**5. Role of the biofuel use on secondary pollutants formation**

Although the levels of primary air pollutants have decreased over the past 30 years in the MASP, secondary pollutants, like ozone and fine particles, frequently exceed established international and local air quality standards. Ozone and fine particles are secondary pollutants, products of VOCs, NO(x), and sunlight in the case of ozone and also sulfur oxides, ammonia, nitrogen oxides in the case of fine particles. Thus, it is very difficult to elaborate efficient strategies for their reduction. Since the frequent episodes of high concentrations of ozone occurring in the MASP are primarily associated with vehicular emissions, some reports in literature examined the impact that the implemented control program for mobile emissions (PROCONVE) had on tropospheric ozone concentrations and evaluate the impact of using reformulated gasoline-ethanol blend (gasohol) and of hydrous ethanol on the ozone formation [88-93]. Studies related to the formation of fine particles are more sparse, Albuquerque et al. [94] analyzed the impact of reducing the sulfur content in the diesel and the secondary formation of sulfate-nitrate-amonnium system, showing that an increase in the nitrateammonium formation may occur while the decrease in the sulfate-ammonium system occurs.

Sanchez-Ccoyllo et al. [92] evaluated the impact of the PROCONVE implementation based on an episode of high surface ozone concentrations that occurred in the MASP during March, 2000, employing the California Institute of Technology/Carnegie Mellon University threedimensional photochemical model (CIT model [95-97]). In this work, different scenarios of emissions were considered according to the implementation of the PROCONVE. Scenario 1 assumed that all vehicles in the fleet were operating within PROCONVE guidelines. Scenarios 2 and 3 considered hypothetical situations in which PROCONVE had not been implemented. Scenario 2 established the premise that all vehicles were using pre-1989 technology, whereas scenario 3 allowed the existence of technological advances. That work showed that tropo‐ spheric ozone concentrations predicted for scenario 2 (vehicles pre-1989 technology) were higher than those predicted for scenarios 1, 3, and for the base case. The authors demonstrated that the PROCONVE had great influence on the improvement of air quality concerning ozone in MASP. In another studied developed by Sanchez-Ccoyllo et al. [93], the analysis was concentrated in the meteorological impacts on the ozone formation. Three meteorological variables: mixing height, wind speed, and air temperature, were considered in the study. The study also evaluated the role of having a reactive hydrocarbon (RHC) limitation or NOx limitation configuration on ozone formation in the MASP. In addition, NOx and RHC emission inventory reductions were used to evaluate their sensitivities with the CIT model. The results showed that changes in mixing height, wind speed, and air temperature input files have the greatest effect on peak ozone production, and the isolated effect of RHC emission reduction lead to 26% lower ozone levels compared to the base case. Based on the results of this study, the authors concluded that reductions on RHC emission could provide the best scenario for promoting lower ozone concentrations in the MASP. In agreement to these results, Orlando et al. [91] showed that the occurrence of high ozone production is more dependent on VOC in a study employing the trajectory model OZIPR together with the SAPRC chemical mechanism. In that work, five base-cases were created to verify the variation in maximum ozone concen‐ tration related to the ozone formation potential of each VOC that was injected in the airbase. The NO(x) and VOC emissions were independently and simultaneously reduced by 5%, 10%, 20%, and 30% to induce variations in ozone formation and the results showed that the most frequent compounds found among the ten main ozone precursors in Sao Paulo, using the reactivity scales created from the five base-cases, were formaldehyde, acetaldehyde, propene, isoprene, cis-2-butene, and trans-2-butene, with formaldehyde being always the main ozone precursor. In addition, the simulations data showed that an efficient strategy to decrease ozone concentrations in the MASP would be to reduce the total VOC emissions, while the same strategy is not possible for NOx, since the reduction of these pollutants would increase ozone concentrations.

The impact of the use of reformulated gasoline/ethanol blend (gasohol) or hydrous ethanol on the ozone formation was evaluated in a study conducted by Martins and Andrade [89]. In this work, a three-dimensional photochemical model was employed to estimate the sensitivity of ozone formation and evaluate the implementation of emission scenarios, considering various fuel formulations in the MASP. Six scenarios were analyzed in the work; scenarios 1 to 5 involved different reduction scenarios in the compounds found in gasohol at different proportions related to the base case emission inventory, and the scenario 6 specified that the entire LDV running in the MASP would burn hydrous ethanol. The results showed that in scenario 3 (reductions in olefins, aromatics, and benzene) and scenario 5 (reductions in the five species that are associated with higher ozone sensitivity), ozone concentrations were below the national standard only at the air quality monitoring stations (not domain-wide). In addition, these results suggest that implementing scenario 6 (entire LDV fleet burn hydrous ethanol) would improve air quality in the MASP. In contrast, the work developed by Salvo and Geiger [98] suggest that the use of ethanol in flex-fuel vehicles has a negative effect on air quality as regards the formation of ozone. Their work reports the consequences of a shift in fuel use in the MASP, brought on by large-scale fluctuations in the price of ethanol relative to gasohol between 2009 and 2011. The work uses highly spatially and temporally resolved observations of road traffic levels, meteorology and pollutant concentrations, together with a consumer demand model, to show that ambient ozone concentrations felt by about 20% as the share of flex-fuel vehicles burning gasohol rose from 14% to 76%. On the other hand, in this work data suggest that NO and CO concentration increased. Thus, considering that tropo‐ spheric ozone production over MASP is hydrocarbon-limited, high NOx emissions (from gasohol burning) resulted in reductions in ambient ozone. These results are consistent with modeling studies previously, which concluded that the atmosphere in the MASP is VOClimited [89, 91-93].
