Knowledge of the origin and fate of tropospheric ozone (O₃) is critical for several reasons:

• It is the primary source (through photolysis by solar UV radiation) of hydroxyl radicals (OH) which, in turn, provide the mechanism by which pollutants such as carbon monoxide (CO) and methane (CH₄) are removed from the lower atmosphere.
• Through its reactions with hydrocarbons, ozone is the source of PAN (peroxyacetylnitrate), a major ingredient of urban photochemical smog and a powerful lacrimator.
• Ozone itself is hazardous to the health of both plants and animals since it is a powerful oxidant.
• Ozone is a significant greenhouse gas, especially in the upper troposphere.
• Thus while some tropospheric ozone is essential for human health, too much is deleterious (the EPA-allowed maximum is 105 ppbv for any 1 hour per day).

The Sources of Tropospheric Ozone

The sources of ozone are two-fold: some is transported down from the stratosphere (the exact amount being controversial) and some is created in situ through the photolysis of nitrogen dioxide (NO₂) to nitric oxide (NO) which, in turn, reacts with CO and hydrocarbons to form O₃. The sum of NO + NO₂ is termed NOx and its presence is essential for ozone formation. Note that the NOx is not consumed in this process (i.e., it is catalytic) and becomes available for further ozone production. Thus urban areas, where combustion and automobile exhausts provide prolific sources of NOx, are particularly prone to atmospheric pollution (especially in areas of abundant sunlight). However, many rural areas (for example, the southeastern US) occasionally fail the EPA standard for reasons that are somewhat mysterious. Finally, the sink of NOx is nitric acid (HNO₃) which is rained out to the surface (and is, incidentally, a component of acid rain).

Tropospheric Ozone and its Precursors

In order, therefore, to understand tropospheric ozone, one must do more than just measure O₃ itself. It is essential to make collocated measurements of NOx, HNO₃, CO and hydrocarbons (of which CH₄ is by far the most abundant) on a global scale because atmospheric pollution is no respecter of political boundaries.

OH

The OH radical provides the main sink for a number of environmentally-important atmospheric gases (see above figure) including CH₄, other hydrocarbons, CO, NOx, and HCFC's (i.e., CFC substitutes). OH is produced in the atmosphere by photolysis of ozone, yielding atomic oxygen in the O(1D) state, which then reacts with water vapor. Model calculations suggest that OH concentrations may have decreased globally by 20% since preindustrial times, largely due to increased emissions of CH₄ and CO. There is concern that a catastrophic depletion of OH could occur if anthropogenic emissions of CH₄ and CO continue to increase. However, anthropogenic increases of ozone and NOx stimulate OH production, complicating the assessment of human influence. Although tropospheric OH concentrations are too low to be measurable by TES (or, indeed, any other known space-borne remote sensor), it will be possible to infer its distribution by profiling the concentrations of the regulating species and combining these with knowledge of the solar UV flux.

Ozone

Ozone is the principal precursor of OH. Trial retrievals with simulated TES nadir data suggest that 3 levels in the troposphere can be measured with 20% (or better) accuracy. Mapping of the lower atmosphere is critical because OH production peaks at 1-3 km altitude. Furthermore, some tropospheric ozone (possibly as much as 50%) originates in the stratosphere, and is transported down at mid and high latitudes in so-called "tropopause fold events." Limb viewing of ozone is necessary to quantify this phenomenon.

H₂O

Water vapor is required to convert O(1D) produced by ozone photolysis to OH. The major determinant of the OH production rate is the covariance of ozone and water-vapor concentrations. Simultaneous retrieval of these two species will provide a precious input to photochemical models. Our current best estimate is that TES water-vapor profiles should be accurate to a few percent.

CH₄

Methane is an important OH sink and, in turn, OH is the main sink for methane. Increases in CH₄ emissions can therefore provide a positive feedback mechanism for OH depletion. Unfortunately, owing to its long lifetime in the troposphere (about 7 years), there is a large permanent background of methane so changes will be difficult (but not impossible) to measure.

CO

CO is a major OH sink and, in turn, OH is the major sink for CO. The lifetime of CO is relatively short (.2 months). Tropospheric concentrations are therefore quite variable (from 50 to 1000 ppbV). Profile accuracies of 10% or better are expected except in the boundary layer, where lack of thermal contrast renders measurement difficult.

NOx

NOx helps to maintain a high oxidizing power in the atmosphere by recycling the OH consumed in the oxidation of trace gases. The source of NOx is both anthropogenic (combustion) and biogenic (biota, lightning). The atmospheric concentration is regulated in part by chemical cycling with HNO₃ and organic nitrates (in particular PAN - peroxyacetylnitrate). TES will measure NO by limb-viewing to 20-30% in the mid and upper troposphere (it is too heavily blanketed by water vapor to be measured in the lower troposphere). HNO₃ (also measured by limb-viewing) can be measured to a few percent throughout the troposphere, at least down to the cloud deck. The detectabilities of NO₂ and PAN are uncertain at this time, but will certainly require limb-viewing.

Other Hydrocarbons

The most abundant non-methane hydrocarbon, ethane (C₂H₆) will be determinable to about 10% in the free troposphere by limb sounding, and TES should be capable of at least a detection of C₂H₂. The detectability/measurability of other hydrocarbons is conjectural due to the inadequacy of the spectral databases for such species.