A baseline study of atmospheric ammonia concentration and fluxes for forest

Abstract:

Natural ammonia emissions are difficult to address because most measurement sites are influenced by nearby anthropogenic ammonia sources. Furthermore, measuring the net exchange of ammonia is challenging due to bi-directionality of the flux and the high reactivity of ammonia. In this study we present two months of half-hourly ammonia fluxes and concentrations measured above the remote forest site Morgan-Monroe State Forest (MMSF) in the central Midwest in USA. Measurements are conducted during the late summer and autumn 2013 using the Relaxed Eddy Accumulation Method with the purpose is to quantify the baseline concentration and exchange of ammonia in a natural forest ecosystem and to understand the controlling processes. Combined with additional ammonia studies for MMSF a seasonal baseline of the ammonia concentration above the forest of spring = 0.39±0.39, summer = 0.30±0.39, autumn = 0.20±0.26, and winter = 0.26±0.1 μg m^-3 was concluded. The mean concentration of ammonia measured in this study was 0.23 μg m^-3 but shorter periods with concentration higher than 1 μg m-3 were also seen. The fluxes were mainly upward (emission) of up to 0.11 μg m^-2 s^-1, however, when the atmospheric concentration was higher, downward fluxes (deposition) up to -0.07 μg m^-2 s^-1 occurred. Air mass back trajectories from the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model did not identify any specific source area causing the higher ammonia concentrations measured affirming that the atmospheric ammonia is surface controlled. The largest fluxes was found shortly after rain events indication that the humidity of the forest ecosystem in crucial in controlling both deposition and emission of atmospheric ammonia. Measurements of size resolved aerosol concentrations and nitric acid gas concentrations and fluxes indicated that gas-particle phase partitioning occurred and influenced the exchange of ammonia. At last, a clear diurnal pattern in the ammonia concentration and fluxes was observed. Higher concentrations during nighttime and lower concentrations during daytime were found to some extent to be related to the diurnal development of the atmospheric boundary layer.