Nanjing study finds summer haze now driven mostly by secondary particles

By AI, Created 12:20 PM UTC, May 25, 2026, /AGP/ – Researchers in Nanjing found that nearly 70% of summer PM1 pollution comes from chemical formation in the atmosphere, not direct emissions. The study shows weather and sunlight now play a major role in shaping urban haze, with implications for how China tackles fine-particle pollution.

Why it matters: - Summer fine-particle pollution in Nanjing is increasingly shaped by chemistry in the air, not just tailpipes, smokestacks or other direct sources. - The shift means air-quality controls focused only on cutting emissions may miss a growing share of the pollution problem. - The findings may help policymakers target haze episodes more effectively during different weather conditions.

What happened: - Researchers at Nanjing University of Information Science and Technology measured submicron particles, or PM1, in Nanjing during the summer of 2022. - The study was published online March 27, 2025 in Volume 159, Issue 1 of the Journal of Environmental Sciences. - The paper is titled “Chemical characteristics of fine aerosols and associated speciated organic compounds in summer Nanjing, China.” - The DOI is https://doi.org/10.1016/j.jes.2025.03.044.

The details: - Secondary pollutants made up nearly 70% of the measured PM1 mass. - Sunlight-driven photochemical reactions strongly promoted sulfate and secondary organic aerosol formation. - Humidity played a major role in nitrate formation. - The team used advanced real-time aerosol mass spectrometry to track secondary organic aerosols, sulfate, nitrate, organic nitrogen compounds, organosulfates and polycyclic aromatic hydrocarbons, or PAHs. - The researchers identified two types of secondary organic aerosols. - One type formed rapidly in the early afternoon through fresh photochemical reactions. - The second type reflected older, more oxidized particles that accumulated later in the day. - Primary organic particles from traffic, cooking and industrial activity can chemically transform into more oxidized secondary aerosols as they age in the atmosphere. - Sulfate formation was linked to photochemical oxidation under strong sunlight. - Nitrate formation depended more on humidity, heterogeneous reactions and gas-to-particle partitioning. - Cooler, more humid periods were associated with higher nitrate concentrations and heavier PM pollution episodes. - Organic nitrogen species originated largely from traffic and industrial emissions. - Organosulfates appeared to form mainly through weak aqueous-phase reactions under humid conditions. - PAHs were primarily linked to vehicle emissions, especially diesel traffic. - Sunlight-driven oxidation may help remove PAHs from the atmosphere during afternoon hours and convert them into secondary organic aerosol products.

Between the lines: - The study points to a broader shift in urban air pollution as direct emissions fall and secondary formation becomes more important. - Weather is not just a background condition here. It changes the chemistry that determines which pollutants build up. - That creates a moving target for regulators, because the same city can face different pollution drivers depending on sunlight, humidity and temperature.

What’s next: - The researchers say future mitigation needs to address atmospheric chemistry, not only direct emissions. - Pollution-control strategies may need to adapt dynamically to changing weather conditions. - The results could inform more precise air-quality planning in rapidly developing cities across China and beyond.