Aromatic VOCs at major road junctions of a metropolis in India: Measurements using TD-GC-FID and PTR-TOF-MS instruments

Abstract

Ambient mass concentrations of benzene and toluene were measured at 12 different road junctions of Ahmedabad city in India during the pre-monsoon season of year 2015. A Thermal Desorption-Gas Chromatography-Flame Ionization Detector (TD-GC-FID) technique was used for the analysis of aromatic volatile organic compounds (VOCs) in air samples. In each of both inner and outer ring roads, air samples were collected at 6 sites to investigate the spatial variation of benzene and toluene. The mass concentrations of benzene and toluene show strong site-to-site and day-to-day variations. The average mass concentration of benzene varied in the ranges of 11–35 µg m–3 and 4–12 µg m–3 along the inner and outer roads, respectively. The mass concentration of toluene varied in the ranges of 43–142 µg m–3 and 11–28 µg m–3 along the inner and outer roads, respectively. Overall, the mass concentrations of VOCs along the inner road were 3–5 times higher than those measured along the outer road.The mass concentrations of benzene and toluene show good correlation suggesting their common emission sources (mostly vehicular). However, the enhancement ratios of ∆Toluene/∆Benzene (~4.0 µg µg–1) along both the roads were higher than the typical ratios (1.5–3.5 µg µg–1) reported for vehicular emissions. The higher values of ∆Toluene/∆Benzene are due to the emissions of VOCs also from industrial and other non-traffic sources. During the daytime, the lower mass concentrations of VOCs and lower ∆Toluene/∆Benzene (~2 µg µg–1) indicate the role of photochemical aging. The combined diurnal trend of ∆Toluene/∆Benzene agrees well with that measured at central Ahmedabad using the proton-transfer-reaction time of flight mass spectrometer (PTR-TOF-MS). However, compared to weekdays, the mass concentrations of VOCs show reduction and increase during the Sunday and Saturday, respectively. The mass concentration of VOCs and their ratio were towards the higher side of data reported for different urban sites of the world.