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Study of Volatile Organic Compounds Precursors Ozone in the Metropolitan Area of Sao Paulo
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The metropolitan area of São Paulo (MASP), with about 20 million inhabitants, is the third largest conurbation of the world. In the MASP in the air quality problems occur mainly due to pollutants from vehicles, reason by which it is emphasized the importance of vehicular emissions controll.
Ozone (O3) is a pollutant that represents a great concern in terms of air quality in the MASP. In 2012 it was observed 98 days of exceedances of the standard value (160 μg m-3 ) for hourly averages for this pollutant in the MASP. To decrease the concentrations of ozone in the atmosphere of the MASP it is necessary to reduce and control ozone precursors, which are nitrogen oxides (NOx) and volatile organic compounds (VOCs) both producers of this pollutant by photochemical processes.
It was employed the OZIPR (Ozone isopleth Package for Research, Gery and Crouse 1990) coupled to SAPRC (Statewide Air Pollution Research Center) model trajectories, to develop an incremental O3 reactivity scale (IR), in terms of VOC, specific to MASP in order to provide data to assist the development of strategies for pollutant reduction.
For sampling of hydrocarbons (HCs) were used canisters, stainless steel cylinders electropolished internally. The HCs were analyzed by the technique of gas chromatography (GC) with mass spectrometry detector (MS) and flame ionization (FID). For sampling of aldehydes were used cartridges coated of C18 silica through the technique of high performance liquid chromatography with ultraviolet detector. The ethanol samples were collected from Florisil cartridges and analyzed using GC-FID.
The samples were taken at the Company of Environmental Sanitation Technology of the State of the São Paulo (CETESB) IPEN/USP station in University City, 800 m altitude, located in the west of the city of São Paulo during the period September 2011 until August 2012, when 66 hydrocarbons samples were collected, 62 of aldehydes and 42 of ethanol, on weekdays, hours from 07:00 am to 09:00 am, integrating two hours, by the fact that, in this time, the traffic is heavy and the solar radiation is low.
To determine how much, each type of VOC, contributed to the formation of O3 were used the product of VOC concentration in μm-3 and the IR value, that stems from the OZIPR. The top ten most abundant VOCs in the atmosphere found in the study for 2011/2012, the CETESB IPEN/USP station were: ethanol (36.3 ppbv), acetaldehyde (28.7 ppbv), formaldehyde (21.7 ppbv) , acetone (10.9 ppbv), propane (5.56 ppbv), ethene (4.84 ppbv), ethane (2.98 ppbv), butane (2.94 ppbv), 1-ethyl-4-methylbenzene (2.85 ppbv) and 1,2,4-trimethylbenzene (2.43 ppbv). In the year 2011/2012 the aldehydes represented 35.3% of VOCs analyzed, followed by ethanol with 22.6%, aromatics 15.7%, alkanes 13.5%, acetone 6.8%, alkenes 6.0% and alkadienes less than 0.1%.
The Figure 1 shows the mean of the positive incremental reactivity for 4 seasons, obtaining an annual average to 19 VOC ozone precursors in this study.
Considering the concentration of VOCs analyzed and incremental reactivity, showed in the Figure 1, determined by the model OZIPR in the atmosphere of MASP for the year 2011/2012, the aldehydes were responsible for 74% of the ozone formation in the atmosphere (61.2% of this number corresponds to acetaldehyde), followed by aromatics 14.5%, that were alkenes 10.2%, alkanes 1.3% and alkadienes (isoprene) 0.03%. Despite alkanes represent 13.5% by mass in the atmosphere, it was responsible for the formation of only 1.3% of ozone due to their low reactivity. This highlight the importance to work with the local scene to determine the incremental reactivity. In the incomplete combustion of ethanol, various gases are emitted, such as his own, which has low reactive to ozone formation when compared with aldehydes which also are products of combustion of the ethanol.
The Figure 2 shows that the decrease in the concentration of VOCs in the MASP will result in the decrease of the ozone concentration.
The ratio VOC/NOx found during this study for the spring season, summer, autumn and winter were 4, 3, 3 and 2, respectively. In places with relative abundance of NOx in relation to VOC, the ratio VOC/NOx is low and the ozone formation is dependent of the concentration of VOCs, how result this study in the MASP. In the literature ratio 8 to 15, characterized intermediate locations where both the control of VOCs as NOx are effective in decreasing of the O3 ratio above of 15 are considered NOx limiting (Finlayson-Pitts and Pitts 2000). The reduction of the established limit for vehicular aldehydes emissions may contribute to the diminish of O3 concentrations. In Brazil, the current limits for gasohol and ethanol emissions are 20 mg km-1 and there is not established limit for diesel.
These results are important know what measures would be effective in controlling the formation of ozone in the atmosphere MASP.
Reducing all aldehydes quantified in this study to a negligible concentration in the atmosphere occurs a reduction in ozone concentrations around 26% in spring, 30% in summer, 28% in winter and 25% in fall.