The stable carbon isotope ratio of biogenic emissions of isoprene and the potential use of stable isotope ratio measurements to study photochemical processing of isoprene in the atmosphere

A technique was developed that allows the determination of the stable carbon isotope ratio of isoprene in air. The method was used for a limited number of ambient measurements as well as laboratory studies of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis), including the light and temperature dependence. The mean stable carbon isotope ratio (delta(13)C) of isoprene emitted from Velvet Bean (Mucana pruriens L. var. utilis) for all our measurements is -27.7parts per thousand +/- 2.0parts per thousand (standard deviation for 23 data points). Our results indicate a small dependence of the stable carbon isotope ratios on leaf temperature and photosynthetic photon flux density (PPFD). The light dependence is 0.0026 +/- 0.0012parts per thousand/(mumol of photons m(-2) s(-1)) for the studied range from 400 to 1700 mumol of photons m(-2) s(-1). The temperature dependence is 0.16 +/- 0.09parts per thousand/K. On average, the emitted isoprene is 2.6 +/- 0.9parts per thousand lighter than the leaf carbon. An uncertainty analysis of the possibility to use stable carbon isotope ratio measurements of isoprene for estimates of its mean photochemical age suggests that meaningful results can be obtained. This is supported by the results of a small number of measurements of the stable carbon isotope composition of ambient isoprene at different locations. The results range from approximately -29parts per thousand to -16parts per thousand. They are consistent with vegetation emissions of isoprene that is slightly depleted in C-13 relative to the plant material and enrichment of C-13 in the atmosphere due to isotope fractionation associated with the reaction with OH-radicals. The stable carbon isotope ratio of ambient isoprene at locations directly influenced by isoprene emissions is very close to the values we found in our emission studies, whereas at sites located remote from isoprene emitting vegetation we find substantial enrichment of C-13. This suggests that stable carbon isotope ratio measurements will be a valuable, quantitative method to determine the extent of photochemical processing of isoprene in ambient air.