Urgency of controlling agricultural nitrogen sources to alleviate summertime air pollution in the North China Plain

Agricultural nitrogen sources (ANS) have played an increasingly important role in the air quality since ANS emission controls are much weaker than those for fossil fuel combustion sources due to the increasing food demand. However, ANS emissions are highly uncertain due to stochastic agricultural management activities and limited field measurements, and impacts of ANS on the air quality remain elusive. In the study, the WRF-Chem model has been used to investigate ANS shares in near surface air pollutant concentrations during a growing season in the North China Plain (NCP), with ANS emissions constrained by satellite retrievals. Soil NO_X and agricultural NH₃ emissions are about 36% and 92% of their total emissions during the growing season. Sensitivity studies demonstrate that ANS count 16.9 μg m⁻³ (9.9%) of the mean maximum daily average 8-h ozone concentrations (MDA8 [O₃]) and 8.9 μg m⁻³ (31.7%) of fine particulate matter concentrations ([PM_2.5]) on average in the NCP. Additionally, the contributions of ANS to MDA8 [O₃] and [PM_2.5] increase with the deterioration of air pollution in cities. A 50% emission reduction in ANS decreases MDA8 [O₃] ([PM_2.5]) from 4.2% to 8.4% (from 19.7% to 31.9%) when the air quality changes from being lightly to heavily polluted in terms of MDA8 [O₃] (hourly [PM_2.5]). Without fossil fuel combustion emissions, the simulated average MDA8 [O₃] and [PM_2.5] are 111.7 and 8.2 μg m⁻³ in cities of the NCP, respectively, exceeding the new standards from the World Health Organization. Our study highlights important contributions of ANS to air quality and the urgency of ANS emission abatement for air pollution alleviation during summertime in the NCP.