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TROPOMI: Sensing the Troposphere from Space

The growth of human population and the industrialisation in the 19th and 20th century have led to dramatic changes in the Earth system.

The chemical composition of the lowest part of the atmosphere, the troposphere, is changing as a result of human activities. The Earth has entered the ‘Anthropocene’ epoch, where the activities of humans play a key role in air quality and climate change. The rapid development of megacities (see Figure 1) and the strong development in the Asian countries are clear examples of rapid changes that affected the atmosphere in the last decades and will continue to do so in the future. 

Measurements of tropospheric NO2 from OMI on EOS-Aura averaged over 2006.
Measurements of tropospheric NO2 from OMI on EOS-Aura averaged over 2006.

Global changes in the chemical composition of the troposphere is one of the main drivers for changes in the climate and air quality. Especially the global inventory of emission sources plays a key role in understanding and modelling the troposphere in relation to climate change and air pollution. Also regional and long-range transport of pollution, as well as the rapid development of pollution levels during the day, are important for understanding air quality and climate change and their interaction.

Global atmospheric composition measurements from space started in the 70th’s with US sensors SBUV (1) and TOMS (2), focussing on the ozone layer residing in the higher layers in the atmosphere. Sensing the lower atmosphere from space is a recent development in satellite remote sensing, where instruments like GOME-1 (3) (launched in 1995 on board ESA-ERS-2), SCIAMACHY (4) (launched in 2002 on board ESA’s ENVISAT), OMI (5) (launched in 2004 on board NASA’s EOS-Aura) and GOME-2 (6) (launched in 2006 on board EUMETSAT MetOp-A) play a leading role. Unprecedented measurements from space from OMI reveal tropospheric pollution maps on a daily basis with urban-scale resolution. Measurements from Thermal Infrared instruments also provide unique information on the troposphere, providing tropospheric profile information.

Figure 1. Cities with over 5 millions inhabitants as reported by the UN for 1995, superimposed on measurements of tropospheric NO2 from OMI on EOS-Aura averaged over 2006. The development of these megacities will have a major impact on future air quality.
Figure 1. Cities with over 5 millions inhabitants as reported by the UN for 1995, superimposed on measurements of tropospheric NO2 from OMI on EOS-Aura averaged over 2006. The development of these megacities will have a major impact on future air quality.

In the last 15 years satellite instruments and retrieval techniques have been improved, enabling tropospheric measurements from space with considerably improved accuracy (see Figure 2). Current satellite measurements uniquely obtain global coverage of the troposphere with consistent quality. The composition of the troposphere plays a major role in air quality and climate change. Tropospheric satellite measurements therefore provide essential information for the understanding of air quality and climate change. The Dutch initiative for the satellite instrument TROPOMI (Tropospheric Monitoring Instrument) is developed to improve the detection of the tropospheric composition in view of climate change and air pollution research and monitoring.

Figure 2. Tropospheric concentrations of key species over Asia, originating from several instruments. Colour scale concentrations range from red, via yellow and green, to blue, which represent very high, high, medium and low values, respectively. For popu
Figure 2. Tropospheric concentrations of key species over Asia, originating from several instruments. Colour scale concentrations range from red, via yellow and green, to blue, which represent very high, high, medium and low values, respectively. For popu

Tropospheric Monitoring Instrument TROPOMI
TROPOMI is a satellite instrument for remote sensing of the Earth’s atmosphere. Its primary goal is to obtain measurements of the troposphere down to the boundary layer and the Earth’s surface in order to quantify emissions and transport of anthropogenic and natural trace gases and aerosols, which impact air quality and climate. The Sentinel 5 precursor (S5p)/TROPOMI will allow continuing the 15 year satellite data sets started with GOME, OMI and SCIAMACHY and will form the bridge towards the ESA sentinel 4 and 5 missions. The S5p/TROPOMI mission was advised by EU and ESA expert groups (6,7).

To address the key user requirements, TROPOMI will measure the main tropospheric pollutants (O3, NO2, CO, formaldehyde (HCHO) and SO2) and two major greenhouse gases (tropospheric O3 and methane (CH4)). In addition, it will measure important parameters of aerosols (aerosol scattering, absorption and type identification), which play a key role in climate change as well as in tropospheric pollution.

