By Maya George, Juliette Hadji-Lazaro, and Cathy Clerbaux; LATMOS-IPSL, UPMC, Paris, France
Lieven Clarisse, Daniel Hurtmans, and Pierre-Fran̤ois Coheur; Spectroscopie de l’Atmosph̬re, Chimie quantique et Photophysique, UniversitÌ© Libre de Bruxelles, Brussels, Belgium
Figure 1: Animation showing MetOp-A and MetOp-B flying simultaneously. Half an orbit (~50 minutes) is separating the two satellites. Image Credit: Maya George/LATMOS, image made using IXION/IPSL software.
An unprecedented spatial coverage
IASI performs measurements covering the globe twice daily, which makes it an ideal instrument for monitoring the evolution of important compounds in the atmosphere. Launched in October 2006, onboard the polar-orbiting MetOp-A satellite, IASI scans along its swath and records about 1.3 million spectra per day. In September, a second IASI instrument was launched onboard MetOp-B, and will allow four overpasses above the same location on the globe every day (Figure 1). A third instrument onboard MetOp-C is scheduled for launch in 2016.
Figure 2: Examples of different atmospheric compounds retrieved from the IASI measurements: Ozone (for partial and total columns), nitric acid (HNO3), formic acid (HCOOH), methanol (CH3OH) and ammonia (NH3). Image Credit: Maya George/LATMOS
Due to its broad spectral range and its low radiometric noise, IASI detects long-lived species including CO2, N2O, CFC-11, CFC-12, HCFC-22, and OCS; medium-lived species including water vapor and its isotopologues, CH4, O3, CO and nitric acid (HNO3); and short-lived species including ammonia (NH3), trace gases from fires (methanol (CH3OH), formic acid (HCOOH)), SO2 in volcanic plumes, and aerosols (Figure 2). Processing tools have been developed at ULB and LATMOS to provide near real-time retrievals and operational products. For example, the CO and SO2 processing is performed in near real time, and the gas concentrations are available about three hours after observation.
IASI CO used to improve pollution forecasts
IASI CO data (Figure 3) are assimilated daily in the European Centre for Medium-Range Weather Forecasts (ECMWF) model to provide operational CO pollution forecasts. This collaboration takes place in the framework of the GMES/MACC project. Typically, IASI CO data are available for ECMWF assimilation less than three and a half hours after observation, allowing the model to integrate new data every 30 minutes. Carbon monoxide is an important ozone precursor and an important tracer for air pollution, with important connections to climate, and it is produced by incomplete combustion of fossil and bio-fuels caused predominantly by industrial processes and biomass burning. Carbon monoxide contributes to climate change through its effect on ozone and methane chemistry, and is currently regulated by air quality standards.
Figure 3: IASI CO distributions for August 2011. High CO concentrations are represented in yellow and red. The high CO levels above central Africa are due to biomass burning. The CO above China is related to industrial processes. Image Credit: Maya George/LATMOS
Because IASI data are acquired in near real time, a continuous analysis and a systematic surveillance of high SO2 levels caused by volcanic eruptions is possible (Figure 4). An automatic system is currently in operation, which sends an alert-email to the Volcanic Ash Advisory Centers (VAAC) when high SO2 levels are detected. Detection of eruptions from space is useful for aviation safety. Aircraft encounters with volcanic plumes are hazardous: Fine ash can melt inside the engine, and potentially cause it to clog up. Ash and sulfate aerosols also can cause abrasion to the exterior of the airplane. For this reason, the International Civil Aviation Organization monitors international air corridors, and maintains nine VAAC, in charge of specific zones of the Earth. Each VAAC is responsible to locate and monitor the evolution of the volcanic plumes within its zone. For this, they use a combination of pilot reports, ground-based measurement and satellite observations. The latter relies mostly on the SO2 spectral signal, as SO2 is for most eruptions an excellent indicator of the plume extent and transport. The recent episodes detected by IASI can be seen online. The IASI instrument also allows for direct detection of volcanic ash and ice, including microphysical properties such as particle sizes thanks to the high spectral resolution.
Figure 4: Detection of the SO2 plume of the Soufriere Hills eruption, which took place on 12 February 2010 (9:30pm local time). The units are DU. Image Credit: Lieven Clarisse/ULB
More information about the IASI instrument and the MetOp satellites can be found on the dedicated sites of CNES and Eumetsat and the following papers.
Clerbaux et al. (2009) and Clarisse et al. (2011) give an overview of the capability of IASI and the 24 species that can be monitored by the instrument.
Clarisse et al. (2008, 2010) for publications related to SO2 detected by IASI.
George et al. (2009), Turquety et al. (2009) and Pommier et al. (2010) for publication related to CO.
Publications related to IASI can be found online.
Daily CO (and ozone) maps with a choice of several projections (globe, Europe and North Pole) are available online.
Acknowledgements
The IASI mission is a joint mission of Eumetsat and the Centre National dÛ_Etudes Spatiales (CNES, France). The IASI L1 data are distributed in near real time by Eumetsat through the Eumetcast system distribution. The authors acknowledge the French Ether atmospheric database for providing the IASI L1C data. The French scientists are grateful to CNES for scientific collaboration and financial support.