Measured Cloud Condensation Nuclei Concentrations Research Articles

Cloud Condensation Nuclei Closure Study Using Airborne Measurements Over the Southern Great Plains

AbstractAirborne measurements of non‐refractory bulk aerosol chemical composition, aerosol size distributions, and cloud condensation nuclei (CCN) were conducted onboard a research aircraft during the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems field campaign in the spring and summer of 2016. A CCN closure study for the entire campaign period was performed where measured CCN concentrations at 0.24% and 0.46% supersaturation were compared with the predicted CCN concentrations calculated using κ‐Köhler theory, three different assumptions of aerosol mixing state, and various assumptions about hygroscopicity, density, and the insoluble fraction of organic particles. We found that the Closure Ratio (CR), calculated as the ratio of predicted to measured CCN concentrations, was equal to one under two different aerosol mixing state assumptions: (a) all particles are composed of 100% organic particles, and (b) particles are externally mixed and composed of pure sulfates, nitrates, and organic particles assuming hygroscopicity values for organic particles () between 0.04 and 0.17. The use of internal mixing state assumption often led to overprediction of CCN concentrations but the agreement within ±20% with the measured CCN concentrations was observed under certain closure permutations. A similar agreement, that is, within ±20%, was also observed using permuted parameters concerning density (1 and 1.5 g cm−3) and an insoluble fraction (0% and 20%) of organic particles. These findings may provide constraints on to predict CCN concentrations at a remote continental Southern Great Plains site.

Characterization of submicron aerosols and CCN over the Yellow Sea measured onboard the Gisang 1 research vessel using the positive matrix factorization analysis method

Aerosol size distributions and cloud condensation nuclei (CCN) number concentrations were measured in spring 2017 over the Yellow Sea on board the research vessel Gisang 1. The average number concentration of particles larger than 10 nm and CCN at 0.65% supersaturation (S) were 7622 ± 4038 cm−3 and 4821 ± 1763 cm−3, respectively. Characteristics of aerosol size distribution data were analyzed using a positive matrix factorization (PMF) method. It was found that only 6 Factors could explain the aerosol size distribution reasonably well. Factors 1 and 2 indicated nucleation mode particles, Factor 3 indicated Aitken mode particles, and Factors 4, 5, and 6 indicated accumulation mode particles. The concentrations of nucleation and Aitken mode particles showed a clear wind direction dependence; high under westerly winds due to the high concentrations of particles and precursor gases in eastern China. Meanwhile, the concentration of larger particles and CCN showed no significant wind direction dependence. Aerosol size distribution was also significantly influenced by meteorology. Small particles were predominant during clear days. In contrast during mist or fog days, the aerosol size distribution shifted to larger sizes. A CCN closure experiment was conducted using results of the PMF analysis. The assumption of internally mixed particles led to overestimation of predicted CCN concentrations but agreement was significantly better when externally mixed particles were considered. The logarithmic curve fit of NCCN(S) = 4825 ∗ log S + 4933 was found to very well explain measured CCN concentrations at a few S over the Yellow Sea, and therefore is recommended as input CCN spectral data for numerical models that explicitly treat CCN activation.

Measurement of cloud condensation nuclei (CCN) and CCN closure at Mt. Huang based on hygroscopic growth factors and aerosol number-size distribution

The number of cloud condensation nuclei (CCN) may indirectly influence the radiative balance of the atmosphere by changing the number of cloud droplets, which in turn changes the albedo, longevity and precipitation intensity of clouds. The spatial and temporal distribution of the CCN concentrations and the influence of particles on CCN activation spectra have received much attention. Measurements of CCN concentrations, aerosol number-size distribution and hygroscopic growth factors were conducted during the periods June 30 to July 17 and July 24 to 28, 2014, at the peak of Mt. Huang (1840 m above sea level). The results show that the CCN concentration were 419 ± 414 cm−3, 806 ± 720 cm−3, 1292 ± 905 cm−3, 1380 ± 873 cm−3, and 1506 ± 867 cm−3 at supersaturation levels of 0.1%, 0.2%, 0.5%, 0.7%, and 1%, respectively. The equation Nccn = N0(1-exp(-BSk)) fits the average CCN spectrum over the observation period. The CCN concentrations were calculated from the hygroscopic growth factors and the aerosol number-size distribution. The calculated CCN concentrations and measured CCN concentrations show close correlation and high accuracy. An analysis of the variation in the particle number-size distribution and hygroscopic growth factors indicates that the change in particle number-size distribution is the primary factor affecting the CCN concentrations.

