The Impact of Nocturnal Boundary Layer Height on Daytime Surface Particulate Nitrate Increases by Vertical Transport From the Residual Layer

Abstract Particulate nitrate (pNO 3 − ) has become a critical air‐quality issue, with pronounced morning increases in surface concentrations attributed to downward transport from upper layer. However, how the nocturnal boundary layer evolution modulates this process remains unclear. Here, we use a one‐dimensional model, constrained by vertical observations at the Shenzhen tower, to quantify how the morning surface pNO 3 − enhancement responds to nocturnal boundary‐layer height. Model revealed that the downward transport contribution is greater on days with morning pNO 3 − increases (90%) than on non‐increase days (70%). The reason is that the lower nocturnal boundary layer weakens vertical mixing and creates a steeper vertical gradient. Sensitivity tests demonstrated that reducing the nocturnal boundary‐layer height by 50% increases the maximum vertical transport rate (0.56–1.21 μg m −3 h −1 ) and correspondingly raises the maximum surface pNO 3 − concentration (0.38–2.41 μg m −3 ). These results highlight the role of boundary‐layer evolution in regulating surface pNO 3 − pollution.