Cloud Condensation Nuclei Production Research Articles

Bursts of cloud condensation nuclei (CCN) by dissipating clouds at Palmer Station, Antarctica

We present here a case study of cloud‐mediated production of cloud condensation nuclei (CCN) recorded at Palmer Station (64°46′S, 64°05′W), Antarctica on 20 Jan 1994. Four instances of CCN bursts occurred on Jan 17, 19, 20 and Feb 7, 1994 when cloud base descended to the surface and dissipated under prevailing meteorological conditions. The most spectacular event occurred on Jan 20 when the CCN concentration was enhanced by a factor of four at 0.25% supersaturation (with respect to water) compared to the pre‐event concentration. At 1.25% supersaturation, the corresponding enhancement was by a factor of seven. This indicated a larger production of aerosol particles in smaller size ranges. The elevated CCN concentrations were measured for over fifteen hours. The CCN activity spectrum during the event resembled the ones that are typical of previous measurements in the urban plumes of St. Louis and Denver.

Aerosol production and growth in the marine boundary layer

The dependence of cloud condensation nuclei (CCN) production on the marine dimethylsulfide (DMS) flux is modeled with a dynamic description of the gas, aerosol, and aqueous phase processes in a closed air parcel. The results support the conclusion reached in previous work with a steady state model that an approximately linear dependence exists between CCN concentration and DMS flux under typical remote marine conditions. This linearity does not hold for low DMS fluxes (the threshold is typically near 2.5 μmol m−2 day−1) because the seasalt particles heterogeneously convert the available SO2 to sulfate inhibiting the creation of new particles. The conditions under which this linear relationship holds are investigated by a series of sensitivity studies, focusing particular attention on the impact of the timing and frequency of cloud events. We consider the regimes of the model's semiempirical parameters, showing that the uncertainty associated with two such parameters, namely, the nucleation rate scaling factor and the sulfuric acid accommodation coefficient, is sufficient to change the predicted CCN production due to DMS from over 300 cm−3 day−1 to none. This sensitivity accounts for most of the range of results predicted by previous models of the DMS‐CCN system.

Reassessing the dependence of cloud condensation nucleus concentration on formation rate

MARINE stratocumulus clouds play an important role in the Earth's radiation budget. The albedo of these clouds depends on the cloud droplet size distribution, and therefore, in part, on the number density of cloud condensation nuclei (CCN)1. It has been postulated2 that a positive feedback loop between increased CCN concentrations and decreased drizzle gives rise to a bistable system, in which there are two equilibrium CCN concentration regimes. According to this model, CCN concentration is only weakly dependent on the CCN production rate within the stable regimes, but very strongly dependent on this rate in the transition region between the regimes. If correct, this strong dependence implies that a small increase in the production of CCN over the oceans could drastically increase the planetary albedo. Using a more sophisticated model3 than that used previously, we find no evidence for bistability. However, we find that CCN concentrations are generally strongly dependent on their production rate, so that changes in the latter would influence the Earth's albedo. We also find that the timescale for reducing high CCN concentrations can be as long as several days, which implies that high CCN concentrations can persist in clouds advected to regions of lower CCN production rate.

Dissipation of Marine Stratiform Clouds and Collapse of the Marine Boundary Layer Due to the Depletion of Cloud Condensation Nuclei by Clouds

When the production of cloud condensation nuclei in the stratocumulus-topped marine boundary layer is low enough, droplet collisions can reduce concentrations of cloud droplet numbers to extremely low values. At low droplet concentrations a cloud layer can become so optically thin that cloud-top radiative cooling cannot drive vertical mixing. Under these conditions, model simulations indicate that the stratocumulus-topped marine boundary layer collapses to a shallow fog layer. Through this mechanism, marine stratiform clouds may limit their own lifetimes.

CCN formation from DMS oxidation without SO2 acting as an intermediate

An alternate pathway for generating cloud condensation nuclei (CCN) in the remote marine boundary layer (MBL) from the oxidation of biogenically‐derived dimethyl sulfide (DMS) is proposed. This pathway invokes the production of H2SO4 from DMS without SO2 serving as an intermediate in the oxidation sequence. The H2SO4 thus produced then undergoes bimolecular nucleation to form tiny sulfate condensation nuclei (CN). These CN rapidly grow to sizes sufficiently large to act as CCN. While CCN production from SO2 appears to be inefficient, model calculations indicate that the alternate pathway is capable of generating significant numbers of new CCN in the MBL within a two to three day time period. The alternate pathway also suggests a strong coupling between oceanic emissions of DMS and the number of CCN in the MBL.

