Owing to their sensitivity in large range of altitudes down to the surface, nadir UV/visible satellite sensors are well suited to monitor the atmospheric composition in both the troposphere and stratosphere. Emissions, which to a large part control the chemical state of the atmosphere, can originate from both natural and anthropogenic sources. Whilst man-made pollution and poor air quality is a major environmental concern, there are many natural sources of pollution which are often greater than their man-made counterparts and sometimes may be a source of hazards for humans on Earth. Relevant trace gases which can be measured by UV/visible spectroscopy are sulfur dioxide (SO₂), formaldehyde (H₂CO), glyoxal (CHOCHO), NO₂, ozone, absorbing aerosols and the halogen oxides BrO and IO.

A major natural source of SO₂ and particles is related to volcanic activity. Volcanic eruptions emit large amounts of ash and gases which can sometimes have severe implications for life on Earth and human society. Volcanic clouds can also have a significant impact on atmospheric chemistry and climate, both locally and on a global scale. For obvious reasons, volcanic ash and – to a lesser extent – sulfuric gases are major hazards to aviation. As a result, near-real-time satellite measurements of SO₂ and aerosol index (a good proxy for volcanic ash) have developed as an essential component of operational early warning systems such as the SACS (Support to Aviation Control System, http://sacs.aeronomie.be), which provides support to Volcanic Ash Advisory Centres (VAACs) designated to advise civil aviation authorities in case of volcanic eruptions. UV/visible space nadir observations have also contributed to a better understanding of the chemical composition of the volcanic plumes, through the unambiguous detection of several halogenated compounds such as BrO, IO and OClO.

Volatile organic compounds (VOCs) are another example of species naturally produced, in this case, by plants and trees as well as by fires and biomass burning processes. Isoprene in particular is a common VOC emitted in large quantities by vegetation, suspected to trigger asthma and other allergic reactions. Natural and anthropogenic emissions of VOCs can be inferred from space-based measurements of H₂CO and glyoxal, which are both short-lived intermediate product common to the degradation of many VOCs. Monitoring the spatial and temporal variability of VOCs is essential since, together with NOx, they control the formation of tropospheric ozone and indirectly also secondary organic aerosols. For these reasons, H₂CO satellite observations (and more recently also glyoxal) have been increasingly used in combination with global chemistry transport models to constrain VOC emissions.

Another important contribution of UV/visible spectroscopy to the monitoring of natural emissions concerns the identification of the role of reactive halogen compounds on polar tropospheric ozone. Following the so-called ‘polar bromine explosion' mechanism which catalytically releases massive amounts of inorganic bromine in the Spring boundary layer of both hemispheres, halogen oxide mixing ratios of up to 30 parts per trillion are produced, sufficient to destroy within one to two days the 30–40 parts per billion of ozone typically present in the boundary layer – leading to a ‘tropospheric ozone hole'. The systematic monitoring by sensors like GOME and SCIAMACHY of the spatial extent and variability of the BrO plumes responsible of the ozone destruction has allowed to significantly progress in the understanding of this phenomenon.

Chapter Editor

Michel van Roozendael (BIRA)

Contributors

Nicolas Theys (BIRA), Isabelle De Smedt (BIRA), Folkard Wittrock (U. Bremen), T. Wagner (MPIC), C. Nowlan (SAO)

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Volcanic eruptions emit plumes of ash and gases into the atmosphere at altitudes relevant to civil aviation. Ash-rich plumes are hazardous for airplanes as ash is very abrasive and easily melts inside their engines. The Support to Aviation Control Service (SACS, http: //sacs.aeronomie.be), is a free online service initiated by ESA for the near-realtime monitoring of volcanic plumes of SO2 and ash. It combines data from UV-visible and infrared spectral imagers, such as SCIAMACHY and GOME-2. This figure shows SO2 and absorbing aerosol index measured by GOME-2 after the eruption of the Icelandic Grímsvötn Volcano (21–28 May). A characteristic feature of the Grímsvötn eruption is that a large amount of SO2 was ejected northwards while the ash cloud went to the south-east (see GOME-2 image, Fig. 21). In this figure, the image of the absorbing aerosols index has been super- imposed onto the SO2 image. The null, low, medium and high level of aerosols/ash detected by GOME-2 corresponds respectively to AAI of under 2, AAI of 2.5, AAI of 3 and AAI over 3.5. Aerosol detection on the north coast of Norway is not related to the Grímsvötn eruption. The SO2 cloud, which travelled over Canada and came back over Europe, was monitored for 3 weeks. From Brenot et al. (2014).

