Hosted by CIESIN, Columbia University
Palisades, New York, June 12-15, 2001

For policy development at the international as well as national level, there is a demand for quantifying the linkages between air quality changes at the regional, hemispheric, and global scales. Understanding processes at the hemispheric scale is important because it sits at the intersection of concerns about regional air quality and concerns about global climate change.

There is well-documented evidence for intercontinental transport of ozone, fine particles, and their precursors. Observations and modeling assessments suggest that:

•	the continuous venting of continental emissions to the troposphere increases the Northern Hemispheric burden of pollutants;
•	the impacts of this increasing burden on regional air quality are further magnified by episodes of rapid transport, which vary in frequency and magnitude of impact by season.

Intercontinental transport of pollutants is important with respect to accumulated pollutant exposure. As air quality standards and objectives are made more stringent, the contribution of intercontinental transport to exceedances of those objectives will increase. While the control of local and regional emission sources can address many air quality problems, the contribution of intercontinental transport to these problems, as well as the contribution of local and regional emission sources to the increasing hemispheric burden, should be considered in air quality management.

Given expected changes in the distribution of emissions in the Northern Hemisphere, the significance of intercontinental transport may increase in the future.

Developing quantitative source-receptor relationships on intercontinental scales is difficult due to the nonlinearity and coupling of the large variety of physical and chemical processes involved. This complexity challenges the limits of the abilities of our current models.

While current models are beginning to provide estimates of these source-receptor relationships, these estimates are highly uncertain (a factor of 2 or more). Designing an observational strategy to evaluate these uncertainties is a significant challenge, but such a strategy is necessary to better quantify the impacts of intercontinental transport.

To improve our understanding of the intercontinental transport of air pollutants in the Northern Hemisphere, we need to develop a systematic approach to model evaluation, long-term monitoring, intensive observational studies, and emissions inventory development. We also need to improve our understanding of the linkages between regional and global air quality and climate change and variability, and we need to improve the communication between the various scientific communities addressing these issues. Specific recommendations in each of these areas are discussed below.

We need to develop a plan for a targeted evaluation of the ability of models to simulate

•	intercontinental transport on an event basis and a long-term, statistical basis and
•	trends in the hemispheric distributions of important chemical species.

The model evaluation plan should involve both model to observation comparisons and model to model comparisons. Model to observation comparisons should test the ability of models to predict past events and historical trends. Model to model intercomparisons should involve comparison of predictions of state variables, estimates of transport contributions/source-receptor relationships, sensitivities to systematic perturbations, and predicted impacts of different meteorological drivers or scenarios.

Long-Term Monitoring

To provide observations to test the models, we must:

•	maintain and enhance existing observation networks and capabilities, especially the WMO's Global Atmospheric Watch program;
•	develop new observation tools and networks, especially to obtain information about conditions above the planetary boundary layer; and
•	make more effective use of existing observational databases, especially those that provide information, such as aerosol chemical composition, that helps identify and characterize transport events and processes.

Sites and observational techniques that are well-suited to characterizing intercontinental transport, such as coastal and high elevation sites and sondes and remote sensors, should be identified and given appropriate priority in resource planning.

New observational efforts should attempt to make effective use of satellite-based and ground-based remote sensors and to capture the potential of observational platforms of opportunity (e.g., commercial aircraft, military aircraft, and ships).

Focused Intensive Field Campaigns

Long-term monitoring (e.g., surface stations and satellites) will only provide part of the information necessary to evaluate models. Focused intensive field campaigns are needed to probe intercontinental transport processes, filling gaps not addressed by long-term monitoring. Such campaigns should involve the integration of:

•	in situ measurements in the free troposphere,
•	models that capture the interaction of meteorology and chemistry,
•	large scale remotely-sensed data to extrapolate from case studies to larger scales, and
•	the development of merged data set

Emissions

Improving our emissions estimates is essential to improving our modeling ability and, ultimately, our understanding of intercontinental transport. For hemispheric air quality modeling, improved emissions information is needed for:

•	developing nations
•	sources in the upper troposphere (e.g., lightning, aircraft, …)
•	ships
•	biogenic and geogenic sources
•	persistent substances (including persistent organic pollutants and heavy metals) that may serve as transport tracers

Observational strategies should be designed to help evaluate and improve emissions inventories that are traditionally developed based on activity data, as opposed to ambient measurements.

We should improve our ability to share emissions data between North America and Europe and improve the global air quality modeling community's access to high-resolution emissions inventories developed for regional and local air quality modeling.

Linkages

We should explore how climate change and variability affects emissions and transport processes, thereby changing the influence of intercontinental transport on regional air quality. We should also recognize the common objectives of climate and air quality research and cooperate to maximize the effectiveness of research investments in the two areas.

Communication and Collaboration

To improve communication and collaboration across the various scientific communities interested in the intercontinental transport of air pollutants, we should support communication through a variety of media, including:

•	computer web-based information sharing and discussion sites (e.g., by expanding the workshop website, http://www.ciesin.columbia.edu/pph) and
•	continuing periodic meetings, either individual workshops or sessions at larger conferences (such as IGAC's Annual Meeting).

We should support collaboration through

•	improving the compatibility or documentation of formats for observational, emissions, and modeling data sets,
•	establishing meta-data catalogues and reviews of observational, emissions, and modeling data sets, and
•	supporting the development of accessible, distributed archives and retrieval software.

Another meeting should be arranged in approximately one year that brings together scientists from across the Northern Hemisphere to share progress in understanding intercontinental transport.

Inquiries can be addressed to pph@ciesin.columbia.edu.