Spatial and temporal trends of volatile organic compounds (VOC) in a rural area of northern Spain

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Abstract

Ambient concentrations of volatile organic compounds (VOCs) were measured at 40 rural sampling points in Navarre (northern Spain). Air samples were collected by means of sorbent passive sampling and analyzed by thermal desorption (TD) and gas chromatography/mass-selective detector (GC/MSD). A total of 140 VOCs were identified during the study, which was carried out between May to October 2004 for a total of a 10 biweekly sampling campaigns. Concentrations of benzene, toluene, ethylbenzene, m/p-xylenes, o-xylene (BTEX) and 1,3,5-trimethylbenzene were determined in order to investigate their temporal and spatial distributions. Geostatistical analysis pointed to traffic as the main emission source of these compounds. Supporting this idea, BTEX and nitrogen oxides concentrations were found to be highly significantly correlated (r = 0.495, P = 0.001), whereas a strong negative correlation between BTEX and ozone was also observed (r =  0.355, P = 0.025). The concentrations for the BTEX group were similar to the values that have been previously reported for other rural areas.

Introduction

The role and importance of VOCs produced by human activity in atmospheric chemistry was established about fifty years ago by Haagen-Smit in his studies of Los Angeles smog (Haagen-Smit et al., 1953). Since then, smog has been detected in almost all major urban and industrial centres at levels which exceed internationally agreed criteria values set to protect human health (Derwent et al., 1995). However, elevated emissions of VOCs from various anthropogenic sources have not only reduced the air quality within source regions, but also have altered the composition of the atmosphere in remote areas due to processes of medium and long-distance transport (Koga et al., 2001).

VOCs are released into the atmosphere as a result of human activities, mainly from motor vehicle exhausts, industrial activity, solvent usage, landfilled waste and agriculture. Emissions of benzene, toluene, ethylbenzene, xylenes and trimethylbenzene from motor vehicles probably have the most serious effects on air quality. Nevertheless, ambient concentrations of VOCs are also influenced by biogenic emissions and photochemical degradation (Yassaa et al., 2001).

VOCs constitute an important group of air pollutants to be studied as they contribute to some of the most serious environmental problems. Some VOCs exert direct adverse effects on either human health, vegetation or both (Fernández-Martínez et al., 2001). Among these, benzene is of particular concern since it is known to be a genotoxic carcinogen (WHO, 2000, Hellén et al., 2002). In addition, VOCs also play a significant role in particle formation (Reisell et al., 2003) and, in the presence of NOx, they react with OH radicals to form ozone (Atkinson, 2000) thus modifying the oxidizing capacity of the atmosphere. The potential for VOCs to have direct effects on plants at current ambient levels is, however, considered negligible, mainly on the basis of controlled short-term exposure of vegetation to high concentrations of VOCs (Cape, 2003). However, it is well known that ozone is highly phytotoxic, causing damages to crops and native vegetation (Krupa and Manning, 1988, Nali et al., 2002, Gimeno et al., 2004, Paoletti, in press). Therefore, controls on VOCs and NOx emissions have been implemented in an attempt to decrease the concentrations of secondary pollutants. Consequently, measurement of VOCs in air becomes necessary: to determine the sources of pollutants and the transport mechanics of pollution, and to study health effects and the compliance of regulatory limits. However, despite all this, studies in rural atmospheres are not very frequent (Cooke et al., 2001, Koga et al., 2001, Duane et al., 2002, Cerqueira et al., 2003, Borbon et al., 2004, Guo et al., 2004, Wang et al., 2001).

The fluctuations of VOCs in air depend on time and space (Harper, 2000). Consequently, high monitoring density is necessary to determine spatial and temporal distribution of these pollutants. Intensive monitoring with passive samplers provides data on spatial variation which a few dispersed analysers could never supply. Furthermore, the flexibility of placement makes them an alternative for defining exposures at locations of difficult access.

The aim of this intensive sampling was to characterise the presence of VOCs, especially ozone precursors, in La Ribera of Navarre, a rural area in the north of Spain of high economic importance with 67% of its surface devoted to agricultural exploitation of many ozone-sensitive species. Additionally, spatial and temporal distributions of VOCs and their relation with other atmospheric pollutants such as ozone and nitrogen oxides were also studied. Finally, the identification of possible emission sources of these compounds was attempted.

Section snippets

Study site

Research was carried out in La Ribera of Navarre, Spain (Fig. 1), an area with an extension of 2186 km2 in the high Ebro river basin and which is mainly made up of low lands with an average altitude in the range of 300 to 400 m a.s.l.

Its climatic characteristics correspond to a continental Mediterranean climate with an annual mean temperature of 14 °C and a summer maximum of 31 °C. Precipitations are irregular and scarce, with an annual register of 400–500 mm of which 75 to 100 mm correspond to

Results and discussion

A total of 140 VOCs were identified in La Ribera of Navarre. These compounds can be classified into two groups: anthropogenic compounds emitted from traffic and industries, and biogenic compounds emitted from vegetation (Table 1). Anthropogenic compounds were identified all over the area whereas biogenic compounds were identified at points 1, 3 and 13, where large extensions of cereals were predominant, and at points 12, 21 and 36, where nearby dense forest formations were present (Pinus

Conclusions

VOC levels registered in La Ribera during summer 2004 by the use of passive samplers were low and similar to others registered in rural atmospheres. Geostatistical analysis of BTEX indicated an increment in levels corresponding to the areas situated close to principal roads, pointing to traffic as the main source of these pollutants. However, an industrial origin was considered for 1,3,5-trimethylbenzene.

BTEX levels were found to be related to NO2 and O3 levels. BTEX concentrations were higher

Acknowledgements

This research work was made possible by the financial support of Departamento de Medio Ambiente of Gobierno de Navarra. Also we would like to thank Asociación de Amigos of the Universidad de Navarra for the concession of a research grant.

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