Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States

Reducing surface ozone ( O-3) to concentrations in compliance with the national air quality standard has proven to be challenging, despite tighter controls on O-3 precursor emissions over the past few decades. New evidence indicates that isoprene emissions changed considerably from the mid-1980s to the mid-1990s owing to land-use changes in the eastern United States ( Purves et al., 2004). Over this period, U. S. anthropogenic VOC ( AVOC) emissions decreased substantially. Here we apply two chemical transport models ( GEOS- CHEM and MOZART-2) to test the hypothesis, put forth by Purves et al. ( 2004), that the absence of decreasing O-3 trends over much of the eastern United States may reflect a balance between increases in isoprene emissions and decreases in AVOC emissions. We find little evidence for this hypothesis; over most of the domain, mean July afternoon ( 1300 - 1700 local time) surface O-3 is more responsive ( ranging from - 9 to + 7 ppbv) to the reported changes in anthropogenic NOx emissions than to the concurrent isoprene ( - 2 to + 2 ppbv) or AVOC ( - 2 to 0 ppbv) emission changes. The estimated magnitude of the O-3 response to anthropogenic NOx emission changes, however, depends on the base isoprene emission inventory used in the model. The combined effect of the reported changes in eastern U. S. anthropogenic plus biogenic emissions is insufficient to explain observed changes in mean July afternoon surface O-3 concentrations, suggesting a possible role for decadal changes in meteorology, hemispheric background O-3, or subgrid-scale chemistry. We demonstrate that two major uncertainties, the base isoprene emission inventory and the fate of isoprene nitrates ( which influence surface O-3 in the model by - 15 to + 4 and + 4 to + 12 ppbv, respectively), preclude a well-constrained quantification of the present-day contribution of biogenic or anthropogenic emissions to surface O-3 concentrations, particularly in the high-isoprene-emitting southeastern United States. Better constraints on isoprene emissions and chemistry are needed to quantitatively address the role of isoprene in eastern U. S. air quality.