Spatial variation of ambient volatile organic compounds in Tehran, Iran

Abstract

Introduction: Little is known about ambient concentrations of volatile organic compounds (VOCs) and their spatial variability in Tehran, Iran. We aimed to characterize the spatial distribution of and correlations between benzene, toluene, ethylbenzene, and xylenes (BTEX). Methods: We used Passam passive samplers to measure benzene, toluene, ethylbenzene, para-xylene (1,4-dimethylbenzene), meta-xylene (1,3-dimethylbenzene), and ortho-xylene (1,2-dimethylbenzene) in the ambient air of Tehran. We deployed the samplers during a two-week campaign in the summer 2015 at 177 sites spread over the city (total size ~613 km2), chosen by a novel cluster analysis method. Spatial correlations between BTEX species were calculated using Spearman’s correlation coefficient (rho) and the spatial variability was characterized by universal kriging modeling (UKM). Results: All pollutants had a skewed distribution. The mean (interquartile range) concentrations of benzene, toluene, ethylbenzene, p-xylene, m-xylene, and o-xylene were, respectively, 10.6 (4.7), 33.0 (16.6), 5.8 (2.7), 5.7 (2.5), 11.0 (5.4), and 6.8 (3.3) µg/m3. Almost 47% of xylene was m-xylene isomer. The rho for benzene with other compounds ranged from 0.87 to 0.96, with minimum correlation for o-xylene and maximum for m-xylene. It ranged from 0.87 to 0.93 for toluene, 0.92 to 0.97 for ethylbenzene, and 0.93 to 0.99 for xylenes. The within-city UKM analyses demonstrated similar spatial patterns. Conclusions: The concentrations of toxic VOCs were relatively high during the summer 2015. In addition, we found large contrasts within the city and substantial correlations among BTEX compounds. This may indicate shared sources for all compounds. In a next step, two-week measurements will be repeated to estimate annual means and to develop land use regression models. The estimated long-term concentrations will be used to investigate chronic health effects of VOCs, which may be a relevant marker for local air pollution in Tehran.