Sources and Formation Characteristics of Particulate Nitrate in the Pearl River Delta Region of China: Insights from Three-Year Online Observations

Nitrate (NO3−) has been identified as a key component of particulate matter (PM2.5) in China. However, there is still a lack of understanding regarding its sources and how it forms, especially in the context of high-frequency and long-term data. In this study, NO3− levels were observed on an hourly basis over an almost three-year period at an urban site in the Pearl River Delta (PRD) region, China, from January 2019 to December 2021. The results reveal an average daily NO3− concentration ranging from 0.08 μg m−3 to 61.69 μg m−3, constituting 11.9 ± 12.5 % of PM2.5. This percentage rose to as high as 57 % during pollution episodes, highlighting NO3−'s significant role in pollution formation. The ammonia-rich environment was found to be the most important factor in promoting NO3− formation. Positive Matrix Factorization (PMF) analysis indicates that the primary sources of NO3− in the PRD region were vehicle emissions (43.8 ± 21.2 %) and coal combustion (39.1 ± 21.5 %), with shipping emissions, sea salt, soil dust and industrial emissions + biomass burning following in importance. Regarding source areas, the primary contributor of vehicle emissions was predominantly from the PRD region, whereas the coal combustion, aside from local contributions, also originates from the northern region. From a long-term perspective, NO3− pollution has remained relatively stable since the summer of 2020. Concurrently, coal combustion source has shown a localization trend. These insights derived from the extensive, high-frequency observation presented in this study serve as a valuable reference for devising strategies to control NO3− and PM2.5 in the PRD region and China.