Biogenic and anthropogenic isoprene emissions in the subtropical urban atmosphere of Delhi

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This study examines the seasonal and intraday variations in isoprene, benzene and toluene at different sites in Delhi with a view to estimate the anthropogenic and biogenic fractions of isoprene and their ozone formation potential. For this purpose, measurements of ambient isoprene, benzene and toluene were made at selected vegetative and traffic sites of Delhi using Gas Chromatography-Mass Spectrometry. The study reveals that the average isoprene concentrations were higher at vegetative sites (4.40 ± 3.35 μg/m3) as compared to traffic sites (0.56 ± 0.22 μg/m3). The benzene and toluene levels, on the other hand, were found to be higher at the traffic sites (23.22 ± 10.94 and 60.58 ± 20.27 μg/m3) as compared to vegetative sites (8.01 ± 4.56 and 24.75 ± 10.86 μg/m3) respectively. The isoprene concentrations at vegetative sites were found to be the highest in summer (6.51 ± 3.56 μg/m3) followed by post-monsoon (4.80 ± 3.18 μg/m3) and winter (1.82 ± 0.57 μg/m3). In the context of intraday variation, isoprene concentrations attained maximum values during the afternoon period. However, no significant intraday/seasonal variation was observed in isoprene concentration at traffic sites. Estimation of anthropogenic and biogenic fractions of isoprene revealed that anthropogenic contribution varied between 60 and 70% at traffic sites and less than 10% at the vegetative sites. Upon computation of ozone formation potential of isoprene, benzene and toluene it is seen that isoprene's contribution towards ozone formation is higher at vegetative sites as compared to traffic sites. But, the anthropogenic isoprene's contribution towards ozone formation is found to be minimal in the present study. In view of the existing vegetation cover (~20%) of Delhi, however, the role of isoprene in surface ozone formation cannot be understated.