Relationships between particles, cloud condensation nuclei and cloud droplet activation during the third Pallas Cloud Experiment

Abstract. Concurrent measurement of aerosols, cloud condensation nuclei (CCN) and cloud droplet activation were carried out as a part of the third Pallas Cloud Experiment (PaCE-3) which took place at a ground based site located on northern Finland during the autumn of 2009. In this study, we investigate relationships between the aerosol properties, CCN and size resolved cloud droplet activation. During the investigated cloudy periods, the inferred number of cloud droplets (CDNC) varied typically between 50 and 150 cm−3 and displayed a linear correlation both with the number of particles having sizes over 100 nm and with the CCN concentrations at 0.4% supersaturation. Furthermore, the diameter corresponding to the 50% activation fraction, D50, was generally in the range of 80 to 120 nm. The measured CCN concentrations were compared with predictions of a numerical model which used the measured size distribution and size resolved hygroscopicity as input. Assuming that the droplet surface tension is equal to that of water, the measured and predicted CCN concentrations were generally within 30%. We also simulated size dependent cloud droplet activation with a previously developed air parcel model. By forcing the model to reproduce the experimental values of CDNC, adiabatic estimates for the updraft velocity and the maximum supersaturation reached in the clouds were derived. Performed sensitivity studies showed further that the observed variability in CDNC was driven mainly by changes in the particle size distribution while the variations in the updraft velocity and hygroscopicity contributed to a lesser extent. The results of the study corroborate conclusions of previous studies according to which the number of cloud droplets formed in clean air masses close to the Arctic is determined mainly by the number of available CCN.

Direct Comparison of the Hygroscopic Properties of Ammonium Sulfate and Sodium Chloride Aerosol at Relative Humidities Approaching Saturation

Holographic optical tweezers are used to make comparative measurements of the hygroscopic properties of single component aqueous aerosol containing sodium chloride and ammonium sulfate over a range of relative humidity from 84% to 96%. The change in RH over the course of the experiment is monitored precisely using a sodium chloride probe droplet with accuracy better than ±0.09%. The measurements are used to assess the accuracy of thermodynamic treatments of the relationship between water activity and solute mass fraction with particular attention focused on the dilute solute limit approaching saturation vapor pressure. The consistency of the frequently used Clegg-Brimblecombe-Wexler (CBW) treatment for predicting the hygroscopic properties of sodium chloride and ammonium sulfate aerosol is confirmed. Measurements of the equilibrium size of ammonium sulfate aerosol are found to agree with predictions to within an uncertainty of ±0.2%. Given the accuracy of treating equilibrium composition, the inconsistencies highlighted in recent calibration measurements of critical supersaturations of sodium chloride and ammonium sulfate aerosol cannot be attributed to uncertainties associated with the thermodynamic predictions and must have an alternative origin. It is concluded that the CBW treatment can allow the critical supersaturation to be estimated for sodium chloride and ammonium sulfate aerosol with an accuracy of better than ±0.002% in RH. This corresponds to an uncertainty of ≤1% in the critical supersaturation for typical supersaturations of 0.2% and above. This supports the view that these systems can be used to accurately calibrate instruments that measure cloud condensation nuclei concentrations at selected supersaturations. These measurements represent the first study in which the equilibrium properties of two particles of chemically distinct composition have been compared simultaneously and directly alongside each other in the same environment.

The hygroscopicity parameter (κ) of ambient organic aerosol at a field site subject to biogenic and anthropogenic influences: relationship to degree of aerosol oxidation

Abstract. Cloud condensation nuclei (CCN) concentrations were measured at Egbert, a rural site in Ontario, Canada during the spring of 2007. The CCN concentrations were compared to values predicted from the aerosol chemical composition and size distribution using κ-Köhler theory, with the specific goal of this work being to determine the hygroscopic parameter (κ) of the oxygenated organic component of the aerosol, assuming that oxygenation drives the hygroscopicity for the entire organic fraction of the aerosol. The hygroscopicity of the oxygenated fraction of the organic component, as determined by an Aerodyne aerosol mass spectrometer (AMS), was characterised by two methods. First, positive matrix factorization (PMF) was used to separate oxygenated and unoxygenated organic aerosol factors. By assuming that the unoxygenated factor is completely non-hygroscopic and by varying κ of the oxygenated factor so that the predicted and measured CCN concentrations are internally consistent and in good agreement, κ of the oxygenated organic factor was found to be 0.22±0.04 for the suite of measurements made during this five-week campaign. In a second, equivalent approach, we continue to assume that the unoxygenated component of the aerosol, with a mole ratio of atomic oxygen to atomic carbon (O/C) ≈ 0, is completely non-hygroscopic, and we postulate a simple linear relationship between κorg and O/C. Under these assumptions, the κ of the entire organic component for bulk aerosols measured by the AMS can be parameterised as κorg=(0.29±0.05)·(O/C), for the range of O/C observed in this study (0.3 to 0.6). These results are averaged over our five-week study at one location using only the AMS for composition analysis. Empirically, our measurements are consistent with κorg generally increasing with increasing particle oxygenation, but high uncertainties preclude us from testing this hypothesis. Lastly, we examine select periods of different aerosol composition, corresponding to different air mass histories, to determine the generality of the campaign-wide findings described above.

