Abstract
Abstract. Aerosol nucleation occurs frequently in the atmosphere and is an important source of particle number. Observations suggest that nucleated particles are capable of growing to sufficiently large sizes that they act as cloud condensation nuclei (CCN), but some global models have reported that CCN concentrations are only modestly sensitive to large changes in nucleation rates. Here we present a novel approach for using long-term size distribution observations to evaluate a global aerosol model's ability to predict formation rates of CCN from nucleation and growth events. We derive from observations at five locations nucleation-relevant metrics such as nucleation rate of particles at diameter of 3 nm (J3), diameter growth rate (GR), particle survival probability (SP), condensation and coagulation sinks, and CCN formation rate (J100). These quantities are also derived for a global microphysical model, GEOS-Chem-TOMAS, and compared to the observations on a daily basis. Using GEOS-Chem-TOMAS, we simulate nucleation events predicted by ternary (with a 10−5 tuning factor) or activation nucleation over one year and find that the model slightly understates the observed annual-average CCN formation mostly due to bias in the nucleation rate predictions, but by no more than 50% in the ternary simulations. At the two locations expected to be most impacted by large-scale regional nucleation, Hyytiälä and San Pietro Capofiume, predicted annual-average CCN formation rates are within 34 and 2% of the observations, respectively. Model-predicted annual-average growth rates are within 25% across all sites but also show a slight tendency to underestimate the observations, at least in the ternary nucleation simulations. On days that the growing nucleation mode reaches 100 nm, median single-day survival probabilities to 100 nm for the model and measurements range from less than 1–6% across the five locations we considered; however, this does not include particles that may eventually grow to 100 nm after the first day. This detailed exploration of new particle formation and growth dynamics adds support to the use of global models as tools for assessing the contribution of microphysical processes such as nucleation to the total number and CCN budget.
Highlights
Ocean Science over one year and find that the model slightly understates the observed annual-average cloud condensation nuclei (CCN) formation mostly due to bias in the nucleation rate predictions, but by no more than 50 % in the ternary simulations
The quantities chosen for model–measurement comparison are formation rate (J3), growth rate (GR), survival probability from 3–50 and 100 nm (SP50 and SP100, respectively), and 50 and 100 nm particle formation rates (J50 and J100)
For the CCN formation rate and survival probability panels of the cumulative distribution functions (CDF) figures, days where the nucleation mode does not grow to the particular cutoff size (50 or 100 nm) are not included in the figure
Summary
Ocean Science over one year and find that the model slightly understates the observed annual-average CCN formation mostly due to bias in the nucleation rate predictions, but by no more than 50 % in the ternary simulations. Westervelt et al.: Formation and growth of nucleated particles into cloud condensation nuclei number concentrations, a subset of which act as cloud condensation nuclei (CCN), brighter and potentially longer-lived clouds are formed. In order for aerosols to exert these influences on clouds, they are either introduced into the atmosphere by direct emission or gas-to-particle conversion (nucleation) where they may grow to sufficiently large sizes to act as CCN (Kerminen et al, 2005; Pierce and Adams, 2007; Kuang et al, 2009). In order for particles formed via nucleation to act as CCN, they must grow by condensation while avoiding loss by coagulation for a longer amount of time and through a larger range of sizes than primary emissions. Coagulation is very efficient between fresh nuclei and larger particles, compounding the increased time that nucleated particles require to grow to CCN sizes. Ambient measurements presented in Kuang et al (2009) highlight the importance of coagulation as at least 80 % of the nucleated particles on average are lost by coagulation before the nucleation mode reached CCN sizes in the cases that they studied, even during days with high growth rates
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