Formation and growth of nucleated particles into cloud condensation nuclei: model–measurement comparison

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 (<i>J</i>₃), diameter growth rate (GR), particle survival probability (SP), condensation and coagulation sinks, and CCN formation rate (<i>J</i>₁₀₀). 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^&minus;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

Forest canopy interactions with nucleation mode particles

“…Regional-scale simulations indicate boundarylayer nucleation significantly contributes to CCN-sized particles over eastern North America (Luo and Yu, 2011). Further, while nucleation in the free-troposphere likely makes a larger contribution to CCN production, observationally based estimates of survival and growth rates of nucleation particles in the planetary boundary-layer imply a non-negligible fraction reach 100 nm diameter within 24 h and thus may contribute to the CCN concentration (Pierce et al, 2014;Westervelt et al, 2013). However, the cause-effect relationships are complex, and the role of BVOC emissions remains uncertain.…”

Modeling the role of highly oxidized multifunctional organic molecules for the growth of new particles over the boreal forest region

“…In many studies (e.g., Bergström et al, 2012;Farina et al, 2010;Fountoukis et al, 2014;Hodzic et al, 2009;Lane et al, 2008;Murphy et al, 2012) the vapors are assumed to be semi-volatile and in equilibrium with the (liquid, well-mixed) particles, making it possible to model formation of BSOA by simple gas-particle equilibrium partitioning (Pankow, 1994). In other studies (e.g., Scott et al, 2015;Spracklen et al, 2008;Tunved et al, 2010;Westervelt et al, 2013) the vapors are assumed to be nonvolatile and the irreversible particle growth is only limited by the collision rate between the vapor molecules and the particles. Recently, large-scale model studies (Jokinen et al, 2015;Langmann et al, 2014;Li et al, 2015;Riipinen et al, 2011;Yu, 2011) have included both mechanisms to be able to treat semi-volatile and nonvolatile vapors, which have yielded a better agreement between model results and observations.…”

The importance of interstitial particle scavenging by cloud droplets in shaping the remote aerosol size distribution and global aerosol-climate effects

“…Any coagulation between the droplets and the interstitial particles will lead to a reduction of interstitial particle number and an increase in the size of the CCN-sized particles after the cloud dries. This coagulation may impact climate in two ways: (1) the removed interstitial particles that may have otherwise grown to CCN sizes via condensation and increased CCN concentrations (Pierce and Adams, 2007;Westervelt et al, 2013Westervelt et al, , 2014. Thus, this coagulation may lower CCN concentrations and lead to a warming through a reduction in the magnitude of the aerosol indirect effect.…”