Background concentrations and fluxes of atmospheric ammonia overa deciduous forest

Natural ammonia (NH₃) fluxes over terrestrial ecosystems are difficult to quantify because most measurement sites are influenced by nearby anthropogenic NH₃ sources. Furthermore, measuring the net exchange of NH₃ is challenging due to bi-directionality of the flux and the high reactivity of NH₃. In this study, we present two months of half-hourly NH₃ fluxes and concentrations measured using a Relaxed Eddy Accumulation system during late summer and fall 2013 above a remote forest site in the central Midwest in USA. Supplementary nitric acid (HNO₃) flux and size-resolved aerosol-N measurements areused to quantify the phase-partitioning and diagnose possible causes of upward NH₃ fluxes. Data from 2013 combined with previous NH₃ studies at this site indicate a seasonal background NH3concentrationof spring: 0.92 ± 0.95, summer: 0.30 ± 0.39, autumn: 0.20 ± 0.26, and winter: 0.26 ± 0.1 g NH₃-N m⁻³.Air mass back trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model did not indicate any directional bias in the measured NH₃ concentration confirming the absence of strong local sources of NH₃. The NH₃ fluxes were mainly upward (emission) and had a magnitude of up to 0.11 g NH₃-N m⁻² s⁻¹. The wetness of the forest surfaces (assessed using a proxy of time since precipitation) was found to be crucial in controlling both deposition and emission of atmospheric NH₃. Size resolved aerosol concentrations (of NH₄⁺, NO₃⁻, Cl⁻and SO₄²⁻) indicated that the aerosol and gas phase concentration of ammonia/ammonium were of similar magnitude but that the aerosol phase typically dominated. Nitric acid flux measurements showed periods of apparent upward fluxes, but they were not preferentially associated with upwards NH₃ fluxes indicating they may not have a common causality.