In following paragraphs more details can be found on the species we detect. For some of them, concrete examples are given in plots, with data from Ile de la Réunion. More plots and time series are available on the infrared data pages.
1. Greenhouse gases
When solar radiation reaches the Earth, 30% of this radiation is reflected back into space
by clouds (20%), by the atmosphere (6%) and by the Earth’s surface (4%). The other 70%
is absorbed by several elements and sent back to space in the form of infrared (IR) radiation.
Greenhouse gases then reflect / absorb part of these IR emissions that were emitted by the Earth, spread the recuperated energy from this radiation and in turn also emit IR radiation. Part of this radiation returns...
... to the surface, causing a second warming up after originally being warmed up by the sun. This results in a temperature increase in the troposphere, known as the greenhouse effect, and a temperature decrease in the stratosphere.
One of the consequences of this temperature increase in the troposphere is a meltdown of the polar icecaps and consequently higher ocean levels. As most greenhouse gases have a long atmospheric lifetime, they can have a significant impact on climate.
The Earth’s greenhouse gases are
- H2O (= most important, i.e., 63%)
- CO2 (= second important, i.e., 20%)
- CFC’s (under which HCFC’s)
Among these, the atmospheric abundances of CO2, SF6, CFC’s and CH4 have increased significantly mainly because of human activity, causing the so-called anthropogenic greenhouse effect, in addition to the natural greenhouse effect. This anthropogenic greenhouse effect is most clearly visible in the Northern Hemisphere (NH).
The goal of the Kyoto Protocol (d.d. 1997) is to globally limit the further increase of greenhouse gases in the atmosphere.
Primary greenhouse gases measured by our FTIR spectrometer:
- closed system in which water circulates from land and oceans into the atmosphere and back, through evaporation, condensation and precipitation
Sinks: reaction with the hydroxyl radical OH (tropospheric as well as stratospheric)
H2O has only a short lifetime of a few days. H2O stimulates the production of polar stratospheric clouds (PSC’s) and it influences the radiative heating equilibrium.
- Natural: oceans
- Anthropogenic: energy and industry (fossil fuels: petroleum and coal), cement production, deforestation
Sinks: photosynthesis by plants and trees, uptake by oceans
CO2 is very stable, has small variations in time and its mixing ratio’s are constant over large altitude ranges. It has a large atmospheric lifetime of about 100 years or more.
CO2 has currently not yet been retrieved from our 120 M FTIR spectrometer data, but it will be analysed based on our 125 HR FTIR spectrometer from 2011 onward, within the frame of the Total Carbon Column Observing Network (TCCON).
- Natural: wetlands, soils, forests, oceans, hydrates, geological sources (gas seepage and volcanoes), wild animals (mainly cattle), termites, wildfires
- Anthropogenic: energy and industry (fossil fuels: gas, petroleum and coal), landfills and waste treatment, manure management (mainly cattle), rice cultivation, field burning of agricultural residues (mainly corn and soybeans), biomass burning
Sinks: uptake by soils, reaction with OH (tropospheric as well as stratospheric) and with stratospheric Cl, and O(1D),
CH4 is the main hydrocarbon, followed by C2H6 and it has a large lifetime of about 10 years. It is a precursor for ozone production in the troposphere, it is a source of water vapor in the stratosphere, it terminates stratospheric ozone loss cycles by forming CH3 and OH or HCl, and it affects stratospheric ozone through OH.
In the troposphere, CH4 is relatively uniformly distributed along different latitudes, namely about 1400 ppb, in the NH as well as in the SH, although in the last few years an increasing trend can be observed of about 0.7%. Yet there is a clear interhemispheric gradient, with present-day VMR values of about 1.7 ppmv in the SH (Southern Hemisphere) and about 1.85 ppmv in the NH (Northern Hemisphere). This gradient reflects the larger sources of methane in the NH.
- Natural: photochemical reactions, El Niño
- Anthropogenic: industry, traffic
Sinks: reaction with OH, NOx (= NO + NO2), HCHO, Cly, CO and with hydrocarbons
Depending on the altitude O3 has a different lifetime, due to the solar radiation, namely about 90 days in the troposphere, about 1 to 3 years around 20 km, about 1 week around 30 km and about 5 to 20 hours around 50 km.
Whereas tropospheric ozone contributes to the greenhouse effect and has a negative influence on crops and on health, stratospheric ozone is very useful in order to stop harmful solar UV-B radiance to reach the Earth’s surface.
More particular, between 0 and 2 km, we talk about ozone smog, which is very poisonous.
