Ammonia, a poorly understood smog ingredient, could be key to limiting deadly particulate pollution. Despite its abundance, the role of the colorless, sharp-smelling, and eye-watering gas in deadly air pollution is poorly understood. In part, that's because it is notoriously difficult to track. Ammonia molecules are “sticky” and eagerly combine with other compounds, making it difficult for monitoring instruments to capture them. And the gas can have a very short life span—sometimes just a few days. “Ammonia is awful,” says environmental engineer Mark Zondlo of Princeton University. “It's truly one of the worst gases to measure in the atmosphere.”

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