Composition

AIRS and Composition Science

The research focus area known as Atmospheric Composition is focused on studying changes in Earth's atmospheric chemistry, especially over time. Research is specifically geared toward creating a better understanding of the following science areas:

• Changes in atmospheric composition and the timescales over which they occur
• Forcings (man-made and natural) that drive the changes
• The reaction of trace components in the atmosphere to global environment change and the resulting effects on the climate
• The effects of global atmospheric chemical and climate changes
• Air quality

AIRS data are used in many areas of atmospheric composition study.

Ozone

AIRS measures the total column and profile of ozone with approximately two pieces of information in the tropopause transition region. This makes the AIRS ozone ideal for studies of stratospheric-tropospheric exchange during severe convection events and the global transport of ozone through the Brewer-Dobson circulation. AIRS ozone is visible during polar night allowing a good view of formation and decline of the annual polar ozone hole.

AIRS ozone data have undergone rigorous validation using aircraft data and ozone-sondes. In addition, AIRS ozone data have been shown to compare well with the spaceborne instruments IASI (Infrared Atmospheric Sounding Interferometer) and OMI (Ozone Monitoring Instrument). In particular, all three capture the vertical and horizontal variability very well in the upper-troposphere/lower-stratosphere.

Carbon Monoxide

AIRS 1600 kilometer cross-track swath and cloud-clearing retrieval capabilities provide daily global carbon monoxide maps over approximately 90% of the Earth. Validation indicates AIRS carbon monoxide retrievals are approaching the 15% accuracy target set by pre-launch simulations. AIRS has become a regular source for carbon monoxide data along with MOPITT, TES and SCIAMACHY and was used to estimate the global emissions of carbon monoxide at approximately 1350 Tg/year, which is much higher than bottoms up estimates. The difference is hypothesized to be from residential heating and vehicle cold starts. AIRS carbon monoxide products also play a key role in multiple studies of wildfire plumes

Carbon Dioxide

The most significant trace gas retrieved by AIRS for the study of anthropogenic effects on climate is carbon dioxide in the Mid-Troposphere (5-7 km). The AIRS mid-tropospheric carbon dioxide data have been shown to be accurate to ±1.20 ppmv when compared to aircraft observations at the same altitude. Global monthly maps of mid-tropospheric carbon dioxide have identified global transport patterns in the mid-troposphere. These results aid climate modelers in parameterization of mid-tropospheric transport processes of carbon dioxide and other gases.

Analysis suggests the influences of El Niño events and the polar vortex on carbon dioxide concentration are apparent in the AIRS data. During El Niño, mid‐tropospheric carbon dioxide is enhanced in the central Pacific Ocean and diminished in the western Pacific Ocean. In the polar region, mid‐tropospheric carbon dioxide is diminished if the polar vortex is strong and polar mid‐tropospheric carbon dioxide is enhanced if the polar vortex is weak. Please note: AIRS CO2 data is not produced for AIRS Version 6 data. It is available for Version 5 data, and only for a portion of the mission.

Methane

AIRS retrieves atmospheric methane in the upper troposphere. AIRS methane has peak sensitivity around 200 hPa, and the data have small enough biases to provide the ability to map seasonal variation of methane in the mid-upper troposphere. The uncertainty estimate for AIRS methane is 20%, including accuracy and precision.

AIRS scientists have observed a significant enhancement of methane in the mid to upper troposphere in the summer season associated with upwelling caused by the Asian monsoons. Results obtained with AIRS data are consistent with model predictions. AIRS observations also reveal methane variability during dry years in the Amazon related to biomass burning. AIRS data has now been used to study regional and global increases in mid-upper tropospheric methane at the relatively long timescale of the mission lifetime.

Dust

The AIRS data are particularly useful for understanding the global distribution, transport, and radiative forcing of atmospheric dust. In one study, AIRS thermal infrared radiance data are used with a fast infrared scattering radiative transfer model to physically retrieve the dust column amount and dust height over both ocean and land. AIRS‐derived dust top heights compare favorably with CALIPSO data and provide estimates of dust longwave radiative forcing: about +1.5 and +4.5 W/m² per unit visible optical depth over ocean and land, respectively, compared to a shortwave forcing estimate of −50 W/m² over ocean.

In an independent study, very good agreement is found between AIRS-retrieved Aerosol Optical Depths (AODs) and MODIS Aqua during the summer dust season. Both instruments show a large annual transport of dust from Africa (240 Tg) to the Atlantic (140 Tg), Amazon (50 Tg), Caribbean (50 Tg), and Europe (20 Tg). Finally, AIRS Water Vapor data was used with MODIS AOD to determine the relationship between water vapor and AOD. Little correlation was found, indicating these two variables can fluctuate independently of each other.

Sulfur Dioxide

SO₂ retrievals based on AIRS brightness temperatures, as well as SO₂ and ash indices based on AIRS brightness temperature differences, have been used to analyze the transport of volcanic plumes and inter-annual variability of Upper Troposphere Lower Stratosphere (UTLS) SO₂ associated with volcanic eruptions.