Unraveling the diurnal atmospheric ammonia budget of a prototypical convective boundary layer.

We investigate diurnal variability of the atmospheric ammonia (NH 3) 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 NH 3 diurnal cycle: surface-atmosphere exchange, entrainment, advection and chemical gas-aerosol transformations. The observations contain new NH 3 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 NH 3 diurnal variability and the individual contributions of the processes governing the ammonia budget. These experiments only differ in their representation of the NH 3 surface exchange. First, a fitted function through the observed NH 3 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 NH 3 molar fraction closely fits the observations. Two regimes are identified in the NH 3 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 NH 3 surface flux. As a result, the model requires an unrealistic budget representation to achieve this close fit, e.g. high free tropospheric NH 3. Our findings on the NH 3 budget, based on integrating modeling and observations, paves the way for future research on the NH 3 surface-atmosphere exchange at the subdaily scales. [ABSTRACT FROM AUTHOR]