Assessment of Organic Compounds as Vehicular Emission Tracers in the Aburrá Valley Region of Colombia

The Aburrá Valley region in Colombia, with Medellín as its main city, is an urban centre with about three million people. An investigation was carried out to determine a set of baseline concentrations for VOC compounds associated with diesel fuel and gasoline, as vehicular emission tracers in the region. The VOC measurement campaigns, based on TENAX tube sampling and analysis according to TO-17 EPA method, were done in areas of low and high vehicular flow as well as on-board measurements covering major Medellín road networks during 24 hours. The results showed that there was a relation between VOCs concentrations and vehicular activity. The diesel fuel sulfur content was also found as an important factor on VOC hydrocarbon formation.


Introduction
VOCs tend to be polluting considering both their inhalation and contact effects and as a source of secondary pollutants.For the present study, they were classified into two (2) groups: poly-nuclear aromatic hydrocarbons (PAHs) and aliphatic hydrocarbons (AH).
Table 1 shows the list of VOCs studied and some basic characteristics.
The studied VOCs behave differently, following the two main types (alkanes [AH], and PAH) and this has to do with their molecular weight, as shown in the behavior of their vapor pressure and boiling point, properties that have to do with their presence in the atmosphere (Figure 1 and Figure 2).

Molecular weight Alkanes PAH
A review [1] [2] of the health impact and occupational limits of the studied VOCs is shown in Table 2.
In general, VOCs play an important role in environmental problems by their accumulation and persistence in the environment [3].Some VOCs, especially those of high molecular weight, resist oxidation processes and become persistent, being adsorbed on particles and transported over long distances [4], powering the global greenhouse effect.
So far no studies of these compounds have been done locally, so it is deemed important to carry out an exploratory work, in parallel with the fact that sulphur content of diesel fuel is undergoing changes at the time, from 2000 to 50 ppm and it is desired to correlate those changes with the said VOCs concentrations.
Alkanes tend to be emitted by vehicles, as they are components of fuels.Table 3 shows typical contents of studied VOCs in low sulfur diesel and gasoline [5].The other studied PAHs come from oil and coal tars and incomplete combustion, including wood combustion; Phenanthrene is also associated with cigarette and marihuana smoke and charcoal broil.

Materials and Methods
The apparatus set up for the sampling is described in Figure 3.
The sampling method applied was EPA TO-17 using 90 mm length, 5 mm diameter stainless steel TENAX adsorption tubes filled with appropriate sorbent materials, prepared and supplied by the DRI (Desert Research Institute at Reno, Nevada, USA).The chemical analysis of the studied VOCs was also done at the DRI, using the Agilent Thermal Desorption-Gas Chromatograph/Mass Spectrometer (TD-GC/MS) system.
The environmental samples were taken in a measurement campaign conducted in three sites, two of them with heavy traffic, the other one with low or inexistent traffic, from July to August 2011, with sampling periods of 24 hours.Each zone was evaluated during a week.
Additional samples were taken in the discharge of a diesel motor working under standardized laboratory conditions with diesel fuel of variable sulfur content.Run cycles followed standard ECE-M2 at 2420 rpm. Figure 4 shows a scheme for the run cycle.

Urban and Rural Sites
The VOC measurement campaign was conducted in three sites, two of them with heavy traffic (Poblado zone and Botanical Garden Park), the other one with low or inexistent traffic (Arví Park), from July to August 2010, with sampling periods of 24 hours.Each zone was evaluated during a week.Another sample was taken sampling during 24 hours continuously within a vehicle moving through designed zones in the city (On Board test).In the Figure 5, the sampling sites are located in the Aburrá Valley map.Table 4 shows the concentrations in the urban and rural zones, and in the on-board 24 hour samples.It is clear that rural areas have lower VOCs concentrations than urban ones.thalene.The on-board sample shows higher VOC's concentrations than the other sampling sites.Probably because the on board test environment was in continuous contact with mobile sources.Figure 7 shows comparative result for all the zones in terms of total VOCs.It is clear in Figure 7 that the concentrations of VOCs in urban areas are clearly greater than the ones in rural areas.Urban total VOCs were about 25 times greater than the rural area.All concentrations, as expected, are clearly much lower than the reported occupational limits for the substances shown in Table 2.

A Review of VOC Concentrations in Urban and Rural Areas around the World
Following results reported elsewhere [2] [6]- [9], Table 5 was prepared to compare values reported around the world with the ones found in the present study, in order to have a comparative measure of the magnitude of the problem in the Medellín region.
This analysis shows, in general, local values lower than the typical ones reported around the world.The typical values shown were chosen by the authors after elimination, at their own criteria, of extreme high values in the reported data.

Diesel Engine Exhaust Concentrations
At the time of the study, S content of diesel fuel used in the region was undergoing changes, from 2000 ppm to 50 ppm and it was desired to correlate those changes with VOCs concentrations in the exhaust gases coming from a diesel motor working under standardized laboratory conditions.
Table 6 shows the results, which show a clear effect of the sulfur content of the fuel on the emissions of the studied VOCs.

Conclusions
 The region atmosphere shows presence of VOCs.
 The concentrations in urban areas are clearly greater than the ones in rural area.In

Figure 1 .
Figure 1.VOCs studied and their vapour pressures as related to molecular weight.

Figure 2 .
Figure 2. VOCs studied and their boiling points as related to molecular weight.

Figure 6 Figure 3 .
Figure 6 compares the VOC concentrations found for the zones.It was found that the higher concentrations of the studied tracers correspond to pentadecane and naph-

Figure 6 .
Figure 6.Comparative results for VOC concentrations in all studied zones.

Figure 7 .
Figure 7.Total VOC concentrations for the studied zones.
the average urban total VOCs are 27 times larger. The values found in rural and urban areas tend to be smaller than the typical values reported, around the world, in the literature.Total VOCs show values around 15% of the reported typical values for urban areas and about 5% for rural areas. The VOC concentrations are related to vehicle emissions, especially to diesel fuel vehicle emissions. An initial baseline has been established which should be useful for future work and public policy in relationship to vehicle related pollution control. Reducing S content on diesel fuel has been a beneficial step in this direction.

Table 2 .
Occupational data for some of the VOCs studied.studied

Table 3 .
Typical contents of studied VOC in low sulfur diesel and gasoline

Table 4 .
Concentrations in urban and rural zones, and in the on-board 24-hour samples studied.

Table 5 .
VOC concentrations (range and typical) around the world and in this study.

Table 6 .
VOC concentrations in diesel fuel motor exhaust and sulfur content in diesel fuel.