Underestimated industrial ammonia emission in China uncovered by material flow analysis

Ammonia (NH₃) is crucial in fine particulate matter (PM_2.5) formation, but past estimations on industrial NH₃ emissions remain highly uncertain. In this study, the flow of NH₃ within air pollution control devices (APCDs) were investigated basing on material flow analysis with in-situ measurements of NH₃ concentrations at the inlets and outlets of each APCD. Then, by combing emission factors updated with recent in-situ measurements and provincial-level activity data from statistical yearbooks and associated reports, NH₃ emissions from various industrial sources, as well as their spatial distribution across China in 2020, were evaluated. It was estimated that, on average, over 70% of NH₃ at APCD inlets is captured by fly ash, about 25% adheres to the inner walls and desulfurization slurry, and less than 5% is directly emitted. The captured filterable NH₃ is later released during subsequent treatment processes, contributing 34.48% to the NH₃ entering APCDs. In total, 38.82% of the NH₃ entering APCDs is ultimately emitted as gaseous NH₃, in which direct and indirect emission accounts for 11.26%and 88.74%, respectively. In 2020, the industrial NH₃ emissions in China were calculated to be 268.02 × 10⁴ t, constituting about 21.19% of total anthropogenic NH₃ emission, and nearly 3-10 times higher than previous estimations in current bottom-up inventories. Flue gas denitrification accounts for the majority (∼60%) of the industrial emission. Regional distribution demonstrates significant spatial variations, with North, East, Central and South China having the highest emission intensities, generally exceed 0.60 t km^-2·a^-1, more than twice the national average. This study suggests that industrial NH₃ emissions may play a more critical role in air pollution than previously expected, and underscores the importance to reassess the industrial NH₃ emission in China as well as its environmental impacts.