A backward modeling study of intercontinental pollution transport using aircraft measurements

Abstract

[1] In this paper we present simulations with a Lagrangian particle dispersion model to study the intercontinental transport of pollution from North America during an aircraft measurement campaign over Europe. The model was used for both the flight planning and a detailed source analysis after the campaign, which is described here with examples from two episodes. Forward calculations of emission tracers from North America, Europe, and Asia were made in order to understand the transport processes. Both episodes were preceded by stagnant conditions over North America, leading to the accumulation of pollutants in the North American boundary layer. Both anthropogenic sources and, to a lesser extent, forest fire emissions contributed to this pollution, which was then exported by warm conveyor belts to the middle and upper troposphere, where it was transported rapidly to Europe. Concentrations of many trace gases (CO, NO y , CO 2 , acetone, and several volatile organic compounds; O 3 in one case) and of ambient atmospheric ions measured aboard the research aircraft were clearly enhanced in the pollution plumes compared to the conditions outside the plumes. Backward simulations with the particle model were introduced as an indispensable tool for a more detailed analysis of the plume's source region. They make trajectory analyses (which, to date, were mainly used to interpret aircraft measurement data) obsolete. Using an emission inventory, we could decompose the tracer mixing ratios at the receptors (i.e., along the flight tracks) into contributions from every grid cell of the inventory. For both plumes we found that emission sources contributing to the tracer concentrations over Europe were distributed over large areas in North America. In one case, sources in California, Texas, and Florida contributed almost equally, and smaller contributions were also made by other sources located between the Yucatan Peninsula and Canada. In the other case, sources in eastern North America, including moderate contributions from forest fires, were most important. The plume's maximum was mainly caused by anthropogenic emissions from the New York area. To our knowledge, this is the first case reported where a pollution plume from a megacity was reliably detected over another continent.

Rapid intercontinental air pollution transport associated with a meteorological bomb

“…Therefore, in order to avoid ambiguities, the original question posed at the beginning of this discussion can be changed to: What is the climatological relevance of air pollution transport in intercontinental express highways? Stohl et al (2002a) presented a 1-year climatology of the pathways and timescales of ICT, which has been extended recently by Eckhardt et al (2003) to a 15-year period, and which is used here to answer the above question. For the climatology, FLEXPART was used to calculate the global transport of six passive continental tracers.…”

“…An infinite tracer lifetime was used in the simulations, but tracer decay can be approximately determined for any lifetime using the discrete age class information available in the model output. For details of the transport climatology, the reader is referred to the papers of Stohl et al (2002a) and Eckhardt et al (2003).…”

“…One aim of this project was to make measurements in the outflow of polluted North Atlantic WCBs. Due to successful tracer model forecasts (Lawrence et al 2003;Stohl et al 2003), it was indeed possible to probe pollution plumes from North America on three occasions, allowing, for the first time, a detailed chemical characterisation of such plumes over Europe . After the campaign, tropospheric NO 2 columns retrieved from spectral data of the GOME satellite sensor (Burrows et al, 1999) were used as supplementary information on the transport of pollution plumes across the Atlantic.…”

“…According to FLEXPART, the leading tip of the NO x tracer filament had travelled from south of Greenland to Scandinavia, more than 50 • of longitude (or almost 3000 km at 60 • N) in only 20 hours, equivalent to average wind speeds above 40 m s −1 . Age spectra of the NO x tracer (see Stohl et al, 2003, for an explanation how age spectra were obtained from FLEXPART) suggest that most of the NO x in the leading part of the filament northeast of Great Britain was emitted in North America 2-3 days before, but a significant fraction was even younger than 2 days.…”

“…The episode of interest started on 8 November 2001, allowing a sufficiently long model spin-up of 11 days. The simulations were described in more detail by Stohl et al (2003). Note that, because FLEXPART does not explicitly simulate chemical processes, quantification of the NO x transported is difficult and must be constrained with the GOME measurements.…”