# Fast particulate nitrate formation via N 2O 5 uptake aloft in winter in Beijing

Particulate nitrate (inline-formula $M2inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimgcef943f6f87bf3bf2b1a023aa0f3fd51 acp-18-10483-2018-ie00001.svg32pt16ptacp-18-10483-2018-ie00001.png ) is an important component of secondary aerosols in urban areas. Therefore, it is critical to explore its formation mechanism to assist with the planning of haze abatement strategies. Here we report vertical measurements of inline-formulaNOx and inline-formulaO3 by in situ instruments on a movable carriage on a tower during a winter heavy-haze episode (18 to 20 December 2016) in urban Beijing, China. Based on the box model simulation at different heights, we found that inline-formula $M5inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimgbe1d83314dd78397928ad372d41ea35e acp-18-10483-2018-ie00002.svg32pt16ptacp-18-10483-2018-ie00002.png formation via inline-formulaN2O5 heterogeneous uptake was negligible at ground level due to inline-formulaN2O5 concentrations of near zero controlled by high NO emissions and NO concentration. In contrast, the contribution from inline-formulaN2O5 uptake was large at high altitudes (e.g., > 150 m), which was supported by the lower total oxidant (inline-formulaNO2inline-formula+inline-formulaO3) level at high altitudes than at ground level. Modeling results show the specific case that the nighttime integrated production of inline-formula $M12inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimg48839bfe031139d539e943933d3a567c acp-18-10483-2018-ie00003.svg32pt16ptacp-18-10483-2018-ie00003.png for the high-altitude air mass above urban Beijing was estimated to be 50 inline-formulaµg minline-formula−3 and enhanced the surface-layer inline-formula $M15inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimgd77a9ba05dac57580168ff923000814c acp-18-10483-2018-ie00004.svg32pt16ptacp-18-10483-2018-ie00004.png the next morning by 28 inline-formulaµg minline-formula−3 through vertical mixing. Sensitivity tests suggested that the nocturnal inline-formulaNOx loss by inline-formulaNO3inline-formulaN2O5 chemistry was maximized once the inline-formulaN2O5 uptake coefficient was over 2 inline-formula× 10inline-formula−3 on polluted days with inline-formulaSa at 3000 inline-formulaµminline-formula2 cminline-formula−3 in wintertime. The case study provided a chance to highlight the fact that inline-formula $M28inlinescrollmathmlchemp{\mathrm{normal NO}}_{normal 3}^{-}$ 32pt16ptsvg-formulamathimg407c7b3a4ba3c83dba4a595f4ce640cf acp-18-10483-2018-ie00005.svg32pt16ptacp-18-10483-2018-ie00005.png formation via inline-formulaN2O5 heterogeneous hydrolysis may be an important source of particulate nitrate in the urban airshed during wintertime.

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Wang, Haichao / Lu, Keding / Chen, Xiaorui / et al: Fast particulate nitrate formation via N2O5 uptake aloft in winter in Beijing. 2018. Copernicus Publications.

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