Photochemical air quality modeling in São Paulo, Brazil

Modeling urban scale air pollution is essentially the problem of describing the formation and transport of chemically reactive species in the atmospheric boundary layer. The city of São Paulo, Brazil, is located near the ocean in a plateau with several river valleys and a complicated topography dominated by high hills. The synoptic situation and its interaction with local circulation influence airflow. The city is highly populated and has an important vehicular fleet, which is responsible for almost the whole emissions scenario, characterized by high urban carbon monoxide, hydrocarbons and nitrogen oxide emissions.

An eulerian three-dimensional model has been used to study the transport and formation of pollutants in the São Paulo area. The model incorporates photochemical reaction mechanisms, turbulent diffusion and dry deposition removal processes. Meteorological and air quality field measurements were available to perform model simulations over the period 16-19 February 1989. This four-day interval belongs to an experimental campaign, which took place in February 1989, during the Southern Hemisphere summer. The period has been chosen because of the favorable meteorological conditions for photochemical pollutants built-up. The numerical simulations performed were based on the official emission inventory.

An improved method for calculating vertical diffusivities was introduced. The vertical exchange is represented by a nonlocal profile that describes the turbulent state of the atmospheric boundary layer in terms of characteristic parameters. The alternative parameterization is well behaved and overcomes the disadvantage of the original one, which does not give a continuous transition between the different regimes in the atmospheric boundary layer. Of the species being modeled, particular attention was paid to carbon monoxide, nitrogen oxides and ozone, because of their suitability to study transport, turbulent diffusion and reaction mechanisms. Three-dimensional distribution and temporal behavior was studied and relative changes in concentrations due to turbulence parameterization were analyzed.

Model results, along with observations, indicate that the urban area is in a nitrogen oxide-inhibited region, that is, the local nitrogen oxide emissions tend to decrease local ozone. On the other hand, the density of the emissions in the area is high enough to lead to increased amounts of ozone downwind. The highest ozone levels may occur well outside the São Paulo downtown area. Photochemical species production and consumption is well described by model simulations. Emissions and wind fields are the most important in regards to model sensitivity. Diffusion field, in turn, is more influenced by the spatial and temporal variability of the surface wind speed. Less reactive and primary pollutants show higher impact of the turbulence parameterization than ozone, which is governed by chemical mechanisms.

*Currently Visiting Professor at the Department of Atmospheric Sciences, Institute of Astronomy and Geophysics, University of São Paulo, São Paulo, Brazil, with the finantial support of FAPESP (Fundaçao de Amparo à Pesquisa do Estado de São Paulo, (96/01403-4)).