A comparison of isoprene nitrate concentrations at two forest-impacted sites

We report measurements of isoprene nitrates as part of the Southern Oxidants Study during the summer of 1999 at a rural/forest site in Tennessee. Average midday concentrations of the isoprene nitrates were ∼ 115 ppt. This is ∼ 10 times greater than the previously reported concentrations during the Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) study in 1998 at Pellston, Michigan, representing as much as 5% of NO_y Here we investigate the possible factors for the large difference in concentrations. To investigate the role of the NO_x concentration on the isoprene nitrate production chemistry at the two sites, [OH] was calculated using a simple steady state model. The results of this calculation help explain the difference in magnitude of the isoprene nitrate concentrations between the two field sites in terms of the [NO_x]-dependent behavior of the OH-initiated oxidation of isoprene and the subsequent isoprene peroxy radical reactions with NO_x. However, it is also clear that the large apparent differences in the photochemical ages of the air masses sampled at the two sites significantly impacted the observed concentrations.