Isoprene Chemistry in the Southeastern United States Constrained By GEOS-Chem Chemical Transport Model Interpretation of Aircraft Observations from the 2013 NASA SEAC4rs Campaign
Wednesday, 17 December 2014: 8:30 AM
We use airborne observations of a detailed suite of trace gases from the 2013 NASA SEAC4RS aircraft campaign, interpreted using a high-resolution chemical transport model (GEOS-Chem), to evaluate and improve our understanding of isoprene chemistry. The SEAC4RS campaign conducted in August-September 2013 over the Southeast US offers an unprecedented dataset to improve our understanding of isoprene oxidation mechanisms in both low-NOx and high-NOx chemical environments. A nested high-resolution (0.25°x0.3125°) version of the GEOS-Chem chemical transport model including isoprene oxidation by multiple mechanisms provides a framework for testing these mechanisms and exploring their sensitivity to chemical drivers. Here, we compare GEOS-Chem output to SEAC4RS aircraft observations, focusing on isoprene and its oxidation products including methyl vinyl ketone (MVK), methacrolein (MACR), formaldehyde (HCHO), isoprene peroxides, and isoprene nitrates, among others. The observations indicate a strong correlation between HCHO and organic nitrates, and we show that GEOS-Chem is able to reproduce this relationship. We interpret this and other observed species-species correlations using detailed model results and sensitivity simulations, focusing on changes in isoprene chemistry between low-NOx and high-NOx environments. In the boundary layer, the standard GEOS-Chem isoprene mechanism shows good predictive capability for isoprene and some oxidation products including HCHO and MVK+MACR. We use the simulation to test the sensitivity of the HCHO yield to NOx concentration in the context of improved interpretation of satellite HCHO observations. The standard GEOS-Chem simulation has less success representing the variability of first and second generation isoprene nitrates, and we evaluate our current understanding of high-NOx isoprene chemistry in the context of these discrepancies. We find that including uptake of isoprene nitrates by aerosol improves agreement with the aircraft observations, and we evaluate the influence of this uptake on NOx and aerosol budgets.