Particle Cloud Condensation Nuclei Research Articles

Exploring the excess of cloud condensation nuclei and rain suppression using a minimal three-dimensional Boussinesq model with bulk cloud microphysics

Over the years, there have been discussions about the possibility of air pollution affecting the process of rain formation. In this study, we have developed a simplified model that represents the atmospheric dynamics and cloud microphysics to explore the impact of pollution on rain formation. We used an existing three-dimensional minimal model consisting of five equations, for which we added a simple bulk parametrization that represents the role of cloud condensation nuclei (CCN) in cloud formation processes. We conducted numerical tests using two CCN profiles, with either one or two accumulation layers and modified their abundance to explore the effects of different CCN concentrations and distributions. We conducted four numerical tests corresponding to the two aforementioned profiles with polluted and low-polluted scenarios. The numerical simulations suggested that a layer with high CCN concentration close to the surface tends to suppress precipitation, while the same concentration distributed over two layers tends to enhance the efficiency of rain formation. The simulations also showed that CCN particles far from the surface produced higher cloud tops and longer events, consistent with previous research. Although the model includes a stable representation of precipitating turbulent convection with bulk cloud microphysics, we expect its simplicity and conservation properties to allow for deeper theoretical analyses that can help us better understand the physical processes involved in the studied phenomenon. We hope this model will serve as a tool to explore different aerosol-related scenarios within the context of minimal models.

Detrainment Dominates CCN Concentrations Around Non‐Precipitating Convective Clouds Over the Amazon

AbstractWe investigated the relationship between the number concentration of cloud droplets (Nd) in ice‐free convective clouds and of particles large enough to act as cloud condensation nuclei (CCN) measured at the lateral boundaries of cloud elements. The data were collected during the ACRIDICON‐CHUVA aircraft campaign over the Amazon Basin. The results indicate that the CCN particles at the lateral cloud boundaries are dominated by detrainment from the cloud. The CCN concentrations detrained from non‐precipitating convective clouds are smaller compared to below cloud bases. The detrained CCN particles from precipitating cloud volumes have relatively larger sizes, but lower concentrations. Our findings indicate that CCN particles ingested from below cloud bases are activated into cloud droplets, which evaporate at the lateral boundaries and above cloud base and release the CCN again to ambient cloud‐free air, after some cloud processing. These results support the hypothesis that the CCN around the cloud are cloud‐processed.

Airborne measurements of cloud condensation nuclei (CCN) vertical structures over Southern China

In this study, the vertical structures of cloud condensation nuclei (CCN) over southern China are investigated with two flight datasets during the period of 16–17 July 2019. The results show that the CCN number concentration (NCCN) increases first with the increasing height, peaking near 1.3 km above the ground, and then decreases sharply to a low value above 2.5 km. Good agreement is obtained by employing a segmentation fitting approach to describe the vertical CCN structure. More specifically a simple linear function is used to fit the NCCN below 1.3 km; a polynomial function is utilized to describe the measurements between 1.3 km and 2.5 km, and, a constant value of 7 cm−3 is assigned above 2.5 km. The CCN particle size distribution (CPSD) presents a unimodal mode with the peak that changes to the direction of larger diameter with growing water vapor supersaturation (SS). As for the effects of SS, NCCN, as well as the diameter of CCN increases with the increasing SS. The CCN spectrum is well represented by the classical Twomey power-law function of NCCN(SS) = C·SSk. Although the values of parameter C are 72 cm−3 and 150 cm−3 in the two measurements, respectively, the paramer k changes a little with its values being 1.5 or so. The results gained herein might be helpful for cloud microphysical parameterization in numerical models and provide a guide for weather modification.

A study of the effect of regenerated CCN on marine stratocumulus cloud development using the WRF-LES model with spectral bin microphysics scheme

Impacts of regenerated cloud condensation nuclei (CCN) on daytime marine stratocumulus clouds (MSC) and stratocumulus-topped boundary layer (STBL) are investigated, by implementing five different CCN regeneration methods in the spectral bin microphysics (SBM) scheme of the Weather Research and Forecasting (WRF) large eddy simulation (LES) model. Without the CCN regeneration process, the model shows unrealistic reduction of total (cloud drops + CCN) particle concentrations but produces very large cloud drops, compared to observations. Such reduction of particle concentrations is remedied when CCN regeneration process is implemented in the model. Evaluation of the model performance with aircraft measurement data confirms that CCN regeneration is critical for realistic simulation of MSC development. However, depending on the selected CCN regeneration method, results vary significantly especially at a later hour of simulation time (5 h). Such variabilities of the microphysical properties and dynamical structures are caused mainly by the different ways of regenerating CCN particles after entrainment of warm and dry air and subsequent evaporation of cloud drops near cloud top, which, in turn, affects cloud radiative properties such as SW heating and LW cooling that drives STBL convection. We are confident that this study can provide a guidance for simulating more realistic three-dimensional MSC cloud development by including the crucial process of CCN regeneration in the SBM scheme. All in all, we recommend the methods, which primarily replenish the depleted CCN with large CCN.

