Unraveling The Diurnal Atmospheric Ammonia Budget Of A Prototypical Convective Boundary Layer

We investigate diurnal variability of the atmospheric ammonia (NH3) budget over unfertilized grassland by combining observations with a conceptual atmospheric boundary layer model. Our combined approach of diurnal observations and modeling enables us to identify the contribution of the four governing processes to the NH3 diurnal cycle: surface-atmosphere exchange, entrainment, advection and chemical gas-aerosol transformations. The observations contain new NH3 flux and molar fraction measurements obtained using the Differential Optical Absorption Spectroscopy (DOAS) remote sensing technique, eliminating problems related to inlet tubing. Using strict filter criteria, 22 days with clear-sky summer conditions are selected. From this selection, we analyze a single representative day characterized by prototypical convective boundary layer conditions, using the boundary layer model constrained by meteorological observations.We design two numerical experiments to study the NH3 diurnal variability and the individual contributions of the processes governing the ammonia budget. These experiments only differ in their representation of the NH3 surface exchange. First, a fitted function through the observed NH3 flux is prescribed to the model. In the second numerical experiment, the surface flux is solved following the DEPosition of Acidifying Compounds (DEPAC) parameterization. With a prescribed surface flux, the modeled NH3 molar fraction closely fits the observations. Two regimes are identified in the NH3 diurnal cycle: the morning, where boundary layer dynamics dominate the budget through entrainment, and the afternoon, where multiple processes are of importance. A similarly close fit to the observed molar fraction is achieved in the second experiment, but we identify a mismatch between the observed and parameterized NH3 surface flux. As a result, the model requires an unrealistic budget representation to achieve this close fit, e.g. high free tropospheric NH3. Our findings on the NH3 budget, based on integrating modeling and observations, paves the way for future research on the NH3 surface-atmosphere exchange at the subdaily scales.