David Parrish
email: parrish@al.noaa.gov
NOAA Aeronomy Lab
325 Broadway R/AL7
Boulder, CO 80305-3328
USA

Kathy Law
email: kathy.law@aero.jussieu.fr
IPSL Service Aéronomie Boite 102
Université Pierre et Marie Curie
4 Place Jussieu
75252 Paris Cedex 05
FRANCE

An overarching goal of the ITCT program is to understand the chemical transformation and removal processes of aerosols, oxidants and their precursors during the intercontinental transport process. To make this understanding possible, measurements from a Lagrangian platform would be ideal; i.e., a platform that moves with an air mass during the total transport process. Such an ideal is not possible due to the limited range and endurance of existing aircraft. A practical approximation to this ideal, is a “psuedo-Lagrangian” study, where one or more aircraft make multiple, sequential sampling flights into the same air mass during the time required for the intercontinental transport of the air mass. Such a pseudo-Lagrangian study constitutes the IGAC Task described here.

In the Summer of 2004 a large international group of scientists will conduct a field program in the North Atlantic region. The program will focus on the study of emissions of aerosol and ozone precursors over North America, their chemical transformations and removal during transport to and over the North Atlantic, and their impact upon Europe. The program is currently being organized and funded by several agencies including NOAA (U.S.) through the NEAQS - ITCT 2004 program, and NASA (U.S.) through the INTEX-NA program. The European contribution, Intercontinental Transport of Pollutants (ITOP), includes contributions from a UK university consortium (funded by NERC), DLR (Germany) and CNRS (France). Each of these programs has its own regionally focused goals and deployments, but together they provide coverage from the source regions on North America, through the transport pathways over the North Atlantic, and over the receptor regions of Europe. The ITCT-Lagrangian-2k4 Task is an organizational and analysis effort that will, within the individual project goals, coordinate the disparate programs into a pseudo-Lagrangian framework. That is, we will combine data from the multiple observational platforms collected at different stages during the transit of a polluted air mass across the North Atlantic.

The ultimate goal of ITCT is the direct observation of the evolution of the aerosols, oxidants and their precursors from emission over North America, trans-Atlantic transformation and transport, and im-pact on aerosol and oxidant levels over Europe. Within this overall objective there are several distinct sub-objectives:

• Determination of the photochemical oxidant and aerosol formation potentials in polluted air masses originating in North American emission regions and their chemical evolution as they are transported out over the North Atlantic. This will involve quantification of physical and chemical loss processes of emitted species over North America, and detailed characterization of radical chemistry, VOCs and their degradation properties, the NOx/NOy budget and physical and chemical aerosol properties in the exported air masses. The contribution of natural sources (e.g. wild fires, biogenics, stratosphere, lightning) to oxidant and aerosol distributions will also be investigated.
• Characterization of the dynamical processes responsible for pollutant transport out of the North American planetary boundary layer (cyclonic air streams, fronts, convection, land-sea breezes, orographic effects) and the importance of mixing/layering processes in the long-range transport and chemical transformation of pollutant plumes as they cross the North Atlantic.
• Quantification of the export of North American pollutants to the background atmosphere, their subsequent fate over the North Atlantic and beyond (e.g. over Europe) and possible impact on climate. This encompasses questions such as: are polluted layers producing or destroying ozone? Do these ageing air masses contain significant levels of anthropogenic aerosols and how is their chemical composition changing as they age?
• Determination of the possible import of North American, and possibly Asian or other, pollutants into the boundary layer over Europe where they may be contributing to background levels of pollutants and regional air quality.

These seven aircraft will also be coordinated with as many as five other research aircraft over North America. These include: the Canadian MSC Convair 580 and the Cal Tech/ONR/CIRPAS Twin Otter conducting detailed aerosol and cloud microphysical studies; the NSF/Harvard/COBRA program (operating the Wyoming King Air) conducting a carbon budget study; the U.S. Dept. of Energy G-1; and the University of Maryland Aztec aircraft. This suite of aircraft will be coordinated with a variety of ground sites including the AIRMAP network in New Hampshire, Harvard Forest in Massachusetts, a site on the southern tip of Nova Scotia, several other U.S. state networks, the PICO-NARE site on the Azores, and the German ATMOFAST lidar and high altitude surface sites.

