Sonia M. Kreidenweis

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than -36 °C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼103 to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m-2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.

Effects of mixing on extinction by carbonaceous particles

Reported values for the absorption cross section of particulate carbon per unit mass range from under 4 to over 20 m2/g, and the intermediate value of 10 m2/g is used by many as a standard gram-specific absorption cross section for atmospheric soot. In order to better understand the possible variations in absorption by atmospheric carbon, we reevaluated its optical properties in terms of the material composition and morphology of soot and the electrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to low-density air/graphite volume mixtures are considered. The effects on extinction efficiency of aggregation and of internal mixing of carbon with sulfate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several homogeneous mixing rules (effective medium approximations), On the basis of our modeling of the optical properties of aggregates of graphitic carbon grains, we conclude that 10 m2/g may be over 50% too high in many cases, and we suggest that the mass absorption coefficient for the light-absorbing carbon in diesel soot at a wavelength of 0.550 μm may often be less than 7 m2/g, although variations in optical constants and, especially, the specific gravity of the absorbing material make it difficult to assign a specific numerical value. Adhesion of carbon grains to sulfate droplet surfaces is expected to enhance their absorption by no more than about 30%. Soot randomly positioned within droplets, however, can display averaged absorption enhancement factors of about 2.5–4 for hosts with refractive indices ranging from 1.33–1.53, respectively, and radii ≳0.20 μm. Nonetheless, calculations indicate that for realistic dry particle populations, αa < 10 m2/g for graphitic carbon in the atmosphere unless (1) most of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 μm (which corresponds to a mass median diameter of 0.34 μm). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment when ascribing characteristic values for their absorption and scattering efficiencies.

Measurements of the concentration and composition of nuclei for cirrus formation

This article addresses the need for new data on indirect effects of natural and anthropogenic aerosol particles on atmospheric ice clouds. Simultaneous measurements of the concentration and composition of tropospheric aerosol particles capable of initiating ice in cold (cirrus) clouds are reported. Measurements support that cirrus formation occurs both by heterogeneous nucleation by insoluble particles and homogeneous (spontaneous) freezing of particles containing solutions. Heterogeneous ice nuclei concentrations in the cirrus regime depend on temperature, relative humidity, and the concentrations and physical and chemical properties of aerosol particles. The cirrus-active concentrations of heterogeneous nuclei measured in November over the western U.S. were <0.03 cm–3. Considering previous modeling studies, this result suggests a predominant potential impact of these nuclei on cirrus formed by slow, large-scale lifting or small cooling rates, including subvisual cirrus. The most common heterogeneous ice nuclei were identified as relatively pure mineral dusts and metallic particles, some of which may have origin through anthropogenic processes. Homogeneous freezing of large numbers of particles was detected above a critical relative humidity along with a simultaneous transition in nuclei composition toward that of the sulfate-dominated total aerosol population. The temperature and humidity conditions of the homogeneous nucleation transition were reasonably consistent with expectations based on previous theoretical and laboratory studies but were highly variable. The strong presence of certain organic pollutants was particularly noted to be associated with impedance of homogeneous freezing.

Influence of sea-salt on aerosol radiative properties in the Southern Ocean marine boundary layer

There has been considerable debate about the relative importance of sea-salt and sulphate from non-sea-salt sources in determining aerosol radiative effects in the marine boundary layer. In the marine boundary layer, the most numerous aerosols are volatile sulphate particles smaller than about 0.08 µm (ref. 1) and most of the aerosol mass is in a few sea-salt particles larger than 1 µm. Yet intermediate-size aerosols between about 0.08 and 1 µm diameter are the most relevant to the radiative forcing of climate because they efficiently scatter solar radiation and also serve as cloud nuclei. Indeed, Charlson et al. hypothesized that oceanic production of sulphate aerosols from the oxidation of dimethyl sulphide could be a powerful feedback in the climate system. It is generally assumed that marine aerosols smaller than about 1 µm are non-sea-salt sulphate, but a recent review cites indirect evidence that many aerosols in the sub-micrometre range contain at least some sea-salt,. Here we present direct observational evidence from a remote Southern Ocean region that almost all aerosols larger than 0.13 µm in the marine boundary layer contained sea-salt. These sea-salt aerosols had important radiative effects: they were responsible for the majority of aerosol-scattered light, and comprised a significant fraction of the inferred cloud nuclei.

