How do the little ones grow? Solving the puzzling occurrence of new particle formation in megacities

The process of new particle formation from gas-phase precursors holds significant importance in Earth's atmosphere and introduces a notable source of uncertainty in climate change predictions. Typical conditions for new particle formation are moderate temperatures, clear sky and low background aerosol contamination. This general paradigm was challenged by the puzzling observation of frequent new particle formation in megacities. The pre-existing aerosol loadings in such environments seemed to be too high to allow clusters of being formed and grow fast enough before they encounter a collision with a background particle and get lost from the number budget.

Here, we show how nanoparticle growth in urban atmospheres is facilitated enabling efficient survival of nanoclusters providing an explanation for the occurrence of NPF in heavily polluted environments. We outline the tool set, which we have developed over the recent years to address this puzzle. Significant uncertainty in the particle number size distribution measurements and growth rate estimates were addressed through new instrumentation and analysis approaches. At the same time, we refined growth models to account for the challenges of a wide variety of potentially condensable vapors and updated our understanding of particle survival in the atmosphere.

We could demonstrate that new particle formation takes a decisive role in air quality issues in megacities, especially as nanoparticles seem to grow at surprisingly constant rates even when no new particle formation is observed. The “unique atmospheric experiment” of the Covid-19 lockdowns finally provided the chance to estimate how sensitive the urban environment is to changes in the atmospheric chemistry, especially with respect to new particle formation. While we speculated that other condensable vapors than previously thought could be part of the puzzle, we can finally show that also the population dynamics are crucial for more efficient nanoparticle survival than previously thought. However, severe challenges remain, as the outlined methodological improvements also revealed that sometimes the little ones even grow slower than expected.