It is hoped that other programs making measurements on commercial aircraft (i.e. MOZAIC, CARIBIC) will also participate in this task by making data available for the period of the measurement intensive. In the case of MOZAIC at least 1-2 flights are made daily across the Atlantic between European cities and, for example, New York or Boston collecting high temporal resolution data on O₃, CO and NOy concentrations. CARIBIC makes less frequent flights (1-2 per month) but with a more comprehensive instrument package that includes measurements of NOx, non-methane hydrocarbons, halogenated species and aerosols. Data from current and planned satellite platforms will contribute to the post-campaign analyses, and in some cases will be used for flight planning. Table 1 lists the satellite data sets that provide measurements of tropospheric species and biomass fire information. In addition, the near-real time visible and infrared imagery from GOES and METEOSAT and the winds derived from this imagery will provide flight-planning guidance. Satellite data that have been particularly useful for flight planning include aerosol products from TOMS, SEAWIFS, and MODIS, O₃ products from TOMS, and the fire data from MODIS. In 2004 we plan to utilize GOME and SCIAMACHY NO₂ columns and perhaps MOPPIT CO columns as well.

Many theoretical groups will be involved in forecasting for flight planning and in modeling for interpretation of results. These include global models (e.g. Harvard, NOAA, U. Iowa, U. Cambridge, MPI-Mainz, CNRS), regional models (U. New Hampshire, Envi-ronment Canada) and Lagrangian trajectory models (NOAA, U. Cambridge/Leeds, CNRS).

Coordination of a program to synthesize results from a pseudo-Lagrangian experiment over the North Atlantic requires an international research framework appropriate for IGAC; that coordination is central to the ITCT Task.

A goal of IGAC is to understand how atmospheric chemistry might evolve under climate change. However, the field project being used as the basis for the ITCT-Lagrangian-2k4 Task will, by necessity, cover only a very limited time window – approximately 6 weeks in Summer. The choice of season was made for several reasons: over North America it is the time of maximum photochemical production of oxidants and aerosols, of maximum biogenic emissions of hydrocarbons that provide much of the fuel for this photochemistry, of enhanced stagnation episodes that allow the primary emissions, oxidants and aerosols to collect over the source regions, and a time of consistent northeastward transport of continental air masses to the North Atlantic troposphere. This narrow time window necessarily limits the investigation of natural variability on time scales longer than weeks. Contrasts with previous North Atlantic Regional Experiment (NARE) studies, which were conducted in the early Spring, late Summer and early Fall, will provide some information on seasonal variability.

However, in the interpretation of the results, it is necessary to consider the possibility of significant inter-annual variability. Importantly, the largest source of year-to-year variability of winter weather in the extra-tropical Northern Hemisphere is the North Atlantic Oscillation. Its influence on the variability of transport in the Summer over the North Atlantic is relatively weak, but still significant. To assess inter-annual variability, longer-term datasets, such as those from ozone sonde networks, AERONET and programs on commercial aircraft (MOZAIC, CARIBIC) will be used together with analysis of model results. Analysis of satellite data will also make an important contribution to our understanding of inter-annual variability of long-range transport of pollutants over the North Atlantic region.

The ITCT-Lagrangian-2k4 Task team has estab-lished a steering group (SG) to oversee pre-campaign planning, development of a coordinated flight strategy and post-campaign data analysis. The SG will include the task coordinators, representatives from the participating aircraft and forecasting and modeling groups, and other interested scientific participants.

The organization and analysis outlined above comprises four steps: review of previous results, instrument comparison activities (to ensure that measurements on the disparate platforms can be accurately integrated without confounding measurement uncertainties), flight coordination during the field deployment, and post-deployment analysis.

