Figure 1. World map of the averaged tropospheric NO2 column measured by OMI in the period May 2006 till February 2007.
Figure 1. World map of the averaged tropospheric NO2 column measured by OMI in the period May 2006 till February 2007.
Nitrogen oxides (NOx, the collective name for NO and NO2) play an important role in atmospheric chemistry and in the climate system as precursor of tropospheric ozone, an important greenhouse gas. The most important source of NOx is the combustion of fossil fuels, mainly by traffic and large power plants, but also the burning of biomass, soil emissions and lightning are important contributors. Close to the ground the lifetime of NOx is short (a few hours). Therefore, NO2 concentrations will be highest close to the source. Nitrogen oxides affect the health of humans and animals. They irritate the lungs and lead to lower resistance to respiratory infections such as influenza. Frequent exposure to high concentrations may cause acute respiratory illness. Another serious problem caused by nitrogen oxides is the formation of aerosols and tropospheric ozone (i.e. smog), which has also harmful health effects. In addition, increasing tropospheric ozone affects climate.

Sulphur dioxide, SO2, enters the atmosphere as a result of natural phenomena as well as anthropogenic activities, e.g. combustion of fossil fuels, oxidation of organic material in soils, volcanic eruptions, and biomass burning. Coal burning is by far the largest man-made source of SO2 (about 50%), with oil burning accounting for a further 25 to 30%. Sulphur dioxide reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidised within airborne water droplets, producing sulphuric acid. This acidic pollution can be transported by wind over large distances, and is deposited as acid rain. Changes in the abundance of SO2 have an impact on atmospheric chemistry and hence on the climate. Consequently, global observations of SO2 are important for atmospheric and climate research. Volcanic eruptions are important sources of ash and SO2 in the atmosphere, which may have an impact on air traffic. Near-real time retrieval of SO2 concentrations enables monitoring of such events and assists in aviation control. Off-line retrieval, on the other hand, is more suitable for monitoring anthropogenic pollution aspects.

Tropospheric nitrogen dioxide

Satellite observations
Satellite instruments (such as GOME, SCIAMACHY and OMI) use spectroscopy to retrieve atmospheric trace gas concentrations in the atmosphere. By comparing the measured spectrum of the backscattered light from the Earth’s atmosphere with a reference spectrum, the column density of nitrogen dioxide along the light path can be determined. The NO2 stratospheric column is deduced from a chemistry-transport model assimilation run of the NO2 column data. Subsequently, the assimilated stratospheric column is subtracted from the retrieved total column, resulting in a tropospheric column. Information about the global tropospheric NO2 columns is publicly available on the TEMIS website. More details about the satellite observations and the retrieval technique can be found in several publications1,2,3).
Figure 2. The yearly averaged tropospheric NO2 column measured by SCIAMACHY for 2004 in China. High values are measured above the major cities. The industrial area around the Yellow River (Huang He) is also noticeable and highlights the river stream.
Figure 2. The yearly averaged tropospheric NO2 column measured by SCIAMACHY for 2004 in China. High values are measured above the major cities. The industrial area around the Yellow River (Huang He) is also noticeable and highlights the river stream.

NO2 has been monitored by satellite since 1995 with GOME, since 2002 with SCIAMACHY, and since 2004 with the OMI instrument; the latter two instruments having the advantage of a high spatial resolution. In Figure 1 the mean tropospheric NOx, is shown as measured by OMI in the period May 2006 till February 2007. Clearly visible are the industrial regions in China, Europe, South-Africa and the USA. The yearly averaged NO2 column for 2005 measured with SCIAMACHY zoomed-in over China can be seen in Figure 2. It shows high concentrations of NO2 above highly populated regions like Beijing, Shanghai, Hong Kong and South Korea. It can also be seen that the satellite detects the emissions around the Yellow river (Huang He). Over the sparsely populated western part of China, low NO2 concentrations are observed, except over the large city Urumqi in the Northwest.
Figure 3. Comparisons between annual means of, a) NO2 SCIAMACHY tropospheric columns (1015molecules cm-2), b) NO2 CHIMERE tropospheric columns obtained by using the averaging kernels, c) emissions (EMEP) of nitrogen oxides (NOx) over Western Europe for 10h00 UTC for 1998, and d) NO2 CHIMERE tropospheric columns computed without using the averaging kernels. The emissions are derived from data given by EMEP, interpolated on the CHIMERE grid domain, unit 10^10 molecules cm-2 s-1.
Figure 3. Comparisons between annual means of, a) NO2 SCIAMACHY tropospheric columns (1015molecules cm-2), b) NO2 CHIMERE tropospheric columns obtained by using the averaging kernels, c) emissions (EMEP) of nitrogen oxides (NOx) over Western Europe for 10h00 UTC for 1998, and d) NO2 CHIMERE tropospheric columns computed without using the averaging kernels. The emissions are derived from data given by EMEP, interpolated on the CHIMERE grid domain, unit 10^10 molecules cm-2 s-1.


