Causes and importance of new particle formation in the present‐day and preindustrial atmospheres

Abstract

New particle formation has been estimated to produce around half of cloud‐forming particles in the present‐day atmosphere, via gas‐to‐particle conversion. Here we assess the importance of new particle formation (NPF) for both the present‐day and the preindustrial atmospheres. We use a global aerosol model with parametrizations of NPF from previously published CLOUD chamber experiments involving sulfuric acid, ammonia, organic molecules, and ions. We find that NPF produces around 67% of cloud condensation nuclei at 0.2% supersaturation (CCN0.2%) at the level of low clouds in the preindustrial atmosphere (estimated uncertainty range 45–84%) and 54% in the present day (estimated uncertainty range 38–66%). Concerning causes, we find that the importance of biogenic volatile organic compounds (BVOCs) in NPF and CCN formation is greater than previously thought. Removing BVOCs and hence all secondary organic aerosol from our model reduces low‐cloud‐level CCN concentrations at 0.2% supersaturation by 26% in the present‐day atmosphere and 41% in the preindustrial. Around three quarters of this reduction is due to the tiny fraction of the oxidation products of BVOCs that have sufficiently low volatility to be involved in NPF and early growth. Furthermore, we estimate that 40% of preindustrial CCN0.2% are formed via ion‐induced NPF, compared with 27% in the present day, although we caution that the ion‐induced fraction of NPF involving BVOCs is poorly measured at present. Our model suggests that the effect of changes in cosmic ray intensity on CCN is small and unlikely to be comparable to the effect of large variations in natural primary aerosol emissions.

Cosmic rays, aerosols, clouds, and climate: Recent findings from the CLOUD experiment

“…A majority of the particles in the atmosphere are formed by nucleation from the clustering of low-volatility vapors, as opposed to primary emissions where particles are emitted directly to the atmosphere (from combustion sources, wind-blown dust, or sea spray) [Gordon et al, 2017]. However, until recently there has been considerable uncertainty involving which vapor species participate in aerosol nucleation.…”

“…While the work of the CLOUD team has made a huge impact on aerosol nucleation, growth, and their overall connection to clouds and climate, they had not yet addressed a GCR-cloud connection. In their recent article in JGR [Gordon et al, 2017], the CLOUD team present their first estimates of the effect of GCR modulations over the solar cycle on CCN (a major step toward the GCR climate impacts through the ion-aerosol clearsky effect). In the following two sections I highlight (1) the CLOUD team's impact on our understanding of nucleation and growth as well as related results done outside of CLOUD and (2) the CLOUD team's recent GCR results.…”

“…The CLOUD experiment subsequently unlocked many of the mysteries of nucleation and growth in our atmosphere, and it has improved our understanding of human influences on climate. Their most recent publication (Gordon et al 2017, https://doi.org/10.1002/2017JD026844) provides their first estimate of the GCR-CCN connection, and they show that CCN respond too weakly to changes in GCR to yield a significant influence on clouds and climate. …”

Resolving nanoparticle growth mechanisms from size- and time-dependent growth rate analysis

“…Despite their small sizes these particles might be of climate relevance through the indirect aerosol-cloud effect (Twomey et al, 1984). Modeling results suggest that this secondary aerosol formation mechanism contributes roughly 50 % of particles to the budget of cloud condensation nuclei (Spracklen et al, 2008;Merikanto et al, 2009;Gordon et al, 2017). New particle formation (NPF) has been the subject of numerous studies for several decades.…”

Temperature uniformity in the CERN CLOUD chamber

“…Most of the increased aerosol has resulted from anthropogenic precursor vapours that, after oxidation in the atmosphere, can form particles which may then grow into new cloud condensation nuclei (CCN). By current estimates, more than half of all CCN originate from nucleation rather than being emitted directly into the atmosphere (Gordon et al, 2017), but the vapours and mechanisms responsible remain relatively poorly known.…”