Cloud condensation nuclei measurements in the marine boundary layer of the Eastern Mediterranean: CCN closure and droplet growth kinetics

Abstract. Measurements of cloud condensation nuclei (CCN) concentrations (cm−3) between 0.2 and 1.0% supersaturation, aerosol size distribution and chemical composition were performed at a remote marine site in the eastern Mediterranean, from September to October 2007 during the FAME07 campaign. Most of the particles activate at ~0.6% supersaturation, characteristic of the aged nature of the aerosol sampled. Application of Köhler theory, using measurements of bulk composition, size distribution, and assuming that organics are insoluble resulted in agreement between predicted and measured CCN concentrations within 7±11% for all supersaturations, with a tendency for CCN underprediction (16±6%; r2=0.88) at the lowest supersaturations (0.21%). Including the effects of the water-soluble organic fraction (which represent around 70% of the total organic content) reduces the average underprediction bias at the low supersaturations, resulting in a total closure error of 0.6±6%. Using threshold droplet growth analysis, the growth kinetics of ambient CCN is consistent with NaCl calibration experiments; hence the presence of aged organics does not suppress the rate of water uptake in this environment. The knowledge of the soluble salt fraction is sufficient for the description of the CCN activity in this area.

Assessment of the relative importance of atmospheric aging on CCN activity derived from field observations

The effect of atmospheric aging on the cloud condensation nuclei (CCN) activity of atmospheric aerosols was studied by comparing different air masses with different degrees of aging along the southern coast of California over the Pacific Ocean during a research cruise on the R/V Roger Revelle from 2–19 November 2004. Activation diameters ( D act) were calculated using the measured CCN concentrations, condensation nuclei (CN) concentrations, and particle size distributions. Measurements of single particle size and chemistry, as well as black carbon (BC) concentrations with an aethalometer, were made to provide further insight into aerosol chemistry. A gradient of aerosol concentrations was encountered: along the coast of California, the highest BC and CN concentrations (1000–6000 ng m −3 and 2000–15,000 cm −3) were measured which decreased as the ship moved away from shore to much lower values (<100 ng m −3, ∼300 cm −3). In all regions, external mixtures of organic carbon, elemental carbon, sea salt, and dust aerosols frequently associated with nitrate and sulfate were observed. A correlation plot between the CCN/CN ratio and D act exhibits a clear linear correlation, showing a distinct relationship between the extent of anthropogenic aging and CCN activity with the most highly aged air masses showing the highest CCN activity and smallest D act. These results show changes in aerosol chemistry due to atmospheric aging that play an important role in determining the CCN activity of atmospheric aerosols. The present study demonstrates that variations in aerosol chemistry must be taken into account in models to adequately account for the physicochemical properties of atmospheric aerosols and their CCN activity.

Cloud‐nucleating properties of the Amazonian biomass burning aerosol: Cloud condensation nuclei measurements and modeling

The cloud‐nucleating properties of the atmospheric aerosol were studied in an area under strong influence of vegetation burning. The measurements were part of Large‐Scale Biosphere Atmosphere Experiment in Amazonia–Smoke Aerosols, Clouds, Rainfall and Climate (LBA‐SMOCC) and were carried out at a ground site located in the state of Rondônia in southwestern Amazonia, Brazil, September to November 2002, covering the dry season, a transition period, and the onset of the wet season. The concentrations of cloud condensation nuclei (CCN) were measured with a static thermal gradient CCN counter for supersaturations ranging between 0.23 and 1.12%. As a closure test, the CCN concentrations were predicted with a time resolution of 10 min from measurements of the dry particle number size distribution (3–850 nm, Differential Mobility Analyzer (DMPS)) and hygroscopic growth at 90% relative humidity (Hygroscopic Tandem Differential Mobility Analyzer (H‐TDMA)). No chemical information was needed. The predicted and measured CCN concentrations were highly correlated (r2 = 0.97–0.99), and the predictions were only slightly lower than those measured, typically by 15–20%. Parameterizations of the predicted CCN concentrations are given for each of the three meteorological periods. These are based on averages taken during the afternoon hours when the measurements at ground level were representative for the aerosol entering the base of convective clouds. Furthermore, a more detailed parameterization including the mixing state of the aerosol is given, where the hygroscopic properties are expressed as the number of soluble ions or nondissociating molecules per unit volume dry particle.

