Yalei Chen

Single particle analyses of ice nucleating aerosols in the upper troposphere and lower stratosphere

A newly developed instrument was deployed on the NASA DC-8 airborne laboratory during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS, Spring 1996) to detect and collect heterogeneous ice nucleating particles (IN) in the upper troposphere and lower stratosphere. The elemental compositions of both ambient particles and IN were determined with single particle analysis using analytical electron microscopy. IN collected during flights on May 4 and 8 had enhanced number fractions of metallic, crustal, and carbonaceous particles, compared with the ambient aerosol population, and were relatively deficient in sulfur-containing particles. IN sampled within aircraft exhaust and contrails had higher number fractions of metallic particles, which includes those rich in Zn, Al, and Ti, than the IN sampled in air that was not immediately affected by aircraft exhaust.

Ice formation by black carbon particles

Ice formation by black carbon particles was studied using a continuous flow thermal diffusion chamber. Submicron particles were treated to uptake H2SO4 in amounts estimated to range from zero to several percent by weight. The particles were processed at constant temperatures (−40 to −60°C), and humidity from ice saturation to values exceeding water saturation. Conditions required for ice nucleation were determined for different acid exposures. Untreated soot particles showed activity as deposition/sorption ice nuclei. Particles with approximate monolayer equivalent coverage by H2SO4 nucleated ice at humidities for which it was inferred that the acid was dilute enough to freeze homogeneously on soot surfaces. Particles with multi-layer equivalent H2SO4 coverage froze as solution droplets most readily. Heterogeneous freezing of the latter particles was observed to occur in preference to the homogeneous freezing of an externally mixed population of H2SO4 droplets at lower temperatures.

A Continuous-Flow Diffusion Chamber for Airborne Measurements of Ice Nuclei

Abstract A continuous-flow thermal gradient diffusion chamber was developed for operating in an aircraft and detecting ice nucleating aerosol particles in real time. The chamber volume is the annular space between two vertically oriented concentric cylinders. The surfaces of the chamber are coated with ice and held at different temperatures, thus creating a vapor supersaturation. Upstream of the chamber, all particles in the sample air larger than 2-μm diameter are removed with inertial impactors. The air then flows vertically downward through the chamber, where ice crystals nucleate and grow on active ice nuclei to between ∼3- and 10-μm diameter in 3–10 s of residence time. At the outlet of the chamber, an optical particle counter detects all particles larger than ∼0.8 μm. Those particles larger than 3 μm are assumed to be the newly formed ice crystals and comprise the ice nucleus count. This paper describes the principles of operation, hardware and construction, data system, calibration, operational pro...

Measurements of ice nucleating aerosols during SUCCESS

Instruments on NASAs DC-8 Airborne Laboratory were used to obtain measurements of heterogeneous ice nucleating aerosols (IN) and CN (condensation nuclei, particles >0.012 µm diameter) in background and aircraft-affected air during the April–May 1996 SUCCESS project (Subsonic Aircraft: Contrail and Cloud Effects Special Study). IN were measured with a continuous flow diffusion chamber over a range of processing conditions from about −15 to −40°C and from ice saturation to ∼15% water supersaturation. IN concentrations ranged from <0.1 to ∼500 l−1, being generally greater at colder temperatures and higher supersaturations. Within limited sample periods, IN concentration related strongly to sampling temperature and supersaturation, but overall, there was wide variability. IN and CN concentrations did not generally correlate. During penetrations of aircraft exhaust plumes, CN exhibited a very strong response, but there was no strong evidence that exhaust is a significant IN source for the temperature and supersaturation conditions of our measurements.

The role of heterogeneous freezing nucleation in upper tropospheric clouds: Inferences from SUCCESS

A temperature spectrum of heterogeneous freezing nuclei concentrations in continental air in the upper troposphere was determined based on airborne measurements. Numerical model simulations incorporating ice formation by heterogeneous and homogeneous freezing of deliquesced soluble aerosol particles were performed to investigate the effect of the heterogeneous process on the microphysics of upper tropospheric clouds. Heterogeneous freezing nuclei were predicted to cause lower maximum concentrations of ice particles formed in clouds. These nuclei also initiate the first ice formation and act to broaden ice crystal size distributions in upper tropospheric clouds. Observations of ice formation in an orographic wave cloud supported these predictions.

