Size Distribution and Secondary Formation of Particulate Organic Nitrates in Winter in a Coastal Area

Understanding the size distribution, variation patterns, and potential formation mechanisms of particulate organic nitrates (PONs) is crucial for assessing their influences on atmospheric chemistry, air quality, and the regional climate. This study investigates PONs in the coastal atmosphere of Qingdao, China by collecting size-resolved particulate matter samples and analyzing six types of organic nitrates, namely pinene keto nitrate (PKN229), monoterpene hydroxyl nitrate (MHN215), monoterpene dicarbonyl nitrate (MDCN247), oleic acid hydroxyl nitrate (OAHN361), oleic acid keto nitrate (OAKN359), and pinene sulfate organic nitrate (PSON295), using ultrahigh-performance liquid chromatography(mass spectrometry). The mean total concentration of organic nitrates in fine particles reaches 677 ng m-3. The predominant compound is MHN215, followed by PKN229 and MDCN247. All six organic nitrates exhibited distinct concentration peaks in the droplet mode, with MDCN247 and OAHN361 showing a minor peak in the condensation mode. In addition, an apparent concentration peak is observed in the coarse mode for OAKN359. Comparative analyses under various conditions highlight the significant influences of primary emissions and secondary formation processes on the abundance and size distribution of organic nitrates. For instance, both firework displays during festivals and high NOx emissions from fuel combustion significantly increase the concentrations of condensation-mode organic nitrates, whereas dust particles enhanced the heterogeneous formation of coarse-mode particles. Furthermore, the high humidity of the coastal area promotes aqueous formation in droplet-mode particles.