1. Retrospective Analysis of Previous Pseudo-Lagrangian Results.
During the NARE 1993 and NARE 1997 studies aircraft were operated on both the North American and European sides of the Atlantic. It was not the primary aim of these experiments to perform a Lagrangian study. These missions were more exploratory in nature, but events may have occurred during both studies when anthropogenically influenced air masses were sampled before and after transport across the Atlantic. However, examination of the data sets for such cases has been rather limited. The first effort of the ITCT-Lagrangian-2k4 task is to coordinate more extensive analysis. There are two periods to be examined. First, during NARE 1993 the NCAR King Air, operated in the Gulf of Maine by NOAA, characterized an air mass leaving the U.S. Trajectory calculations suggest the British C-130 may have sampled this same air mass during its return flight from Halifax to Britain. Second, during NARE 1997 the NOAA WP-3D characterized air masses off the U.S. east coast. In one case the British C-130 flying from the Azores may have sampled a characterized air mass after substantial transport. The DLR Falcon flying over Europe may also have sampled one of these air masses over Europe. Analysis of other datasets (e.g. MOZAIC, lidar, aircraft) may also reveal further interesting insights into the transport pathways and chemical signatures of pollutant plumes transported across the North Atlantic. The analysis of these earlier data has two primary goals: First, to see if measured levels of relatively inert tracers, such as carbon monoxide and long-lived NMHCs, support the indications from trajectory calculations that the same air mass was sampled. Second, to see if indications of processing can be discerned in the measured levels of more reactive species such as shorterlived NMHCs, oxides of nitrogen, ozone, and other oxidants. The successes and problems identified in these analyses of the early studies will serve as a valuable guide to the field implementation during Summer 2004.

2. Instrument Comparison Activities.
For the pseudo-Lagrangian approach to be suc-cessful, it is essential that the aircraft involved make measurements that are equivalent within quantified uncertainties. Quantifying measurement uncertainty establishes an objective, defensible basis upon which the pseudo-Lagrangian analysis can be built. In effect, a unified observation system is created. Comparison exercises will take place before, during, and after the 2004 field mission. The three general phases envisioned for the comparisons are outlined below, followed by a proposed strategy for their implementation.
Evaluation of standards (pre, during, post-mission): Comparison of compressed gas standards should be performed at least once for the different in-situ gas phase instruments on the participating platforms for NO, CO, CO₂, SO₂ and volatile organic compounds (VOCs). Comparison of ozone standards should also be performed. Effort will also be made to evaluate instrumental sensitivities to HNO₃ between the aircraft and shipborne instrumentation by sampling from a characterized permeation device. Also, Ion Chromotography (IC) standards will be exchanged between investigators utilizing IC aerosol composition measurement systems such as the Particle In Liquid Sample (PILS) system.
Direct comparison of measurements (pre, during, post-mission): Prior to field deployment, running instruments in the lab or in the field side-by-side is an excellent way to test performance. This is more easily done for some instruments than others, so this will be up to the various investigators to arrange as desired. During the mission joint flights of two aircraft, over-flights of the ship, Ronald H. Brown, and overflights of ground sites will provide data from which instrument performance may be critically assessed. Such overflights provide an opportunity to compare a large variety of gas-phase, aerosol-phase, meteorological, and radiative parameters.
Indirect comparison of measurements (during and post-mission) While sampling in close physical proximity provides useful information, other opportunities exist to evaluate instrument performance by examining data taken during normal flight procedures. For example, the CO/CO₂ ratio should be approximately conserved for some time during transport over water, suggesting that aircraft and ship data in an urban plume might be usefully compared in this regard. Further, free tropospheric ozone levels should be comparable between the in situ and remote measurements if taken within a well-defined volume and relative short time of one another. These comparisons-of-opportunity can provide useful additional data with which to evaluate instrumental performance between the various participating platforms.
Proposed Strategy:
- Species: With regard to the pseudo-Lagrangian analysis, the measurements of most importance to compare include the following gas-phase oxidant and aerosol precursor and tracer species: CO, CO₂, NOx, NOy, O₃, SO₂ and VOCs including oxygenates. For aerosols, likely comparisons include size-resolved number density and chemical composition as measured by PILS and aerosol mass spectrometer systems. Although perhaps secondary to the pseudo-Lagrangian analysis, the opportunity will be taken to compare the measurements of other important species as well, including the HOx family (OH, HO₂, and RO₂), peroxides and carbonaceous aerosol.
- Organization: A small group with representation from each major organizational group (NASA, NOAA, etc.) will attend to logistical details for comparisons. IGAC will play an important role in the organization of this group and its activities.
- Formality: After a side-by-side comparison, data sets will be reduced by the investigators and submitted independently to a referee. Data sets will include estimated uncertainties, allowing for a proper quantitative comparison. Once all data sets for a specific comparison are submitted they will be released to all study participants. Referees will encourage comparison participants to look for non-recoverable errors.
- Develop comparison matrix: A "transfer standard" concept will be used so that each platform is tied to the others through at least comparison with another, mutual platform. Ideally, each comparison will occur at least twice, and will be made over a range of important parameters (i.e., altitude, water vapor, possibly interfering pollutants.) Wingtip-to-wingtip aircraft comparisons, aircraft fly-bys of surface stations (ground-based or ship), and US-vs-European comparisons will all be employed.

