Nitroaromatic Compounds from Secondary Nitrate Formation and Biomass Burning Are Major Proinflammatory Components in Organic Aerosols in Guangzhou: A Bioassay Combining High-Resolution Mass Spectrometry Analysis

The limited characterization and detection capacity of unknown compounds hinder our understanding of the molecular composition of toxic compounds in PM_2.5. The present study applied Fourier transform ion cyclotron resonance mass spectrometry coupled with negative and positive electrospray ionization sources (ESI^–/ESI⁺ FT-ICR-MS) to probe the molecular characteristics and dynamic formation processes of the effective proinflammatory components in organic aerosols (OAs) of PM_2.5 in Guangzhou for one year. We detected abundant proinflammatory molecules in OAs, mainly classified as CHON compounds (compounds composed of C, H, O, and N atoms) in elemental and nitroaromatic compounds (NACs) in structures. From the perspective of the formation process, we discovered that these proinflammatory molecules, especially toxic NACs, were largely driven by secondary nitrate formation and biomass burning (in emission source), as well as SO₂ (in atmospheric evolution). In addition, our results indicated that the secondary processes had replaced the primary emission as the main contributing source of the toxic proinflammatory compounds in OAs. This study highlights the importance of community measures to control the production of nitroaromatic compounds derived from secondary nitrate formation and biomass burning in urban areas.