Urban Cloud Condensation Nuclei Spectral Flux

The cloud condensation nuclei (CCN) spectral flux and the condensation nuclei (CN) flux from an urban area are determined from in situ aircraft measurements at Denver, Colorado. The concentration differences between upwind and downwind cross sections at several altitudes are used to estimate anthropogenic fluxes. The CCN and CN production rate estimates are consistently three times larger than a similar 1966 study by Squires, also at Denver. On a per capita basis, the present measurements are only a factor of 2 higher. This is probably within the seasonal and day-to-day variability and/or may be result of somewhat different measurement techniques. The instantaneous CCN spectra produced flux estimates over the range of critical supersaturation Sc of 0.04%–0.9%. Some of the estimated particle production rates for Denver are 1 × 1018 s−1 for CN, 1.7 × 1017 s−1 for CCN at 0.9% Sc, 1.16 × 1017 s−1 for CCN at 0.5% Sc, 3 × 1015 s−1 for CCN at 0.1% Sc, and 5 × 1014 s−1 for CCN at 0.04% Sc.

A model study of the formation of cloud condensation nuclei in remote marine areas

A theoretical study is presented of the mechanisms by which freshly nucleated sulfuric acid particles can be grown to the size of cloud condensation nuclei (CCN) in remote marine areas. Three mechanisms are considered: (1) growth by the scavenging and subsequent aqueous phase oxidation of atmospheric SO2, (2) growth by coagulation, and (3) growth by scavenging of atmospheric sulfuric acid. While model calculations indicate that the first two mechanisms are too slow to represent a significant source of CCN in the remote marine atmosphere, scavenging of sulfuric acid can be an effective mechanism for transforming tiny sulfuric acid particles into CCN provided the concentration of preexisting CCN is less than some critical concentration. For conditions typical of the marine boundary layer with ambient SO2 concentrations of about 30 parts per trillion by volume (pptv), we estimate this critical concentration to be quite low (that is, about 1 cm−3). However, under conditions with enhanced SO2 the value of this critical concentration also increases and is estimated to reach a level of about 10 cm−3 when SO2 is equal to 100 pptv. Our calculations suggest that coupling between dimethyl sulfide (DMS) emissions and CCN production in the marine boundary layer can only exist when the existing CCN concentrations at a specific locale fall below this critical concentration.

Observations of anthropogenic cloud condensation nuclei

Cloud condensation nuclei (CCN) concentrations and spectral measurements obtained with the DRI instantaneous CCN spectrometer (Hudson, 1989, J. atmos. ocean. Technol. 6, 1055–1065) are presented. Anthropogenic CCN production mechanisms were readily identifiable while natural CCN production could not be detected.

Heterogeneous production of cloud condensation nuclei in the marine atmosphere

Model calculations are presented which indicate that newly created particles in the marine atmosphere will commonly only be able to achieve sizes, by means of gas‐phase processes, sufficient to activate in cumuliform clouds. Aqueous sulfate production in such clouds will be generally necessary to grow them large enough to activate in marine stratus clouds.

Production of Cloud Condensation Nuclei in Mexico City

As the first step in a long-term program oriented toward understanding the influence of air pollution on weather and climate in Mexico City, we measured the hourly variation of the atmospheric aerosol concentration active as cloud condensation nuclei (CCN) at supersaturation of 0.75%. The tests were performed at ground level using a horizontal thermal diffusion chamber during 1–5 April 1984. The results showed a pattern characterized by a maxima CCN concentration with values between 6500 and 3500 CCN cm−3 during the first hours of sunlight, until the concentration reaches values fluctuating between 2000 and 600 CCN cm−3 during the afternoon. Analysis of the results shows how the interplay of anthropogenic sources and local meteorological conditions determine a characteristic pattern of the CCN concentration.

