Abstract
Abstract. Aerosol nucleation is an important source of particle number in the atmosphere. However, in order to become cloud condensation nuclei (CCN), freshly nucleated particles must undergo significant condensational growth while avoiding coagulational scavenging. In an effort to quantify the contribution of nucleation to CCN, this work uses the GEOS-Chem-TOMAS global aerosol model to calculate changes in CCN concentrations against a broad range of nucleation rates and mechanisms. We then quantify the factors that control CCN formation from nucleation, including daily nucleation rates, growth rates, coagulation sinks, condensation sinks, survival probabilities, and CCN formation rates, in order to examine feedbacks that may limit growth of nucleated particles to CCN. Nucleation rate parameterizations tested in GEOS-Chem-TOMAS include ternary nucleation (with multiple tuning factors), activation nucleation (with two pre-factors), binary nucleation, and ion-mediated nucleation. We find that nucleation makes a significant contribution to boundary layer CCN(0.2%), but this contribution is only modestly sensitive to the choice of nucleation scheme, ranging from 49 to 78% increase in concentrations over a control simulation with no nucleation. Moreover, a two order-of-magnitude increase in the globally averaged nucleation rate (via changes to tuning factors) results in small changes (less than 10%) to global CCN(0.2%) concentrations. To explain this, we present a simple theory showing that survival probability has an exponentially decreasing dependence on the square of the condensation sink. This functional form stems from a negative correlation between condensation sink and growth rate and a positive correlation between condensation sink and coagulational scavenging. Conceptually, with a fixed condensable vapor budget (sulfuric acid and organics), any increase in CCN concentrations due to higher nucleation rates necessarily entails an increased aerosol surface area in the accumulation mode, resulting in a higher condensation sink, which lowers vapor concentrations and growth rates. As a result, slowly growing nuclei are exposed to a higher frequency of coagulational scavenging for a longer period of time, thus reducing their survival probabilities and closing a negative feedback loop that dampens the impact of nucleation on CCN. We confirm quantitatively that the decreases in survival probability predicted by GEOS-Chem-TOMAS due to higher nucleation rates are in accordance with this simple theory of survival probability.
Highlights
Aerosols affect climate directly by scattering incoming solar radiation and indirectly by modifying cloud properties
The TER5 simulation was already considered in Westervelt et al (2013), but we add in the TER3 and TER simulations to allow us to look at how incremental changes in the nucleation rates effect growth rates, coagulation sink, condensation sink, survival probabilities, and cloud condensation nuclei (CCN) concentrations
We have shown in the previous two sections that the condensation sink increases with faster nucleation rates to diminish survival probabilities, we have yet to directly link condensation sink to growth rates, coagulation sinks, and survival probability
Summary
Aerosols affect climate directly by scattering incoming solar radiation and indirectly by modifying cloud properties. Lee et al (2013) compiled 28 model parameters covering several important aerosol processes and ran Monte Carlo simulations to determine the magnitude of uncertainty in CCN concentrations caused by each parameter They find that while 45 % of CCN are attributed to nucleation, the CCN are generally insensitive to the details of the nucleation rates across a wide, but sensible, range of boundary layer and freetropospheric nucleation assumptions. We test the hypothesis that higher nucleation rates will lead to a lower survival probability and will dampen CCN concentrations using global modeling results and the nucleated particle analysis code presented in Westervelt et al (2013). We propose a simple theory for the hypothesized CCN dampening in which particle survival probability is inversely related to condensation sink and show that model results are generally consistent with this simple theory
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