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.