In recent years we have been involved in the following series of field missions:

GTE
The Global Tropospheric Experiment (GTE) is a series of ongoing global airborne measurement campaigns to study the impact that humans are having on the global troposphere. It utilizes large extensively instrumented aircraft as primary research tools. GTE also draws heavily upon satellite observations of meteorology, land use, and atmospheric chemical species to aid in experiment design and in the scientific analyses of results obtained from aircraft and ground-based measurements.

Within GTE several mission categories with a primary focus are defined. The Chemical Instrumentation Test and Evaluation (CITE) experiments were initiated to evaluate our ability to measure critical tropospheric species. The field studies known as Atmospheric Boundary Layer Experiment (ABLE) have been conducted to study large ecosystems that are known to exert a major influence on global chemistry and, in some cases, are undergoing profound changes. The impact of long range transport of continental emissions, natural and anthropogenic, have been studied through focused missions such as TRACE-A (Transport and Chemistry near the Equator in the Atlantic), Pacific Exploratory Missions (PEM) A and B, PEM-Tropics, and TRACE-P (Transport and Chemical Evolution over the Pacific).

SONEX
Global air traffic has more than doubled in the last 15 years and this trend is projected to continue at least into the early part of the twenty-first century. Aircraft exhaust emissions of NO_X and particles have the potential to modify the chemistry and microphysics of the upper troposphere and lower stratosphere. The SASS (Subsonic Assessment) Ozone and NO_XExperiment (SONEX), was an airborne field campaign conducted in October-November 1997 in the vicinity of the North Atlantic Flight Corridor. A key objective of SONEX was "to assess the effect of emissions from subsonic aircraft on nitrogen oxides and ozone." A fully instrumented NASA DC-8 aircraft was used as the primary SONEX platform. SONEX activities were closely coordinated with the European POLINAT-2 (Pollution from Aircraft Emissions in the North Atlantic Flight Corridor) program, which used a Falcon-20 aircraft. Upper troposphere/"lowermost" stratosphere (UT/LS) was the region of greatest interest.

Results from the SONEX/POLINAT-2 campaign have been published in Special Sections of Geophysical Research Letters (October, 1999) and J. of Geophysical Research (February, 2000). SONEX/POLINAT-2 successfully collected a comprehensive body of data over the north Atlantic that can be used to further test and validate global models of transport and photochemistry, and identified areas of uncertainty. We conclude that increased aircraft NO_Xemissions in the future will likely lead to additional O₃ formation, but the rate will vary greatly depending on the state of the atmosphere.

INTEX-NA
The Intercontinental Chemical Transport Experiment (INTEX) field campaign is being proposed as an integrated field experiment that uses surface, airborne, and satellite platforms, together with models to assess the impact of human induced emissions on the composition and chemistry of the atmosphere in the Northern Hemisphere (NH). Its primary objectives are:

• to quantify the export and chemical evolution of radiatively and chemically important trace gases and aerosols from eastern north America to the western Atlantic (Phase A) and elucidate the mechanisms and pathways associated with these transport processes.
• to quantify the transcontinental impact of Asian pollution on the eastern Pacific as input to North America (Phase B) and elucidate the mechanisms for these transport processes.

A parallel objective is to develop the airborne database to test and evaluate satellite observations.

INTEX is a result of activities following the NASA Snowmass workshop held in August 1999. The central premise of this workshop was that within the next three or so years, satellites would allow near global atmospheric chemistry measurements permitting an integrated view of the atmosphere. Satellite and aircraft missions naturally complement each other and a challenging task is to design campaigns that take full advantage of this synergy. INTEX is designed with this perspective in mind. INTEX also supports the new IGAC (International Global Atmospheric Chemistry) Project initiative on the Intercontinental Transport and Chemical Transformation (ITCT) of pollution (ITCT, 2000). INTEX also has elements that integrate well with the US Carbon Cycle Program and the ongoing NASA SASS Program.

INTEX is being driven by the following overarching hypotheses that were recognized as central to studies in the troposphere:

• Concentrations of trace gases and aerosols will continue to rise and impact global/regional climate.
• Changes in anthropogenic emissions and changes in climate will affect atmospheric oxidant and aerosol concentrations on a global scale.
• Regional air pollution will remain a major environmental problem in the developed world and will increasingly affect the developing world.

The NH continents are a major global source for many environmentally important gases and aerosols. There is substantial evidence that pollution from these continents can travel over thousands of miles and impact air quality in downwind regions. Our quantitative knowledge of the export fluxes of gases and aerosols to the global atmosphere is poor. There is a clear need, from a policy and societal perspective, to quantify these exports from Asia and North America to the global atmosphere and to assess their impact on global air quality and climate.

We envision that INTEX will take place in two intensive phases. Phase A intensive will focus on the eastern seaboard of North America where most of the chemical outflow takes place. Phase B intensive will focus on the eastern Pacific where the impact of Asian emissions is maximum. The ideal timing for Phase A is during summer when biogenic sources/sinks of NMHC and greenhouse gases, anthropogenic influence on tropospheric ozone and OH, and the climatic effect of midlatitude aerosols are all at or near their maximum. The ideal timing for Phase B is late spring when the Asian impact is expected to be at its peak. Suitable bases of operation during Phase A are Bangor, Maine and Bermuda/or Azores (alternate). Phase B could be best executed from a continental site on the West Coast of the US (e. g. Seattle) and Hawaii. It is expected that the primary INTEX objectives can be met through the use of the DC-8 and P-3 aircraft.

Essential to the success of INTEX will be the combined perspective afforded by aircraft and satellite measurements on the chemical outflow from continents to the oceans. The AURA and ENVISAT polar-orbiting satellites, expected to be operational during INTEX, will provide global and continuous measurements of tropospheric O₃, CO, CH₄, CH₂O, SO₂, NO_X, HNO₃, and aerosols. We recognize that operation over the oceans in cloud-free environments provide the best environment for maximum synergy between airborne and satellite measurements.

We expect INTEX to generate considerable interest from the international scientific community especially from Asia and Europe. This is expected to manifest itself through independent European and Asian supported campaigns based in the Eastern/Central Atlantic and the western/central Pacific respectively, that could be closely coordinated with INTEX. NOAA already has plans that complement this proposed activity. In addition, involvement of satellite science communities is expected. A steering committee has been assembled to facilitate collaboration and maximize science returns.