Air quality monitoring
Blond and co-authors4) have shown that SCIAMACHY provides detailed information on the nitrogen dioxide content in the planetary boundary layer. The cloud free satellite observations were compared with surface measurements and simulations over Western Europe performed with the regional air-quality model CHIMERE (shown in Figure 3). The model has a resolution of 50 km, similar to the satellite observations. CHIMERE seems to underestimates surface NO2 concentrations for urban and suburban stations but this is mainly attributed to the low representativeness of point observations. No such bias is found for rural locations. The yearly-average SCIAMACHY and CHIMERE spatial distributions of NO2 show a high degree of quantitative agreement over rural and urban sites: a bias of 5% (relative to the retrievals) and a correlation coefficient of 0.87 (n=2003). The consistency of both SCIAMACHY and CHIMERE outputs over sites where surface measurements are available gives confidence in evaluations of the model over large areas not covered by surface observations. The NO2 columns show a high daily variability. Still, the daily NO2 pollution plumes observed by SCIAMACHY are often well described by CHIMERE both in extent and in location. This result demonstrates the capabilities of a satellite instrument such as SCIAMACHY to accurately monitor the NO2 concentrations over large areas on a regular basis. It provides evidence that present and future satellite missions, in combination with a regional air quality model and surface data, will contribute to improve quantitative air quality analyses at a continental scale.
Figure 4. World map of the linear trend per year for tropospheric NO2 in the period 1996 till 2005 derived from satellite observations by GOME and SCIAMACHY. For the light grey areas no significant trend has been found in the time series. For the dark grey areas not enough observations were available to construct a time series of tropospheric NO2.
Figure 4. World map of the linear trend per year for tropospheric NO2 in the period 1996 till 2005 derived from satellite observations by GOME and SCIAMACHY. For the light grey areas no significant trend has been found in the time series. For the dark grey areas not enough observations were available to construct a time series of tropospheric NO2.

Trends in tropospheric NOx emissions
The combined measurement series of both GOME and SCIAMACHY almost span a decade, which allows for a trend analysis of NO2 concentrations. To do so, the averaged monthly tropospheric NO2 columns are fitted with a linear model that also includes a sinus to represent the seasonal variation of NO2. The seasonal variation for anthropogenic NO2 is mainly determined by the changing day-length over the year. In absence of sunlight NO2 has a longer lifetime in the atmosphere, which explains that the NO2 columns are on average higher during wintertime. By applying the model to each grid cell a spatial distribution of the fit parameters is calculated. Furthermore the precision of the trend is calculated. It can be concluded that the 10 years long NO2 dataset from GOME and SCIAMACHY can be used for significant trend analysis in most parts of the world. In highly populated and industrialised areas the trend is large enough to be significant. For instance Shanghai had a yearly increase of tropospheric NO2 of about 29% since 1996. Figure 4 shows the derived annual growth in the tropospheric NO2 columns from this analysis. The largest trend is found in Eastern China, where the economic growth is one of the fastest of the world. The fastest growing city with respect to both economy and tropospheric NO2 is Shanghai. It is interesting to note that the growth in the region around Hong Kong is less than for other regions with a high economical activity. This is probably due to the already high level of economic activity in 1996 when our trend study started, and to a package of measures against air pollution in Hong Kong over the last years. Further results of this trend study are published by Van der A and co-authors5).
Figure 5. The difference in ground-level ozone caused by the increase of Chinese NOx emissions between 1997 and 2005.
Figure 5. The difference in ground-level ozone caused by the increase of Chinese NOx emissions between 1997 and 2005.