Versatile aerosol concentration enrichment system (VACES) operating as a cloud condensation nuclei (CCN) concentrator: development and laboratory characterization

Abstract. The ability of atmospheric aerosol particles to act as cloud condensation nuclei (CCN) depends on many factors, including particle size, chemical composition and meteorological conditions. To expand our knowledge of CCN, it is essential to understand the factors leading to CCN activation. For this purpose, a versatile aerosol concentrator enrichment system (VACES) has been modified to select CCN at different supersaturations. The VACES enables sampling non-volatile CCN particles without altering their chemical and physical properties. The redesigned VACES enriches CCN particles by first passing the aerosol flow to a new saturator and then to a condenser. The activated particles are concentrated by an inertial virtual impactor and then can be returned to their original size by diffusion drying. For the calibration, the saturator temperature was fixed at 52 ∘C and the condenser temperature range was altered from 5 to 25 ∘C to obtain activation curves for NaCl particles of different sizes. Critical water vapour supersaturations can be calculated using the 50 % cut point of these curves. Calibration results have also shown that CCN concentrations can be enriched by a factor of approx. 17, which is in agreement with the experimentally determined enrichment factor of the original VACES. The advantage of the redesigned VACES over conventional CCN counters (both static and continuous flow instruments) lies in the substantial enrichment of activated CCN, which facilitates further chemical analysis.

Explaining variance in black carbon's aging timescale

Abstract. The size and composition of particles containing black carbon (BC) are modified soon after emission by condensation of semivolatile substances and coagulation with other particles, known collectively as "aging" processes. Although this change in particle properties is widely recognized, the timescale for transformation is not well constrained. In this work, we simulated aerosol aging with the particle-resolved model PartMC-MOSAIC (Particle Monte Carlo – Model for Simulating Aerosol Interactions and Chemistry) and extracted aging timescales based on changes in particle cloud condensation nuclei (CCN). We simulated nearly 300 scenarios and, through a regression analysis, identified the key parameters driving the value of the aging timescale. We show that BC's aging timescale spans from hours to weeks, depending on the local environmental conditions and the characteristics of the fresh BC-containing particles. Although the simulations presented in this study included many processes and particle interactions, we show that 80% of the variance in the aging timescale is explained by only a few key parameters. The condensation aging timescale decreased with the flux of condensing aerosol and was shortest for the largest fresh particles, while the coagulation aging timescale decreased with the total number concentration of large (D >100 nm), CCN-active particles and was shortest for the smallest fresh particles. Therefore, both condensation and coagulation play important roles in aging, and their relative impact depends on the particle size range.

New Particle Formation and Growth in an Isoprene-Dominated Ozark Forest: From Sub-5 nm to CCN-Active Sizes

Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (∼100 nm) observed during this field campaign. The measured sub-5 nm particles were up to ∼20,000 cm−3 during a typical NPF event. Nucleation rates J1 were relatively high (11.0 ± 10.6 cm−3 s−1), and one order of magnitude higher than formation rates of 5 nm particles (J5). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO2-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period.Copyright 2014 American Association for Aerosol Research

Weakness of the weekend effect in aerosol number concentrations

Weekday related anthropogenic aerosol emissions have been suggested to affect regional climate via indirect aerosol effects. I studied the variability of potential cloud condensation nuclei using measurements of number size distributions of Cloud Condensation Nuclei (CCN)-sized aerosol particles and CCNs measured at several European regional background stations, located at a wide variety of environments. With notably rare exceptions, there were no statistically significant difference between concentrations on different weekdays. I further analysed the concentration time-series of four long-period datasets in Germany and Finland with wavelet analysis . Outside of urban areas, very little weekday-connected variability was found. The lack of 7-day variability outside of cities is in contrast of earlier studies in this field, which used mostly particle mass as the representative measure of aerosol concentration. A time-scale and variability analysis showed that PM 10 and PM 2.5 are more sensitive for the weekly variation than CCN-sized particles. Using mass-based variations as a proxy for short-term variations of CCN particle numbers can thus overestimate the weekend effect for these particles. The results of this study do not support aerosol indirect effects from 50 to 500 nm diameter particles as a major contributor on potential weekday connected variations in European meteorology. ► The weekly variation of cloud condensation nuclei sized aerosol concentrations was studied. ► No clear signal of weekly variation in the number of CCN was found in the European background air. ► Time-scale analysis shows that most mass-based measurements overestimate the weekly variability of CCNs. ► Using PM or optical measurements to evaluate the short-term variability of NCCN is not recommended. ► The indirect aerosol effects are not likely to cause weekly variation in European background.

In Situ Characterization of Cloud Condensation Nuclei, Interstitial, and Background Particles Using the Single Particle Mass Spectrometer, SPLAT II

The aerosol indirect effect remains the most uncertain aspect of climate change modeling, calling for characterization of individual particles sizes and compositions with high spatial and temporal resolution. We present the first deployment of our single particle mass spectrometer (SPLAT II) operated in dual data acquisition mode to simultaneously measure particle number concentrations, density, asphericity, and individual particle size and quantitative composition, with temporal resolution better than 60 s, thus yielding all the required properties to definitively characterize the aerosol-cloud interaction in this exemplary case. We find that particles are composed of oxygenated organics, many mixed with sulfates, biomass burning particles, some with sulfates, and processed sea-salt. Cloud residuals are found to contain more sulfates than background particles, explaining their higher efficiency to serve as cloud condensation nuclei (CCN). Additionally, CCN sulfate content increased with time due to in-cloud droplet processing. A comparison between the size distributions of background, CCN, and interstitial particles shows that while nearly all CCN particles are larger than 100 nm, over 80% of interstitial particles are smaller than 100 nm. We conclude that for this cloud, particle size is the controlling factor on aerosol activation into cloud-droplets, with higher sulfate content playing a secondary role.