Diurnal Variation in the Basal Emission Rate of Isoprene

Isoprene is emitted from numerous plant species and profoundly influences tropospheric chemistry. Due to the short lifetime of isoprene in the atmosphere, developing an understanding of emission patterns at small time scales is essential for modeling regional atmospheric chemistry processes. Previous studies suggest that diurnal fluctuations in isoprene emission may be substantial, leading to inaccuracies in emission estimates at larger scales. We examined diurnal patterns in the basal emission rate of isoprene in red oak (Quercus rubra), eastern cottonwood (Populus deltoides), and eucalyptus (Eucalyptus saligna) and the influence of light and temperature on the magnitude of these diurnal patterns. Maximum diurnal increases in isoprene emission were large in cottonwood (45%) and oak (25%), with increases exceeding 100% in 1-yr-old cottonwoods. Eucalyptus showed no diurnal variation in emission. All species showed diurnal declines in photosynthesis. Across species, there was a positive correlation between maximum diurnal change in both isoprene emission and photosynthesis. The magnitude of diurnal increase in isoprene emission varied when individual cottonwoods were sampled repeatedly over three days. Temperature and light history, integrated from 1 to 48 hours prior to measurement, did not explain these variations in diurnal emission. Diurnal increases in emission were present when plants were shaded to <7% ambient light. Our results indicate that diurnal fluctuations in emission are large and species specific, and must be considered when estimating emission rates for use in short-term regional atmospheric-chemistry models.