Important Role of NO₃ Radical to Nitrate Formation Aloft in Urban Beijing: Insights from Triple Oxygen Isotopes Measured at the Tower

Until now, there has been a lack of knowledge regarding the vertical profiles of nitrate formation in the urban boundary layer (BL) based on triple oxygen isotopes. Here, we conducted vertical measurements of the oxygen anomaly of nitrate (Δ¹⁷O-NO₃^–) on a 325 m meteorological tower in urban Beijing during the winter and summer. The simultaneous vertical measurements suggested different formation mechanisms of nitrate aerosols at ground level and 120 and 260 m in the winter due to the less efficient vertical mixing under stable atmospheric conditions. Particularly, different chemical processes of nitrate aerosols at the three heights were found between clean days and polluted days in the winter. On clean days, nocturnal chemistry (NO₃ + HC and N₂O₅ uptake) contributed to nitrate production equally with OH/H₂O + NO₂ at ground level, while it dominated aloft (contributing 80% of nitrate production at 260 m), due to the higher aerosol liquid water content and O₃ concentration there. On polluted days, nocturnal reactions dominated the formation of nitrate at the three heights. Particularly, the contribution of the OH/H₂O + NO₂ pathway to nitrate production increased from the ground level to 120 m might be attributed to the hydrolysis of NO₂ to HONO and then further photolysis to OH radicals in the day. In contrast, the proportion of N₂O₅ + H₂O decreased at 260 m, likely due to the low relative humidity aloft that inhibited the N₂O₅ hydrolysis reactions in the residual layer. Our results highlighted that the differences between meteorology and gaseous precursors could largely affect particulate nitrate formation at different heights within the polluted urban BL.