Cross-regional transport of PM nitrate in the Pearl River Delta, China: Contributions and mechanisms.

Cross-regional transport potentially contributes to PM nitrate (pNO), and this can occur as indirect transport, through which pNO precursors are transported to targeted regions, wherein they subsequently react with locally emitted ones to produce pNO. However, the process has been rarely studied, which limits its comprehensive understanding. We applied the CMAQ model to study the contributions and mechanisms of pNO transport during autumn in the Pearl River Delta (PRD), a metropolitan region under the growing influence of cross-regional transport on PM pollution. Results showed that cross-regional transport contributed to 58% pNO monthly in the PRD, and this mostly occurred as indirect transport contributions (accounting for 43% among all contributions). For the first time, we identified the mechanism of indirect pNO transport in the PRD, which mainly involves transported O and locally emitted NO reacting to produce pNO through NO heterogeneous hydrolysis. pNO contributions in different periods and regions indicated differences in the indirect transport contributions to NO heterogeneous hydrolysis under varying O availability conditions, which are determined by wind fields and the intensity of NO emissions. On the regional scale, the pNO level is controlled by both transported O and local NO emissions, but pNO sensitivity to these two precursors varies among cities. This study demonstrates the notable effect and complex process of cross-regional pNO transport in the PRD. Considering the important role of transported O for pNO, O reduction within a larger scale is required to achieve PM pollution control target.