Nitration of Urban PM2.5: A Hidden Driver of Enhanced Human Inflammation.

Nitrogen oxides (NOx) are well-established air pollutants that have been epidemiologically linked to adverse outcomes. However, their complex chemical reactivity obscures the mechanisms through which specific nitrogenous species are responsible for observed health risks. We hypothesize that NOx-driven nitration processes generate nitrated aromatic compounds (NACs) as potential health threat contributors. To test this, we performed a longitudinal, population-based exposomic study in Beijing. ∼1000 NACs were characterized from personal PM2.5, with median concentrations higher than those of traditional polycyclic aromatics (PACs, represented by PAHs). Remarkably, secondary NACs, particularly those formed via aqueous-phase nitration and represented by 3-/5-nitrosalicylic acids, accounted for over 70% of total NAC exposure in summer. Their associations with lung and systemic inflammatory biomarkers of IL-6 were 2.1-232 times stronger than those of PAHs and gaseous NOx. Our findings provided a new perspective for considering secondary nitration transformations in urban air quality standards.