The significant contribution of biomass burning to methanol-soluble nitrogenous organics and its evolution during the highly-humid haze event in urban Wuhan

Fine particle (PM_2.5) controls in China have achieved great success these years, but heavy haze pollution still occurs occasionally. The sources and evolution of the complex organic mixtures remain poorly resolved. We collected 46 PM_2.5 samples during a 12-day severe haze episode in Wuhan, a megacity in central China. We analyzed the molecular-level organic composition and functional groups with high-performance liquid chromatography-orbitrap tandem mass spectrometry (HPLC-Orbitrap MS/MS). The methanol-soluble nitrogenous organics (ONs) contributed 87.6% and 52.5% of total signal intensity in positive electrospray ionization (ESI + ) mode and negative electrospray ionization (ESI-) mode, respectively. Through source apportionment and backward trajectory analysis, we found great contribution of biomass burning (~21.2% of ONs in ESI+ mode and 78.2% in ESI- mode) and significant aqueous/heterogenous products (~71.2% of ONs in ESI+ mode and 18.7% in ESI- mode) during the prolonged, highly-humid evolution (relative humidity ~86.8% lasting for 7 days). We identified C6–C12 amines and C6-C9 nitrophenols as possible molecular markers for biomass burning emissions, while C17-C19 amine oligomers and dicarboxylic acids as secondary markers from heterogeneous/aqueous reactions. The evolution may include three pathways: (1) polymerization of amines with an oxygenated molecule to form long-chain oxygenated amine oligomers; (2) ring-opening fragmentation of nitrophenols to form dicarboxylic acids; (3) ring-retaining functionalization of nitroaromatics to form nitroaromatic carboxylic acids. Pathway 3 has rarely been observed in ambient air, which is possibly related with the extremely high humidity during the periods and deserves further studies. Lastly, we found that ~39.3% of ON compounds had isomers, and ~43.7% of these isomers had distinct timeseries and originated from different sources, strengthening the necessity of measuring isomers. With the help of tandem high-resolution mass spectrometry, this study provides valuable datasets for the molecular and structural information on sources and evolutions of ONs under the highly-humid urban atmospheres.