relative humidity, wind speed and direction), with required spatial and temporal coverage. Data on aerosol chemical composition are essential and must include particle carbon, particulate matter, sulfate, organic carbon, and elemental carbon. The requirements for spatial and temporal resolution will be specific to a given urban area because they are a sensitive function of topography and chemical composition, but diurnal data must be routinely secured as a function of altitude from the ground to above the boundary layer, with airborne platforms that are guided by real-time observations and the associated trajectory calculations defining air mass motion and residence times.

Production of Infrared Active Gases that Control Climate

Ozone is recognized to be an important component in the radiative balance of the Earth in the critical region of the tropopause.14 It is also recognized to be increasing in the upper troposphere 15 and to be strongly linked to photochemical production via hydrogen and nitrogen radicals.16 The upper troposphere is thus central to the link between the production of infrared gases and climate. From the perspective of scientific strategy, the upper troposphere is ideally suited for critical photochemical experiments because it provides an in situ laboratory with chemistry representative of the troposphere and yet is simple enough to reach closure on a range of experiments testing key hypotheses. There are several regions that should receive particular attention in the selection of trajectories. For example, the region that lies between Africa and Brazil, dominated by biomass-burning products, constitutes a profoundly different source region than the largely pristine region of the western tropical Pacific. The polluted continental regions and their wake regions in the Pacific rim are distinct from the Arctic upper troposphere. It has been clearly demonstrated that using the large dynamic range in species afforded by these regional differences, with the proper complement of tracers and careful real-time analysis of the meteorological fields, provides decisive causal links to be tested and established.

Priority: It is currently hypothesized (1) that ozone is catalytically produced in the upper troposphere via cycles involving radicals in the nitrogen and hydrogen families; (2) that NOx is supplied by organic nitrates (PAN, etc.) and converted to nitric acid on roughly the overturning timescales of the upper troposphere; and (3) that HOx is supplied by photolysis of relatively insoluble organic precursors such as acetone and methyl hydroperoxide. These hypotheses must be tested. Observations are therefore required of the short-lived and catalytically active radicals (OH, HO2, CH3O2, CH3C(O)O2, NO, NO2, and NO3); the precursor species that may also be products (H2O, acetone, CH2O, CH3OO3, PAN, CH4, and solar ultraviolet spectrally resolved); the product/reservoir species (HOOH, HONO, HONO2, HOONO2, N2O5); the sink species that are also local tracers

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