VOCs CONCENTRATIONS IN THE AMBIENT AIR OF BOGOTA CITY: SOURCE IDENTIFICATION AND APPORTIONMENT

Bogota city, the capital of Colombia, and one of the largest cities in Latin America, is also considered one of the more polluted cities in the region (figure 1).  Despite the efforts conducted during the last few years, a lot is yet to be done to better understand the city´s air pollution. While most of the research has been focused on the study of particles and ozone, very few studies have been conducted to understand the spatial and temporal distribution of Volatile Organic Compounds (VOC).  These species are a group of toxic pollutants (some of them carcinogenic) that are also precursors of tropospheric ozone.

The implementation of effective strategies to control the air pollution in a city depends mainly on the correct identification of the sources of the pollution. In Bogota city, the VOC concentrations, their trends, and their spatial and temporal distributions are unknown. Moreover, the sources of the VOC are not identified. The lack of this information comprises a significant limitation for developing an effective air quality management program for the city. In this project, we developed an on-line monitoring of 20 C2-C6 VOC from November 2008 to June 2009.  VOC were characterized in the ambient air of the city at three different monitoring sites within the urban perimeter of Bogota (figure 2). VOC measurements were conducted using a Synstech Spectras portable gas chromatograph (GC) with a temporal resolution of 30 minutes (figure 3).

Table 1 shows the average VOC concentrations registered at each of the sampling sites. Such concentrations were higher in sites 2 and 3, characterized by intensive industrial and commercial activities. Analyses of the diurnal behavior of the total VOC showed that concentrations tend to be higher in the morning (at around 8:00 a.m.). Another VOCconcentration peak was present in the evening, at around 10:00 p.m. Such trends are consistent with the road traffic activity in the surroundings of the measuring sites.

Figure 4 shows the distribution of the VOC concentrations documented in this study, classified by their chemical structure. According to these results, alkanes are the most abundant hydrocarbons in the ambient air in Bogota. Species such as pentane and butane isomers (n-pentane, i-pentane, n-butane, i-butane) are classified in this category. This condition is observed at the three sampling sites where these compounds represent about 80% of total VOC. Alkenes (i.e. ethene, propene, pentene) represent only 15% of total VOC. We computed Ozone Formation Potential (OFP) of the measured VOC using the Maximum Incremental Reactivity (MIR) concept developed by Carter (1994). Since VOC react in the atmosphere to produce O3, we calculate the OFP to evaluate the role of each VOC in the O3 formation. Table 1 shows the calculated OFP. These results show that ethene, propene, n-butane, i-pentane and isoprene are the species with the highest OFP (note that high concentrations are not necessarily linked with elevated O3 production).

Finally, we used two receptor models (Principal Components Analysis – PCA, and Positive Matrix Factorization - PMF) to identify the sources of the different VOC. Our Results showed that most of the compounds included in this study are emitted by road traffic. The receptor modeling also shows that the top-five compounds in terms of ozone production come mainly from the exhaust of gasoline vehicles.  This demonstrates the importance of adopting strategies to reduce the emissions of these pollutants in the city in order to improve the overall air quality in the city.

 

 

Figure 1. Bogota city during a pollution episode; these days are common in summer when VOCs react with other substances to produce pollution.

 

                         
Figure 2. Location of the measuring sites in Bogota city

 

 

             
Figure 3. Overview of the on-line Syntech Spectras Gas Chromatograph and other devices used for the measurements

 

            
Figure 4. VOCs classified according to their chemical structure

 

 

                       
Table 1. VOCs concentrations and ozone forming potential

 

Participants

Ecole Polytechnique Fédérale de Lausanne – EPFL
Luis Carlos Belalcazar, PhD candidate: luis.belalcazar@epfl.ch
Dr. Alain Clappier, Thesis director: alain.clappier@epfl.ch

Universidad de los Andes
Jose Alonso Pacheco, M.Sc. student: jos-pach@uniandes.edu.co
Juan Felipe Franco, Researcher: jufranco@uniandes.edu.co
Dr. Eduardo Behrentz, project director: ebehrent@uniandes.edu.co