A preliminary study of the chemical modification of cloud condensation nuclei in stratiform clouds at Whiteface Mountain, New York

Results are presented for pH, nitrate and sulfate ion content of cloud water collected at Whiteface Mountain in northern New York. Evidence exists for the production of sulfate in possibly one of the case studies and for nitrate production in all three of the case studies examined. The results are not entirely consistent with the hypothesis that chemical modification of cloud condensation nuclei (CCN) takes place in stratiform clouds. For only one case is CCN production evident and total aerosol number concentration is enhanced. The measured nitrate production cannot be accounted for by the soluble fraction of the CCN aerosol and may result from the absorption of interstitial HNO 3(g). This might account for the change in cloud condensation nuclei, indicated by the results, when sulfate production was not evident. The CCN spectrum can also be altered by agglomeration of captured particles upon cloud droplet evaporation. It is not possible to separate the two mechanisms (chemical vs physical) at this time.

Cloud-Active Nuclei from Coal-Fired Electric Power Plants and Their Interactions with Clouds

Abstract The concentrations of cloud condensation nuclei (CCN) in the plumes from coal-fired electric power plants are generally about 2 to 5 times greater than in the ambient air unaffected by the plumes. However, if the ambient air is very clean, the concentrations of CCN in a coal power plant plume can be up to ∼80 times greater than in the ambient air. The rates of production of CCN due to gas-to-particle (g-to-p) conversion in the plume from one of the plants studied were measured on different occasions to be ∼2 × 1015 and ∼5 × 1013 CCN h−1 per mole of SO2. The maximum current of CCN to be expected in the plume from a coal power plant is ∼1017 CCN s−1. After a travel time of ∼1 h, most of the CCN in power plant plumes have been produced by g-to-p conversion rather than emitted directly from the stack. The concentrations of ice nuclei in the plumes did not differ significantly from those in the ambient air. The materials in a plume may be transported rapidly in the vertical if the plume is entrained i...

Particle Emissions From a Large Kraft Paper Mill and Their Effects on the Microstructure of Warm Clouds

Airborne measurements of cloud condensation nuclei (CCN), total particle counts, the size distribution of particles, and the microstructure of warm clouds have been made in the vicinity of a large Kraft pulp and paper mill. The plume of CCN and total particles from the mill is well-defined at distances up to 40 km downwind and the concentrations of these particles reach values which exceed those in the ambient air by a factor of about 4. A large peak in the concentrations of CCN was measured in the plume at a distance downwind of the mill corresponding to a travel time of about 4 hr. The total particle count in the plume peaked after about 2 hr of travel time downwind. The particles in the plume were measured over the size range 0.3 to 8 μm; their concentrations were much higher than in the ambient air, particularly those with diameters <1 μm. At points downwind in the plume corresponding to travel times ≳6 min, a secondary peak appeared in the concentration of particles with diameters just in excess of 0.3 μm and it persisted for travel times up to about 4 hr. The rates of production of CCN and particles >0.3 μm in diameter in the plume is estimated to be about 0.05 cm−3 sec−1. It is suggested that this is due to gas-to-particle conversions.Clouds that form in the plume from the paper mill had a much broader spectrum of droplet sizes than those which form in the ambient air. Since the former clouds contain a significant number of droplets with diameters >30 μm, as well as a large number of smaller droplets, precipitable drops should form in them very efficiently by the coalescence mechanism. These observations support the suggestion made by Hobbs et al. that the effluents from paper mills are probably responsible for the anomalously high annual precipitation amounts which have occurred in the vicinity of such mills in Washington State in recent years.

Cloud Condensation Nuclei from Industrial Sources and Their Apparent Influence on Precipitation in Washington State

Measurements of the concentrations of cloud condensation nuclei (CCN) in the air in Washington State have shown that pulp and paper mills, and certain other industries, are prolific sources of CCN. The rate of production of CCN from large paper mills can be as high as 1019 sec−1. Direct observations show that clouds often form downwind of these mills and, in some cases, these clouds produce precipitable particles very efficiently. A comparison of precipitation and streamflow records in Washington for the period 1929–46 with those for 1947–66 shows that a number of areas have had a mean annual precipitation during the second period more than 30% greater than that in the first period. In nearly all cases these areas are in the vicinity of large industrial sources of CCN and it is suggested that the higher precipitation in recent years is a consequence of the increased numbers of very efficient CCN emitted into the atmosphere by these sources.