Sulfur-Nitrogen Chemistry Shift Reshapes Airborne Particles, Single-Particle Study Shows

New single-particle research from Yangzhou shows that uneven emission cuts change aerosol chemistry in ways bulk models miss. When COVID lockdowns cut nitrogen oxides far more than sulfur dioxide, the gas-phase sulfur-to-nitrogen ratio rose sharply, and particles grew larger with more sulfate coating. The effect was strongest on mixed black carbon and organic particles, and it flipped with humidity: below 55% relative humidity particle types stayed distinct, above 85% they converged toward a uniform mix. The study tracked over 324,000 individual particles and used causal analysis plus machine learning to show that gas-phase chemistry drives particle-level partitioning. That matters for air quality policy and climate modeling. Cutting NOx faster than SO2 can raise ozone by around 15% and change how soot absorbs sulfate, which affects water uptake, light scattering, and cloud formation. The authors argue future models need particle-type heterogeneity and humidity-dependent chemistry instead of bulk averages.