Abstract. Atmospheric remote sounding from satellites is an essential component of the observational strategy deployed to monitor atmospheric pollution and changing composition. The IASI nadir looking thermal infrared sounder onboard MetOp will provide 15 years of global scale observations for a series of key atmospheric species, with unprecedented spatial sampling and coverage. This paper gives an overview of the instrument's capability for measuring atmospheric composition in the perspective of chemistry and air quality. The assessment is made in terms of species, accuracy and vertical information. Global distributions are presented for CO, CH 4 , O 3 (total and tropospheric), HNO 3 , NH 3 , and volcanic SO 2 . Local distributions of organic species measured during fire events, such as C 2 H 4 , CH 3 OH, HCOOH, and PAN are also shown. For each species or process, the link is made to specialized papers in this issue.

Formic acid contributes significantly to acid rain in remote environments1, 2. Direct sources of formic acid include human activities, biomass burning and plant leaves. Aside from these direct sources, sunlight-induced oxidation of non-methane hydrocarbons (largely of biogenic origin) is probably the largest source3, 4. However, model simulations substantially underpredict atmospheric formic acid levels5, 6, 7, indicating that not all sources have been included in the models. Here, we use satellite measurements of formic acid concentrations to constrain model simulations of the global formic acid budget. According to our simulations, 100-120 Tg of formic acid is produced annually, which is two to three times more than that estimated from known sources. We show that 90% of the formic acid produced is biogenic in origin, and largely sourced from tropical and boreal forests. We suggest that terpenoids-volatile organic compounds released by plants-are the predominant precursors. Model comparisons with independent observations of formic acid strengthen our conclusions, and provide indirect validation for the satellite measurements. Finally, we show that the larger formic acid emissions have a substantial impact on rainwater acidity, especially over boreal forests in the summer, where formic acid reduces pH by 0.25-0.5. Stavrakou, T., Muller, J. F., Peeters, J., Razavi, A., Clarisse, L., Clerbaux, C., Coheur, P., Hurtmans, D., De Maziere, M., Vigouroux, C., Deutscher, N., Griffith, D., Jones, N. & Paton-Walsh, C. (2012). Satellite evidence for a large source of formic acid from boreal and tropical forests. Nature Geoscience, 5 (1), 26-30. lower than in a recent modelling study [7] . The major part of this secondary 51 flux is due to isoprene oxidation by OH (8.9 Tg) and by ozone (3.9 Tg), 52followed by monoterpene oxidation (3 Tg). high-latitude areas (Fig. 1, Supplementary Figure S4) 157In an attempt to evaluate the IASI-derived source, we have conducted ex- 179We quantify the global impact of the IASI-constrained HCOOH source 180 on precipitation acidity using the calculated wet deposition fluxes of nitrate, 181 sulphate, ammonium, formate and acetate ions (Supplementary Section 7). 182The inferred decrease in pH due to the additional HCOOH source is es- Figure S9). 185Our model simulations predict that formic acid alone accounts for as much 186as 60-80% of the rainwater acidity over Amazonia, in accordance with in 187 situ measurements [29] , but also over boreal forests during summertime. Its the biota [30] . 197In this letter we have revisited the formic acid global distribution and 198 budget, using source inversion constrained by space observations. We found where H(f) is the model operator acting on the control variables, y is the introduced through the off-diagonal elements [24] . The matrix E is assumed

