Cross-regional transport potentially contributes to PM_2.5 nitrate (pNO₃), and this can occur as indirect transport, through which pNO₃ precursors are transported to targeted regions, wherein they subsequently react with locally emitted ones to produce pNO₃. However, this process has been rarely studied, which limits its comprehensive understanding. We applied the CMAQ model to study the contributions and mechanisms of pNO₃ transport during autumn in the Pearl River Delta (PRD), a metropolitan region under the growing influence of cross-regional transport on PM_2.5 pollution. Results showed that cross-regional transport contributed to 58% pNO₃ monthly in the PRD, and this mostly occurred as an indirect transport contribution (accounting for 43% among all contributions). For the first time, we identified the mechanism of indirect pNO₃ transport in the PRD, which mainly involves transported O₃ and locally emitted NO_x reacting to produce pNO₃ through N₂O₅ heterogeneous hydrolysis. pNO₃ contributions in different periods and regions indicated differences in the indirect transport contributions of N₂O₅ heterogeneous hydrolysis under varying O₃ availability conditions, which are determined by wind fields and the intensity of NO_x emissions. On a regional scale, the pNO₃ level is controlled by both transported O₃ and local NO_x emissions, but pNO₃ sensitivity to these two precursors varies among cities. This study demonstrates the significant effect and complex process of cross-regional pNO₃ transport in the PRD. Considering the important role of transported O₃ for pNO₃, O₃ reduction within a larger scale is required to achieve PM_2.5 pollution control target.

跨区域运输可能促成PM _2.5硝酸盐（pNO ₃），这可以作为间接运输发生，通过该运输，pNO ₃前体被运输到目标区域，随后它们与局部排放的前者反应生成pNO ₃。但是，这个过程很少被研究，这限制了它的全面理解。我们应用CMAQ模型研究了珠江三角洲（PRD）秋季pNO ₃传输的贡献和机制，珠江三角洲是一个大都市区，在跨区域运输对PM _2.5污染的影响日益增加的情况下。结果表明，跨区域运输贡献了58％的pNO ₃在珠三角地区每月一次，这主要是作为间接运输贡献（占所有贡献的43％）。首次，我们确定间接PNO的机构₃传输在PRD，它主要包括输送ö ₃和本地所发射NO _X反应以产生PNO ₃至N ₂ ö ₅异构水解。不同时期和地区的pNO ₃贡献表明，在不同的O ₃条件下，N ₂ O ₅非均相水解的间接转运贡献有所不同可用性的条件下，它们通过风场确定和NO的强度_X排放。在区域范围内，pNO _3的水平受运输的O ₃和局部NO _x排放的控制，但是pNO ₃对这两种前体的敏感性在城市之间有所不同。这项研究证明了跨区域pNO ₃转运在珠三角中的重要作用和复杂过程。考虑到运输的O ₃对于pNO ₃的重要作用，为了实现PM _2.5污染控制目标，需要在更大范围内还原O ₃。