Experimental Evidence for Trans-Atlantic Transport of Air Pollution
Contributed by Andreas Stohl, Technical University of Munich, Germany, and Thomas Trickl, Fraunhofer-Institut für Atmosphärische Umweltforschung, Germany.

A Note from the Chair

NARE

Introduction

AEROCE

Transport in the MBL

Synoptic scale transport

Intercontinental transport

Model studies of O3 sources

Anthropogenic influences,
central N. Atlantic

PICO-NARE

Downloadable PDF version of IGACtivities, Issue No. 24.

Downloadable PDF version of NARE references

Introduction

With increasingly costly measures taken to reduce ambient concentrations of air pollutants at a regional level, pollution sources far upwind are receiving more and more attention. There is particular concern that rising emissions in regions with growing industrialization could offset the positive effects on regional air quality of emission reductions made in other parts of the world. It is suggested that Eurasian pollution transported by westerly winds across the Pacific Ocean basin can increase concentrations of CO, peroxyacetyl nitrate (PAN), O3 and aerosols in North America [Jaffe et al., 1999; Wilkening et al., 2000]. Model studies indicate the contribution of Asian emissions to pollutant concentrations over North America will be significant in the future [Berntsen et al., 1999; Jacob et al., 1999; Yienger et al., 2000].

Trans-Atlantic transport of pollution is of equal concern for Europe as trans-Pacific transport is for North America. In fact, because of the smaller dimension of the North Atlantic as compared to the North Pacific, intercontinental transport between North America and Europe may be even more important. This paper summarizes the experimental evidence for trans-Atlantic air pollution transport.

Evidence for trans-Atlantic transport of O3 and its precursors

The NARE campaigns have provided comprehensive evidence of the strong influence that pollution export from North America has on the air chemistry over the western part of the Atlantic Ocean, both within and above the boundary layer [Petersen et al., this issue].

Detection of air pollution from North American sources is more difficult as one proceeds eastwards over the Atlantic Ocean. Parrish et al. [1998] reported a clear influence on O3 and CO at 1km altitude over the Azores in the central Atlantic Ocean. Correlation of O3 with tracers of anthropogenic pollution (PAN, CO, VOCs, etc.) at Izaña on Tenerife led Schmitt [1994] to suggest a common anthropogenic source of these substances, but due to the complexity of the transport processes a clear attribution to North American emissions was impossible. During the OCTA campaign, polluted air with relatively high concentrations of O3, CO and NOy was found both at low levels and in the middle troposphere off the coast of Portugal [Wild et al., 1996]. While the lower-level pollution could be attributed to European sources, the higher-altitude pollution was likely of North American origin.

Detection of North American pollution at surface sites in Europe has been particularly elusive. At Mace Head on the west coast of Ireland an unequivocal attribution to North American sources is difficult even for inert chlorofluorocarbons (CFCs). Comparing Mace Head measurements with modeled transport of CFCs, Ryall et al. [1998] estimated "that, on average, North American sources may account for only a few per cent of the CFC-11 enhancements over the Northern Hemisphere baseline values. During long-range transport events, North American sources may contribute CFC peaks at Mace Head which are about one order of magnitude smaller in concentration than those from European sources." During such episodes, CO concentrations at Mace Head are also enhanced [Jennings et al., 1996]. However, few such episodes have been identified, and corresponding O3 enhancements are marginal. Derwent et al. [1998] therefore concluded "that either the North American O3 and CO plume does not intersect the European coastline over Mace Head or that this plume had become merged into the Northern Hemisphere background in transit." Similar difficulties in the definite detection of pollutant plumes from North America have been faced at other stations, for instance at Porspoder, on the French Atlantic coast, where a North American influence on PAN and VOC concentrations was suggested, especially in spring-time [Dutot et al., 1997; Fenneteaux et al., 1999].

Despite these difficulties in detecting an influence of North American emissions on European O3, model studies clearly indicate the possibility of O3 transport across the North Atlantic [Wild et al., 1996; Schultz et al., 1998], and suggest that reductions in North American NOX emissions would decrease the O3 concentrations throughout the North Atlantic basin [Atherton, 1996]. Schultz et al. [1998] have shown that O3 is destroyed en route. The O3 destruction rate depends on both the chemical and meteorological conditions of the transport but is, in any case, slow enough to allow intercontinental O3 transport. Wild et al. [1996] demonstrated the possibility of even net O3 production during transit. The studies of Schultz et al. [1998] and Wild et al. [1996] agree on the key factor that favors intercontinental O3 transport: Lifting of the polluted air to higher altitudes. The wind speeds are much faster there so transport to Europe takes less time, plus the photochemical environment and thermal changes favor release of photochemically active NOX from PAN and HNO3, thereby allowing O3 formation during transport. Ultimately, the ozone-forming capacity of an airmass is determined by the emission input of NOY and the amount of NOY removed by dry deposition and by wet deposition during the ascending phase of the transport. As wet removal of NOY is highly efficient upon export from the boundary layer [Stohl et al., 2001], it is likely that most of the O3 is formed in the North American boundary layer and is subsequently slowly destroyed en route, as suggested by Schultz et al. [1998].

