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. Nitrogen-containing organic compounds (NOCs) substantially contribute tolight-absorbing organic aerosols, although the atmospheric processesresponsible for the secondary formation of these compounds are poorlyunderstood. In this study, seasonal atmospheric processing of NOCs isinvestigated using single-particle mass spectrometry in urban Guangzhou from2013 to 2014. The relative abundance of NOCs is found to be strongly enhancedwhen they are internally mixed with photochemically produced secondary oxidizedorganics (i.e., formate, acetate, pyruvate, methylglyoxal, glyoxylate,oxalate, malonate, and succinate) and ammonium ( NH 4 + ). Moreover, both the hourlydetected particle number and the relative abundance of NOCs are highlycorrelated with those of secondary oxidized organics and NH 4 + . Therefore, it is hypothesized that the secondary formation of NOCs is most likely linked to oxidized organics and NH 4 + . Results from both multiple linearregression analysis and positive matrix factorization analysis further showthat the relative abundance of NOCs could be well predicted ( R2  > 0.7, p NH 4 + . Interestingly, the relative abundance of NOCs is inversely correlated with NH 4 + , whereas their number fractions are positively correlated. Thisresult suggests that although the formation of NOCs does require theinvolvement of NH 3 / NH 4 + , the relative amount of NH 4 + may havea negative effect. Higher humidity and NOx likely facilitates the conversionof oxidized organics to NOCs. Due to the relatively high oxidized organicsand NH 3 / NH 4 + , the relative contributions of NOCs in summer andfall were higher than those in spring and winter. To the best of ourknowledge, this is the first direct field observation study reporting aclose association between NOCs and both oxidized organics and NH 4 + .These findings have substantial implications for the role of NH 4 + in theatmosphere, particularly in models that predict the evolution and depositionof NOCs. Highlights. NOCs were highly internally mixed with photochemically produced secondary oxidized organics NOCs could be well predicted by the variations of these oxidized organicsand NH 4 + Higher relative humidity and NOx may facilitate the conversion of theseoxidized organics to NOCs

High secondary formation of nitrogen-containing organics (NOCs) and its possible link to oxidized organics and ammonium