Tracey A. Rissman

Aerosol-cloud drop concentration closure in warm cumulus

Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASAs Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). An “aerosol-cloud” closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm^(−3), drives large microphysical differences (250–2300 cm^(−3)) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties (effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosols effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study.

Toward aerosol/cloud condensation nuclei (CCN) closure during CRYSTAL‐FACE

concentrations were 233 cm 3 (at S = 0.2%) and 371 cm 3 (at S = 0.85%). Three flights during the experiment differed from this general trend; the aerosol sampled during the two flights on 18 July was more continental in character, and the observations on 28 July indicate high spatial variability and periods of very high aerosol concentrations. This study also includes a simplified aerosol/CCN closure analysis. Aerosol size distributions were measured simultaneously with the CCN observations, and these data are used to

Cloud condensation nuclei activity, closure, and droplet growth kinetics of Houston aerosol during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

In situ cloud condensation nuclei (CCN) measurements were obtained in the boundary layer over Houston, Texas, during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) campaign onboard the CIRPAS Twin Otter. Polluted air masses in and out of cloudy regions were sampled for a total of 22 flights, with CCN measurements obtained for 17 of these flights. In this paper, we focus on CCN closure during two flights, within and downwind of the Houston regional plume and over the Houston Ship Channel. During both flights, air was sampled with particle concentrations exceeding 25,000 cm^(−3) and CCN concentrations exceeding 10,000 cm^(−3). CCN closure is evaluated by comparing measured concentrations with those predicted on the basis of measured aerosol size distributions and aerosol mass spectrometer particle composition. Different assumptions concerning the internally mixed chemical composition result in average CCN overprediction ranging from 3% to 36% (based on a linear fit). It is hypothesized that the externally mixed fraction of the aerosol contributes much of the CCN closure scatter, while the internally mixed fraction largely controls the overprediction bias. On the basis of the droplet sizes of activated CCN, organics do not seem to impact, on average, the CCN activation kinetics.

Chemical Amplification (or Dampening) of the Twomey Effect: Conditions Derived from Droplet Activation Theory

Cloud droplet number concentrations are controlled by both meteorological and microphysical factors. Microphysical factors include aerosol number concentration and composition. This paper examines the importance of microphysical phenomena compared to the sensitivity with respect to parcel updraft velocity in the activation of aerosols to become cloud droplets. Of the compositional (chemical) factors that can influence droplet number concentration, the effect of organics is examined through their ability to alter droplet surface tension and to contribute solute. A recent parameterization of aerosol activation (by Abdul-Razzak et al.) is extended to obtain analytical expressions for the sensitivity of activation to microphysical factors relative to updraft velocity. It is demonstrated that, under some conditions, the droplet number concentration can be as much as 1.5 times more sensitive to changes in aerosol composition than to updraft velocity. Chemical effects seem to be most influential for size distributions typical of marine conditions and decrease in importance for strongly anthropogenically perturbed conditions. The analysis indicates that the presence of surface-active species can lead to as much uncertainty as results from variations in updraft velocity. The presence of surfactant species also drastically changes the response of the cloud condensation nuclei to changes in the updraft velocity spectrum. Conditions are found under which an increase in dissolved organic compounds can actually lead to a decrease in cloud droplet number, a “contra-Twomey effect.” Results presented have more general implications than just for organic compounds and can apply, in principle, for any chemically induced activation effect.

Use of in situ cloud condensation nuclei, extinction, and aerosol size distribution measurements to test a method for retrieving cloud condensation nuclei profiles from surface measurements

If the aerosol composition and size distribution below cloud are uniform, the vertical profile of cloud condensation nuclei concentration can be retrieved entirely from surface measurements of CCN concentration and particle humidification function and surface-based retrievals of relative humidity and aerosol extinction or backscatter. This provides the potential for long-term measurements of CCN concentrations near cloud base. We have used a combination of aircraft, surface in situ, and surface remote sensing measurements to test various aspects of the retrieval scheme. Our analysis leads us to the following conclusions. The retrieval works better for supersaturations of 0.1% than for 1% because CCN concentrations at 0.1% are controlled by the same particles that control extinction and backscatter. If in situ measurements of extinction are used, the retrieval explains a majority of the CCN variance at high supersaturation for at least two and perhaps five of the eight flights examined. The retrieval of the vertical profile of the humidification factor is not the major limitation of the CCN retrieval scheme. Vertical structure in the aerosol size distribution and composition is the dominant source of error in the CCN retrieval, but this vertical structure is difficult to measure from remote sensing at visible wavelengths.

