Ozone modeling in an ethanol-, gasoline- and diesel- fuel environment: the Metropolitan area of São Paulo, Brazil

The MASP (Metropolitan Area of São Paulo) is the largest industrialized region in Latin America. It has an area of 8.051 km2 with a population of 16,3 million inhabitants. Currently there are approximately 5 million automotive vehicles: 300000 heavy-duty diesels, and 4.7 million light-duty vehicles. Approximately 3.7 million of the light duty cars are fueled with a mixture containing 78-80% (v/v) gasoline and 20-22% ethanol (G80E20), that is referred to as gasohol and 1.0 million fueled with hydrated ethanol (E95). Over the past several years, in the MASP, ambient ozone concentrations have reached over five times the concentration considered protective of public health by the World Health Organization, with routine occurrence of levels that exceed Brazil's 1 hour National Ambient Air Quality Standard (160 mg/m3). Approximately 90% of the ozone precursors are emitted to the atmosphere by the vehicular fleet. According to the official state inventory, 21% of hydrocarbons are emitted by diesel vehicles, 19% by gasohol and 9% by ethanol; 81% of NOx is emitted by diesels, 10% by gasohol, and 5% by ethanol (popularly called. in Brazil as "alcohol").

An Urban Eulerian photochemical Model developed at the California Institute of Technology and Carnegie-Mellon University, denominated CIT, was applied to simulate the concentrations of ozone and to evaluate the impact of using ethanol as a fuel and a gasoline additive in the MASP. The CIT grid based Eulerian airshed model describes the formation and transport of chemically reacting species in the turbulent planetary boundary layer, including the formation of ozone and PAN. The three basic modules of the model include: the dispersion in the planetary boundary layer, the chemistry, and the emission inventories, with spatial and temporal distribution. We have simulated hourly ozone concentrations for the February 16-19, 1989 period. We decided to use data from 1989 because we have detailed meteorological information obtained in an experiment involving sampling with captive and pilot balloons and radiosondings. The role of initial and boundary conditions in the estimated concentration of ozone was evaluated. Simulations were performed considering four different scenarios: 1) a light duty fleet (LD) fueled with ethanol (E95) and the heavy duty with diesel; 2) a LD fleet fueled with gasoline (G100) and the heavy duty with diesel, 3) the actual 1989 fleet (E95, E20G80, and diesels), and 4) only the diesel fleet. For each of the LD cases, the 1989 diesel fleet contribution was also included. Under the assumed scenarios diesel emissions appeared to play a very important role in the estimated ozone profiles, and the light fleet fueled with ethanol had minor impact on the formation of atmospheric ozone.

*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)).