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Urban air quality in China has been declining substantially in recent years due to severe haze episodes. The reduction of sulfur dioxide (SO 2 ) and nitrogen oxide (NO x ) emissions since 2013 does not yet appear to yield substantial benefits for haze mitigation. As the reductions of those key precursors to secondary aerosol formation appears not to sufficient, other crucial factors need to be considered for the design of effective air pollution control strategies. Here we argue that ammonia (NH 3 ) plays a - so far - underestimated role in the formation of secondary inorganic aerosols, a main component of urban fine particulate matter (PM 2.5 ) concentrations in China. By analyzing in situ concentration data observed in major cities alongside gridded emission data obtained from remote sensing and inventories, we find that emissions of NH 3 have a more robust association with the spatiotemporal variation of PM 2.5 levels than emissions of SO 2 and NO x . As a consequence, we argue that urban PM 2.5 pollution in China in many locations is substantially affected by NH 3 emissions. We highlight that more efforts should be directed to the reduction of NH 3 emissions that help mitigate PM 2.5 pollution more efficiently than other PM 2.5 precursors. Such efforts will yield substantial co-benefits by improving nitrogen use efficiency in farming systems. As a consequence, such integrated strategies would not only improve urban air quality, but also contribute to China's food-security goals, prevent further biodiversity loss, reduce greenhouse gas emissions and lead to economic savings. [ABSTRACT FROM AUTHOR]