Fast particulate nitrate formation via N₂O₅ uptake aloft in winter in Beijing

Particulate nitrate (pNO₃ ^-) 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 NO_x and O3 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 pNO₃ ^- formation via N₂O₅ heterogeneous uptake was negligible at ground level due to N₂O₅ concentrations of near zero controlled by high NO emissions and NO concentration. In contrast, the contribution from N₂O₅ uptake was large at high altitudes (e.g., > 150 m), which was supported by the lower total oxidant (NO₂ + O₃) level at high altitudes than at ground level. Modeling results show the specific case that the nighttime integrated production of pNO₃ ^- for the high-altitude air mass above urban Beijing was estimated to be 50 μg m^-3 and enhanced the surface-layer pNO₃ ^- the next morning by 28 μg m^-3 through vertical mixing. Sensitivity tests suggested that the nocturnal NO_x loss by NO₃-N₂O₅ chemistry was maximized once the N₂O₅ uptake coefficient was over 2 × 10^-3 on polluted days with Sa at 3000 μm² cm^-3 in wintertime. The case study provided a chance to highlight the fact that pNO₃ ^- formation via N₂O₅ heterogeneous hydrolysis may be an important source of particulate nitrate in the urban airshed during wintertime.