3. Flight Coordination.
The pseudo-Lagrangian approach to trans-Atlantic transport involves three arenas of flight operations: over the source and outflow region of North America, over the mid-Atlantic and over the inflow region of Europe. Flight coordination will differ in each of these regions. On the North American side of the Atlantic, ITCT-Lagrangian-2k4 will not attempt to formulate flight plans or coordinate flights of the various aircraft. Each of the aircraft will have a variety of program goals to meet and will conduct flights according to those goals. However, these goals are such that they ensure that air masses with strong anthropogenic influence leaving North America will be well characterized during multiple aircraft flights as well as via measurements from other platforms. The responsibility of the ITCT- Lagrangian-2k4 team will be to closely monitor the flights that are made in the source region, to monitor forecast trajectories for these air masses, and to alert aircraft in the central and eastern Atlantic of possible interception opportunities. On the European side, efforts will be coordinated under the ITOP umbrella. A coordinated strategy for flight planning will be developed in collaboration with North American participants.

The ITCT-Lagrangian-2k4 team will coordinate the development of planning tools and procedures for identifying potential events that aircraft over the mid-Atlantic and Europe can mutually intercept. Analysis of trajectories and tracer calculations over periods of several years will be part of the process used to establish the most suitable locations for the mid-Atlantic and eastern Atlantic flights. The Task team will be sure that a strategy for coordinating flights is part of the overall planning for the Summer 2004 study.

4. Post-Campaign Analysis.
A combination of data analysis and modeling will be used to address the scientific objectives of individual programs and also the wider objectives of ITCT-Lagrangian-2k4. The SG will communicate with the separate science teams to assure that the Lagrangian-related data sets are available to all of the science teams involved, and they will coordinate the analysis of the data from the pseudo- Lagrangian point of view. Other data such as those collected on commercial aircraft, by sondes/lidar, and by satellites will also become part of the ITCT-Lagrangian-2k4 analysis data set where they have utility. The SG will organize joint workshops to discuss the results and identify possible papers to describe key findings.

Quality assurance is discussed in detail above. The schedule for the Data Plan is:

• A “preliminary” data archive will be created during the field deployment. This will gradually evolve into the “final” data archive as the data reduction process is completed.
• A “final” data archive with merging of data sets on different time bases will be created 9 months after the completion of the field deployment.
• A data workshop will be held within 12 months of the completion of the field deployment.
• The “final” data archive will be released to the public domain 6 months after the workshop.

This task seeks to perform organizational and analysis efforts that will coordinate disparate, pre-existing programs. As such it does not have educational and capacity building efforts separate from those of the component programs. However, each of these components has its own efforts. For example, the PICO-NARE program is a joint project between the Universidade dos Açores and Michigan Technological University. The interactions will expand the educational capacity of both institutions.

A Core Project of the International Geosphere-Biosphere Programme (IGBP)

In Cooperation with IAMAS Commission on Atmospheric Chemistry and Global Pollution

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