- Natural: lightning, oxidation of NH3, (tropical) soils, oceans, forests
- Anthropogenic: industry (nylon production, fossil fuel combustion), manure management (mainly cattle), biomass burning, field burning of agricultural residues (mainly corn and soybeans), chemical conversion of nitrogen in fertilizers, agricultural soil management (mainly cropland and grassland), sewage treatment, adipic acid and nitric acid production
Sinks: uptake by soils, photolysis, reaction with stratospheric O(1D),
Its photochemical lifetime varies from approximately 100 years at 20 km and below, to 1 year at 33 km and 1 month at 40 km. As these lifetimes are longer than dynamical time scales, the global distribution of N2O is primarily governed by the Brewer-Dobson circulation. This makes it a useful tracer in the stratosphere.
Secondary greenhouse gases measured by our FTIR spectrometer:
Apart from their indirect effect on climate change, CO and C2H6 play a central role in tropospheric chemistry through their reactions with OH. They are emitted primarily by anthropogenic sources, and they can be used as tracers of tropospheric pollution and transport (e.g., transport of emissions from biomass burning), because they have relatively high tropospheric abundances and long tropospheric lifetimes.
- Natural: fires, biogenic sources, oceans, photo-oxidation of CH4 and non-methane hydrocarbons, CO2 photo-dissociation
- Anthropogenic: biomass burning, traffic, heating, industry (fossil fuel combustion)
Sinks: reaction with OH (tropospheric as well as stratospheric)
CO has a lifetime of several weeks to a few months, allowing the characterization of both emission sources and atmospheric transport of pollution plumes. Surface CO has a bad influence on health.
- Natural: gas losses from land and oceans
- Anthropogenic: production and transmission of fossil fuels, biofuel use, biomass burning
Sinks: reaction with tropospheric OH and with stratospheric Cl
It has a short lifetime of about 2 to 4 months. Because C2H6 has the same sources and sinks as CO, and consequently the same properties, both are strongly correlated.
C2H6 varies strongly with latitude: in the SH it is uniformly distributed, on average 0.5 ppb, whereas it has a strong gradient in the NH, increasing towards the North to about 1.1 ppb on average.
2. Stratospheric gases
Hydrogen chloride (HCl) is a toxic, colourless, strongly acidic gas that dissolves readily in water to form hydrochloric acid. HCl is an inorganic chlorine molecule that is abundant in the higher stratosphere, together with chlorine nitrate (ClONO2). These molecules are called "chlorine reservoirs" because they do not themselves react with ozone (O3), but they generate a small amount of chlorine-containing radicals, such as Cl, ClO, ClO2, and related species, referred to collectively as the "ClOx family", which do react with O3. An increase in the concentration of the chlorine reservoirs leads to an increase in the concentration of the ozone-destroying radicals. E.g., the immediate cause of the Antarctic ozone hole is an unusual sequence of reactions, catalyzed by polar stratospheric clouds (PSCs), that "empty" these chlorine reservoirs and produce high concentrations of ozone-destroying ClOx radicals [1, 2].
Over the past few decades an enormous effort has been devoted to identifying sources and sinks of stratospheric chlorine. The concentrations of the major species have been measured as a function of altitude, by in-situ methods (e.g., collection filters carried on planes and balloons) and remote methods (e.g., ground based and satellite spectroscopic measurements). From all this work it was possible to clarify the processes that carry chlorine through the stratosphere.
In the troposphere, the HCl mixing ratio decreases markedly with increasing altitude. In the stratosphere, on the other hand, it increases with altitude, rapidly up to about 35 km, and then more slowly up to 55 km and beyond [3,4].
ClONO2, the other important inorganic chlorine compound in the stratosphere, also increases rapidly in the lower stratosphere, and then falls off at higher altitudes. These and other results strongly suggest that HCl in the stratosphere is being produced there and hence not drifting up from below.
Based on many observations and studies, it has been shown that HCl is a by-product of the destruction of chlorofluorocarbons (CFC's) by UV radiation in the stratosphere. In the stratosphere HCl increases with altitude exactly where CFC's decrease. Growth rate of HCl has decreased because of reduced CFC production following the Montreal Protocol. The atmospheric lifetime of CFC's is about 100 year.
A few percent of HCl is produced by volcanic eruptions.
 R. P. Wayne, Chemistry of Atmospheres, 2nd Ed., Oxford, (1991)
 F. S. Rowland, "Chlorofluorocarbons and the depletion of stratospheric ozone", Am. Sci. 77, 36 (1989).
 C. B. Farmer, O. F. Raper, and R. H. Norton, "Spectroscopic detection and vertical distribution of HCl in the troposphere and stratosphere", Geophys. Res. Lett. 3, 13 (1975).
