429 research outputs found
Water activity and activation diameters from hygroscopicity data - Part I: Theory and application to inorganic salts
International audienceA method is described that uses particle hygroscopicity measurements, made with a humidified tandem differential mobility analyzer (HTDMA), to determine solution water activity as a function of composition. The use of derived water activity data in computations determining the ability of aerosols to serve as cloud condensation nuclei (CCN) is explored. Results for sodium chloride and ammonium sulfate are shown in Part I. The methodology yields solution water activities and critical dry diameters for ammonium sulfate and sodium chloride in good agreement with previously published data. The approach avoids the assumptions required for application of simplified and modified Köhler equations to predict CCN activity, most importantly, knowledge of the molecular weight and the degree of dissociation of the soluble species. Predictions of the dependence of water activity on the mass fraction of aerosol species are sensitive to the assumed dry density, but predicted critical dry diameters are not
Water activity and activation diameters from hygroscopicity data - Part II: Application to organic species
International audienceA method has been developed for using particle hygroscopicity measurements made with a humidified tandem differential mobility analyzer (HTDMA) to determine water activity as a function of solute weight percent. In Part I, the method was tested for particles composed of sodium chloride and ammonium sulfate. Here, we report results for several atmospherically-relevant organic species: glutaric acid, malonic acid, oxalic acid and levoglucosan. Predicted water activities for aqueous dicarboxylic acid solutions are quite similar in some cases to published estimates and the simplified predictions of Köhler theory, while in other cases substantial differences are found, which we attribute primarily to the semivolatile nature of these compounds that makes them difficult to study with the HTDMA. In contrast, estimates of water activity for levoglucosan solutions compare very well with recently-reported measurements and with published data for aqueous glucose and fructose solutions. For all studied species, the critical dry diameters active at supersaturations between 0.2 and 1% that are computed with the HTDMA-derived water activities are generally within the experimental error (~20%) estimated in previously-published direct measurements using cloud condensation nuclei counters. For individual compounds, the variations in reported solution water activity lead to uncertainties in critical dry diameters of 5-25%, not significantly larger than the uncertainty in the direct measurements. To explore the impact of these uncertainties on modeled aerosol-cloud interactions, we incorporate the variations in estimates of solution water activities into the description of hygroscopic growth of aerosol particles in an adiabatic parcel model and examine the impact on the predicted drop number concentrations. For the limited set of initial conditions examined here, we find that the uncertainties in critical dry diameters for individual species lead to 0-21% changes in drop number concentration, with the largest effects at high aerosol number concentrations and slow updraft velocities. Ammonium sulfate, malonic acid and glutaric acid have similar activation behavior, while glutaric acid and levoglucosan are somewhat less hygroscopic and lead to lower drop number concentrations; sodium chloride is the most easily activated compound. We explain these behaviors in terms of a parameter that represents compound hygroscopicity, and conclude that this parameter must vary by more than a factor of 2 to induce more than a 15% change in activated drop number concentrations. In agreement with earlier studies, our results suggest that the number concentration of activated drops is more sensitive to changes in the input aerosol size and number concentrations and the applied updraft velocity than to modest changes in the aerosol composition and hygroscopic properties
Cladoceran birth and death rates estimates
I. Birth and death rates of natural cladoceran populations cannot be measured directly. Estimates of these population parameters must be calculated using methods that make assumptions about the form of population growth. These methods generally assume that the population has a stable age distribution.
2. To assess the effect of variable age distributions, we tested six egg ratio methods for estimating birth and death rates with data from thirty-seven laboratory populations of Daphnia pulicaria. The populations were grown under constant conditions, but the initial age distributions and egg ratios of the populations varied. Actual death rates were virtually zero, so the difference between the estimated and actual death rates measured the error in both birth and death rate estimates.
3. The results demonstrate that unstable population structures may produce large errors in the birth and death rates estimated by any of these methods. Among the methods tested, Taylor and Slatkin's formula and Paloheimo's formula were most reliable for the experimental data.
