The broad strokes of what they found came as little surprise. The haze was mostly composed of tiny particles, less than 2.5 microns in diameter (PM2.5), which can lodge in the lungs and contribute to premature death. Some of the particles were dust, smoke, or soot, but about three-quarters were made up of ammonium nitrate. It forms when nitrogen oxides produced by vehicles, furnaces, and industrial equipment combine with ammonia, which typically wafts from farms that use ammonia-based liquid fertilizers or produce heaps of animal manure. The researchers were startled, however, by the levels of pure ammonia they measured.