Background concentrations and fluxes of atmospheric ammonia over a 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 are used 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 NH3 concentration of spring: 0.92 ± 0.95, summer: 0.30 ± 0.39, autumn: 0.20 ± 0.26, and winter: 0.26 ± 0.1 μg 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 μg 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.