Atmospheric reactive nitrogen conversion kicks off the co-directional and contra-directional effects on PM2.5-O3 pollution.

As the two important ambient air pollutants, particulate matter (PM_2.5) and ozone (O₃) can both originate from gas nitrogen oxides. In this study, applied by theoretical analysis and machine learning method, we examined the effects of atmospheric reactive nitrogen on PM_2.5-O₃ pollution, in which nitric oxide (NO), nitrogen dioxide (NO₂), gaseous nitric acid (HNO₃) and particle nitrate (pNO₃^-) conversion process has the co-directional and contra-directional effects on PM_2.5-O₃ pollution. Of which, HNO₃ and SO₂ are the co-directional driving factors resulting in PM_2.5 and O₃ growing or decreasing simultaneously; while NO, NO₂, and temperature represent the contra-directional factors, which can promote the growth of one pollutant and reduce another one. Our findings suggest that designing the suitable co-controlling strategies for PM_2.5-O₃ sustainable reduction should target at driving factors by considering the contra-directional and co-directional effects under suitable sensitivity regions. For co-directional driving factors, the design of suitable mitigation strategies will jointly achieve effective reduction in PM_2.5 and O₃; while for contra-directional driving factors, it should be more patient, otherwise, it is possible to reduce one item but increase another one at the same time.