4. Further analyses of three of the methods were made using computer simulations of growth of age-structured populations with initially unstable age distributions. These analyses show that the time interval between sampling strongly influences the reliability of birth and death rate estimates. At a sampling interval of 2.5 days (equal to the duration of the egg stage), Paloheimo's formula was most accurate. At longer intervals (7.5â10 days), Taylor and Slatkin's formula which includes information on population structure was most accurate
Size-resolved aerosol composition and its link to hygroscopicity at a forested site in Colorado
Aerosol hygroscopicity describes the ability of a particle to take up water
and form a cloud droplet. Modeling studies have shown sensitivity of
precipitation-producing cloud systems to the availability of aerosol
particles capable of serving as cloud condensation nuclei (CCN), and
hygroscopicity is a key parameter controlling the number of available CCN.
Continental aerosol is typically assumed to have a representative
hygroscopicity parameter, Îș, of 0.3; however, in remote locations
this value can be lower due to relatively large mass fractions of organic
components. To further our understanding of aerosol properties in remote
areas, we measured size-resolved aerosol chemical composition and
hygroscopicity in a forested, mountainous site in Colorado during the
six-week BEACHON-RoMBAS (Bio-hydro-atmosphere interactions of Energy, Aerosols,
Carbon, H<sub>2</sub>O, Organics and NitrogenâRocky Mountain Biogenic
Aerosol Study) campaign. This campaign followed a year-long
measurement period at this site, and results from the intensive campaign
shed light on the previously reported seasonal cycle in aerosol
hygroscopicity. New particle formation events were observed routinely at
this site and nucleation mode composition measurements indicated that the
newly formed particles were predominantly organic. These events likely
contribute to the dominance of organic species at smaller sizes, where
aerosol organic mass fractions were between 70 and 90%. Corresponding
aerosol hygroscopicity was observed to be in the range Îș = 0.15â0.22,
with hygroscopicity increasing with particle size. Aerosol chemical
composition measured by an aerosol mass spectrometer and calculated from
hygroscopicity measurements agreed very well during the intensive study, with
an assumed value of κ<sub>org</sub> = 0.13 resulting in the best
agreement
Irreversible loss of ice nucleation active sites in mineral dust particles caused by sulphuric acid condensation
During the FROST-2 (FReezing Of duST) measurement campaign conducted at the
Leipzig Aerosol Cloud Interaction Simulator (LACIS), we investigated changes
in the ice nucleation properties of 300 nm Arizona Test Dust mineral
particles following thermochemical processing by varying amounts and
combinations of exposure to sulphuric acid vapour, ammonia gas, water
vapour, and heat. The processed particles' heterogeneous ice nucleation
properties were determined in both the water subsaturated and supersaturated
humidity regimes at â30 °C and â25 °C using Colorado State
University's continuous flow diffusion chamber. The amount of sulphuric acid
coating material was estimated by an aerosol mass spectrometer and from
CCN-derived hygroscopicity measurements. The condensation of sulphuric acid
decreased the dust particles' ice nucleation ability in proportion to the
amount of sulphuric acid added. Heating the coated particles in a
thermodenuder at 250 °C â intended to evaporate the sulphuric acid
coating â reduced their freezing ability even further. We attribute this
behaviour to accelerated acid digestion of ice active surface sites by heat.
Exposing sulphuric acid coated dust to ammonia gas produced particles with
similarly poor freezing potential; however a portion of their ice nucleation
ability could be restored after heating in the thermodenuder. In no case did
any combination of thermochemical treatments increase the ice nucleation
ability of the coated mineral dust particles compared to unprocessed dust.
