Organic nitrate chemistry and its implications for nitrogen budgets in an isoprene- and monoterpene-rich atmosphere: constraints from aircraft (SEAC 4 RS) and ground-based (SOAS) observations in the Southeast US

Formation of organic nitrates (RONO ) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NO ), but the chemistry of RONO formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO ) in the GEOS-Chem global chemical transport model with ∼25 × 25 km resolution over North America. We evaluate the model using aircraft (SEAC RS) and ground-based (SOAS) observations of NO , BVOCs, and RONO from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50% of observed RONO in surface air, and we find that another 10% is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10% of observed boundary layer RONO were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO accounts for 60% of simulated gas-phase RONO loss in the boundary layer. Other losses are 20% by photolysis to recycle NO and 15% by dry deposition. RONO production accounts for 20% of the net regional NO sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NO emissions. This segregation implies that RONO production will remain a minor sink for NO in the Southeast US in the future even as NO emissions continue to decline.