TROPOMI measures in the UV/VIS wavelength range (270 to 490 nm) like OMI, and in addition in two bands, one in the near infrared (710-790 nm) for dedicated cloud detection, and one in the short wave infrared (around 2300 nm) for CO and methane. The instrument targets 7 × 7 km2 pixel size, with daily global coverage. TROPOMI will be launched on ESA’s Sentinel 5 precursor in 2015 in an afternoon orbit in loose formation with the future (2013) NOAA Polar Orbiting Earth Operational Environmental Satellites.

TROPOMI is a Dutch initiative building upon the successes of SCIAMACHY and OMI. This new instrument combines all innovative aspects of the previous instruments and improves on most specifications. Notably improved are horizontal resolution and the accuracy of the tropospheric columns, due to improved cloud and surface albedo characterization capabilities. By flying in an afternoon orbit (overpass time is 13:30 local time), the TROPOMI measurements can be combined with the GOME-2 measurements flying in the morning (9:30 am) to obtain information on the diurnal cycle of several of the trace gases. It will be for the first time that the diurnal cycle will be measured from space on a daily basis.

The instrument is designed (see Figure 3 for an impression) by TNO and Dutch Space in the Netherlands, who were also involved to great extent in TROPOMI’s predecessors OMI, SCIAMACHY, GOME-1 and -2. KNMI is the Principal Investigator (PI) Institute of TROPOMI, SRON fulfils the co-PI position. Within KNMI there are several divisions that will cooperate and benefit from the TROPOMI mission. The Climate Observations division hosts the PI and Chair of ESA’s S5p/TROPOMI Mission Advisory Group. The Chemistry and Climate division supported the formulation of the scientific objectives of TROPOMI and will be a user of the data. The Information and Observation Services and Technology department will help to design the ground segment. Use of the data by others within KNMI is envisaged.

Scientific themes of TROPOMI
TROPOMI aims at providing data for the scientific and operational community dealing with climate and weather. In order to fulfil the requirements of these groups, the following four TROPOMI scientific objectives have been identified:

To better constrain the strength, evolution, and spatiotemporal variability of the sources of trace gases and aerosols impacting air quality and climate

For a proper understanding of air quality and emissions of trace gases contributing to climate change it is needed to quantify the strength, distribution and variability of emissions of NOx, CO, aerosols, SO2, CH4 and volatile organic compounds, and to identify the contribution of anthropogenic emissions, such as fossil fuel burning and agriculture, and natural emissions, such as lightning. Currently emission inventories are characterized by large error bars and incomplete estimates of inter-annual, seasonal and diurnal variability. TROPOMI will provide state-of-the art measurements of the above mentioned compounds to improve on the emission inventories.

To improve upon the attribution of climate forcing by a better understanding of the processes controlling the lifetime and distribution of methane, tropospheric ozone, and aerosols

Unlike CO2 and other major well-mixed greenhouse gases including CH4 and N2O, aerosols and tropospheric ozone are inhomogeneously distributed climate forcing agents. Together, CH4 and tropospheric O3 contribute about 30% to the present-day total forcing due to anthropogenic greenhouse gases compared to the pre-industrial situation, while CO2 accounts for 53%. Poorly quantified is the transformation of gaseous precursors (NOX, SO2, CO and VOCs) into radiatively active constituents, including CO2, O3 and secondary aerosols. TROPOMI will contribute to understanding the attribution of climate forcing by detecting aerosols and tropospheric ozone and methane, which have a direct radiative effect on climate. TROPOMI also measures precursors (especially NO2, SO2, CO and VOCs) of radiatively active constituents, including CO2, O3 and secondary aerosols 8), which will help to improve the understanding of the lifetime and distribution of methane, tropospheric ozone and aerosols.

Figure 3. Impression of the TROPOMI instrument, seen from different directions. The yellow beam enters the solar calibration port, while the orange beam represents the Earth measurements wide angle swatch providing daily global coverage. The blue/purple t
Figure 3. Impression of the TROPOMI instrument, seen from different directions. The yellow beam enters the solar calibration port, while the orange beam represents the Earth measurements wide angle swatch providing daily global coverage. The blue/purple t

A specific challenging research topic is the connection between air quality and climate. In a warmer climate, air pollution episodes will become more severe as a result of higher temperatures, promoting photochemical ozone formation. At the same time, air pollutants are often radiatively active (e.g. O3), or a precursor for a radiatively active constituent (e.g. NO2), and thus also have a direct impact on climate.