Global implications
The fast growing emissions in China lead locally to rapidly increasing NO2 concentrations, which affects the local ozone concentrations. Clearly these large increases will have severe consequences for the local air quality, but even effects on global scale can be expected, because the lifetime of tropospheric ozone is much larger than the lifetime of NO2. Therefore, ozone can be transported over large distances by the wind. Using a chemical transport model the change in ozone due to increasing emissions in China can be calculated. Figure 5 shows increasing ozone concentrations in the Northern hemisphere caused by the growing Chinese emissions in the period 1997-2005. In this period of eight years the global averaged tropospheric ozone column has increased with 0.54 %6). The largest growth in tropospheric ozone we find in a plume reaching from China to the East along the direction of the prevailing winds. From the Figure we conclude that the tropospheric ozone concentrations in the entire Northern hemisphere are increased due to the growing emissions in China. These increases seem small, but are still important. In Europe, the air pollution has been increased as a result of intercontinental transport. In addition, since ozone is a strong greenhouse gas, the effects on climate change cannot be neglected.
Figure 6. OMI observations of SO2 from the eruption of the Manam volcano in New Guinea on 27 January 2005 (source: NASA).
Figure 6. OMI observations of SO2 from the eruption of the Manam volcano in New Guinea on 27 January 2005 (source: NASA).

Volcanic eruptions
Volcanic eruptions may eject large amounts of ash (aerosols) and trace gases such as SO2 into the atmosphere. These ejecta can have considerable impact on the safety of air traffic and on human health. Ground-based monitoring is carried out at only a limited number of volcanoes: most volcanoes, in particular remote ones, are not monitored on a regular basis. Global observations of SO2 and aerosols derived from satellite measurements in near-real-time may therefore provide useful complementary information to assess possible impacts of volcanic eruptions on air traffic control and public safety. Observations of SO2 are performed with UV-VIS satellite instruments such as GOME, SCIAMACHY and OMI. The technique used to retrieve the SO2 column is called Differential Optical Absorption Spectroscopy (DOAS). Volcanic emissions and strong pollution events are clearly detected with this approach, which makes it very suitable for use in a near-real-time service, with an automatic warning system for exceptional SO2 emission levels. Within the ESA projects TEMIS and PROMOTE this system provides information to the London and Toulouse VAAC (Volcanic Ash Advisory Centre), which issues warnings to the airline companies in case of observed volcanic ash or SO2 clouds. The London and Toulouse VAAC are responsible for the air traffic in Europe, Africa and part of Asia.

Figure 6 shows the OMI observations of SO2 from the eruption of the Manam volcano in New Guinea on 27 January 2005.

Conclusions
A decade of satellite observations of nitrogen dioxide in the atmosphere has been used to derive trends in anthropogenic emissions in China. As expected the nitrogen dioxide concentration is growing most rapidly in Eastern China, where the economic growth is largest. By feeding the derived trends to a global chemical transport model of the atmosphere the effects on the concentrations of world-wide tropospheric ozone can be determined. According to the model the background concentration of ozone has increased in the entire northern hemisphere as a result of the growing emissions in China. Another trace gas observed by satellites is SO2, which occurs in large quantities in areas with large air pollution or with volcanic eruptions. Observation of these volcanic eruptions in near-real time helps the Volcanic Ash Advisory Centres in warning airline companies for potentially dangerous areas.

References

  1. Eskes, H.J. and K.F. Boersma, 2003. Averaging Kernels for DOAS total-column satellite retrievals. Atm. Chem. Phys., 3, 1285 – 1291.
  2. Boersma, K.F., H.J. Eskes and E.J. Brinksma, 2004. Error analysis for tropospheric NO2 retrieval from space. J. Geophys. Res., 109, 4311, doi: 10.1029/2003JD003961.
  3. Boersma, K.F., 2005. Satellite observations of tropospheric nitrogen dioxide; retrieval interpretation and modelling, Ph.D. Thesis, Universiteitsdrukkerij Technische Universiteit Eindhoven, Eindhoven, The Netherlands.
  4. Blond, N., K.F. Boersma, H.J. Eskes, R.J. van der A, M. van Roozendael, I. De Smedt, G. Bergametti and R. Vautard, 2007. Intercomparison of SCIAMACHY nitrogen dioxide observations, in-situ measurements, and air quality modeling results over Western Europe. J. Geophys. Res, in press.
  5. A, R.J. van der, D.H.M.U. Peters, H.J. Eskes, K.F. Boersma, M. van Roozendael, I. de Smedt and H.M. Kelder, 2006. Detection of the trend and seasonal variation in tropospheric NO2 over China. J. Geophys. Res., 111, D12317, doi:10.1029/2005JD006594.
  6. Kuenen, J.J.P., 2006. Anthropogenic NOx emission estimates for China based on satellite measurements and chemistry-transport modeling, KNMI Technical Report TR-288, 62pp.