Large biogenic contribution to boundary layer O3-CO regression slope in summer

Strong correlation between O-3 and CO was observed during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) aircraft experiment in July 2011 over the Washington-Baltimore area. The observed correlation does not vary significantly with time or altitude in the boundary layer. The observations are simulated well by a regional chemical transport model. We analyze the model results to understand the factors contributing to the observed O-3-CO regression slope, which has been used in past studies to estimate the anthropogenic O-3 production amount. We trace separately four different CO sources: primary anthropogenic emissions, oxidation of anthropogenic volatile organic compounds, oxidation of biogenic isoprene, and transport from the lateral and upper model boundaries. Modeling analysis suggests that the contribution from biogenic isoprene oxidation to the observed O-3-CO regression slope is as large as that from primary anthropogenic CO emissions. As a result of decrease of anthropogenic primary CO emissions during the past decades, biogenic CO from oxidation of isoprene is increasingly important. Consequently, observed and simulated O-3-CO regression slopes can no longer be used directly with an anthropogenic CO emission inventory to quantify anthropogenic O-3 production over the United States. The consistent enhancement of O-3 relative to CO observed in the boundary layer, as indicated by the O-3-CO regression slope, provides a useful constraint on model photochemistry and emissions.