Glendon Frick

Marine boundary layer measurements of new particle formation and the effects nonprecipitating clouds have on aerosol size distribution

Measurements of aerosol size distributions (0.005 < r < 20 μm), cloud droplet spectra, SO2, O3, CN, and other supporting quantities were made in the cloud-topped and clear marine boundary layer (MBL) from an airship operating within about 50 km of the Oregon coast. Comparison of size distribution of interstitial aerosol within the cloud with the size distribution below the cloud clearly indicates that the processing of the aerosol through (nonprecipitating) stratus can lead to increased mass of the subset of particles which had served as cloud condensation nuclei (CCN). This increase in mass in the CCN results in a distinct “cloud residue” mode in the size distribution measured below the cloud. In all cases the aerosol mass in the cloud residue mode greatly exceeded the mass in the interstitial mode, even though the number concentration of interstitial particles sometimes exceeded the CCN concentration. Evidence of new particle formation in clear air was also found on numerous occasions. Analyses of the data indicate that the growth of newly formed particles into the observed size range is consistent with gas phase oxidation of SO2 to sulfate and subsequent condensation on the aerosol. However, the exact nucleation process, whether by homogeneous nucleation, ion-assisted nucleation, or heterogeneous nucleation on precursor embryos, is still an open question.

Emissions from Ships with respect to Their Effects on Clouds

Emissions of particles, gases, heat, and water vapor from ships are discussed with respect to their potential for changing the microstructure of marine stratiform clouds and producing the phenomenon known as ‘‘ship tracks.’’ Airborne measurements are used to derive emission factors of SO 2 and NO from diesel-powered and steam turbine-powered ships, burning low-grade marine fuel oil (MFO); they were ;15‐89 and ;2‐25 g kg21 of fuel burned, respectively. By contrast a steam turbine‐powered ship burning high-grade navy distillate fuel had an SO2 emission factor of ; 6gk g 21. Various types of ships, burning both MFO and navy distillate fuel, emitted from ;4 3 1015 to 2 3 1016 total particles per kilogram of fuel burned (;4 3 1015‐1.5 3 1016 particles per second). However, diesel-powered ships burning MFO emitted particles with a larger mode radius (;0.03‐0.05 mm) and larger maximum sizes than those powered by steam turbines burning navy distillate fuel (mode radius ;0.02 mm). Consequently, if the particles have similar chemical compositions, those emitted by diesel ships burning MFO will serve as cloud condensation nuclei (CCN) at lower supersaturations (and will therefore be more likely to produce ship tracks) than the particles emitted by steam turbine ships burning distillate fuel. Since steam turbine‐powered ships fueled by MFO emit particles with a mode radius similar to that of diesel-powered ships fueled by MFO, it appears that, for given ambient conditions, the type of fuel burned by a ship is more important than the type of ship engine in determining whether or not a ship will produce a ship track. However, more measurements are needed to test this hypothesis. The particles emitted from ships appear to be primarily organics, possibly combined with sulfuric acid produced by gas-to-particle conversion of SO 2. Comparison of model results with measurements in ship tracks suggests that the particles from ships contain only about 10% water-soluble materials. Measurements of the total particles entering marine stratiform clouds from diesel-powered ships fueled by MFO, and increases in droplet concentrations produced by these particles, show that only about 12% of the particles serve as CCN. The fluxes of heat and water vapor from ships are estimated to be ;2‐22 MW and;0.5‐1.5 kg s21, respectively. These emissions rarely produced measurable temperature perturbations, and never produced detectable perturbations in water vapor, in the plumes from ships. Nuclear-powered ships, which emit heat but negligible particles, do not produce ship tracks. Therefore, it is concluded that heat and water vapor emissions do not play a significant role in ship track formation and that particle emissions, particularly from those burning low-grade fuel oil, are responsible for ship track formation. Subsequent papers in this special issue discuss and test these hypotheses.

The Impact of Ship-Produced Aerosols on the Microstructure and Albedo of Warm Marine Stratocumulus Clouds: A Test of MAST Hypotheses 1i and 1ii

Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks. This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.

