Concentration variation of isoprene and its implications for peak ozone concentration

Atmospheric isoprene and other selected hydrocarbons were measured continuously at a suburban site and an urban site in order to study their possible effects on ozone formation in different seasons. The summer measurements at both the urban and suburban sites revealed that atmospheric mixing ratios of isoprene were synchronous with solar radiation, concentrations reaching a maximum at noon and decreasing to near zero in the evening. In contrast, wintertime atmospheric abundance of isoprene showed little concurrency with solar radiation. As the biogenic source subsided in winter, the anthropogenic source started to play a dominant role in controlling the atmospheric abundance of isoprene. Using propane as a reference compound, the source characteristics of isoprene were able to be deconvoluted from meteorological modulation confirming the insignificant biogenic contribution in winter. Implication of isoprene on noontime ozone formation was discussed not only from the aspect of reactivity but also from the time characteristics of the diurnal cycle. This study demonstrates that although the reactivity is important, the timing of the flux, which has been shown to have drastic difference between the cold and warm seasons, could play an even more critical role in shaping the daily ozone profile. This investigation shows that the midday emissions of isoprene in warm seasons could produce a much larger photochemical loss and therefore, a stronger ozone formation ability than emitted in other hours of a day due to the coherence of isoprene and OH diurnal cycles, both of which are sunlight driven. (c) 2006 Elsevier Ltd. All rights reserved.