China has produced roughly half of the world's steel in recent years, but the country's iron and steel industry is a major source of air pollutants, especially particulate matter, SO2 and NOX emissions. To reduce such emissions, China imposed new emission standards in 2015 and promoted ultralow emission standards in 2019. Here, we use measurements from China's continuous emissions monitoring systems (covering 69-91% of national iron and steel production) to develop hourly, facility-level emissions estimates for China's iron and steel industry. In turn, we use this data to evaluate the emission reductions related to China's increasingly stringent policies. We find steady declines in emission concentrations at iron-and steelmaking plants since the 2015 standards were implemented. From 2014 to 2018, particulate matter and SO2 emissions fell by 47% and 42%, respectively, and NOX increased by 3%, even as the production increased by 14%. Moreover, we estimate that if all facilities achieve the ultralow emission standards, particulate matter, SO2 and NOX emissions will drop by a further 50%, 37% and 58%, respectively. Our results thus reveal the substantial benefits of the Chinese government's interventions to curb emissions from iron and steel production and emphasize the promise of ongoing ultralow emission renovations. [200 words]China's iron and steel industry dominates the global market, producing 45-53% of crude steel worldwide between 2010 and 2019 1-5 , and the country's crude steel production has grown faster than global production over the same period (at an average annual rate of 5% compared with 3% globally) 1 . As an energy-intensive industry, such production represents similarly large amounts of fossil fuel consumption; between 2010 and 2018, iron and steel production accounted for 7-9% of coal use in China 3,4,6,7 . In turn, the industry has greatly contributed to China's haze pollution [3][4][5][6][8][9][10] , accounting for 7-25%, 7-12% and 1-6% of the country's anthropogenic emissions of particulate matter (PM, including all PM size categories) 8,9 , sulfur dioxide (SO2) 8-10 and nitrogen oxide (NOX) 8-10 , respectively, between 2010 and 2015. In an effort to reduce emissions from iron-and steelmaking, China introduced emission standards for the major stationary sources (generating facilities, units, boilers and machines) in 2012 [11][12][13][14][15] , which are defined as the allowed upper limits of the emission concentrations in flue gas (in mg m -3 ; Supplementary Table 1) 2 . In 2015, China strengthened these standards by lowering the limits as much as 60%, 67% and 40% for PM, SO2 and NOX, respectively [11][12][13][14][15] . In addition, stricter local standards were designed for Shandong 16 and Hebei provinces 17 , reducing the limits in those provinces up to 50% and 40% lower than the national standards, respectively.In April 2019, China announced a set of even stricter ultralow emissions (ULE) standards 18 that reduce the 2015 standards by yet another 50-80%, 40-83% and 25-83% for PM, SO2 a...
