Spatial and temporal variability of benzene, toluene, ethylbenzene and xylene (BTEX) in an urban area of complex terrain

Abstract

Benzene, toluene, ethylbenzene and xylene (BTEX) are primarily emitted into the atmosphere from combustion sources which are the dominant source of hydrocarbons in most urban areas. A number of volatile organic compounds (VOCs) emitted from vehicle sources, including BTEX, have a significant impact on human health. However, we currently have a poor understanding of the temporal and spatial pattern in atmospheric concentration of these compounds due to the expense and limited availability of air quality monitors capable of resolving these trends. Recent advances in instrumentation are driving this field forward in terms of size, cost and simplicity of new devices for higher density monitoring. In this study a novel, low-cost instrument for continuous part-per-billion by volume (ppbv) measurement of hydrocarbons in ambient air (temperature programmed desorption, TPD) was used to characterise BTEX concentrations across five sites in Auckland. The sites were characterised by urban, industrial and rural emission profiles. Meteorological conditions, source behaviour and relative position of the monitoring instrument were examined to determine the influence of these variables on observed ambient concentrations of BTEX at these sites. The results show that ambient concentrations of BTEX at rural sites around the Auckland region were very low (less than 1 ppbv) and urban background concentrations were also low (3 – 4 ppbv). However, at a roadside site in Auckland (Khyber Pass) ambient concentrations of BTEX were significantly higher (3 – 10 ppbv, with peaks of greater than 25 ppbv). Analysis revealed that ambient concentrations were influenced by local emission source strength; in this case periods of high vehicle movement resulted in the highest concentrations. Meteorological parameters were found to influence the concentration of BTEX to a lesser degree than emission sources; those with the most significant impact were wind speed and wind direction. A relation between source strength and temperature could not be determined indicating that local chemistry may not be a significant determinant on concentrations. The most significantly correlated concentrations of observed ambient pollutants to BTEX were: carbon monoxide (CO); trimethylbenzene; 1,3 butadiene; ethene; propene; propanal and Peroxyacetyl nitrate (PAN). However, ambient concentrations of BTEX could not be predicted effectively using the commonly used surrogate of CO (R² = 0.585) and other compounds could only be measured using more expensive instrumentation than the TPD. However, a simple semi-empirical model (based on wind speed and direction) trained to the dataset could provide a reasonable estimate of hourly concentrations (R² = 0.634).