Response of South Asia PM_2.5 pollution to ammonia emission changes and associated impacts on human health

Countries in South Asia are suffering severe PM_2.5 pollution with rapid economic development, impacting human health and the environment. Whilst much attention has been given to understanding the contribution of primary emissions, the contribution of agriculture to PM_2.5 concentrations, especially from agricultural ammonia (NH₃) emissions, remains less explored. Using an advanced regional atmospheric chemistry and transport modelling system (WRF-EMEP) with a new estimate of anthropogenic NH₃ emissions inputs, we estimate the influence of agricultural NH₃ emissions on surface PM_2.5 in South Asia and evaluate the health impacts and the economic losses attributable to PM_2.5 in 2018. Results show that WRF-EMEP can reproduce magnitudes and variations of PM_2.5 well, with a high annual mean PM_2.5 concentration that exceeds 120 µg/m² and mainly appeared in the Indo-Gangetic Plain. We estimate 2,228,000 (95 % Confidence Interval: 2,052,000-2,400,000) premature deaths and US$ 596,000 (95 % CI: 549,000-642,000) million in economic losses are attributable to total ambient PM_2.5 under the current emissions. We calculate that NH₃ emissions are associated with 11 % of the annual average PM_2.5 concentrations across South Asia. Changes in PM_2.5 concentrations follow a non-linear response to NH₃ emissions reductions, highlighting increased efficiency with 70 %-100 % reductions in NH₃ emissions reductions. We estimate that 247,000 (227,000-265,000) premature deaths and US$ 66,000 (61,000-71,000) million economic losses through this pathway can be attributed to NH₃ emissions. These findings confirm that in the current NH₃-rich chemical environment of South Asia, the efficiency of PM_2.5 reduction is only moderately sensitive to the reduction in intensity of NH₃ emissions until emissions are cut very severely. Thus, SO₂, NO_x and NH₃ emissions controls need to be considered jointly for greater mitigation of ambient secondary PM_2.5 in South Asia.