Characterization of ambient aerosol from measurements of cloud condensation nuclei during the 2003 Atmospheric Radiation Measurement Aerosol Intensive Observational Period at the Southern Great Plains site in Oklahoma

Measurements were made by a new cloud condensation nuclei (CCN) instrument (CCNC3) during the Atmospheric Radiation Measurement (ARM) Programs Aerosol Intensive Observational Period (IOP) in May 2003 in Lamont, Oklahoma. An inverse aerosol/CCN closure study is undertaken, in which the predicted number concentration of particles available for activation (N_P) at the CCNC3 operating supersaturations is compared to that observed (N_O). N_P is based on Kohler Theory, with assumed and inferred aerosol composition and mixing state, and the airborne aerosol size distribution measured by the Caltech Dual Automatic Classified Aerosol Detector (DACAD). An initial comparison of N_O and N_P, assuming the ambient aerosol is pure ammonium sulfate ((NH_4)_2SO_4), results in closure ratios (N_P/N_O) ranging from 1.18 to 3.68 over the duration of the IOP, indicating that the aerosol is less hygroscopic than (NH_4)_2SO_4. N_P and N_O are found to agree when the modeled aerosol population has characteristics of an external mixture of particles, in which insoluble material is preferentially distributed among particles with small diameters (<50 nm) and purely insoluble particles are present over a range of diameters. The classification of sampled air masses by closure ratio and aerosol size distribution is discussed in depth. Inverse aerosol/CCN closure analysis can be a valuable means of inferring aerosol composition and mixing state when direct measurements are not available, especially when surface measurements of aerosol composition and mixing state are not sufficient to predict CCN concentrations at altitude, as was the case under the stratified aerosol layer conditions encountered during the IOP.

Effect of Angle of Attack on the Performance of an Airborne Counterflow Virtual Impactor

Abstract A three-dimensional model has been developed within the framework of the commercial computational fluid dynamics program, FLUENT®, to investigate the collection efficiency of an airborne counterflow virtual impactor (CVI). The model assumes steady-state, isothermal, compressible, and turbulent flow. Particle trajectories are computed based on the Lagrangian discrete phase model (DPM). In addition to predicting the effects of flight velocity and counterflow rate on the particle collection efficiency, as do prior models, the model quantifies the effect of flight attack angle on the particle collection efficiency. With an angle of attack as small as 5ˆ, the CVI collection efficiency drastically degrades at large particle sizes, and only particles with intermediate sizes are collected. Smaller particles do not have sufficient inertia to fight the counterflow, and larger particles tend to impact the CVI inner walls and are lost to the CVI walls. The modeling results show that the alignment between the free stream flow and the CVI inlet is critical to the performance of the CVI.

A Multi-Year Data Set on Aerosol-Cloud-Precipitation-Meteorology Interactions for Marine Stratocumulus Clouds

Airborne measurements of meteorological, aerosol, and stratocumulus cloud properties have been harmonized from six field campaigns during July-August months between 2005 and 2016 off the California coast. A consistent set of core instruments was deployed on the Center for Interdisciplinary Remotely-Piloted Aircraft Studies Twin Otter for 113 flight days, amounting to 514 flight hours. A unique aspect of the compiled data set is detailed measurements of aerosol microphysical properties (size distribution, composition, bioaerosol detection, hygroscopicity, optical), cloud water composition, and different sampling inlets to distinguish between clear air aerosol, interstitial in-cloud aerosol, and droplet residual particles in cloud. Measurements and data analysis follow documented methods for quality assurance. The data set is suitable for studies associated with aerosol-cloud-precipitation-meteorology-radiation interactions, especially owing to sharp aerosol perturbations from ship traffic and biomass burning. The data set can be used for model initialization and synergistic application with meteorological models and remote sensing data to improve understanding of the very interactions that comprise the largest uncertainty in the effect of anthropogenic emissions on radiative forcing.