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Tropospheric Chemistry

As trace gases and pollutants are transported through the atmosphere they are subject to photolysis, oxidation, and reaction with other trace gases. The processes of gas transport, photolysis, and chemical reaction, and the resulting concentration of these gases and pollutants varies with altitude, weather, terrain, and time of day.

Since the lifetime of ozone (O3) is strongly dependent on its altitude, measurements of boundary layer O3 formation are necessary in order to understand O3 formation in polluting regions, and subsequent venting into the free troposphere. It is important to know, on a global scale, the percentage of lower tropospheric O3 that originates from the upper and middle troposphere, and the percentage of lower tropospheric O3 that reaches the upper and middle troposphere.

Whether vented upwards from the boundary layer, or chemically formed within the free troposphere, once there O3 can be globally transported, because the lifetime of O3 in the middle and upper troposphere is on the order of weeks to months. Ozone in the free tropospheric can then subside into the boundary layer where it can add significantly to the local pollution burden. Since O3 undergoes a variety of chemical reactions and reaction rates at different altitudes, understanding the transport of O3 and its precursors – from the surface and the boundary layer to the free atmosphere, and from the stratosphere to the upper troposphere – is vital for understanding atmospheric processes and dynamics.

Co-located Profiles of Ozone and Carbon Monoxide from TES

When carbon monoxide (CO) is lofted into the free troposphere, the results include longer lifetimes of O3 precursors and enhanced O3 in convection regions in the middle and upper troposphere. TES is helping to provide the fractional contribution of source regions to the zonal mean O3 distribution for global climate and chemistry models. Co-located O3 and CO profiles in the free troposphere provide critical information for studying boundary layer exchange, ozone chemistry and transport in the troposphere, and the global radiation balance.

Distribution of Mid-Tropospheric Ozone

When TES O3 distributions are assimilated into Chemical Transport Models or Global Climate-Chemistry Models, significant improvements in mid-tropospheric ozone distribution can be observed. Incorporating TES data into meteorologically-driven models can also help to separate the effects of the chemical verses dynamical processes on the observed behavior of ozone.

Ozone and Water Vapor in the Upper-Troposphere Lower-Stratosphere

TES co-located ozone and water vapor retrievals show features typical of stratosphere-troposphere exchange in the Upper Troposphere-Lower Stratosphere. This information is needed to study the jet stream and the dynamics at the boundary between the troposphere and the stratosphere, to identify stratospheric intrusions, investigate their three-dimensional structure, and estimate cross-tropopause exchange. Understanding the dynamics and chemistry at the boundary between the troposphere and the stratosphere is also needed because reactions involving pollutants are often most pronounced at these higher altitudes.

Distribution of Lower Tropospheric Ozone

Except during the summer (when TES is sensitive to boundary layer ozone if the surface temperatures are over 300 K and there is significant thermal contrast between the ground and the air), current satellite instruments exhibit a general inability to resolve ozone in the boundary layer for typical atmospheric conditions. However, when TES infrared profiles are analyzed simultaneously with ultraviolet retrievals from OMI, the sensitivity to pollutants in the boundary layer and free troposphere is improved by a factor of 2 or more. Combining OMI and TES radiance measurements allows for dramatic improvement (between 30% to 400%) in the vertical resolution of ozone estimates in the boundary layer, and substantial improvement (30% to 60%) in the free troposphere. The increased sensitivity results from a combination of the vertical resolution from TES’s high-resolution (0.1 cm-1) infrared measurements, and the sensitivity to the boundary layer from OMI’s ultraviolet radiance measurements. Read the full paper by Worden et al., 2007.

Stratospheric Ozone Trends

Since TES is not designed to focus on the stratosphere (although it does produce a total ozone column product), refer to instruments such as MLS and SAGE for information on stratospheric ozone trends.

Back: Ozone

Ozone’s impact is all about location. High in the stratosphere, ozone shields us from the sun’s harmful UV rays. But beneath that, at the top of the troposphere, it acts as a greenhouse gas and contributes to global warming. In the middle of the troposphere, it plays a key role in a chemical process that cleans the air of certain pollutants. But at the bottom of the troposphere, where we live and breathe, it contributes to smog and is toxic to plants and animals. TES measurements allow scientists to track the abundance, creation, destruction, and movement of this critical chemical at various altitudes throughout the atmosphere.

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