Figure 1. Three-dimensional trajectories with strong ascent that visualize the WCB. The upper part of the figure shows a horizontal projection, the lower part shows a time-height profile of the trajectories. The black line marks the leg of the MOZAIC aircraft that flew through the WCB and measured about 100 ppb O3.

The model studies cited above suggest that the best chances of detecting O3 plumes from North America over Europe are not at the surface, but in the upper troposphere. Stohl and Trickl [1999] presented an example of such an episode. Polluted, ozone-rich (almost 100ppbv at the east coast) air left North America and was rapidly transported to the upper troposphere by a warm conveyor belt (WCB) [see Cooper et al., this issue, for an explanation of WCBs] over the North Atlantic (Figure1). Measurements aboard a commercial aircraft flying through the WCB showed O3 maxima of about 100ppbv. The polluted air then rode the jet stream to Europe, arriving after 2 days. Lidar measurements in the outflow of the WCB over Europe again showed O3 maxima up to 100ppbv in moist air, located above a stratospheric intrusion (Figure2).

Other examples of intercontinental O3 transport can be found in Stohl and Trickl [2001] and Kreipl et al. [2001]. A common feature of these episodes is that the polluted air from North America arrives in the upper troposphere over Europe in the outflow of a WCB associated with a low-pressure system tracking over the Atlantic. It overrides the dry intrusion airstream of a preceding cyclone. Thus the normal atmospheric stratification is inverted: Boundary layer air from North America is found in the European upper troposphere, while air of stratospheric origin is found in the lower troposphere. Eventually the polluted air descends, although we have no evidence yet that it reaches the surface.

Figure 2. Time-height section of ozone as captured by lidar measurements in Garmisch-Partenkirchen. The white areas correspond to thin cloud layers, for which no useful data have been obtained. The stratospheric intrusion is labeled with "S", the North American boundary layer air with "USA".

The reason that these episodes are quite frequent is that North American anthropogenic emissions occur at relatively low latitudes, from where WCBs draw their inflow [Stohl, 2001, and Cooper et al., this issue]. In contrast, emissions released at more northerly latitudes are less affected by WCB transport and tend to remain in the lower troposphere. Indeed, Forster et al. [2001] recently found an example where emissions from boreal forest fires in Canada were transported to Europe at lower altitudes. Aerosols caused a dense haze layer over Germany that weather observers erroneously described as cirrus clouds. O3 concentrations in the polluted airmass were 25ppbv higher than in the unpolluted airmasses above and below. Model simulations carrying passive CO tracers from both European and North American anthropogenic emissions and from the forest fires showed that the largest contribution to European CO above the baseline value came from the forest fires. At Mace Head (see Figure3) North American anthropogenic emissions had little influence on the measured CO, whereas Canadian forest fire emissions dominated the CO variations. This was not so much because the CO emissions of the fires were larger than the anthropogenic emissions, but because anthropogenic emissions from more southerly latitudes were transported to the upper troposphere while the forest fire emissions partly remained in the lower troposphere. Tropospheric NO2 columns derived from satellite data showed that NOX was also transported across the North Atlantic during this episode [Spichtinger et al., 2001].

Figure 3. Measured (light blue curves in each panel) and simulated CO concentrations in ppb at Mace Head during August 1998. (a) Model results for the American anthropogenic CO tracer (dotted curve) and the American plus European anthropogenic CO tracer (dark blue curve). (b) Modeled CO concentrations for the forest fire emissions (black curve). (c) Modeled CO concentrations from American and European emissions plus forest fires (dark blue curves). A background of 97 ppb was added to the simulated CO concentrations.

Conclusions

There is little evidence that North American anthropogenic emissions exert a strong influence on the concentrations of air pollutants at European surface sites. Some episodes have been identified, but still lack unequivocal attribution to North American sources. In contrast, in the European upper troposphere several cases were found with significantly enhanced O3 concentrations that could be attributed clearly to transport from North America. During August 1998, boreal forest fires in Canada had a strong influence on European air chemistry. The climatological relevance of intercontinental transport, however, still lacks quantification, and the timescale of mixing into the hemispheric background is also unknown.

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