Abstract. This study provides improved methanol emission estimates on the global scale, in particular for the largest methanol source, the terrestrial biosphere, and for biomass burning. To this purpose, one complete year of spaceborne measurements of tropospheric methanol columns retrieved for the first time by the thermal infrared sensor IASI aboard the MetOp satellite are compared with distributions calculated by the IMAGESv2 global chemistry-transport model. Two model simulations are performed using a priori biogenic methanol emissions either from the new MEGANv2.1 emission model, which is fully described in this work and is based on net ecosystem flux measurements, or from a previous parameterization based on net primary production by Jacob et al. (2005). A significantly better model performance in terms of both amplitude and seasonality is achieved through the use of MEGANv2.1 in most world regions, with respect to IASI data, and to surface-and air-based methanol measurements, even though important discrepancies over several regions are still present. As a second step of this study, we combine the MEGANv2.1 and the IASI column abundances over continents in an inverse modelling scheme based on the adjoint of the IMAGESv2 model to generate an improved global methanol emission source. The global optimized source totals 187 Tg yr −1 with a contribution of 100 Tg yr −1 from plants, only slightly lower than the a priori Correspondence to: T. Stavrakou (jenny@aeronomie.be) MEGANv2.1 value of 105 Tg yr −1 . Large decreases with respect to the MEGANv2.1 biogenic source are inferred over Amazonia (up to 55 %) and Indonesia (up to 58 %), whereas more moderate reductions are recorded in the Eastern US (20-25 %) and Central Africa (25-35 %). On the other hand, the biogenic source is found to strongly increase in the arid and semi-arid regions of Central Asia (up to a factor of 5) and Western US (factor of 2), probably due to a source of methanol specific to these ecosystems which is unaccounted for in the MEGANv2.1 inventory. The most significant error reductions achieved by the optimization concern the derived biogenic emissions over the Amazon and over the Former Soviet Union. The robustness of the derived fluxes to changes in convective updraft fluxes, in methanol removal processes, and in the choice of the biogenic a priori inventory is assessed through sensitivity inversions. Detailed comparisons of the model with a number of aircraft and surface observations of methanol, as well as new methanol measurements in Europe and in the Reunion Island show that the satellite-derived methanol emissions improve significantly the agreement with the independent data, giving thus credence to the IASI dataset.

Abstract. Atmospheric remote sensing from satellite is an essential component of the observational strategy deployed to monitor atmospheric pollution and changing composition. The IASI nadir looking thermal infrared sounder onboard MetOp will provide 15 years of global scale observations for a series of key atmospheric species, with unprecedented spatial sampling and coverage. This paper gives an overview of the instrument's capability for measuring atmospheric composition in the perspective of chemistry and air quality studies. The assessment is made in terms of species, accuracy and vertical information. Global distributions are presented for CO, CH4, O3 (total and tropospheric), HNO3, NH3 and volcanic SO2. Local distributions of organic species measured during fire events, such as C2H4, CH3OH, HCOOH, and PAN are also shown. For each species or process, the link is made to specialized papers in this issue.

Abstract.In this paper we demonstrate the potential of the infrared Fourier transform spectrometer IASI in analysing volcanic eruptions, using the September 2007 eruption at Jebel at Tair as an illustrative example. Detailed radiative transfer calculations are presented, simulating IASI-like transmittance spectra for a variety of volcanic plumes. We analyse the sensitivity of IASI to SO 2 at different altitudes and demonstrate that IASI is in principle capable of sensing SO 2 down to the surface. Using the brightness temperature difference of well chosen SO 2 channels as a filter, we are able to track the plume of the Jebel at Tair eruption for 12 days, on a par with state of the art UV sounders. A method is presented for quickly estimating the altitude of a volcanic plume based on the relative intensities of the SO 2 absorption lines. Despite recent advances, it is still very challenging to retrieve vertical profiles of SO 2 from nadir viewing satellites. Currently the most accurate profiles in nadir are retrieved using backtracking of the plume with atmospheric transport models. Via full inverse retrievals using the optimal estimation method, we show the possibility of extracting medium coarse vertical profiles from IASI data. The retrieval allows us to present an evolution of the total mass of SO 2 in the plume for the Jebel at Tair eruption. An analytical relation is derived between brightness temperature differences and concentrations, which fits the experimental data very well. The spectral range of IASI also allows retrieval of volcanic aerosols. In the initial plume of the Jebel at Tair eruption, volcanic aerosols were found in the form of ice particles, for which we derived particle sizes.