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Air Pollution

image of polluted city

Tropospheric ozone and its precursors

Ozone in the troposphere, particularly near the surface, is harmful to humans and all of the Earth’s ecosystems. Ozone in the troposphere is produced by photochemical oxidation of carbon monoxide (CO) and volatile organic compounds (VOCs) in the presence of nitrogen oxides (NO + NO2 --> NOx). Carbon monoxide is an ozone precursor and a long-lived tracer of combustion. Generally, as NOx emissions increase, ozone pollution also rises, although when the ratio of VOCs to NOx is low, then reductions in VOCs will be effective in reducing ozone. Similarly, when the ratio of VOCs to NOx is high, VOC controls may be ineffective.

On the positive side, ozone reacts with water vapor and sunlight to form OH (the hydroxyl radical), which is known as the "detergent" molecule because it reacts with air pollutants and controls the atmosphere's capacity to cleanse itself. But on the other hand, ozone near the surface is toxic to plants and animals, including people. It is the source of photochemical smog, which is becoming a worldwide problem. And in the upper troposphere, ozone acts as a very efficient greenhouse gas.

It is difficult to determine how best to address the problem of ozone in the troposphere because the processes of ozone formation and destruction are exceedingly complex. The global distribution of ozone remains largely unknown and is greatly complicated by the troposphere's turbulent weather systems. TES is helping to fill this important knowledge gap.

What are we learning from TES?

As part of the United States Clean Air Act, the EPA has defined the National Ambient Air Quality Standards for six key air pollutants (ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, lead, and particulate matter) that are harmful to human health and the environment. The EPA sets the standards that states must meet for these pollutants; however it is up to each individual state to insure compliance. Many states perform extensive computer modeling of the air quality in their region and use the results from these models to develop strategies to comply with EPA standards. The states also rely on ozone measurements from surface monitor networks to verify the results from their models.

TES routinely makes collocated measurements of ozone (O3), carbon monoxide (CO) and other atmospheric parameters that can be used as validation for the results from air quality models. Such data can also provide information about pollution transport from locations outside state boundaries into the state. This is how TES data can help improve the tools that provide information and develop compliance strategies, which are in turn used by policy makers to improve air quality. TES data can be applied in similar ways to studies of understanding the effects of different climate forcings and linkages between air quality and climate change. In these ways, TES data can help policy makers to identify practical actions and research activities that can improve air quality, slow the growth of climate forcings, and improve our understanding of the linkages between air quality and climate change.

Visualization - Regional timetrend browse plots for ozone, carbon monoxide, methane, temperature and water vapor provide a quick way to understand trends. For example, during summer CO tends to be lower (due to higher OH abundance) and O3 tends to be higher (because more photolysis is taking place).

Texas Air Quality Study (GoMACCS / TexAQS)

TES participates in all of the Texas Air Quality studies, which provide opportunities to test the synthesis of chemistry and transport models such as the real-time air quality modeling system (RAQMS), which serves as a critical link between satellite and in situ observations. A case study (PDF, 951 KB) undertaken in August 2006, found that TES retrievals of CO and ozone vertical profiles, in conjunction with the RAQMS global model, provide a means of investigating the impact of distant sources on the background concentrations over Texas.

TES monitors ammonia and methanol over Beijing

TES is conducting Special Observations over Beijing throughout the 2008 Summer Olympic season. One such observation was conducted over the Beijing area in northeast China on July 10, 2007. This is the first time that ammonia (NH3) and methanol (CH3OH), have been detected by space-based nadir viewing measurements that penetrate into the lower atmosphere. A preliminary analysis of the TES spectra was conducted, and residual spectral radiances reveal surprisingly strong features attributable to enhanced concentrations of NH3 and CH3OH, well above the normal background levels. Very little is known about the regional and temporal variations of these molecules.

Although ammonia and methanol are not highly correlated (plant growth is the source for methanol, whilst livestock and fertilizer are the sources for ammonia), the two pollutants were accompanied by high concentrations of both CO and O3 (up to 200 ppb). Since the burning of crop residues is common in this area and season, it is conjectured that the sources for both these molecules is local. (Methanol is an important chemical in the global budget of tropospheric O3, and deposition of ammonia is important because the formation of ammonium nitrate may be a significant source of aerosol formation and could play a significant role in atmospheric denitrification.)

TES O3 and CO profile retrievals assimilated into the GEOS-Chem model'

Demonstrating that TES data can be assimilated within atmospheric chemistry and transport models (using inverse modeling and chemical data assimilation techniques), TES CO profiles from November 2004 were ingested into a global three-dimensional (3-D) chemistry and transport model, GEOS-Chem. The TES CO data significantly improved the model simulation of CO in the middle to upper troposphere, along with assimilation of other trace gases, significantly improves the modeled distribution of CO in the southern hemisphere. Good agreement was found between TES assimilated model estimates of CO and in situ measurements. Assimilation of TES data can also help scientists assess the impact of initial conditions on the CO fields in the model.

Next: Nadir and Limb Views

Back: Global Water Cycle

TES is the only remote-sensing instrument currently flying that can distinguish ozone at altitudes where it does harm from altitudes where it is beneficial, TES data can be uniquely valuable to local and regional air-pollution analysis and forecasting. The TES team is working with people at the EPA and with state air quality boards to use the satellite data to shed light on levels of ozone above the Earth's surface and away from surface monitors.

Chemical Clues Ozone Global Climate Change Global Water Cycle Biomass Burning Air Pollution What Is TES Detecting Altitudes Nadir and Limb Views Validation