Ozone transport; beneficial or harmful?

 

In general, the atmosphere acts as a dispersing agent for air pollutants. Dispersion can cause both beneficial pollutant reduction near a source and harmful pollutant increases downwind of a source. Dispersion takes place through horizontal transport and vertical mixing in the lower atmosphere. Horizontal transport depends on location as well as on elevation above ground, and to some extent on time of day. The horizontal transport for elevated emission sources (>100m) is substantially higher than the transport of low-level sources, due to higher wind speeds aloft, and this difference tends to be amplified at night.

 

Examination of dispersion conditions during high-ozone periods across the OTAG domain shows that dispersion in the Southeast is typically poor due to stagnating air masses, whereas the western and northern sections of the domain are typically well-ventilated by southeasterly and westerly winds, respectively. In contrast, low-ozone days exhibit strong flow everywhere within the domain, but coming from areas outside the domain (e.g., Canada and the Gulf of Mexico). One interpretation of these results is that periods of high ozone across OTAG results from a stagnation event somewhere in the domain (particularly central and south) followed by strong unidirectional flow to some other part of the domain (particularly in the upper Midwest and Northeast). While these patterns are not always seen for every ozone episode, they point out the necessary condition for regional ozone events; namely stagnation followed by transport. Such conditions are seen in every episode chosen for OTAG modeling.

 

To summarize, the "good news" about transport is that it can disperse, or clean up, the ozone formed in an area during a stagnation event. The south-central and southeastern portions of the OTAG domain, which experience relatively more stagnation, can benefit from this aspect of transport. The "bad news" about transport is that it can carry high concentrations of ozone from one portion of the domain to another, and this aspect of transport tends to cause more problems in the midwestern and northeastern portions of the domain.


 
Figure 1. Transport wind vector during low ozone days. In all regions low ozone concentrations occur on days with substantial transport vector from outside the region.

 
Figure 2. Transport wind vector during high ozone days. In the Southwest, Southeast, and the Midwest high ozone concentrations occur when the transport wind vectors are short and directionally variable. In the Northeast, however, high ozone is associated with strong unidirectional westerly winds.
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