Large particulate nitrate formation from N<sub>2</sub>O<sub>5</sub> uptake in a chemically reactive layer aloft during winter time in Beijing

Abstract. Particulate nitrate (pNO3−) is a dominant 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. Simultaneous ground-based and tower-based measurements were conducted during a winter heavy haze episode in urban Beijing, China. We found pNO3− formation via N2O5 heterogeneous uptake was negligible at ground level, due to the presence of high NO concentrations limiting the production of N2O5. In contrast, the contribution from N2O5 uptake was larger at higher altitudes (e.g., > 150 m), which was supported by the observed large total oxidant (NO2 + O3) missing aloft compared with ground level. The nighttime integrated production potential of pNO3− for the higher altitude air mass overhead urban Beijing was estimated to be 50 μg m−3, and enhanced the surface pNO3− significantly with 28 μg m−3 after nocturnal boundary layer broken in the next morning. In this case, the oxidation of NOX to nitrate was maximized once N2O5 uptake coefficient over 0.0017, since N2O5 uptake dominated the fate of NO3 and N2O5 with the presence of large aerosol surface concentrations. These results highlight that pNO3− formation via N2O5 heterogeneous hydrolysis at higher altitude air masses aloft could be an important source for haze formation in the urban airshed during winter time. Accurately describing the formation and development of reactive air masses aloft is a critical task for improving current chemical transport models.