Important Role of NO3Radical to Nitrate Formation Aloft in Urban Beijing: Insights from Triple Oxygen Isotopes Measured at the Tower

Until now, there has been a lack of knowledgeregarding the vertical profiles of nitrate formation in the urbanboundary layer (BL) based on triple oxygen isotopes. Here, weconducted vertical measurements of the oxygen anomaly of nitrate(Delta 17O-NO3-) on a 325 m meteorological tower in urban Beijingduring the winter and summer. The simultaneous vertical measure-ments suggested different formation mechanisms of nitrate aerosolsat ground level and 120 and 260 m in the winter due to the lessefficient vertical mixing under stable atmospheric conditions.Particularly, different chemical processes of nitrate aerosols at thethree heights were found between clean days and polluted days in thewinter. On clean days, nocturnal chemistry (NO3+ HC and N2O5uptake) contributed to nitrate production equally with OH/H2O+NO2at ground level, while it dominated aloft (contributing 80% ofnitrate production at 260 m), due to the higher aerosol liquid water content and O3concentration there. On polluted days, nocturnalreactions dominated the formation of nitrate at the three heights. Particularly, the contribution of the OH/H2O+NO2pathway tonitrate production increased from the ground level to 120 m might be attributed to the hydrolysis of NO2to HONO and thenfurther photolysis to OH radicals in the day. In contrast, the proportion of N2O5+H2O decreased at 260 m, likely due to the lowrelative humidity aloft that inhibited the N2O5hydrolysis reactions in the residual layer. Our results highlighted that the differencesbetween meteorology and gaseous precursors could largely affect particulate nitrate formation at different heights within the pollutedurban BL.

Automated summary

This study investigates the vertical profiles of nitrate formation in the urban boundary layer (BL) based on triple oxygen isotopes. The results highlight the different formation mechanisms of nitrate aerosols at different heights and under different atmospheric conditions, emphasizing the importance of meteorology and gaseous precursors in particulate nitrate formation.