Atmospheric particulate pollutants pose threats to both human health and ecosystems. According to Atmospheric China 2025: Best Practices in Air Pollution Prevention and Control in China, although China's annual average PM2.5 concentration now meets the National Ambient Air Quality Standard (35 μg/m3), it remains higher than the World Health Organization's guideline (5 μg/m3).
The formation of aerosols from molecular precursors is a key process driving particulate pollution, making precise characterization of their chemical composition and nucleation mechanisms essential for elucidating formation pathways and informing effective control measures. As priority pollutants in air quality management, nitrogen oxides (NOx = NO + NO2) and sulfur dioxide (SO2) play critical yet incompletely understood roles in secondary organic aerosol (SOA) formation from volatile organic compounds.
In a study published in Environmental Science & Technology, a research team led by Prof. JIANG Ling and Prof. LI Gang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) revealed distinct roles of NOx (NO2 and NO), as well as their synergistic interactions with SO2, in SOA formation during β-myrcene photooxidation.
Schematic illustration of the synergistic effects of NOx and SO2 on β-myrcene photooxidation (Image by ZHAO Ya)
Using a DICP-CAS smog chamber with advanced online analytical instruments, the researchers investigated the SOA formation from β-myrcene photooxidation. The results revealed a significant difference between the effects of NO2 and NO. NO2 enhanced SOA yield and increased the oxygen-to-carbon (O:C) ratio through oxidant amplification. In contrast, NO suppressed particle nucleation and reduced product oxidation states, while promoting particle growth by facilitating the partitioning of organic nitrates into the particulate phase. SO2 exhibited strong synergistic effects with both NO2 and NO, facilitating particle formation and growth.
Furthermore, the researchers developed a novel aerosol mass spectrometer based on a tunable vacuum ultraviolet free electron laser (VUV-FEL). The newly-built instrument enabled the detection of previously unobserved compounds, including organic peroxides, organic nitrate, and organosulfates. Combined with quantum chemical computations, the researchers proposed detailed oxidation pathways for the formation of these compounds.
"The identification and analysis of these novel compounds help to correct the biases in SOA formation predictions and provide more precise data for understanding aerosol formation mechanisms in regions influenced by both anthropogenic and biogenic sources," said Prof. JIANG.