A modeling study of aqueous production of dicarboxylic acids: 1. Chemical pathways and speciated organic mass production

[1] While the formation pathways and thermodynamic properties of inorganic species (e.g., sulphate) in atmospheric aerosols are well understood, many more uncertainties exist about organics. In the present study we present oxidation pathways of organic gas phase species that lead to low volatility organic compounds (C2-C6 dicarboxylic acids, pyruvic acid) in both the aqueous and gas phases. This mechanism is implemented in a cloud parcel model initialized with pure (NH4)2SO4 particles in 10 discrete sizes. Under clean continental conditions a few cloud processing cycles produce a total organic mass addition of � 150 ng m � 3 . Individual resuspended aerosol size classes contain significant organic fractions, sometimes higher than 50%. These are likely upper bound estimates of organic mass production. In a polluted, i.e., SO2-rich scenario, about 400 ng m � 3 organic material is produced after about eight cloud cycles. Since the initial conditions in this latter case favor significant production of sulphate, the organic fraction of the aerosol mass after cloud processing represents a much lower percentage of the total aerosol mass. Oxalic, glutaric, adipic, and pyruvic acids are the main contributors to the organic fraction in both cases. In agreement with observations, the oxalate fraction in processed particles exceeds the fractions of other dicarboxylic acids since it represents an end product in the oxidation of several organic gas phase species. The study suggests that cloud processing may act as a significant source of small dicarboxylic acids, some fraction of which can be retained in the aerosol phase upon drop evaporation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: dicarboxylic acids, hygroscopicity, organic aerosols

Emissions of trace gases and aerosols during the open combustion of biomass in the laboratory

[1] We characterized the gas- and speciated aerosol-phase emissions from the open combustion of 33 different plant species during a series of 255 controlled laboratory burns during the Fire Laboratory at Missoula Experiments (FLAME). The plant species we tested were chosen to improve the existing database for U.S. domestic fuels: laboratory-based emission factors have not previously been reported for many commonly burned species that are frequently consumed by fires near populated regions and protected scenic areas. The plants we tested included the chaparral species chamise, manzanita, and ceanothus, and species common to the southeastern United States (common reed, hickory, kudzu, needlegrass rush, rhododendron, cord grass, sawgrass, titi, and wax myrtle). Fire-integrated emission factors for gas-phase CO2, CO, CH4 ,C 2–4 hydrocarbons, NH3 ,S O2, NO, NO2, HNO3, and particle-phase organic carbon (OC), elemental carbon (EC), SO4� ,N O3 ,C l � ,N a + ,K + , and NH4 generally varied with both fuel type and with the fire-integrated modified combustion efficiency (MCE), a measure of the relative importance of flaming- and smoldering-phase combustion to the total emissions during the burn. Chaparral fuels tended to emit less particulate OC per unit mass of dry fuel than did other fuel types, whereas southeastern species had some of the largest observed emission factors for total fine particulate matter. Our measurements spanned a larger range of MCE than prior studies, and thus help to improve estimates of the variation of emissions with combustion conditions for individual fuels.

The Impact of Giant Cloud Condensation Nuclei on Drizzle Formation in Stratocumulus: Implications for Cloud Radiative Properties

Abstract The impact of giant and ultragiant cloud condensation nuclei (>5-μm radius) on drizzle formation in stratocumuli is investigated within a number of modeling frameworks. These include a simple box model of collection, a trajectory ensemble model (comprising an ensemble of Lagrangian parcel models), a 2D eddy-resolving model, and a 3D large-eddy simulation model. Observed concentrations of giant cloud condensation nuclei (GCCN) over the ocean at ambient conditions indicate that 20-μm radius haze particles exist in concentrations of between 10−4 and 10−2 cm−3, depending on ambient wind speed and seastate. It is shown that these concentrations are sufficient to move a nonprecipitating stratocumulus into a precipitating state at typical cloud condensation nucleus (CCN) concentrations of 50 to 250 cm−3, with higher concentrations of GCCN being required at higher CCN concentrations. However, at lower CCN concentrations, drizzle is often active anyway and the addition of GCCN has little impact. At high C...