Cloud Activating Properties of Aerosol Observed during CELTIC

Abstract Measurements of aerosol size distribution, chemical composition, and cloud condensation nuclei (CCN) concentration were performed during the Chemical Emission, Loss, Transformation, and Interactions with Canopies (CELTIC) field program at Duke Forest in North Carolina. A kinetic model of the cloud activation of ambient aerosol in the chamber of the CCN instrument was used to perform an aerosol–CCN closure study. This study advances prior investigations by employing a novel fitting algorithm that was used to integrate scanning mobility particle sizer (SMPS) measurements of aerosol number size distribution and aerosol mass spectrometer (AMS) measurements of the mass size distribution for sulfate, nitrate, ammonium, and organics into a single, coherent description of the ambient aerosol in the size range critical to aerosol activation (around 100-nm diameter). Three lognormal aerosol size modes, each with a unique internally mixed composition, were used as input into the kinetic model. For the two smaller size modes, which control CCN number concentration, organic aerosol mass fractions for the defined cases were between 58% and 77%. This study is also unique in that the water vapor accommodation coefficient was estimated based on comparing the initial timing for CCN activation in the instrument chamber with the activation predicted by the kinetic model. The kinetic model overestimated measured CCN concentrations, especially under polluted conditions. Prior studies have attributed a positive model bias to an incomplete understanding of the aerosol composition, especially the role of organics in the activation process. This study shows that including measured organic mass fractions with an assumed organic aerosol speciation profile (pinic acid, fulvic acid, and levoglucosan) and an assumed organic aerosol solubility of 0.02 kg kg−1 still resulted in a significant model positive bias for polluted case study periods. The slope and y intercept for the CCN predicted versus CCN observed regression was found to be 1.9 and −180 cm−3, respectively. The overprediction generally does not exceed uncertainty limits but is indicative that a bias exists in the measurements or application of model. From this study, uncertainties in the particle number and mass size distributions as the cause for the model bias can be ruled out. The authors are also confident that the model is including the effects of growth kinetics on predicted activated number. However, one cannot rule out uncertainties associated with poorly characterized CCN measurement biases, uncertainties in assumed organic solubility, and uncertainties in aerosol mixing state. Sensitivity simulations suggest that assuming either an insoluble organic fraction or external aerosol mixing were both sufficient to reconcile the model bias.

Mapping the Operation of the DMT Continuous Flow CCN Counter

This work thoroughly analyzes a new commercial instrument for measuring Cloud Condensation Nuclei (CCN), the Droplet Measurement Technologies Cylindrical Continuous-Flow Streamwise Thermal Gradient CCN Chamber (CFSTGC). This instrument can measure CCN concentrations at supersaturations from 0.06% to 3% (potentially up to 6%), at a 1 Hz sampling rate that is sufficient for airborne operation. Our analysis employs a fully coupled numerical flow model to simulate the water vapor supersaturation, temperature, velocity profiles and CCN growth in the CFSTGC for its entire range of operation (aerosol sample flow rates 0.25–2.0 L min − 1 , temperature differences 2–15 K and ambient pressures 100–1000 mb). The model was evaluated by comparing simulated instrument responses for calibration aerosol against actual measurements from an existing CCN instrument. The model was used to evaluate the CCN detection efficiency for a wide range of ambient pressures, flow rates, temperature gradients, and droplet growth kinetic...