Holes and Entrainment in Stratocumulus

Aircraft flights through stratocumulus clouds (Sc) during the Dynamics and Chemistry of Marine Stratocumulus II (DYCOMS-II) study off the California coast found narrow in-cloud regions with less liquid water content (LWC) and cooler temperatures than average background values. The regions are named cloud holes and are assumed to be a result of water evaporated by the entrainment of dryer air from above the Sc. While such features have been noted previously, this study provided a unique opportunity to investigate in much greater detail the nature of the holes, as well as their relationship to the entrainment rate, because high-speed temperature and LWC probes with maximum spatial resolution of 10 cm were flown together for the first time. Nine long-duration flights were made through mostly unbroken Sc for which conditional sampling was used to identify the location and size of the holes. The holes are concentrated near cloud top, their average width near cloud top is about 5 m, their relative length distribution is nearly constant for all flights, and they can penetrate hundreds of meters deep into the Sc before being lost by mixing. Entrainment velocities at cloud top are estimated from measurements of fluxes of reduced LWC and vapor mixing ratios in holes, the fraction of cloud area covered by holes, and the total water jump between cloud top and the free atmosphere. Rates as large as 10 mm s 1 are found for nocturnal flights, and these rates are about 3 times larger than for daytime flight segments. The rates correlate best with the size of the buoyancy jump above the Sc; the present conditional-sampling approach for measuring the rates gives larger rates than the “flux jump” rates determined by others for the same flights by a factor of about 2. The stability criterion for all Sc predicts thinning and breakup of the Sc, which does not occur. The minimal amount of cloud-top evaporative cooling caused by entrainment contributes little to the top-down convection dominated by radiative cooling during nocturnal flights; however, evaporative cooling caused by the mixing of holes as they subduct with the large-scale eddy circulation in the Sc may contribute, but with an as-of-yet unknown amount.

A cloud chamber study of the effect that nonprecipitating water clouds have on the aerosol size distribution

When an air parcel in the atmosphere passes through a nonprecipitating cloud cycle, a subset of the aerosol population called cloud condensation nuclei (CCN) is activated and forms cloud droplets. During the cloud phase, trace gases, such as SO2, are dissolved into the droplets and undergo aqueous phase chemical reactions, forming low-volatility products, such as sulfates, that remain as residue when the cloud droplets evaporate. The resulting increase in residual mass can have a dramatic effect on the aerosol size distribution, causing the CCN to grow relative to the smaller particles (interstitial aerosol) which were not activated in the cloud. This process was graphically demonstrated in a series of experiments carried out in the Calspan 600-m3 environmental chamber, under conditions where the precloud reactants could be carefully controlled. Size distributions taken before and after a cloud cycle showed significant conversion of SO2 to H2SO4 and a dramatic change in the aerosol size distribution. Subs...

The Nonequilibrium Character of the Aerosol Charge Distributions Produced by Neutralizes

The mechanisms that control the polar ion concentrations downstream of an ionized region are examined and it is shown that the ratio of positive to negative ion concentrations is not constant. The imbalance in the ionic concentrations caused by unequal diffusion of ions to the walls and to aerosol particles is magnified in the ion ratio as ionic recombination rapidly depletes ions of both polarities equally. Consequently, the aerosol charge distribution is not in equilibrium but is evolving in response to the changing ion environment. The conclusions drawn are supported by numerical modeling and by measurements of ionic concentrations and ratios of negatively to positively charged particles downstream of the ionized region. Several existing neutralizers are evaluated and a prototype ionizer which produces an aerosol with a nearly symmetric equilibrium charge distribution is discussed.

Deducing droplet concentration and supersaturation in marine boundary layer clouds from surface aerosol measurements

Air parcels in the marine boundary layer (MBL) are mixed up through nonprecipitating clouds at the top of the MBL many times (on average) before they can be removed by precipitation scavenging. The equivalent dry size of the particles (cloud condensation nuclei, CCN) upon which droplets are formed increases because of liquid phase oxidation of soluble trace gases during the cloud processing. The observed separation of the submicron size distribution into an interstitial mode and cloud droplet residue mode makes it possible to infer the effective MBL cloud supersaturation and cloud droplet concentrations from surface measurements of the aerosol size distribution during periods when nonprecipitating MBL clouds are present in the back trajectory and the MBL is well mixed. The effect of particle composition on the accuracy of the inferred cloud supersaturations is evaluated. A large database of hundreds of size distributions taken on an Atlantic and a Pacific cruise and an airship flight off the Oregon coast are used to calculate the range of effective MBL cloud supersaturations and droplet concentrations encountered during these expeditions. The inferred droplet concentrations on the Pacific cruise were mostly in the 25 to 150 cm -3 range, whereas they were mostly in the 50 to 500 cm -3 range for the Atlantic cruise. The inferred effective supersaturation in the tropical MBL clouds was typically in the 0.15% to 0.25% range. Recent work of Tang and Munkelwitz [1994] would indicate that particles consisting of mixtures of ammonium sulfate and sulfuric acid would not have recrystalized in the differential mobility analyzer (DMA) within the range of relative humidities (45% to 60%) at which the DMA was operated. At these humidities the hydrated size can be as much as 20% greater than the dry size. Corrections for the hydrated size within the DMA at the time of measurement are included and are also used to correct previous measurements of the relationship between dry size and critical supersaturation made using the Naval Research Laboratory (NRL) DMA and NRL thermal gradient CCN counter.

