Heatwave reveals potential for enhanced aerosol formation in Siberian boreal forest

Siberia is covered by 6 million km(2) of forest, which moderates climate as a carbon sink and a source of aerosol particles causing negative radiative effect. Aerosol particles in boreal forests frequently form via gas-to-particle conversion, known as new particle formation (NPF). Compared to boreal sites at similar latitudes, NPF was reported to occur less often in the Siberian forest. However, factors controlling NPF in Siberia remain unknown. Our results suggest that the combination of biogenic and anthropogenic contributions caused unexpectedly high monthly NPF frequency (50%) at the observatory Fonovaya in the West Siberian taiga during the Siberian 2020 heatwave. High frequency was due to early spring photosynthetic recovery, which boosted biogenic emissions into polluted air masses carrying SO2. After mid-April, high temperatures and cleaner air masses led to less frequent (15%) and less intense NPF despite the increased emissions of natural organic vapors and ammonia. Furthermore, the contrast between the two spring periods was seen in cluster composition, particle-forming vapors (two times difference in sulfuric acid concentration), particle formation (J(3), 2.2 and 0.4 cm(-3) s(-1)) and growth rates (GR(2-3), 1.7 and 0.6 nm h(-1)). Given the strong warming trend, our results suggest that within 25-30 years, the monthly NPF frequency during early spring in the West Siberian taiga can reach 40%-60%, as in the European boreal sites.