“The paper describes the development of the first global tropospheric chemical weather forecasting system and its application to planning flights during three field measurement campaigns in 1999 and 2001.”

This paper presents a chemical weather modeling system which was developed for the purpose of supporting atmospheric chemistry field campaigns, and for an analysis of issues such as long-range pollutant transport. Many of the citations on this paper so far come from studies which have made use of our forecasts for field campaign flight planning, and our hindcasts for analyzing atmospheric chemistry observations made during these campaigns.

I suspect in part this usage comes from our efforts to make the system very user-oriented, with a web interface providing both standard forecast figures for several regions of interest as well as an interface for creating custom-made figures from the output. In addition, several citations are from other researchers who have gotten involved in chemical weather forecasting. This paper was among the first papers published on this topic, and was the first to give an in-depth discussion of forecasting for the troposphere.

Perhaps a final ironic reason has to do with the history of the paper—it was rejected twice based on critical reviews about the value of this work, so by the time it was published in Atmos. Chem. Phys. it had undergone a lot of polishing and orienting towards carefully showing what is really interesting and useful in this line of research.

The paper describes the development of the first global tropospheric chemical weather forecasting system and its application to planning flights during three field measurement campaigns which took place during 1999 and 2001. We actually set up two systems using the same base model, one focusing on ozone-related trace gases—as described in this paper—and one focusing on aerosols (described in Collins et al., J. Geophys. Res., 106, 7313-7336, 2001, and Rasch et al., J. Geophys. Res., 106, 7337-7355, 2001). Prior to the development of our operational modelling systems, only two other similar systems had been set up for field campaign planning; one was stratospheric, and the other was a regional tropospheric model. Our introduction of global tropospheric forecasts has turned out to be especially important in predicting intercontinental pollutant transport, which is the main focus of the examples analyzed in the paper. Through the development of the web interface mentioned above, the output from our chemical weather simulations are easily made use of by colleagues. In the few years since the development of these first global chemical weather forecasts, several similar systems have been developed and applied worldwide.

Our manuscript also includes results from three other chemical weather forecast systems which were employed together with ours in the field campaigns in 2001. The use of multiple chemical weather forecasts in a single campaign has also emerged as a standard, in which models with different strengths provide valuable, complementary information for improved flight planning.

People are generally familiar with meteorological weather forecasts, which they see in the newspaper or the evening news. These forecasts are produced by numerical models running on powerful computers. Our modelling system has taken this a step further by adding four important processes: 1) emissions of important gases such as carbon monoxide, 2) chemical reactions of these gases (which are mainly driven by the energy from sunlight), 3) transport by wind fields (which we get from the weather forecasts), and 4) removal of gasses from the atmosphere, for instance, soluble gases being washed out in forecasted precipitation. This allows us to predict the concentrations of ozone, nitrogen oxides, and other important gases up to 3 days in advance, on a 3-dimensional global grid for the lower part of the atmosphere (known as the "troposphere," which extends from the Earth’s surface up to about 10-15 km). We have mainly used these forecasts to help decide exactly when and where to make measurements in the atmosphere. Before this, such decisions were primarily based on normal meteorological weather forecasts and the intuition of experienced scientists. In turn we use these predictions with the resulting measurements to study various important processes, such as the transport of pollutants from North America to Europe. Other applications for chemical weather forecasts—especially more highly resolved regional forecasts—are predictions of air quality and visibility and other factors which affect human health, agriculture and tourism.

I started working on atmospheric chemistry model development in 1993. In late 1998, we decided to try to take this a step further by making forecasts, rather than just the historical evaluations that had been done until then. After a few months of hard work, together with a few colleagues, we had a running system ready in time for the Indian Ocean Experiment (INDOEX) in early 1999. During this first campaign most of the system was still run "by hand," which meant getting up at 3 A.M. daily for six weeks to make sure everything was operating smoothly, and sending pictures of the output to the operations station in the Maldives via fax; since then, we have developed this into a fully-automated, continually-running system with the web interface mentioned above.

Since the technical side of our global chemical weather forecasting system is primarily aimed at supporting atmospheric measurements campaigns, this will have little direct social impact itself other than saving taxpayer money by helping to make measurements planning more efficient and accurate. However, this system supports research which does have direct social and political implications, in particular intercontinental pollutant transport. Examples showing pollutant transport from North America to Europe in winter and Asia to Europe in summer are discussed in the paper. Furthermore, this paper is a part of the early developments in the larger, growing field of chemical weather research, which is closely connected to several socially-relevant topics such as air quality, health, agriculture, visibility and tourism.

For additional information, the interested reader can see an essay on the field of chemical weather research which we recently published in Lawrence et al., Environ. Chem., 2, 6-8, 2005 (doi:10.1071/EN05014).