Observational relationships between ammonia, carbon dioxide and water vapor under a wide range of meteorological and turbulent conditions: RITA-2021 campaign

Abstract. We present a comprehensive observational approach, aiming to establish relations between the surface-atmosphere exchange of ammonia (NH₃) and the CO₂ uptake and transpiration by vegetation. In doing so, we study relationships useful for the the improvement and development of NH₃ flux representations and their dependences. The NH₃ concentration and flux are measured using a novel open-path miniDOAS measurement setup, taken during the five week RITA-2021 campaign (25 August until 12 October 2021) at the Ruisdael Observatory at Cabauw, the Netherlands. After filtering for unobstructed flow, sufficient turbulent mixing and CO₂ uptake, we find the diurnal variability of the NH₃ flux to be characterized by daytime emissions (0.05 μg m^-2 s^-1 on average) and deposition at sunrise and sunset (-0.05 μg m^-2 s^-1 on average). We first compare the NH₃ flux to the observed gross primary production (GPP), representing CO₂ uptake, and latent heat flux (L_vE), representing to evapotranspiration. Next we study the observations following the main drivers of the dynamic vegetation response, which are photosynthetically active radiation (PAR), temperature (T) and the water vapor pressure deficit (VPD). Our findings show indication of the dominance of stomatal emission of NH₃, with high correlation between the observed emissions and both net L_vE (0.70) and PAR (0.72), as well as close similarities in the diurnal variability of the NH₃ flux and GPP. However, the efforts to establish relationships are hampered due to the amount of diversity of NH₃ sources of the active agricultural region and low data availability after filtering. Our findings show the need to collocate meteorological, carbon and nitrogen studies to advance on our understanding of NH₃ deposition and its representation.