Different Formation Pathways of Nitrogen-containing Organic Compounds in Aerosols and Fog Water in Northern China

While aqueous-phase processing contributes to the formation of nitrogen-containing organic compounds (NOCs), the detailed pathways are not well understood. In this study, the molecular composition of NOCs in both pre-fog aerosols and fog water collected at a suburban site in northern China was characterized using Fourier-transform ion cyclotron resonance mass spectrometry in both negative and positive modes of electrospray ionization (ESI- and ESI+). In both pre-fog aerosols and fog water, NOCs account for number fractions of more than 60 % in all assigned formulas in ESI- mode and more than 80 % in ESI+ mode. By comparing the molecular composition of biomass burning, coal combustion, and vehicle emissions, 72.3 % of NOCs in pre-fog aerosols were assigned as originating from these primary anthropogenic sources (pNOCs), while the remaining NOCs were regarded as secondary NOCs formed in aerosol (saNOCs). On the other hand, the unique NOCs in fog water were regarded as secondary NOCs formed in fog (sfNOCs). According to “precursor-product pair” screening involving 39 reaction pathways, we found that the nitration reaction, the amine pathway and the intramolecular N-heterocycle pathway of NH3 addition reactions contribute to 43.6 %, 22.1 %, and 11.6 % of saNOCs, but 26.8 %, 28.4 %, and 29.7 % of sfNOCs, respectively. Such distinct formation pathways are most likely attributed to the diverse precursors and the aqueous acidity. Correspondingly, saNOCs contain more abundant carbohydrates-like and highly oxygenated compounds with two nitrogen atoms compared to pNOCs, whereas sfNOCs contain more lipids-like with fewer oxygen atoms. The results reveal the disparity in secondary processes that contribute to the richness of NOCs in aerosols and fog water. The findings are valuable for understanding the formation and control of organic nitrogen pollution.