Assessing representativity of NH3 measurements influenced by boundary-layer dynamics and turbulent dispersion of a nearby emission source.

This study presents a fine scale simulation approach to assess the representativity of ammonia (NH₃) measurements in proximity of an emission source. Close proximity to emission sources (< 5 km) can introduce a bias in regionally representative measurements of the NH₃ molar fraction and flux. Measurement sites should therefore be located a significant distance from emission sources, but such requirements are poorly defined and can be difficult to meet in densely agricultural regions. This study presents a consistent criterium to assess the regional representativity of NH₃ measurements in proximity of an emission source, calculating variables that quantify the NH[sub 3] plume dispersion using a series of numerical experiments at a fine resolution (20 m). Our fine scale simulation framework with explicitly resolved turbulence enables us to distinguish between the background NH₃ and the emission plume, including realistic representations of NH₃ deposition and chemical gas-aerosol transformations. We introduce the concept of blending-distance, based on the calculation of turbulent fluctuations, to systematically analyze the impact of the emission plume on simulated measurements, relative to this background NH₃. This sensitivity analysis includes systematic experiments varying meteorological factors, emission/deposition and NH₃ dependences. Considering these sensitivities, we find that NH₃ measurements should be located at a minimum distance of 0.5 - 2.5 km and 1-3.5 km from an emission source, for NH₃ molar fraction and flux measurements respectively. The simulation framework presented here can easily be adapted to local conditions and paves the way for future ammonia research at high spatio-temporal resolution. [ABSTRACT FROM AUTHOR]