AIRS and Composition Science
The research focus area known as Atmospheric Composition is focused on studying changes in the Earth's atmospheric chemistry, especially over time. The research is specifically geared toward creating a better understanding of the following four science areas: the changes in atmospheric composition and the timescales over which they occur, the 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, and air quality. AIRS data are used in many areas of atmospheric composition study.
The ozone profile and total burden are produced from the AIRS retrieval as a necessary part of the temperature and water vapor retrieval. Scientists have successfully retrieved other trace gases including, carbon monoxide, methane, and carbon dioxide from AIRS because of the broad global daily coverage and the paucity of available data sets for methane and carbon dioxide from other platforms.
AIRS measures the total column and profile of ozone with approximately 2 pieces of information in the boundary between the tropopause and the stratosphere. 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 ozonesondes. More recently AIRS Ozone data have been validated with aircraft and 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 well in the upper-troposphere/lower-stratosphere.
AIRS' 1600 km 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.
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 been generated, and identify 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. Recent analysis suggests that the influences of El Niño events and polar vortex on the carbon dioxide concentration are apparent in the AIRS data. During El Niño, mid‐tropospheric carbon dioxide is enhanced in 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.
AIRS also retrieves atmospheric methane in the upper troposphere. The accuracy of AIRS methane is about 1.2-1.5%, with peak sensitivity around 200 mb, which should provide the ability to map seasonal variation of methane and provide valuable information of the global methane distribution in mid-upper troposphere. 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. The results obtained with AIRS data are consistent with model predictions.
The AIRS data are particularly useful for understanding 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, a very good agreement is found between AIRS-retrieved 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 used with MODIS Aerosol Optical Depth (AOD) to determine relationship between water vapor and AOD. Little correlation was found indicating these two variables can fluctuate independently of each other.