Aligning experimental and model perspectives on atmospheric nanoparticle growth

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. The growth of freshly formed molecular clusters should in theory be crucial for the climate impact of new particle formation, influencing the survival probability of these particles exponentially and determining their ability to act as cloud condensation nuclei. However, defining the fundamental aspects of nanoparticle growth is intricate. It involves a complex interplay of condensational and reactive vapor uptake, aerosol coagulation, sink processes, and a diverse array of potential gaseous precursors. Observational nanoparticle growth rates, derived from the evolution of the particle-size distribution, portray growth as a collective phenomenon. However, models often interpret these rates at a single-particle level, integrating them into simplified size-distribution representations (Stolzenburg et al., 2023). dditionally, many models only consider a limited subset of condensable vapors, while recent experimental observations identify an increasing number of potential contributors to new particle growth.

Our objective here is to bridge the gap between experimental and modeling studies on nanoparticle growth. We compare three large-scale models (NorESM, ECHAM, and TM5) regarding their sensitivity to organic nanoparticle growth processes. Surprisingly, we find a much lower sensitivity than anticipated from box models. Through the inclusion of a sectional scheme into NorESM, we demonstrate that representing the complexity of size distribution dynamics leads to significantly different cloud condensation nuclei (CCN) levels. Furthermore, our results suggest that, on regional scales, sensitivity to organic growth is much higher. Inclusion of additional growth processes and/or a scaling of condensable vapor concentrations could yield a significantly altered climate response. In turn, comprehensive experimental observations from e.g. the open oceans are still lacking and we show that continental data exhibit surprisingly little variation in measured particle growth rates. The latter indicates limited sensitivity in current experimental approaches and potential unaccounted multi-phase chemistry in the growth process.

Consequently, we propose specific guidance for future research to address questions regarding the buffered climate response in large-scale models and the unexpectedly low variation observed in global growth measurements. We advocate for more sensitivity studies and improved model-measurement comparisons.

References:

Stolzenburg, D., Cai, R., Blichner, S. M., Kontkanen, J., Zhou, P., Makkonen, R., Kerminen, V.-M., Kulmala, M., and Kangasluoma, J.: Atmospheric nanoparticle growth, Rev. Mod. Phys., 95, 045002, https://doi.org/10.1103/RevModPhys.95.045002, 2023.