Ice Formation by Sulfate and Sulfuric Acid Aerosol Particles under Upper-Tropospheric Conditions

Abstract Ice formation in ammoniated sulfate and sulfuric acid aerosol particles under upper-tropospheric conditions was studied using a continuous flow thermal diffusion chamber. This technique allowed for particle exposure to controlled temperatures and relative humidities for known residence times. The phase states of (NH4)2SO4 and NH4HSO4 particles were found to have important impacts on their ice formation capabilities. Dry (NH4)2SO4 particles nucleated ice only at high relative humidity (RH ≥ 94%) with respect to water at temperatures between −40° and −60°C. This result suggested either an impedance or finite time dependence to deliquescence and subsequent homogeneous freezing nucleation. Ammonium sulfate particles that entered the diffusion chamber in a liquid state froze homogeneously at relative humidities that were 10% lower than where ice nucleated on initially dry particles. Likewise, crystalline or partially crystallized (as letovicite) NH4HSO4 particles required higher relative humidities fo...

Isolating and identifying atmospheric ice-nucleating aerosols: A New Technique

Abstract Laboratory studies examined two key aspects of the performance of a continuous-flow diffusion chamber (CFD) instrument that detects ice nuclei (IN) concentrations in air samples: separating IN from non-IN, and collecting IN aerosols to determine chemical composition. In the first study, submicron AgI IN particles were mixed in a sample stream with submicron non-IN salt particles, and the sample stream was processed in the CFD at −19°C and 23% supersaturation with respect to ice. Examination of the residual particles from crystals nucleated in the CFD confirmed that only AgI particles served as IN in the mixed stream. The second study applied this technique to separate and analyze IN and non-IN particles in a natural air sample. Energy-dispersive X-ray analyses (EDS) of the elemental composition of selected particles from the IN and non-IN fractions in ambient air showed chemical differences: Si and Ca were present in both, but S, Fe and K were also detected in the non-IN fraction.

Laboratory studies of ice nucleation by aerosol particles in upper tropospheric conditions

Ice formation in sulfate, sulfuric acid and black carbon/sulfate aerosol particles under upper tropospheric conditions was studied using a continuous flow thermal diffusion chamber. No clear difference in the homogeneous freezing conditions (temperature, relative humidity) as a function of degree of liquid sulfate neutralization was found, consistent with most other studies. Results support that homogeneous freezing nucleation cannot alone explain observed conditions for cirrus cloud formation. Some types of black carbon (soot) associated with sulfates in mixed particles will induce freezing in preference to the homogeneous process, but only at quite large particle sizes. Small soot produced from burning a particular jet fuel did not show heterogeneous ice nucleation activity until water saturation conditions were exceeded at upper tropospheric temperatures.

Studies of homogeneous and heterogeneous ice formation by aerosols in upper tropospheric cloud conditions

Publisher Summary This chapter studies numerical modeling to help define the potential roles of homogeneous freezing of soluble cloud condensation nuclei (CCN) aerosols and heterogeneous freezing by ice nuclei in determining the composition of cirrus clouds. The apparent sensitivity of cirrus cloud properties to changes in the abundance and sizes of heterogeneous IN is much greater than that for soluble CCN. Consideration of existing data on upper tropospheric aerosol characteristics and cirrus cloud microphysics also suggest that the heterogeneous process dominates. It describes laboratory and field studies underway to address uncertainties in understanding the two ice formation processes. It shows that both the magnitude of ice formation and the relative humidity needed for ice formation are typically overestimated by the homogeneous freezing process compared to available observations from cirrus clouds. These discrepancies do not appear explainable by uncertainties in aerosol composition and abundance and/or a poor knowledge of the freezing behavior of small solution droplets in upper tropospheric conditions. However, it was demonstrated that the presence of insoluble aerosol components acting as heterogeneous ice nuclei could explain the concentrations of ice crystals formed in cirrus and the observed relative humidities of their formation. Laboratory and field studies relevant to these issues are in progress.