REGIONS CONTRIBUTING TO HAZE

Both light extinction associated with particulate sulfate and particulate sulfate attributed to individual source regions varied considerably throughout the BRAVO study. This information is combined and graphically displayed to illustrate the variations in absolute and percent-fractional contributions to light extinction at Big Bend by sulfur-source regions. The top plot shows the fractional contributions to absolute haze by various sources of particulate sulfate, as well as the total light-extinction level (black line) and Rayleigh light-scattering. The bottom plot shows the fractional contribution to light extinction by the various particulate-sulfate sources and by Rayleigh light-scattering (top most on the plot), which is relatively more important on the clearest days. The contributions to light extinction by particle-free air (i.e., Rayleigh scattering) are shown explicitly, since they represent a natural limit that cannot be improved. From the analysis that produced the graphs, we also know that—

• Contributions to light extinction from particulate sulfate were more than twice as high on the haziest days as compared to the least hazy days (47% compared to 21%).

• Sources of sulfur dioxide from Mexico generally contributed 5–15 Mm -1 to light extinction on most days. However, during some minor haze episodes in July and August, these sources contributed 30–40% to the average light extinction.

• Contributions to light extinction from the Carbón power plants increased from 4% on the least hazy days to 8% on the haziest days. The relative contributions to light extinction from other sources of sulfur dioxide from Mexico were about the same for the haziest days as compared with the least hazy days.

• Sources of sulfur from Texas contributed less than 5 Mm -1 on most days. During one of the few periods of higher contribution, these sources contributed nearly 30 Mm -1, corresponding to about 40% of the light extinction on the haziest day in October.

• Sources of sulfur dioxide in the eastern United States contributed less than 5 Mm -1 on most days; however, during the two haziest episodes in the study, these sources contributed approximately 50 and 30 Mm -1, respectively, corresponding to approximately 50 and 30% of the light extinction.

• Sources of sulfur dioxide in the western United States had a similar low contribution to light extinction on days of least and most haziness.

• Relative contributions of sulfur dioxide to light extinction from Texas and the eastern United States increased from 2–10% and from 3–19%, respectively, on the haziest days as compared to the least hazy days.