Trans-regional NO₂ drives winter nitrate source and formation disparities between the North China Plain and the...

PM_2.5 and its major component nitrate both pose serious threats to human health. Despite progress under China's Clean Air Action Plan since 2013, severe haze events still occur frequently, with nitrate aerosols showing limited response to NOx reductions. Quantifying trans-regional transport's contribution to nitrate pollution remains challenging. Here, we conducted dual isotopes of nitrate collected simultaneously during winter in the North China Plain (NCP) and the Yangtze River Delta (YRD). The results suggested that nocturnal chemistry dominated nitrate formation in both regions. The NO₃+HC process contributed ∼20 % NCP, but negligible in the YRD. While nitrate sources remained stable between clean and haze days in the NCP, local mobile sources showed a higher contribution during haze than during clean days in the YRD. Trans-regional NO₂ from the NCP contributed ∼30 % to the YRD on average (higher on clean days, lower on haze days). Crucially, at the onset of severe YRD haze, transported NCP nitrate particles contributed up to 90 %. Our findings underscore the significant role that trans-regional NO₂ transport from the NCP plays in nitrate aerosol formation in the YRD, emphasizing the need for regional coordination in air quality management.