One of the main advantages of nadir UV/visible satellite observations is their ability to probe the atmosphere down to the surface where air pollution affects human health directly. Although the number of species that can be observed in the UV/vis spectral region is limited, it turns out that several key parameters of air quality can be detected, including NO2, ozone, formaldehyde, sulphur dioxide and aerosols.

Historically, air pollution first was a problem linked to fires, used outdoors for land clearing and agriculture and indoors for cooking. With the onset of industrialisation, sulphur dioxide became important as result of smeltering and the burning of sulphur rich coal for energy and heating. In combination with humidity, sulphuric acid is formed which lead to the infamous winter smog (smoke and fog) in London, killing more than 4000 people in London in just 5 days in 1952. A completely different type of smog is formed in summer in warm and sunny conditions when active photochemistry of nitrogen oxides and volatile organic compounds leads to the formation of ozone and other photo-oxidants. As the London smog, this so called Los Angeles smog has adverse effects on humans but also on plants.

Technical measures to limit sulphur dioxide emissions have much reduced the occurrence of winter smog in the industrialised countries. This was supported by a switch from coal to cleaner fuels and a displacement of heavy industry towards Asia. Summer smog on the other side remained a problem as an ever growing number of cars and increasing energy need leads to high NOx levels in spite of attempts to reduce both, NOx and VOC emissions. Over the last two decades, rapid changes happened, in particular in Asia where strong economic developments in China and in other countries lead to an unprecedented increase in air pollution. At the same time, air quality in Europe, the US and Japan improved significantly as environmental regulation continuously tightened air quality standards. In recent years, China has undertaken large efforts to follow this path and first signs of improvement in air quality have been reported.

Nitrogen dioxide is the most studied air quality related satellite data product, and many exciting and novel applications of satellite data have been demonstrated on this species. This success is based on the strong and structured absorption signal which can be easily retrieved in the blue part of the spectrum where Rayleigh scattering is not as large as in the UV. This is important as the sensitivity of nadir satellite observations decreases towards the surface as result of scattering, and this effect is more pronounced at shorter wavelengths. While the change in sensitivity can be accounted for in the retrieval, the fact that for most species, no vertical distribution can be derived but only an integrated tropospheric column is a challenge for the use of satellite data in air quality applications. On the other hand, integrated columns have several advantages for atmospheric chemistry applications in comparison to in-situ observations as they do not depend on changes in boundary layer height and properly account for elevated pollution layers. 

Chapter Editor

Andreas Richter (U. Bremen)

Contributors

Steffen Beirle (MPIC), Andreas Hilboll (U. Bremen), S.-W. Kim (NCAR), Randall Martin (U. Dalhousie), Ronald  van der A (KNMI)

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Mean annual vertical columns of tropospheric NO2 normalized to 1996 for the regions central East Coast US, Western Europe, US, east-central China, Japan, Middle East, and north-central India. Values for 1996–2002 are from GOME; values from 2003–2011 are from SCIAMACHY measurements. The first five regions are defined as in Richter et al. (2005). The y-axis has been modified to make relative changes above and below 1 more comparable (values larger than 1 have been scaled to y => 2− 1 / y). Very large changes are observed over just 15 years both over the western industrialised countries where NO2 burdens have decreased markedly and over China, the Middle East and India where NO2 is increasing. From Hilboll et al. (2013).

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Tropospheric NO2 VCDs over the US East Coast (top) and Europe (bottom) averaged separately for different weekdays, based on GOME measurements for the time period 1996-2001. In Europe or the US, NO2 is significantly reduced over weekends due to less traffic and industrial activities. This is different in other regions of the world: In Islamic cities, a Friday minimum is observed, while in Israel, Saturday shows lowest values. In China, NO2 pollution stays on the same level throughout the week at least in GOME data. More recent satellite data indicates that a weekend effect is developing in China as well as traffic emissions become more important.The weekly cycle of NO2 holds important information on source type (traffic, industry etc.), but also the lifetime of nitrogen oxides (which controls e.g. in how far the Monday levels are influenced by the Sunday reductions). From Beirle et al. (2003).

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Mean tropospheric VCD of NO2 from SCIAMACHY 2003-2007 after masking out continents and applying a high-pass filter to remove the tropospheric NO2 background. From space, the major shipping routes can clearly be identified, where large cargo vessels release their exhausts in the otherwise clean oceanic atmosphere. As international trade volume has increased over the last decade, so has the importance of ship emissions on air quality close to shipping lanes at coastal regions and in particular near harbors. Update from Beirle et al. (2004) and Richter et al. (2004).

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Eastern US Power Plant NOx Controls Detected by Satellite. Since late 1990’s, pollution control programs in the US have focused on coal-burning power plants by utilizing improved burner technology and post-burner ammonia scrubbers. (Left) The model NO2 columns in the Ohio River Valley where power plants dominate total NOx emissions were as large as that in the northeastern US cities before pollution controls (using the emissions in 1999). (Middle) After the pollution controls, the model columns in the Ohio River Valley utilizing the measured power plant NOx emissions in 2004 were substantially reduced. (Right) SCIAMACHY NO2 measurements in 2004 saw the impact of the US power plant emission controls. Large changes in NO2 columns occurred between late 1990’s and 2004. From Kim et al. (2009).

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A time series has been constructed of tropospheric NO2 columns derived from GOME and SCIAMACHY observations in the 10-year period 1996-2006. By analysing the seasonal cycle, annual variability and the height of NO2 column, the dominant source type (anthropogenic, soil, biomass burning or lightning) of the measured NO2 can be identified. From van der A et al. (2008).

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Martin et al. (2004) finds that the ratio of formaldehyde columns to tropospheric NO2 columns is an indicator of the relative sensitivity of surface ozone to emissions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). The attached figure shows monthly mean tropospheric HCHO/NO2 column ratio retrieved from GOME for North America, Europe, and East Asia. Ratios greater than 1 tend to be NOx-limited. Ratios below 1 tend to be NOx-saturated. White areas indicate remote regions (observed tropospheric NO2 columns less than 2.5x1015 molecules cm2) and regions below the HCHO detection limit of 4x1015 molecules cm2. The satellite-derived ratios indicate that surface ozone is more sensitive to emissions of NOx than of VOCs throughout most continental regions of the Northern Hemisphere during summer. Exceptions include Los Angeles and industrial areas of Germany. A seasonal transition occurs in the fall when surface ozone becomes less sensitive to NOx and more sensitive to VOCs.

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