly enhances the signal, allowing methane and carbon dioxide remote sensing over oceans, and even at high latitudes.

• COMEX will characterize sunglint advantages for methane remote sensing over marine and lake sources.

Using passive reflected light to measure a trace gas requires determining the surface reflectance or albedo for each pixel. Where the scene is complex, a pixel can have many components. Naturally, the smaller the pixel the better; however, all instruments have tradeoffs between spatial resolution, spectral resolution, and sensitivity. COMEX focuses on both urban and rural scenes to better understand sub-pixel complexity – also known as spectral clutter.

• COMEX will characterize the effect of surface spectral reflectance heterogeneity on successful trace gas remote sensing for moderate and broad spectral resolution spectrometers.

There are methane emission spectral features in the longwave (thermal) infrared spectrum that can be used for remote sensing. Thermal infrared remote sensing has advantages – no need for sunlight – and complexity – there is sensitivity to temperature contrast. The thermal infrared imaging spectrometer, Mako, will collect methane maps based on emissivity features for COMEX target sites for comparison.

• COMEX will identify synergies from combined near infrared and thermal infrared remote sensing for trace gas plume remote sensing.

The CIRPAS Twin Otter supports a comprehensive science package for atmospheric measurements including temperature, 3D winds, humidity, and aerosols. Also, CIRPAS flies the MAMAP sensor and two in situ analyzers, one for methane and carbon dioxide and one for carbon dioxide and its isotopes. These analyzers use Cavity Ring-Down Spectrometry (CRDS), and Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS), respectively. Meanwhile, AJAX supports a CRDS analyzer, while AMOG houses multiple OA-ICOS analyzers.

The experimental design focuses on the CIRPAS airplane, which alternates between remote sensing surveys above the boundary layer – about 2000 – 5000 ft above ground - and in situ surveys within. In coordination, AVIRIS NG flies above on a separate Twin Otter swath mapping the plumes and sources. These relatively low altitude flights are coordinated with the ER2 and AVIRIS Classic to collect remote sensing data from the stratosphere. The AlphaJet and Mako on a Twin Otter airplane are de-conflicted in time. Supporting surface wind, greenhouse gas concentration and other measurements are made by the AMOG Surveyor (AutoMObile greenhouse Gas), for near surface data, below where airplanes can fly.

COMEX leverages the sunny skies and strong methane sources of Southern California in summer 2014, targeting strong emission sites in the South San Joaquin Valley and Los Angeles Basin and the Santa Barbara Channel. COMEX is collecting data on landfills, oil refineries, oil production fields, natural seepage sources – both terrestrial and marine, and dairies.

For more information on COMEX components:

AVIRIS-C  - Airborne Visual InfraRed Imaging Spectrometer
AVIRIS NG - Airborne Visual InfraRed Imaging Spectrometer Next Generation
MAMAP - Methane Airborne Mapper
Mako - Mako Long Wave InfraRed Imaging Spectrometer
OA-ICOS - Off-Axis Integrated Cavity Output Spectroscopy
CRDS - Cavity Ring-Down Spectroscopy
AlphaJet (AJAX) - Alpha Jet Atmospherier eXperiment
CIRPAS - Center for Interdisciplinary Remotely-Piloted Aircraft
ER-2 -  Earth Research 2 airplane
HyspIRI - Hyperspectral InfraRed Imager
CarbonSat - CarbonSat