Cloud resolving simulations of Arctic stratus: Part II: Transition-season clouds

Abstract Two-dimensional simulations of transition (fall and spring) season Arctic stratus clouds (ASC) were conducted using a sophisticated cloud resolving model with bin microphysics coupled to a two-stream radiative transfer model. The impacts of temperature variation and various ice microphysical processes on the evolution of the simulated mixed-phase ASC layer are studied. Cloud layers either collapse through rapid glaciation and ice precipitation from the cloud layer or maintain a quasi-steady state. Sensitivity studies show that the stability of the mixed-phase cloud layer is dependent upon the temperature, ice concentration, and the habit of the ice crystals. In particular, cloud layer stability is shown to be most strongly dependent upon the concentration of ice forming nuclei (IFN). In addition, it is shown that ice production and sedimentation can assist the formation of a second, lower cloud layer suggesting a new mechanism of multiple-layer formation in the Arctic.

A study of new particle formation and growth involving biogenic and trace gas species measured during ACE 1

Measurements are presented of ambient nanoparticle distributions (2.7 to 10 nm diameter) in regions of high biogenic emissions encountered during the First Aerosol Characterization Experiment (ACE 1), November 15 to December 14, 1995. Large numbers of newly formed nanoparticles were observed directly downwind of penguin colonies on Macquarie Island (54.5thinsp{degree}S, 159.0thinsp{degree}W). In these regions, nanoparticle concentrations were also correlated with sulfuric acid (H{sub 2}SO{sub 4(g)}) gas concentrations. The measurements show that biogenic species, possibly ammonia (NH{sub 3}), either by itself or with H{sub 2}SO{sub 4}, nucleated to form new particles at rates much higher than bimolecular H{sub 2}SO{sub 4}/H{sub 2}O nucleation. Nanoparticle distributions evolved as air was advected away from the island showing clear evidence of growth of the newly formed particles. Observed growth rates were in the range of 2 to 5 nmthinsph{sup {minus}1} and were about a factor of 4 to 17 times higher than the growth by condensing H{sub 2}SO{sub 4(g)} and associated water. The cause for fast growth of the newly formed particles is unknown. {copyright} 1998 American Geophysical Union

The susceptibility of ice formation in upper tropospheric clouds to insoluble aerosol components

Ice may form by both homogeneous and heterogeneous freezing nucleation processes in clouds at temperatures below -35°C. Most investigations have focused on the former process. This paper presents results from adiabatic parcel model calculations that include the effects of both freezing processes in unactivated solution droplets. Uncertainties in predicting the homogeneous freezing rates are discussed and used to select solution drop composition and freezing characteristics that bracket those expected in the upper troposphere. The heterogeneous freezing rates of insoluble atmospheric aerosols are parameterized based on published freezing rates of carbonaceous particles. Process model simulations show that the potential variability in ice formation in cirrus clouds is much greater if heterogeneous freezing nucleation is considered in addition to homogeneous freezing. The impact of insoluble aerosols on ice formation is inferred to increase with insoluble particle size and with the fraction of soluble aerosols containing insoluble components. The maximum impact of heterogeneous nucleation is indicated for vertical motions less than 0.2 m s -1 and for insoluble components being associated with at least 10% of all soluble aerosols. The wide range of ice crystal concentrations observed in cirrus is most consistent with the occurrence of both heterogeneous and homogeneous ice formation processes. These conclusions are partially supported by existing observations of aerosols and cloud microphysical characteristics in upper tropospheric clouds but require new measurements for confirmation.