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Tropospheric ozone (O3)
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23 January 2004
   
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Tropospheric ozone

Ozone is an important trace gas in the troposphere. It is not directly emitted into the troposphere, but chemically produced by NOx, CO, CH4 and other hydrocarbons. These ozone precursors are emitted in large quantities due to human activities such as traffic and industry. Ozone in the troposphere plays various important roles:
  • The enhanced production of ozone in the summer can cause photochemical smog. Excessive amounts of ozone near the surface are toxic to ecosystems, animals and man.

  • Ozone is a primary source of hydroxyl radicals, which are the detergents of the troposphere, initiating almost all oxidation processes. Ozone changes in the troposphere have a large impact on the tropospheric composition.

  • Ozone in the upper troposphere acts as a greenhouse gas by absorbing long-wave terrestrial radiation.
 

Satellite observations of tropospheric ozone

Whereas ground-based measurements of tropospheric ozone give limited data in time and space, observations from space platforms offer the possibility to measure the distribution of tropospheric ozone over large areas, and to study the large scale temporal and spatial behavior. This is of great importance since ozone formed over source regions, where large amounts of ozone precursors are emitted, can be transported over great distances and affects areas far from the source.

The GOME and SCIAMACHY satellite instruments improve the capabilities to observe ozone in the troposphere. These instruments provide three ozone products, namely a total ozone column, a nadir view ozone profile and a limb view ozone profile (SCIAMACHY only). Tropospheric ozone can be derived in several ways using these products.

Tropospheric ozone Sep. 2001

 

Tropospheric ozone in the tropics

The GOME and SCIAMACHY total ozone column product is the integrated ozone amount from the ground to the top of the atmosphere. The tropospheric column typically contributes only about 10% to the total column. Furthermore, the sensitivity of these instruments to ozone in the troposphere is depending on the altitude of the ozone and other aspects such as ground albedo, cloud characteristics and aerosols (this dependence is a common characteristics of all tropospheric trace gas retrievals). These dependencies are corrected in the air-mass factor computation. The AMF depends on climatological a-priori information, which is a potential source of errors. The convective-cloud-differential (CCD) method is an approach to derive a tropospheric column for tropical regions from the total column in combination with cloud information.

The original CCD method uses TOMS total ozone measurements over highly reflecting, high altitude clouds. In some regions, especially the tropical western Pacific, these high-reflectivity clouds are often associated with strong convection and cloud tops near the tropopause. The tropospheric column can be obtained at cloud-free pixels by subtracting the above-cloud stratospheric ozone amount from TOMS total ozone. Because cloud height information is not measured by TOMS, the primary assumption in the CCD method is that the high-reflectivity clouds often have cloud tops at the tropopause.

With the GOME and SCIAMACHY instruments, the CCD method can be improved by using both cloud fraction and cloud height information, as determined with the FRESCO cloud algorithm from the NIR wavelength region. The retrieved cloud information showed that for the GOME-CCD method, the tropical atmosphere should be divided in three layers: a tropospheric layer below the convective cloud tops (about 14 km), a transition layer (between the cloud-top height and the tropopause) and a stratospheric layer (above the tropopause). With the GOME-CCD method, monthly tropospheric ozone columns (0-14 km) have been calculated for the tropical region between 20°N and 20°S, on a 5 degree resolution, for the whole GOME period from 1995 till present.

 

Global tropospheric ozone using data assimilation

The GOME ozone profile product also contain information about tropospheric ozone. Recent developments on a faster radiative transfer model (LIDORT) have resulted in the possibility to derive tropospheric ozone using retrieved ozone profiles. The practical vertical resolution of the ozone profiles is however limited: 5 km in the stratosphere, and about 8 km below (lower stratosphere and troposphere) and the averaging kernels of the tropospheric levels have a substantial tail into the stratosphere. This implies that it is difficult to estimate the tropospheric column based on the profile retrieval alone. In order to obtain a global ozone field with high vertical resolution, the retrieved ozone profiles, the derived averaging kernels and meteorological data are combined using data assimilation. The resulting 3-dimensional ozone fields can be used, possibly in combination with the existing ozone column, to derive a global tropospheric ozone column.
 

Improvements with SCIAMACHY

The same algorithms used for GOME can be applied also to the SCIAMACHY nadir observations. The advantage of using SCIAMACHY data is the higher spatial resolution, and therefore, a higher probability of cloud-free pixels. Alternating with the nadir observations, the SCIAMACHY instrument will observe stratospheric ozone profiles in limb mode. Sensitivity studies show that retrieving tropospheric ozone from the combined nadir and limb observations of SCIAMACHY will strongly improve the precision of the retrieved ozone profiles.