To better estimate long-term trends in the troposphere related to air quality and climate from the regional to the global scale

Long-term measurements are showing that human activities change the composition of the Earth’s atmosphere. These changes resulted in the development of the ozone hole, the increase of greenhouse gases, smog episodes, acid rain, air pollution, brown clouds, intercontinental transport of pollutants, changes in atmospheric oxidation efficiency, etc. Long-term observational records are essential to quantify the impact of the atmospheric composition on climate. In order to understand changes in tropospheric composition, local and global data records are needed. Local air pollution is determined not only by local sources and sinks, but also by long-range transport of pollutants. Therefore, in order to have a full picture and an in-depth understanding of air pollution and tropospheric composition, both local and global observations are needed. Satellite measurements can quantify the (inter-)continental transport and dispersion of pollution plumes, by which important information on the non-local contribution to air pollution is obtained.

To develop and improve air quality model processes and data assimilation in support of operational services including air quality forecasting and protocol monitoring

Operational applications that will benefit from TROPOMI satellite data include air quality forecasts and environmental hazard warnings. At present mainly space-borne tropospheric NO2 and aerosol measurements are used to improve air quality forecast applications. O3 observations are used for UV warnings and the improvement of the operational weather forecast. CO, aerosols and tropospheric NO2 measurements are being applied for operational monitoring of forest fires, and SO2 observations are used for aviation control warnings and monitoring of volcanic eruptions. TROPOMI will be able to continue the near-real-time services already existing for OMI and SCIAMACHY and will provide important data for the GMES EU project Monitoring Atmospheric Composition and Climate (MACC), lead by ECMWF.

The main advantage of TROPOMI type satellite measurements for measuring the troposphere
There are basically two methods used to measure trace gases in the troposphere, both based on passive remote sensing techniques: the solar backscatter technique and the thermal infrared technique. In the solar backscatter technique the instrument measures the solar radiation that has been absorbed and scattered by the atmosphere. This so-called Earth radiance spectrum contains the specific absorption features of the molecules of interest. The Solar backscatter instruments usually provide tropospheric columns of the trace gases. This technique has the advantage to be sensitive to the surface, since the atmosphere is transparent in this visible wavelength range. Therefore, this method is best suited to determine emission sources, which is the main objective of the TROPOMI instrument. In the thermal infrared technique the thermal emission of the Earth-atmosphere system is measured, revealing the specific absorption features of the trace gases. With the thermal infrared technique some vertical information can be obtained, approximately two layers in the troposphere, but the sensitivity to the surface is less, so that accurate total column amounts are considerable more difficult to obtain.

Challenges in measuring the troposphere from space
In order to address the scientific themes related to the troposphere, new instruments need to improve in horizontal and vertical resolution, diurnal information, amount of collocated measurements, and cloud and surface albedo detection capabilities. TROPOMI makes major steps forward on horizontal resolution, amount of collocated cloud-, and albedo measurements and, combined with the morning measurements of GOME-2, also on the diurnal cycle.

Figure 4. Tropospheric NO2 around Mexico City on 20 January 2005. The image on the left shows OMI measurements recorded in the so-called ‘zoom-in mode’, resulting in ground pixels of the order 12 × 13 km2. De numbers refer to: 1 = Mexico City and suburbs;
Figure 4. Tropospheric NO2 around Mexico City on 20 January 2005. The image on the left shows OMI measurements recorded in the so-called ‘zoom-in mode’, resulting in ground pixels of the order 12 × 13 km2. De numbers refer to: 1 = Mexico City and suburbs;

Conclusion
Information on the tropospheric composition is of fundamental importance for understanding climate change and air quality. In the last decade a rapid development took place in sensing the troposphere from space, enabling measurements of tropospheric columns and providing the first tropospheric profile retrievals. TROPOMI, to be launched in 2015 on ESA’s Sentinel 5 precursor, will improve on current capabilities in terms of horizontal pixel size (see Figure 4), cloud and surface albedo detection and correction algorithms and will enable daily diurnal cycle measurements in combination with GOME-2. TROPOMI’s measurements will lead to improved emission estimates which are of paramount importance for climate and air quality modelling.

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