 J. Eyre and H. Roscoe, "Radiometric measurement of stratospheric HCl", Nature, 266, 243 (1977).
Hydrogen fluoride (HF) is a toxic, colourless, strongly acidic gas that dissolves readily in water to form hydrofluoric acid. It is less reactive than chlorine. Hydrogen fluoride is a stable by-product of chlorofluorocarbons (CFC's) destruction by UV radiation in the stratosphere. In the stratosphere HF increases with altitude exactly where CFC's decrease.
The total amount of HF in the stratosphere increased by a factor of 3 to 4 between 1978 and 1989 [1, 2]. The fluorine budget, as a function of altitude, adds up in much the same way as the chlorine budget .
Growth rate of HF has decreased because of reduced CFC production following the Montreal Protocol. It is also produced by volcanic eruptions.
 R. Zander, M.R. Gunson, J.C. Foster, C.P. Rinsland, and J. Namkung, "Stratospheric ClONO2, HCl, and HF concentration profiles derived from ATMOS/Spacelab 3 observations - an update", J. Geophys. Res. 95, 20519 (1990).
 C. P. Rinsland, J. S. Levine, A. Goldman, N. D. Sze, . K. W. Ko, and D. W. Johnson, "Infrared measurements of HF and HCl total column abundances above Kitt Peak, 1977-1990: Seasonal cycles, long-term increases, and comparisons with model calculations", J. Geophys. Res. 96, 15523 (1991).
 R. Zander, M. R. Gunson, C. B. Farmer, C. P. Rinsland, F. W. Irion, and E. Mahieu, "The 1985 chlorine and fluorine inventories in the stratosphere based on ATMOS observations at 30 degrees North latitude", J. Atmos. Chem. 15, 171, (1992).
Hydrogen cyanide (HCN) is a colourless, volatile, and extremely poisonous chemical compound whose vapours have a bitter almond odour.
HCN main source in the troposphere is the biomass burning. It is a tropospheric source gas that enters in the stratosphere and is slowly destroyed there. HCN is not only an indicator of biomass burning events, it also acts as a tracer to observe the dynamics of the middle atmosphere. Its primary loss mechanism is reaction with the OH radical. Its atmospheric residence time appears to be about 2.5 years, although 1.5 years is a possible range.
Nitric acid (HNO3) is a very water soluble, acidic gas.
In the atmosphere it is formed by the conversion of nitrogen monoxide into nitrogen dioxide, and ultimately into nitric acid. It readily reacts with atmospheric water to produce acidic precipitation. Nitric acid also reacts with gaseous ammonia to form particulate or aerosol nitrate, which in turn is removed by wet and dry deposition of the particles. In the clean background troposphere, its removal in precipitation acts as a sink for odd hydrogen and nitrogen compounds and limits the formation of ozone.
HNO3 is chemically destroyed by photolysis and oxidation by OH.
HNO3 is a reservoir species for nitrogen in the stratosphere and therefore plays an important role in stratospheric ozone chemistry. It can sequester the more reactive NOx species, thereby reducing ozone destruction. HNO3 indirectly regulates the magnitude, spatial extent, and duration of ozone destruction in the stratosphere.
3. Biomass burning products
Ile de la Réunion is located only 700 km from the East coast of Madagascar and about 2000 km from southeastern Africa, a region with large biogenic NMVOC emissions as well as extensive vegetation fires during the May-November period. Therefore, it is an excellent location to study compounds that are direct and/or indirect biomass burning products: CO , C2H6, HCN, C2H2, HCHO, HCOOH,…
Formaldehyde is one of the most abundant carbonyl compounds and a central component of the oxidation of volatile organic compounds (VOC). Both NOx and VOC concentrations determine the production of ozone in the troposphere. The main sources of HCHO in the atmosphere are the photochemical oxidation of methane and non-methane volatile organic compounds (NMVOCs), among which are biogenic VOCs (isoprene), and anthropogenic hydrocarbons. HCHO is also released during biomass burning. The sinks of formaldehyde are photolysis, oxidation by OH and dry and wet deposition [Stavrakou et al., 2009]. HCHO is a major source of CO. Due to its short lifetime of only a few hours, its global distribution closely resembles the distribution of its sources. Far away from the emission regions, e.g. over oceans, formaldehyde observations might provide an opportunity to test our current knowledge regarding methane oxidation, and possibly also to quantify the effect of long-range transport of NMVOCs from source regions.
Our FTIR measurements of HCHO have been compared to ground-based Multi-Axis Differential Absorption Spectroscopy (MAXDOAS) and satellite measurements (SCIAMACHY/ENVISAT, described in detail in De Smedt et al., 2008); but also to simulations from the 3D chemistry transport model IMAGESv2 (Stavrakou et al., 2009). All details can be found in Vigouroux et al. (2009). We obtain a good agreement between the various data sets. We have learned more about the long-range transport of Non-methane Volatile Organic Compounds (NMVOCS, precursors of HCHO) and deficiencies in the model. It was shown, with the help of the Lagrangian particle dispersion model FLEXPART that fast, direct transport of NMVOCS from Madagascar has a significant impact on the HCHO abundance and its variability at Reunion Island, and that this is underestimated in the model.
