Air Pollution as a Climate Forcing: A Workshop
Day 1 Presentations
Ozone Transport to and Formation in the Remote Troposphere
NOAA/Aeronomy Lab, Boulder, CO, U.S.A.
Ozone plays a central role in the physical and chemical processes that control the radiation balance of the earth. In addition, ozone is a harmful pollutant and long-range transport of ozone elicits significant concern about regional air quality. For these reasons, understanding the continental sources of ozone and the chemical and physical processes responsible for ozone formation and transport from the continents to the remote troposphere is critical for managing regional air quality and global climate change. The aim of this report is to indicate the present understanding of the export of ozone from the continents to the remote troposphere, the processes responsible for that export, and the implications of this for predicting the ozone distribution with climate variability and change.
Export Estimates. Measurements have documented the importance of ozone transported from the continent to the remote marine troposphere. In summer, photochemically produced O3 attributable to North American emissions dominates the O3 budget in the lower troposphere over the western North Atlantic (Parrish et al., 1993). Flow of ozone from North America into the North Atlantic is illustrated in Figure 1. More refined calculations estimate that the flux of ozone from North America to the North Atlantic in summer is 1.0 to 1.6 Gmol/day (Chin et al., 1994). Likewise, significant export of ozone from Asia to the remote Pacific has also been observed (Akimoto et al., 1996). It is clear that the direct export of ozone photochemically produced on the continents significantly influences the ozone distribution on intercontinental scales during summer. Analysis of recent measurements suggests that O3 export is much less important in other seasons (Parrish et al., 1999).
Mechanisms for Long-range Transport. The mechanism responsible for long-range transport is not continuous but is associated with rapid transport events occurring along frontal zones. It has been proposed that airstreams formed by mid-latitude cyclones and the associated frontal systems govern the transport of trace gases. A conceptual model has been developed for transport from North America to the western North Atlantic (Cooper et al., 2001). A diagram illustrating the conceptual model for these airstreams is shown in Figure 2. Airstreams in other regions are comparable.
Contribution of Continental Nitrogen Oxides. Aside from direct ozone export it is also necessary to assess the importance of the formation of ozone from ozone precursors exported from the continents. For ozone production in the global atmosphere, export of reactive nitrogen oxides from the continental boundary layer is critical, since photochemical ozone formation during transport is NOx (NO + NO2) limited (Fehsenfeld and Liu, 1993). Consequently, transport of NOx into the free troposphere over the continents contributes significantly to ozone formation in more remote regions. Figure 3 illustrates the chemical evolution of the oxides of nitrogen in the boundary layer and their eventual export into the free troposphere. CO has been used as a conservative tracer to estimate how much reactive nitrogen oxide is exported. Preliminary results indicate that only a small fraction (< 25%) of emitted nitrogen oxides is exported. The NOy exported to the free troposphere is predominately nitric acid, with lesser amounts (<= 10%) of NOx and PAN that would be available for ozone formation (Parrish, private communication). Most of this NOx is transported by turnover in the boundary layer due to frontal passage or is associated with deep convection.
Emissions from Forests. It is important to identify the main sources of ozone precursors and the factors that mediate these emissions. In the discussion of ozone and its connection to climate change, the emission of ozone precursors that are associated with natural (biogenic sources) are of particular interest since they are very sensitive to meteorological changes. In this regard, it is well known that natural (biogenic) emissions, VOCs from forests and NOx from soils, play an important role in ozone formation over continental areas (Fehsenfeld et al., 1992; Williams et al., 1992). Much headway has been made in identifying the processes responsible for those emissions and providing estimates of the emission from those sources.
In a like fashion, forest fires, initiated by lightning or human-caused, are important contributors to emissions of carbon monoxide, nitrogen oxides and aerosols (Trainer and Wotawa, 2001). These fires burn millions of acres each year. In 1995, it is estimated that the CO emitted by fires was equal to 37% of the annual anthropogenic emissions of CO in the United States. The enhancement in CO from forest fires was also observed in NOx-enhanced photochemical ozone production (McKeen et al., 2001). Although the emission of NOx from these fires is much less than that of CO, the buoyant lifting produced by the fires provides a very efficient mechanism for transporting NOx into the free troposphere. Regarding intercontinental transport, NOx emissions from Canadian fires have been observed over Ireland (Spichtinger, et al, 2001). Finally, although random, the frequency and intensity of these fires is associated with climate variability.
Implications. The export of ozone from the continents can be a significant factor in determining the summertime global ozone distribution. In addition, the export of nitrogen oxides can make significant contributions to the formation of ozone in the remote troposphere. The escape of these compounds from the planetary boundary layer over the continents is governed by deep convection in and frontal passage through the source regions. Subsequent long-range transport of these compounds is governed by airstreams associated mid-latitude cyclones. The frequency and intensity of these events strongly depends on how the physical processes that control them respond to global climate variability and change. In addition, the emissions from natural biogenic sources as well as those associated with forest fires, whether natural or human-caused, is directly influenced by meteorology. Drought stressed forests and soils have much lower emissions, while drought greatly increases the frequency and severity of forest fires. Hence, these emissions will be very sensitive to forcing by global change. The variability in the random, less probable and/or extreme meteorology that influence the emissions of ozone precursors and are responsible for the delivery and disseminations of ozone and ozone precursors will be very difficult to predict. Hence, global climate change can greatly influence the tropospheric ozone distribution, and changes in that distribution in turn will feed back into the climate system.
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