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Under volcanic conditions, SO2 plumes can be emitted at high latitude in the troposphere or even be injected into the lower stratosphere. Based on GOME-2 measurements, this study explores the potential of UV/visible spectroscopy to quantify SO2 plume heights and as a result improve the accuracy of the SO2 column measurements. It is shown that under conditions of high SO2 emission, the altitude of the emitted plume can be directly retrieved from the observations. On the left hand side, the figure displays SO2 vertical column density (VCD) and retrieved SO2 plume altitude, and the corresponding total error and retrieval degrees of freedom for signal (DFS) for the Mount Kasatochi SO2 plume on 9 August 2008. The figure on the right hand side also shows the dependency of the altitude DFS as a function of the retrieved SO2 VCD, indicating some information content for altitude retrieval even at low SO2 content, and typically greater than 0.9 for measurements greater than 30 DU. From Nowlan et al. (2011).

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Formaldehyde (H2CO) is a well-established indicator of biogenic, pyrogenic and anthropogenic non-methane volatile organic carbon (NMVOC) emissions. This study presents the first trend analysis performed on H2CO satellite columns, retrieved from the GOME and SCIAMACHY instruments between 1997 and 2009. A linear model with a seasonal component is used to fit the time series of monthly averaged columns. The study focuses on Asia. Statistically significant positive trends of formaldehyde columns are observed over northeastern China (4% yr−1) and India (1.6% yr−1), related to strong increases in anthropogenic NMVOC emissions. From De Smedt et al. (2010).

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This study presents the first global simultaneous observations of glyoxal (CHOCHO) and formaldehyde (HCHO) columns retrieved from measurements by the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) satellite instrument. Yearly averages for (a) glyoxal and (b) formaldehyde are depicted for the year 2005. (c–e) Sub-figures illustrate the ratio between measured CHOCHO and HCHO while (f–g) show the global distribution as calculated by the model. In panel a (inset), CHOCHO is shown during biomass burning in Alaska in June 2004. The area with biomass burning is marked with X signs applying distributions from AATSR (Advanced Along Track Scanning Radiometer) fire counts. From Wittrock et al. (2006).

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Launched on the ERS-2 platform in April 1995, the GOME instrument showed for the first time that tropospheric air masses enriched in BrO are always situated close to sea ice and typically extend over areas of about 300–2,000 km. The BrO abundances remain enhanced for periods of 1 to 3 days initiating ozone depletion in the polar troposphere. This figure published in Nature in 1998 shows GOME observations of the BrO vertical column density over the Arctic from 4/5 to 8/9 September 1996. The areas where elevated tropospheric BrO concentrations build up and decay are clearly visible around the Antarctic continent (no satellite data are available for the white areas on the map). From Wagner and Platt (1998).

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Observed for the first time using the GOME-2 instrument onboard MetOp-A, a large plume of bromine monoxide emitted by the Kasatochi volcano on 7 August 2008 has been followed several days after the eruption. The figure displays the measured BrO total columns during the period from 8-13 August 2008. Further analysis of the results indicate that BrO was directly injected in the upper troposphere/lower stratosphere at altitudes ranging from 8 to 12 km and that the total mass of reactive bromine released in the atmosphere was around 50–120 tons, corresponding to approximately 25% of the previously estimated total annual mass of reactive Br emitted by volcanic activity. From Theys et al. (2009).

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• Brenot, H., Theys, N., Clarisse, L., van Geffen, J., van Gent, J., Van Roozendael, M., van der A, R., Hurtmans, D., Coheur, P.-F., Clerbaux, C., Valks, P., Hedelt, P., Prata, F., Rasson, O., Sievers, K., Zehner, C.: Support to Aviation Control Service (SACS): an online service for near-real-time satellite monitoring of volcanic plumes, Nat. Hazards Earth Syst. Sci., 14, 1099–1123, doi:10.5194/nhess-14-1099-2014, 2014.
• De Smedt, I., T. Stavrakou, J.-F. Müller, R. van der A, and M. Van Roozendael: Trend detection in satellite observations of formaldehyde tropospheric columns, Geophys. Res. Lett., 37, L18808, doi:10.1029/2010GL044245, 2010.
• Nowlan, C. R., X. Liu, K. Chance, Z. Cai, T. P. Kurosu, C. Lee, and R. V. Martin: Retrievals of sulfur dioxide from the Global Ozone Monitoring Experiment 2 (GOME-2) using an optimal estimation approach: Algorithm and initial validation, J. Geophys. Res., 116, D18301, doi:10.1029/2011JD015808, 2011.
• Theys, N., M. Van Roozendael, B. Dils, F. Hendrick, N. Hao, and M. De Mazière: First satellite detection of volcanic bromine monoxide emission after the Kasatochi eruption, Geophys. Res. Lett., 36, L03809, doi:10.1029/2008GL036552, 2009.
• Wagner, T., and U. Platt: Satellite mapping of enhanced BrO concentrations in the troposphere, Nature, 395, 486–490, 1998.
• Wittrock, F., A. Richter, H. Oetjen, J. P. Burrows, M. Kanakidou, S. Myriokefalitakis, R. Volkamer, S. Beirle, U. Platt, and T. Wagner: Simultaneous global observations of glyoxal and formaldehyde from space, Geophys. Res. Lett., 33, 1–5, doi:10.1029/2006GL026310, 2006.