Identification and quantification of particle growth channels during new particle formation

by M. R. Pennington, B. R. Bzdek, J. W. Depalma, J. N. Smith, A. M. Kortelainen, L. Hildebrandt Ruiz, T. Pet??j??, M. Kulmala, D. R. Worsnop, M. V. Johnston show all authors
Atmospheric Chemistry and Physics ()
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Abstract

Atmospheric new particle formation (NPF) is a key source of ambient\nultrafine particles that may contribute substantially to the global\nproduction of cloud condensation nuclei (CCN). While NPF is driven by\natmospheric nucleation, its impact on CCN concentration depends strongly\non atmospheric growth mechanisms since the growth rate must exceed the\nloss rate due to scavenging in order for the particles to reach the CCN\nsize range. In this work, chemical composition measurements of 20 nm\ndiameter particles during NPF in Hyytiala, Finland, in March-April 2011\npermit identification and quantitative assessment of important growth\nchannels. In this work we show the following: (A) sulfuric acid, a key\nspecies associated with atmospheric nucleation, accounts for less than\nhalf of particle mass growth during this time period; (B) the sulfate\ncontent of a growing particle during NPF is quantitatively explained by\ncondensation of gas-phase sulfuric acid molecules (i.e., sulfuric acid\nuptake is collision-limited); (C) sulfuric acid condensation\nsubstantially impacts the chemical composition of preexisting\nnanoparticles before new particles have grown to a size sufficient to be\nmeasured; (D) ammonium and sulfate concentrations are highly correlated,\nindicating that ammonia uptake is driven by sulfuric acid uptake; (E)\nsulfate neutralization by ammonium does not reach the predicted\nthermodynamic end point, suggesting that a barrier exists for ammonia\nuptake; (F) carbonaceous matter accounts for more than half of the\nparticle mass growth, and its oxygen-to-carbon ratio (similar to 0.5) is\ncharacteristic of freshly formed secondary organic aerosol; and (G)\ndifferences in the overall growth rate from one formation event to\nanother are caused by variations in the growth rates of all major\nchemical species, not just one individual species.

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