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

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 is investigated using single-particle mass spectrometry in urban Guangzhou from 2013 to 2014. The relative abundance of NOCs is found to be strongly enhanced when they are internally mixed with photochemically produced secondary oxidized organics (i.e., formate, acetate, pyruvate, methylglyoxal, glyoxylate, oxalate, malonate, and succinate) and ammonium (NH4+). Moreover, both the hourly detected particle number and the relative abundance of NOCs are highly correlated with those of secondary oxidized organics and NH4+. Therefore, it is hypothesized that the secondary formation of NOCs is most likely linked to oxidized organics and NH4+. Results from both multiple linear regression analysis and positive matrix factorization analysis further show that the relative abundance of NOCs could be well predicted (R-2 > 0.7, p < 0.01) by oxidized organics and NH4+. Interestingly, the relative abundance of NOCs is inversely correlated with NH4+, whereas their number fractions are positively correlated. This result suggests that although the formation of NOCs does require the involvement of NH3/NH4+, the relative amount of NH4+ may have a negative effect. Higher humidity and NOx likely facilitates the conversion of oxidized organics to NOCs. Due to the relatively high oxidized organics and NH3/NH4+, the relative contributions of NOCs in summer and fall were higher than those in spring and winter. To the best of our knowledge, this is the first direct field observation study reporting a close association between NOCs and both oxidized organics and NH4+. These findings have substantial implications for the role of NH4+ in the atmosphere, particularly in models that predict the evolution and deposition of NOCs. Highlights. NOCs were highly internally mixed with photochemically produced secondary oxidized organics NOCs could be well predicted by the variations of these oxidized organics and NH4+ Higher relative humidity and NOx may facilitate the conversion of these oxidized organics to NOC