More than 19 million people live in the Metropolitan Area of São Paulo (MASP) nowadays and its vehicular fleet, which is the main emission source of atmospheric pollutants in the region, exceeds the number of 7 million vehicles. The main air quality problems registered in the MASP are caused by ozone concentrations. Hence, the use of photochemical models presents a good form to evaluate the air quality in the area. Over the past few years, different approaches for the representation of the secondary atmospheric pollutants in numerical models have been developed. A current classification of the air quality models defines the inline and the offline models. The first group of models solves simultaneously the meteorology and the atmospheric chemistry. The models referred to the second group make use of meteorological fields previously solved by atmospheric models or observed data in order to calculate the transport of chemical species.

In this work, the models CIT (offline), WRF-Chem (inline) and SPM-BRAMS (inline) had been used to represent these both categories of air quality models. The results were compared to ozone, carbon monoxide and nitrogen oxides concentrations obtained from the air quality monitoring data collected in the Metropolitan Area of São Paulo (MASP). The models simulated the period between October 30 and November 01 of 2006 when the second ozone monitoring campaign occurred. For all models, we considered the same horizontal resolution (5 km) and the same grid (60x30). For the SPM-BRAMS a nesting grid was used. The lower resolution grid had 20 Km and the higher had 5 km, the same used by CIT and WRF-Chem.

Despite the simplified chemistry that counts only with 15 chemical reactions, the SPM-BRAMS shows good results when simulating the ozone concentrations, except when those had exceed the Brazilian National Air Quality Standard. In general, CIT and WRF-Chem registered values above of the observed ones and show a better performance to simulate higher ozone levels, detecting in some case the standard violations. The CIT model did not represent well the higher concentration decrease during the night and in the last campaign day. The three models had not registered good results simulating CO concentrations. WRF-Chem results overestimate the concentrations values while the models CIT and SPM-BRAMS had results at the same magnitude order. The great sensitivity of this pollutant to local emissions changes can be one of the factors for those results. The NOx concentrations were better simulated by CIT. However, the results did not represent correctly the diurnal profile of the pollutant. WRF-Chem and SPM-BRAMS overestimate NOX peak concentrations. In SPM-BRAMS, problems related to the chemical simplification of volatile organic compounds and the bad spatial distribution of the emissions in the region could be responsible for these results. The inadequate representation of the surface and the use of interpolated hourly average meteorological data, by CIT, could impact the numerical representation of several meteorological processes that affect the air quality in the MASP.