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 (inline-formula $M1inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg8aeb386a576ed6c8280ae774099f80e4 acp-20-1469-2020-ie00001.svg24pt15ptacp-20-1469-2020-ie00001.png ). Moreover, both the hourly detected particle number and the relative abundance of NOCs are highly correlated with those of secondary oxidized organics and inline-formula $M2inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg607f7ebf9a4fde3320a23a055c7bd38e acp-20-1469-2020-ie00002.svg24pt15ptacp-20-1469-2020-ie00002.png . Therefore, it is hypothesized that the secondary formation of NOCs is most likely linked to oxidized organics and inline-formula $M3inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimgf83a9f1907f38a5589c34b239e10518b acp-20-1469-2020-ie00003.svg24pt15ptacp-20-1469-2020-ie00003.png . Results from both multiple linear regression analysis and positive matrix factorization analysis further show that the relative abundance of NOCs could be well predicted (inline-formulaR² > 0.7, inline-formulap < 0.01) by oxidized organics and inline-formula $M6inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg6ca56caa63735c5009fe6b299c1a126b acp-20-1469-2020-ie00004.svg24pt15ptacp-20-1469-2020-ie00004.png .

Interestingly, the relative abundance of NOCs is inversely correlated with inline-formula $M7inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg3226c502fdca30fe88bf9305df4b3716 acp-20-1469-2020-ie00005.svg24pt15ptacp-20-1469-2020-ie00005.png , whereas their number fractions are positively correlated. This result suggests that although the formation of NOCs does require the involvement of inline-formula $M8inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 3}}/{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 50pt15ptsvg-formulamathimg309095706beb5b1764d6fb46e0ba6fa3 acp-20-1469-2020-ie00006.svg50pt15ptacp-20-1469-2020-ie00006.png , the relative amount of inline-formula $M9inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg9641cdd414b305565815b5b604dabf23 acp-20-1469-2020-ie00007.svg24pt15ptacp-20-1469-2020-ie00007.png may have a negative effect. Higher humidity and inline-formulaNO_x likely facilitates the conversion of oxidized organics to NOCs. Due to the relatively high oxidized organics and inline-formula $M11inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 3}}/{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 50pt15ptsvg-formulamathimgee7df86abbfea3f6aae29dce7aec2231 acp-20-1469-2020-ie00008.svg50pt15ptacp-20-1469-2020-ie00008.png , 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 inline-formula $M12inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg0cbc4fa826540dbaebe9f7dd184b3bee acp-20-1469-2020-ie00009.svg24pt15ptacp-20-1469-2020-ie00009.png . These findings have substantial implications for the role of inline-formula $M13inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg9d8f7ee8bf88d657d75cdcf077dbe3e2 acp-20-1469-2020-ie00010.svg24pt15ptacp-20-1469-2020-ie00010.png in the atmosphere, particularly in models that predict the evolution and deposition of NOCs.

Highlights.

• d1e397NOCs were highly internally mixed with photochemically produced secondary oxidized organics

• d1e401NOCs could be well predicted by the variations of these oxidized organics and inline-formula $M14inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimgcb88f58b3b25b473f7c5a29ace587a7f acp-20-1469-2020-ie00011.svg24pt15ptacp-20-1469-2020-ie00011.png

• d1e417Higher relative humidity and inline-formulaNO_x may facilitate the conversion of these oxidized organics to NOCs