Seasonal Characterization of Organic Nitrogen in Atmospheric Aerosols Using High Resolution Aerosol Mass Spectrometry in Beijing, China

Despite extensive efforts to characterize organic nitrogen (ON) in atmospheric aerosols, knowledge of the sources and processes of ON in the megacity of Beijing is still limited, mainly due to the complexity of ON species and the absence of highly time-resolved measurements. Here we demonstrate the applications of Aerodyne high-resolution time of-flight aerosol mass spectrometer combined with positive matrix factorization in characterization of ON in submicron aerosols. Our results show that the average nitrogen-to-carbon ratios (N/C) vary from 0.021 to 0.028, and the average ON concentrations range from 0.26 to 0.59 mu g m(-3) during four seasons in Beijing. ON accounts for 7-10% of the total nitrogen (TN) on average, yet the sources vary differently across different seasons. We found that 56-65% of ON was secondary during three seasons except winter when 59-67% was related to primary emissions. Particularly, more oxidized secondary organic aerosol contributes the dominant fraction of ON (39-44%) in spring, summer and autumn, while biomass burning is a more important source of ON in winter (23-44%). These results are consistent with the better positive correlations between N/C and oxygen-to-carbon ratio, a surrogate of organic aerosol aging, during these three seasons than that in winter. N/C also shows a clear increase as a function of relative humidity during all seasons, suggesting that aqueous-phase processing likely played an important role in formation of nitrogen-containing compounds. In addition, the uncertainties and limitations in quantification of ON with aerosol mass spectrometry are illustrated, particularly, ON could be underestimated by similar to 20-42% by ignoring the fragment contributions in NHx+ and NOx+.