Only ammonia reduction is insufficient: Meeting vegetation critical levels but not PM2.5 air quality targets.

Excessive ammonia (NH3) can result in visible foliar injury on vegetation and promote the formation of fine particulate matter (PM2.5), thus it is urgently necessary to reduce NH3 to protect ecosystems, environmental quality and human health. However, we still lack effective strategies to achieve the dual objectives of protecting vegetation and meeting the PM2.5 air quality standard. Here, we used the Community Multiscale Air Quality model to evaluate the NH3 and PM2.5 concentrations and provided synergistic emission reduction strategies to meet both of the above targets in China. Results showed that NH3 concentrations in 2019 in most provinces were much higher than the critical level of 3 μg m-3 for vegetation protection. Nearly 50% of NH3 emissions need to be reduced to meet the critical level, especially in central and northeast China. Meanwhile, provinces in eastern and central China still cannot reach the air quality standard target of PM2.5 (35 μg m-3) even with further reduction of NH3 emissions on this basis. Alternatively, reducing other species of pollutants, such as volatile organic compounds, nitrogen oxides, and sulfur dioxide, with a range of 0-60% will relieve the pressure of NH3 emission reduction in China to meet the air quality targets. The reduction of these pollutants in the industrial, transportation, and fugitive dust sectors should be prioritized for the synergistic management of NH3 and PM2.5 pollution.