<p>Nitrogen-containing organic compounds (NOCs) substantially contribute to light absorbing organic aerosols, although the atmospheric processes responsible for the secondary formation of these compounds are poorly understood. In this study, seasonal atmospheric processing of NOCs were investigated by single particle mass spectrometry in urban Guangzhou from 2013–2014. The relative abundance of NOCs was found to be strongly enhanced by internal mixing with the photochemically produced secondary oxidized organics (such as formate, acetate, pyruvate, methylglyoxal, glyoxylate, oxalate, malonate and succinate). Furthermore, the co-occurrence of NOCs with ammonium was also observed. Interestingly, the relative abundance of NOCs was inversely correlated with ammonium, while their number fractions were positively correlated. Multiple linear regression analysis and positive matrix factorization analysis were performed to predict the relative abundance of NOCs generated from oxidized organics and ammonium. Both results showed close associations (R<sup>2</sup> > 0.7, <i>p</i> < 0.01) between the predicted NOCs and the observed values. Increased humidity and higher particle acidity were found to promote the production of NOCs. Higher relative contributions of NOCs were observed in summer and autumn, in comparison to spring and winter, due to the relatively higher contribution of oxidized organics and NH<sub>3</sub>/NH<sup>+</sup><sub>4</sub> in summer and autumn periods. To the best of our knowledge, this is the first direct field observation study establishing a close association between NOCs and both oxidized organics and ammonium. These findings have substantial implications on the role of ammonium in the atmosphere, particularly in models predicting the evolution and deposition of NOCs.</p>