A Continuous-Flow Streamwise Thermal-Gradient CCN Chamber for Atmospheric Measurements

We have addressed the need for improved measurements of cloud condensation nuclei (CCN) by developing a continuous-flow instrument that provides in situ measurements of CCN. The design presented in this article can operate between 0.1 and 3% supersaturation, at sampling rates sufficient for airborne operation. The design constitutes a cylindrical continuous-flow thermal-gradient diffusion chamber employing a novel technique of generating a supersaturation: by establishing a constant streamwise temperature gradient so that the difference in water vapor and thermal diffusivity yield a quasi-uniformcenterline supersaturation. Our design maximizes the growth rate of activated droplets, thereby enhancing the performance of the instrument. The temperature gradient and the flow through the column control the supersaturation and may be modified to retrieve CCN spectra. The principle of the CCN instrument was validated in controlled laboratory experiments at different operating conditions using a monodisperse aerosols with known composition and size. These experiments yield sharp activation curves, even for those kinetically limited particles that have not exceeded their critical diameter. The performance of the CCN instrument was also assessed using polydisperse laboratory-generated aerosol of known composition and size distributions similar to ambient particulate matter. In all tests, the measured CCN concentrations compared well with predicted values and highlight the instrument’s ability to measure CCN at various size distributions. The full potential of the new design has yet to be explored; however, model simulations suggest that direct measurements in the climatically important range of supersaturations of less than

Physical properties of the sub-micrometer aerosol over the Amazon rain forest during the wet-to-dry season transition - comparison of modeled and measured CCN concentrations

Abstract. Sub-micrometer atmospheric aerosol particles were studied in the Amazon region, 125 km northeast of Manaus, Brazil (-1°55.2'S, 59°28.1'W). The measurements were performed during the wet-to-dry transition period, 4-28 July 2001 as part of the LBA (Large-Scale Biosphere Atmosphere Experiment in Amazonia) CLAIRE-2001 (Cooperative LBA Airborne Regional Experiment) experiment. The number size distribution was measured with two parallel differential mobility analyzers, the hygroscopic growth at 90% RH with a Hygroscopic Tandem Mobility Analyzer (H-TDMA) and the concentrations of cloud condensation nuclei (CCN) with a cloud condensation nuclei counter. A model was developed that uses the H-TDMA data to predict the number of soluble molecules or ions in the individual particles and the corresponding minimum particle diameter for activation into a cloud droplet at a certain supersaturation. Integrating the number size distribution above this diameter, CCN concentrations were predicted with a time resolution of 10 min and compared to the measured concentrations. During the study period, three different air masses were identified and compared: clean background, air influenced by aged biomass burning, and moderately polluted air from recent local biomass burning. For the clean period 2001, similar number size distributions and hygroscopic behavior were observed as during the wet season at the same site in 1998, with mostly internally mixed particles of low diameter growth factor (~1.3 taken from dry to 90% RH). During the periods influenced by biomass burning the hygroscopic growth changed slightly, but the largest difference was seen in the number size distribution. The CCN model was found to be successful in predicting the measured CCN concentrations, typically within 25%. A sensitivity study showed relatively small dependence on the assumption of which model salt that was used to predict CCN concentrations from H-TDMA data. One strength of using H-TDMA data to predict CCN concentrations is that the model can also take into account soluble organic compounds, insofar as they go into solution at 90% RH. Another advantage is the higher time resolution compared to using size-resolved chemical composition data.

Chemical composition and sources of the high Arctic aerosol relevant for cloud formation

The chemical composition, sources, and methods of production of cloud condensation nuclei (CCN) were studied over the pack ice covered Arctic Ocean in summer. After ∼2 days' travel of air over the pack ice, two local CCN sources were identified: particles derived from the bursting bubbles on open water between ice floes (film and jet drops) and particles reaching CCN size by acquisition of oxidation products of dimethyl sulfide. The bubble‐derived particle modes were found to have an exponential dependence on prevailing wind speed. The film drop generation depended less on wind speed and jet drop production depended more on wind speed than particles over open water did. However, the enhanced presence of film drop particles at wind speeds &lt;5 m s−1 suggested a source of bubbles, other than wind, that is most active during clear sky days and ice melting. The film drop particles were mainly organic in nature, but their sea‐salt component increased at high wind speeds &gt;12 m s−1 to dominate both mass and number. The jet drop mode was composed largely of sea salt, making a significant mass contribution already for wind speeds &gt;5 m s−1. However, the smaller‐sized jet drops were not pure sodium chloride but contained a varied and appreciable organic component. The sulfur‐containing particles dominated the CCN population and mass for the most common wind speeds of 5–12 m s−1. Measured CCN concentrations were on average less than would have been expected from either a sulfate or a sea‐salt composition and the observed particle‐size distribution. It is concluded that other components, probably organic, depressed the nucleating ability of the particles. However, on clear sky days, there were a majority of occasions on which CCN concentrations were more than predicted from a sulfate composition and the measured size distribution.