Acetylene is a combustion product (industries, biofuel, biomass burning) and its main sink is the reaction with OH. Its lifetime is about 20 days. The difficulty of retrieving C2H2 is its very weak infrared absorption features. However, we managed to observe C2H2 in our Reunion spectra and we have made comparisons with the CTM IMAGESv2 (Stavrakou et al., 2009). Different IMAGESv2 sensitivity simulations have been made which suggest that the observed underestimation of the model during the dry season might be caused by an underestimation of biomass burning emissions of C2H2.
A publication is in preparation.
The lifetime of HCOOH is about 4 days. Its dominant sinks are wet and dry deposition, and reaction with OH. The direct sources of formic acid are anthropogenic emissions (biofuel), soils, vegetations, biomass burning emissions, but the dominant source is photochemical production (C2H2, C2H4, glycolaldehyde, isoprene, terpenes,…), whose understanding is still very incomplete. It is therefore of strong interest to measure it. It is, as for C2H2, a challenge due to its very weak infrared absorption features.
We compare our FTIR products to the CTM IMAGESv2. We see that the model strongly underestimates the observations, which is consistent with reported literature results (von Kuhlmann et al., 2003, Ito et al. 2007)that point to an underestimation of the HCOOH sources. The most likely missing source is secondary biogenic production. Maximum values are found in the dry season in both FTIR and model results, partly caused by the contribution of biomass burning but also to the longer lifetime due to low precipitation
The tropospheric HCN lifetime is currently estimated as about 5 months. Biomass burning is the main source and the reaction with OH the main sink. Ocean uptake has been suggested also as an additional sink. The correlation with CO is very good.
A publication is in preparation.
B. Validation studies
Our measured FTIR data are used extensively in validation and inter-comparison studies. Due to our capability to probe the entire atmospheric column on a systematic basis, the ground-based FTIR data are very well suited for comparisons with satellite data, certainly when combining the FTIR data with those from a quasi-global network of FTIR stations, such as the ones from the Network for the Detection of Atmospheric Composition Change (NDACC).
We have been leading or involved in coordinated validation studies, using ground-based FTIR data from NDACC.
O3, HNO3, N2O, CH4
We have performed MIPAS (v4.561) validation for the species HNO3 and N2O at five ground-based FTIR stations distributed in both hemispheres: Kiruna (68°N), Jungfraujoch (47°N), Wollongong (34°S), Lauder (45°S), and Arrival Heights (78°S).
This work is described in detail in Vigouroux et al., 2007. The original feature of our validation work is that we have used the stratospheric 4D-VAR data assimilation system BASCOE constrained by MIPAS data, to get around the spatial collocation problem that always exists when one compares two different remote-sensing instruments. We give examples of such comparisons in the following Figure. Quantitative results on the comparisons can be found in Vigouroux et al., 2007.
We have also participated, with data from the same five FTIR stations, to coordinated MIPAS validation studies of O3 (Cortesi et al., 2007), HNO3 (Wang et al., 2007), and N2O and CH4 (Payan et al., 2009).
CH4, CO2, CO, N2O
M. De Mazière is validation coordinator for the SCIAMACHY CO, CH4 and N2O column products.
Past and ongoing studies in this field have performed the validation of 3 distinctive scientific SCIAMACHY retrieval algorithms for CO, CH4 and to al lesser extent N2O and CO2. These retrieval algorithms, namely WFM-DOAS , IMAP-DOAS and IMLM have significantly matured since their first results in 2004. However all teams involved are still finding ways to improve upon the accuracy and precision.
CO, HNO3, CH4
The team is working on the validation of atmospheric chemistry products from the IASI experiment, like CO and HNO3, and the retrieval and validation of IASI CH4 data.
C. Long range transport studies
To interpret our FTIR observations at Ile de la Réunion, we need information on the source regions of the observed species (such as CO and formaldehyde). The model we use for these purposes is FLEXPART, a free, elaborately tested, software package which calculates the Lagrangian trajectories and dispersion of a large number of particles. Lagrangian particle dispersion models are ideal for our purposes as single particle trajectory models such as HYSPLIT and FLEXTRA yield no quantitative information, while Eulerian chemical transport models are often too elaborate and are less suited for allocating specific source areas.
|Another case for which we have used FLEXPART long range transport simulations is the mapping of Volcanic BrO emissions and the assessment of the injection height and total released Br mass.