prove our ability to constrain carbon sources and sinks in inverse models. It would pave the way for construction of national carbon budgets, providing important input for global environmental agreements aimed at mitigating climate change. The challenge is to deliver a measurement with sufficiently high precision to be useful for inverse modeling. A precision of 0.3 ppmv (parts per million by volume) is thought to be necessary (Pak and Prather, 2001; Rayner and O’Brien, 2001). The Orbiting Carbon Observatory (OCO) satellite instrument, planned for launch in 2007, is expected to provide this precision (Crisp et al., 2004). It will measure CO2 column mixing ratios with kilometer-scale spatial resolution by solar backscatter in the 1.58 μm band, with measurements in additional bands to correct for aerosol and surface pressure effects. Simulations with chemical transport models sampled along the OCO orbit track suggest that the measurements should be of great value for constraining carbon fluxes down to a regional scale (Crisp et al., 2004).

Methane concentrations have increased by a factor of 2.5 since the eighteenth century, but the rate of growth began to slow in the 1980s and was close to zero in 1999-2002 (Dlugokencky et al., 2003). The reason for this slowdown is not clear. Changes in agricultural practices, decreased natural gas production in Russia, and increasing OH concentrations (reducing the lifetime of methane) may all have contributed (Khalil and Shearer, 1993; Dentener et al., 2003; Wang et al., 2004). A number of inverse model studies have been conducted to constrain sources of methane using long-term observations from the NOAA CMDL network (Hein et al., 1997; Houweling et al., 1999; Wang et al., 2004), but they do not yield consistent results. Aircraft observations in continental outflow over the northwest Pacific have been used recently to constrain Eurasian sources of methane (Xiao et al., 2004) and halocarbons (Palmer et al., 2003). Satellite measurements of methane and halocarbons have so far been restricted to the stratosphere. There has been great interest in using solar backscatter measurements to constrain the column mixing ratio of methane (Edwards et al., 1999), but efforts so far have been unsuccessful. Similar to CO2, satellite observations of methane with sufficiently high resolution would increase considerably our ability to constrain regional sources.

Ozone

Ozone has a lifetime ranging from days to months in the troposphere and up to years in the lower stratosphere. Its distribution in the atmosphere is thus highly variable, in contrast to the long-lived greenhouse gases. Vertical profiles from ozonesondes provide at present the best characterization of the global distribution of ozone. Their coverage is extensive in the extratropical Northern Hemisphere but relatively sparse in the tropics and the



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