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changes at numerous sites located primarily in urban areas; remote-sensing (satellite) instruments that provide global-scale observations of selected atmospheric species; and a variety of balloon- and aircraft-borne instruments used for in situ measurement campaigns. These different observational platforms vary widely in their scope and degree of analytical rigor. The following sections describe the capabilities and limitations of these different observational systems for addressing the global air quality issues discussed in this report. Gas-phase species and aerosols are discussed separately since each presents unique observational challenges.

Gas-phase Species: In Situ Observations

The temporal and spatial sampling requirements for observations of a gas-phase chemical species are closely linked to the atmospheric lifetime of that species: the required geographical density of sampling locations increases as the atmospheric lifetime of the species decreases. The necessary sampling density also increases with the need for higher levels of measurement precision, a requirement that is often related to the nature of the individual measurement sites. For instance, the more a sampling site is affected by local sources or local meteorology (e.g., upslope/downslope diurnal winds, passing fronts, land/sea breezes), the more frequent the sampling must be in order to resolve these effects. At many sites, hourly or more frequent sampling is often required to resolve measurable changes from background values for some species.

There are a few networks of surface observational sites that are far removed from local sources and sinks and thus can be used to obtain a regionally representative “baseline” of atmospheric composition. This includes U.S.-supported networks operated by the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) and by the Advanced Global Atmospheric Gases Experiment (AGAGE), as well as a number of non-U.S. national and regional efforts (e.g., in Australia, the European Community, Japan, and New Zealand). These networks employ in situ automated measurements as well as flask sampling, which permits centralized analyses of samples from different sites and helps verify the relative calibrations of the in situ measurements.

These types of networks have generally proven successful in characterizing the overall distribution and temporal trends of long-lived gases including CO2, CO, CH4, N2O, and CFCs 1 . In contrast, the atmospheric concentrations of shorter-lived (i.e., more reactive) species including NOx, VOCs, O3, and PM exhibit large spatial and temporal variations, and hence measurements at select baseline sites around the globe are generally far too sparse to characterize their overall

1 Quantifying regional sources/sinks of these compounds requires more intensive measurement strategies, but this issue is beyond the scope of this report.

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