The smoke that affected the city of Buenos Aires and its suburbs (approximate population, 13M) in mid April 2008 was an extreme event without historical precedent. The episode resulted in an increase of health problems among the population (respiratory problems, eye irritation) and, due to poor visibility, led to hazardous driving conditions and accidents that forced the intermittent closure of major highways. The origin of the smoke was in biomass burning practices, which, unfortunately, may become more common in the future.

High resolution model simulations with the WRF-ARW model at 4-km grid spacing indicate that in addition to the persistent northwesterly winds, the event was favored by local circulations. The city of Buenos Aires lies by the La Plata River, which has a width ranging from 38 km upstream to 230 km at its mouth. The large expanse of the river and its cooler surface in April favored a river-to-land circulation at low levels during daytime. Over land, the PBL height ranged from 200-700 m at night and was lifted during the day. But in the neighboring La Plata River, the PBL remained low throughout day and night preventing the smoke from mixing vertically and trapping the smoke at low levels. The regional inland circulation, combined with the low PBL height over the river, channeled and maintained the smoke along the south western coast of the river, and particularly over Buenos Aires.

Five-day forecasts initialized at different times during the episode show that the model was able to predict not only the persistent event but also its initiation. Thus, it is suggested that a high resolution regional model could be used to routinely monitor and predict several days in advance the atmospheric transport of smoke. These results could have policy implications, as preventive measures on biomass burning could be put in effect when smoke from the fires is predicted to affect a densely populated area.