DRAFT 6-29-05

13 Backward trajectories estimate the route that an air parcel took to get to the location in which high ozone

was observed. These trajectories use gridded meteorological data to determine the parcel's trajectory. Because of

the difficulty in representing atmospheric variables, which are continuous in space and time, by discrete gridded data

points, these trajectories should be viewed as uncertain. This uncertainty is difficult to quantify, but 24-72 hour

backward trajectories have potential errors in the hundreds of kilometers. Further, just because an trajectory passed

over a populated area, does not necessarily mean that emissions from that area were loaded into the eventual highozone

parcel. The height of the trajectory along the path as it relates to the mixed layer can help indicate whether

upwind emissions loading takes place.

14 http://www.tceq.state.tx.us/policy/ta/am/TexAQS_II.html

15 http://www.ladco.org/index.html

16 Blanchard, C.L.; Lurmann, F.W.; Roth, P.M.; Jeffries, H.E.; Korc, M. 1999. The use of ambient

data to corroborate analyses of ozone control strategies. Atmos. Environ. 33, 369-381.


Trainer M., Parrish D.D., Buhr M.P., Norton R.B., Fehsenfeld F.C., Anlauf K.G., Bottenheim J.W.,

Tang Y.Z., Wiebe H.A., Roberts J.M., Tanner R.L., Newman L., Bowersox V.C., Meagher J.F., Olszyna K.J.,

Rodgers M.O., Wang T., Berresheim H., Demerjian K.L., and Roychowdhury U.K. (1993) Correlation of ozone with


problem from other regions solely through the use of trajectories.13 However, if other analyses

confirm the importance of upwind emissions, trajectories can be useful in determining which

upwind areas, in particular, are contributing to the ozone problem in the OTA.

2.1.2 Ozone and Ozone Precursor Data: At the simplest level, depending on data

availability, it may be possible in some cases to observe exceedance-levels of 8-hour ozone

being transported into the region of interest. Animations of observed ozone over specific regions

(e.g., as shown on the EPA AIRNow site) may indicate plumes of ozone being generated in

urban areas, then advecting in steady-state over rural areas. It may also be possible to use

animations from fully-evaluated photochemical grid models to detect transport into a particular

region. A more data-intensive approach would be to demonstrate transport by compiling recent

aircraft observations along an upwind boundary. Ozone transport predominantly takes place not

at the surface, but within the entire planetary boundary layer (PBL) during the daytime, and in

the residual layer that exists at night when the surface winds become decoupled from aloft winds

due to the formation of a nocturnal temperature inversion. If aircraft data are available along the

upwind boundaries of an area thought to be affected by significant transport, and those data show

that incoming ozone is equivalent to, or higher than, ozone observed at the surface on days with

exceedances of the NAAQS, that could be construed as evidence of transport. Additionally,

field study analyses such as those completed by the Texas Council of Environmental Quality

(TCEQ) in Houston14 and by the Lake Michigan Air Directors Consortium (LADCO) in the Lake

Michigan region15 can provide the necessary data to quantify the influences of transport on an

area. Finally, it may also be possible through detailed analyses of ozone precursor data to make



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