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

We present a comprehensive observational approach that aims to establish relationships between the surface–atmosphere exchange of ammonia (inline-formulaNH₃) and inline-formulaCO₂ uptake and transpiration by vegetation. In doing so, we study relationships useful for the improvement and development of inline-formulaNH₃ flux representations in models. The inline-formulaNH₃ concentration and flux are measured using a novel open-path miniDOAS (differential optical absorption spectroscopy) measurement setup, taken during the 5-week Ruisdael Land–Atmosphere Interactions Intensive Trace-gas and Aerosol measurement (RITA-2021) campaign (25 August until 12 October 2021) at the Ruisdael Observatory in Cabauw, the Netherlands. After filtering for unobstructed flow, sufficient turbulent mixing and inline-formulaCO₂ uptake, we find the diurnal variability in the inline-formulaNH₃ flux to be characterized by daytime emissions (0.05 inline-formula $M9inlinescrollmathmlunit\mathrm{normal\; \mu }\mathrm{normal\; g}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; s}}^{-\mathrm{normal\; 1}}$ 52pt15ptsvg-formulamathimg2a56a9738d1f1f7d21742b5af8dac2d2 bg-21-557-2024-ie00001.svg52pt15ptbg-21-557-2024-ie00001.png on average) and deposition at sunrise and sunset (inline-formula−0.05 inline-formula $M11inlinescrollmathmlunit\mathrm{normal\; \mu }\mathrm{normal\; g}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; s}}^{-\mathrm{normal\; 1}}$ 52pt15ptsvg-formulamathimg65e618a4ddfdc4b95db070148e07136a bg-21-557-2024-ie00002.svg52pt15ptbg-21-557-2024-ie00002.png on average). We first compare the inline-formulaNH₃ flux to the observed gross primary production (GPP), representing inline-formulaCO₂ uptake, and latent heat flux (inline-formulaL_vE), representing net evaporation. Next, we study the observations following the main drivers of the dynamic vegetation response, which are photosynthetically active radiation (PAR), temperature (inline-formulaT) and the water vapor pressure deficit (VPD). Our findings indicate the dominance of the stomatal emission of inline-formulaNH₃, with a high correlation between the observed emissions and both inline-formulaL_vE (0.70) and PAR (0.72), as well as close similarities in the diurnal variability in the inline-formulaNH₃ flux and GPP. However, efforts to establish relationships are hampered by the high diversity in the inline-formulaNH₃ sources of the active agricultural region and the low data availability after filtering. Our findings show the need to collocate meteorological, carbon and nitrogen studies to advance our understanding of inline-formulaNH₃ surface exchange and its representation.