Abstract
AbstractNew 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.
Highlights
Atmospheric aerosol particles play a key role in regulating Earth’s radiative balance
Concerning causes, we find that the importance of biogenic volatile organic compounds (BVOCs) in new particle formation (NPF) and cloud condensation nuclei (CCN) formation is greater than previously thought
As these numbers are quite similar to the 26% and 41% calculated by removing all BVOCs, we conclude that the role of organics in aerosol formation is most important for the smallest particles, and of secondary importance for larger particles
Summary
Atmospheric aerosol particles play a key role in regulating Earth’s radiative balance. Better mathematical models for the molecular clustering process have been published recently, for example, by Chen et al [2012] These developments have permitted global model studies to simulate the roles of diverse compounds, most notably ammonia and monoterpene oxidation products, in NPF alongside sulfuric acid. Several previous model studies of the present-day atmosphere found that variations in the cosmic ray intensity typical of the solar cycle were very unlikely to produce significant variations in CCN concentrations [Pierce and Adams, 2009b; Snow-Kropla et al, 2011; Yu et al, 2012; Dunne et al, 2012; Kazil et al, 2012; Yu and Luo, 2014a] We update these studies with our improved NPF model, and we explore possible effects in both preindustrial and present-day conditions. We repeat these estimates with our updated NPF model and propose an alternative calculation of the importance of BVOCs in CCN formation
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