Laboratory Studies of the Efficiency of Selected Organic Aerosols as CCN

Abstract Experiments were conducted in the large (600 m 3 ) expansion chamber of the Calspan Corporation to test the efficiency of selected organic aerosols as cloud condensation nuclei (CCN). Solutions of pinonic acid (PA), ammonium sulfate (AS) (as a benchmark), and a mixture of both were nebulized and used for initial CCN spectra, as were products of cyclohexene and α-pinene oxidation by ozone. Measurements of the resulting aerosols were made with both particle-sizing instruments and a cloud condensation nucleus (CCN) counter. Activated cloud drop number concentrations (CDNC) were determined during chamber expansion by standard laser-scattering probes. The results support the contention that the presence of sparingly soluble organic components in aerosols significantly affect the rate at which these aerosols activate to form cloud drops. However, even the organic aerosols eventually fully activated to the extent expected due to size assuming complete solubility. It is only the time necessary to activate them which is altered and the atmospheric significance of this kinetic effect of organics is not yet clear.

Particle formation and growth from ozonolysis of α‐pinene

Observations of particle nucleation and growth during ozonolysis of α-pinene were carried out in Calspans 600 m3 environmental chamber utilizing relatively low concentrations of α-pinene (15 ppb) and ozone (100 ppb). Model simulations with a comprehensive sectional aerosol model which incorporated the relevant gas-phase chemistry show that the observed evolution of the size distribution could be simulated within the accuracy of the experiment by assuming only one condensable product produced with a molar yield of 5% to 6% and a saturation vapor pressure (SVP) of about 0.01 ppb or less. While only one component was required to simulate the data, more than one product may have been involved, in which case the one component must be viewed as a surrogate having an effective SVP of 0.01 ppb or less. Adding trace amounts of SO2 greatly increased the nucleation rate while having negligible effect on the overall aerosol yield. We are unable to explain the observed nucleation in the α-pinene/ozone system in terms of classical nucleation theory. The nucleation rate and, more importantly, the slope of the nucleation rate versus the vapor pressure of the nucleating species would suggest that the nucleation rate in the α-pinene/ozone system may be limited by the initial nucleation steps (i.e., dimer, trimer, or adduct formation).

Experimental and modeling studies of secondary organic aerosol formation and some applications to the marine boundary layer

A series of controlled experiments were carried out in the Calspan Corporations 600 m3 environmental chamber to study some secondary organic aerosol formation processes. Three precursor-ozone systems were studied: cyclopentene-ozone, cyclohexene-ozone, and α-pineneozone. Additionally, SO2 was added to the initial gas mixture in several instances and was likely present at trace levels in the ostensibly organic-only experiments. It was found that all three systems readily formed new submicron aerosols at very low reactant levels. The chemical composition of formed aerosols was consistent with some previous studies, but the yields of organic products were found to be lower in the Calspan experiments. A three-step procedure is proposed to explain the observed particle nucleation behavior: HO · production → H2SO4 formation → H2SO4-H2O (perhaps together with NH3) homogeneous nucleation. It is also proposed that some soluble organic products would partition into the newly formed H2SO4-H2O nuclei, enhance water condensation, and quickly grow these nuclei into a larger size range. While the observations in the two cycloolefin-ozone systems could be well explained by these proposed mechanisms, the exact nature of the nucleation process in the α-pinene-ozone system remains rather opaque and could be the result of nucleation involving certain organics. The results from three simple modeling studies further support these proposals. Their applicability to the marine boundary layer (MBL) is also discussed in some detail. Particularly, such a particle nucleation and growth process could play an important role in secondary aerosol formation and, quite likely, CCN formation as well in certain MBL regions.