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.

通过将观测结果与概念性大气边界层模型相结合，我们研究了未施肥草地上大气氨（NH ₃）预算的日变化。我们将昼夜观测和建模相结合的方法使我们能够确定四个控制过程对NH ₃昼夜循环的贡献：地表-大气交换，夹带，对流和化学气-气溶胶转换。观测结果包含新的NH ₃使用差分光学吸收光谱（DOAS）遥感技术获得的通量和摩尔分数测量结果，消除了与进口管有关的问题。使用严格的筛选条件，选择22天的晴朗天空夏季条件。通过这种选择，我们使用受气象观测约束的边界层模型来分析以典型对流边界层条件为特征的单个代表日。

我们设计了两个数值实验，以研究NH _3的日变化和控制氨预算的过程的各个贡献。这些实验仅在其NH ₃表面交换的表示上有所不同。首先，通过观察到的NH ₃通量拟合函数被指定给模型。在第二个数值实验中，通过酸化化合物的DEPosition（DEPAC）参数化来求解表面通量。在规定的表面通量的情况下，建模的NH ₃摩尔分数非常符合观测值。在NH ₃中确定了两种状态昼夜周期：上午，边界层动力通过夹带作用控制着预算；下午，多个过程至关重要。在第二个实验中获得了与观察到的摩尔分数相似的近似拟合，但我们确定了观察到的和参数化的NH ₃表面通量之间的不匹配。结果，该模型需要不切实际的预算表示来实现这种紧密拟合，例如高自由对流层NH ₃。基于对建模和观测结果的整合，我们对NH ₃预算的发现为亚尺度下NH ₃地表-大气交换的未来研究铺平了道路。