The Impact of Secondary Flow Systems on Air Pollution in the Area of São Paulo

Abstract

The area between the Atlantic Ocean and S?o Paulo is highly polluted due to high emission rates at Cubat?o, a city situated 15 km inland at a steep slope. It was expected that secondary circulations would develop caused by the land?sea contrast and strong orographic changes, which influence the transport and diffusion of air pollutants. In 1994?95, surface stations were operated and radiosonde ascents were performed to analyze the characteristic features of the land?sea-breeze circulation. The stations make evident a land?sea-breeze system that arrived in the suburbs of S?o Paulo in the early afternoon. The upslope winds favor the propagation of the sea breeze at the steep slope. During the measurement period, large-scale northwesterly winds prevailed that advected warm air from the plateau to the coastal area in the afternoon and resulted in a limitation of the boundary layer growth. The data were used to initialize a three-dimensional mesoscale model for calculation of the transport and deposition of SO2 emitted at Cubat?o. The boundary layer height was found to be a limitation for vertical mixing of the air pollutants. However, a step between the coastal boundary layer and the boundary layer over the plateau causes SO2 to be vented into the free atmosphere at the slope and then transported toward the Atlantic Ocean with the large-scale northwesterly winds. Thus, over the coastal area, the SO2 concentrations in the free atmosphere were even higher than within the mixed layer. The deposition, summed up over a day, was calculated and found to be strongest at the slope and over the Atlantic Ocean.