Atmospheric ammonia (NH₃) emanations from Lake Natron's saline mudflats

In a recent global analysis of satellite-derived atmospheric NH₃ data, a hotspot was observed in the vicinity of Lake Natron, Tanzania. The lake is in the centre of an endorheic (limited drainage) basin and has shallow, saline-alkaline waters. Its remote location and the absence of nearby large anthropogenic sources suggest that the observed NH₃ is mainly of natural origin. Here we explore 10 years of IASI NH₃ satellite data and other publicly available datasets over the area to characterize the natural NH₃ emissions in this unique ecosystem. Temporal analysis reveals that the emissions are episodic and linked with the lake's surface area. The largest NH₃ column loadings generally occur at the end of the dry season in September-November over Lake Natron's largest mudflat, that is exposed with receding water levels. The timing is different from the agricultural dominated NH₃ emissions in the wider Natron area, which peak early in the year, after the first wet season. The likely source of NH₃ at Lake Natron is decomposition of organic material, either from rivers and springs or produced in the lake (plankton, bird excreta). High temperatures and alkalinity are known to promote NH₃ losses from soda lakes. We formulate six processes that may explain why the largest losses are observed specifically over concentrated brines and/or exposed sediments. As a by-product, we also show that hyperspectral infrared sounders such as IASI are capable of mapping different types of evaporative minerals such as trona and thermonatrite.