Tropospheric ozone is generated from chemical reactions linked to car traffic and power plants. Automobile and industrial emissions release nitrogen oxide gases (NO_x) and volatile organic compounds (VOC) that are by-products of burning gasoline and coal. NO_x and VOC combine chemically with oxygen to form ozone during the sunny, high-temperature conditions of late spring, summer and early fall. High levels of ozone are usually encountered in the heat of the middle of the day, and dissipate during the cooler nights.

The most prominent band for ozone in the TIR is at 9.7 μm. It extends from 980 to 1100 cm^-1 (see Fig. 2). It is only weakly perturbed by other absorption features of H₂O and CO₂. Previous experience has demonstrated that the TIR measurements provide information on the ozone vertical profile from the ground up to an altitude of about 40 km (Coheur et al., 2005), with a maximum sensitivity located in the mid-troposphere. The tropospheric column is typically measured with 10% accuracy.

Summer 2007 in Europe was unusual, with wet and cold conditions in NW Europe along with extreme and persistent heat in SE Europe (see Fig. 3). The meteorological situation during the 14th to 21st July period was characterised by long-lasting wide areas of high air pressure spread over continental Europe. Pollution episodes, which exceeded the information threshold limits, were observed in northern Italy and at several locations in Austria, Romania and around the Mediterranean.

Fig. 3 illustrates the impact of this exceptional atmospheric situation, which led to the increased formation of tropospheric ozone in southern parts of Europe. A further analysis of this pollution event with the IASI data is provided in Eremenko et al., 2008. It is a first step in using infrared satellite observations to monitor tropospheric ozone and to improve the forecasts of air quality and climate models.

Carbon monoxide is a primary pollutant produced from vegetation burning (for agricultural purposes or from wildfires). Other emission sources include fossil fuel combustion (associated with car traffic, industry and domestic heating) as well as methane and non-methane hydrocarbon oxidation. Its primary sink is oxidation by the hydroxyl radical (OH), which in turn controls the removal of most of the atmospheric pollutants as it is usually the predominant atmospheric oxidant. The CO atmospheric lifetime ranges from a few weeks to a few months depending on location and season, making it particularly suitable as a tracer of pollutant emissions. As result of its relatively long lifetime, it undergoes long-range transport from its sources and mixes both horizontally and vertically.

Carbon monoxide absorbs infrared radiation mainly in its 1-0 vibrational band, centred near 2140 cm^-1 (see Fig. 2). The spectral region, which extends from 2020 to 2250 cm^-1, has interferences from lines of H₂O, CO₂, N₂O, OCS and O₃. Current TIR sounders provide CO vertical profiles in the troposphere, depending on the sounder. The CO tropospheric column is usually measured with 10 % accuracy or better from TIR, and with a maximum of 2 independent pieces of information on the vertical.

The southern European and the Mediterranean basin are regularly affected by forest fires, which can burn thousands of hectares in a few days. The 2007 fire season was devastating for the southern part of Europe. Greece was the most affected country in terms of burned areas and injuries. More than 3000 km² were burned, more than half of this total in the Peloponnese at the end of August 2007 (EFFIS, 2007), destroying several protected natural sites, thousands of buildings and killing 63 people. Although the number of fires was similar to previous years, the severity of the burning events was much larger due to several heat waves in the preceding weeks, extended droughts and strong winds.

A detailed analysis of the regional impact of the record-breaking fires which devastated thousands of square kilometers of forest in Greece during the last week (23–30) of August 2007 is reported in Turquety et al., 2009. Large CO plumes were observed above the Mediterranean basin and North Africa, with CO total columns exceeding 30 molecules/cm² and volume mixing ratio up to 22 ppmv (see Fig. 4). Using the integrated total amount, the increase in CO burden due to these fires is estimated to be 0.327 Tg, ∼40% of the total annual anthropogenic emissions in Greece.

Volcanoes are an important source of atmospheric trace gases and particles. Eruptive events emit, in addition to large amounts of ash, a gaseous plume high into the troposphere or even the stratosphere. Mostly contained within a thin layer (typically 1 km thick or less), the plume is mainly composed of water vapour, carbon dioxide, sulphur dioxide and halogen species. It is transported away from its emission source following the main wind fields in the troposphere or the upper troposphere/lower stratosphere. During transport, chemistry modifies the composition of the plume, in particular sulphur dioxide, which is progressively transformed into sulphuric acid particles. Volcanic emissions pose a risk for populations living close to volcanically active regions and for air traffic as aircraft engines can be damaged by both ash and sulphuric acid. Therefore, rapid detection of volcanic emissions and prediction of their transport pathways is of high interest.

SO₂ absorbs thermal IR radiation in the υ₁ band around 1150 cm^-1, the υ₃ band around 1350 cm^-1, and the υ₁ + υ₃ band around 2500 cm^-1. The υ₃ band is the most prominent but lies in a range where strong absorptions by methane and water vapour occur (Clarisse et al., 2008; Clerbaux et al., 2008).

The eruption of Kasatochi volcano on 7th August 2008 is a good example on how satellite measurements can provide valuable information on an eruption posing a danger to aviation, as in the region of the Aleutian Island chain the density of volcanoes is very high but most of these volcanoes are not regularly monitored. Three major eruptions emitted large amounts of volcanic ash and gas into the atmosphere that rose to a height of at least 10 km, an altitude where they are a major hazard to aircraft. The eruption of Kasatochi volcano caused 44 Alaska Airline flights to be cancelled between Anchorage and the US West Coast leaving about 6000 passengers stranded. Flights from other US airlines had to be cancelled or diverted as well (Rix et al., 2009).

From the analysis of IASI near real time data low resolution SO₂ profiles can be obtained, adding information on the altitude of the ejection. Based on the IASI SO₂ retrieval system ULB is running in near-real time an SO₂ alert system since November 2008. The alerts are sent out by email and complemented by images directly available on the web (http://cpm-ws4.ulb.ac.be/Alerts/index.php). As the retrieval technique is negligibly affected by SO₂ in the boundary layer (e.g. pollution or passive degassing of volcanoes), it is an ideal tool to be used for aviation hazard mitigation. As an illustration, Fig. 5 gives a 10-day integrated view of the SO₂ flag after the eruption of Kasatochi as seen using the IASI observations.