Meteorological simulations of boundary-layer structure during the 1996 Paso del Norte Ozone Study

Meteorological simulations centered around the border cities of El Paso and Ciudad Ju´arez have been performed during an ozone episode that occurred on August 13, 1996 during the 1996 Paso del Norte Ozone Study field campaign. Simulations were performed using the HOTMAC boundary-layer meteorological model using a 1, 2, 4 and 8-km horizontal grid size nested mesh system. Investigation of the vertical structure and evolution of the atmospheric boundary layer for the August 12 14 time period is emphasized in this paper due to its suspected importance in precipitating the ozone episode Sci Total Environ Ž2001. . This period was characterized by a slowly-evolving high pressure system over the region, a persistent upper-level jet at 2500 3500 m above ground level Žagl., deep daytime mixed layer heights of 3500 m depth and unusually deep nighttime stable layers extending up to 2500 m above the ground. The fact that the boundary-layer growth stalled on the morning of August 13 relative to that on August 12 has been suggested as a possible reason for the ozone episode on the 13th. In addition, relatively weak surface-level winds were measured on August 13. Using both model results and experimental data we hypothesize explanations for the slower mixed-layer growth on the morning of the 13th and the stronger surface-level winds found on the 12th and 14th.