Formation of Tar Balls in Wildfire Smoke Driven by Nighttime Chemistry

Wildfires are a major source of atmospheric organic aerosols, including tar balls, which are often abundant in smoke plumes. Tar balls are highly viscous, spherical, and weakly hygroscopic particles that contain light-absorbing brown carbon. As a result, they can exert strong influences on climate, air quality, and human health by absorbing solar radiation, slowing oxidative aging and enabling long-range transport of toxic compounds, and potentially affecting clouds and precipitation as ice-nucleating particles. Despite their importance, the mechanisms responsible for tar ball formation remain unresolved, with proposed pathways including heat shock near fires and polymerization of organic matter initiated by OH radicals.
Here we propose an additional formation pathway for tar balls in wildfire smoke: secondary organic aerosol (SOA) formation from reactions between 〖"NO" 〗_"3" radicals and phenolic compounds. Phenolic species are emitted in large quantities by wildfires and represent a major fraction of SOA precursors, particularly at night.
In this work, we generated SOA from 〖"NO" 〗_"3" reactions with five phenolic compounds: guaiacol, syringol, vanillin, vinylguaiacol, and isoeugenol. These compounds together represent a substantial fraction of phenolic SOA precursors in wildfire smoke. We evaluated whether the resulting particles meet key criteria for classification as tar balls. The particles are spherical, contain brown carbon, and exhibit viscosities exceeding 2〖×10〗^11 Pa s under dry conditions, remaining above 2〖×10〗^9 Pa s at elevated relative humidity.
We further show that chemical markers identified in laboratory-generated phenolic-〖"NO" 〗_"3" SOA are present in nighttime wildfire smoke samples. Together, these results provide evidence that 〖"NO" 〗_"3" radical chemistry can drive tar ball formation at night in wildfire smoke and should be considered in atmospheric models to improve predictions of wildfire impacts on air quality, human health, and climate.