Gaseous ammonia (NH
3) is the most abundant alkaline gas in the atmosphere. In addition, it is a major component of total reactive nitrogen. The largest source of NH
3 emissions is agriculture, including animal husbandry and NH
3-based fertilizer applications. Other sources of NH
3 include industrial processes, vehicular emissions and volatilization from soils and oceans. Recent studies have indicated that NH
3 emissions have been increasing over the last few decades on a global scale. This is a concern because NH
3 plays a significant role in the formation of atmospheric particulate matter, visibility degradation and atmospheric deposition of nitrogen to sensitive ecosystems. Thus, the increase in NH
3 emissions negatively influences environmental and public health as well as climate change. For these reasons, it is important to have a clear understanding of the sources, deposition and atmospheric behaviour of NH
3. Over the last two decades, a number of research papers have addressed pertinent issues related to NH
3 emissions into the atmosphere at global, regional and local scales. This review article integrates the knowledge available on atmospheric NH
3 from the literature in a systematic manner, describes the environmental implications of unabated NH
3 emissions and provides a scientific basis for developing effective control strategies for NH
3.
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