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

Abstract

Formation of organic nitrates (RONO_2) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NO_x), but the chemistry of RONO_2 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_2) in the GEOS-Chem global chemical transport model with  ∼  25  ×  25 km^2 resolution over North America. We evaluate the model using aircraft (SEAC^4RS) and ground-based (SOAS) observations of NO_x, BVOCs, and RONO_2 from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO_2 species measured during the campaigns. Gas-phase isoprene nitrates account for 25–50 % of observed RONO_2 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_2 were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO_3 accounts for 60 % of simulated gas-phase RONO_2 loss in the boundary layer. Other losses are 20 % by photolysis to recycle NO_x and 15 % by dry deposition. RONO_2 production accounts for 20 % of the net regional NO_x sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NO_x emissions. This segregation implies that RONO_2 production will remain a minor sink for NO_x in the Southeast US in the future even as NO_x emissions continue to decline.

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