These first measurements of the effect of identical chemical processing of
dust particles on their ice nucleation ability under both water subsaturated
and mixed-phase supersaturated cloud conditions revealed that ice nucleation
was more sensitive to all coating treatments in the water subsaturated
regime. The results clearly indicate irreversible impairment of ice
nucleation activity in both regimes after condensation of concentrated
sulphuric acid. This implies that the sulphuric acid coating caused
permanent chemical and/or physical modification of the ice active surface
sites; the possible dissolution of the coating during droplet activation did
not restore all immersion/condensation-freezing ability
Black carbon enrichment in atmospheric ice particle residuals observed in lower tropospheric mixed phase clouds
Recommended from our members
Surface modification of mineral dust particles by sulphuric acid processing: Implications for ice nucleation abilities
The ability of coated mineral dust particles to act as ice nuclei (IN) was investigated at LACIS (Leipzig Aerosol Cloud Interaction Simulator) during the FROST1- and FROST2-campaigns (Freezing of dust). Sulphuric acid was condensed on the particles which afterwards were optionally humidified, treated with ammonia vapour and/or heat. By means of aerosol mass spectrometry we found evidence that processing of mineral dust particles with sulphuric acid leads to surface modifications of the particles. These surface modifications are most likely responsible for the observed reduction of the IN activation of the particles. The observed particle mass spectra suggest that different treatments lead to different chemical reactions on the particle surface. Possible chemical reaction pathways and products are suggested and the implications on the IN efficiency of the treated dust particles are discussed
Combined and single effects of pesticide carbaryl and toxic Microcystis aeruginosa on the life history of Daphnia pulicaria
The combined influence of a pesticide (carbaryl) and a cyanotoxin (microcystin LR) on the life history of Daphnia pulicaria was investigated. At the beginning of the experiments animals were pulse exposed to carbaryl for 24 h and microcystins were delivered bound in Microcystisâ cells at different, sub-lethal concentrations (chronic exposure). In order to determine the actual carbaryl concentrations in the water LCâMS/MS was used. For analyses of the cyanotoxin concentration in Daphniaâs body enzyme-linked immunosorbent assay (ELISA) was used. Individual daphnids were cultured in a flow-through system under constant light (16 h of light: 8 h of dark), temperature (20°C), and food conditions (Scenedesmus obliquus, 1 mg of C lâ1). The results showed that in the treatments with carbaryl egg numbers per female did not differ significantly from controls, but the mortality of newborns increased significantly. Increasing microcystin concentrations significantly delayed maturation, reduced size at first reproduction, number of eggs, and newborns. The interaction between carbaryl and Microcystis was highly significant. Animals matured later and at a smaller size than in controls. The number of eggs per female was reduced as well. Moreover, combined stressors caused frequent premature delivery of offspring with body deformations such as dented carapax or an undeveloped heart. This effect is concluded to be synergistic and could not be predicted from the effects of the single stressors.
Ice Nucleating Particle Connections to Regional Argentinian Land Surface Emissions and Weather During the Cloud, Aerosol, and Complex Terrain Interactions Experiment
Here, we present a multi-season study of ice-nucleating particles (INPs) active via the immersion freezing mechanism, which took place in north-central Argentina, a worldwide hotspot for mesoscale convective storms. INPs were measured untreated, after heating to 95°C, and after hydrogen peroxide digestion. No seasonal cycle of INP concentrations was observed. Heat labile INPs, which we define as âbiologicalâ herein, dominated the population active at â5 to â20°C, while non-heat-labile organic INPs (decomposed by peroxide) dominated at lower temperatures, from â20 to â28°C. Inorganic INPs (remaining after peroxide digestion), were minor contributors to the overall INP activity. Biological INP concentration active around â12°C peaked during rain events and under high relative humidity, reflecting emission mechanisms independent of the background aerosol concentration. The ratio of non-heat-labile organic and inorganic INPs was generally constant, suggesting they originated from the same source, presumably from regional arable topsoil based on air mass histories. Single particle mass spectrometry showed that soil particles aerosolized from a regionally common agricultural topsoil contained known mineral INP sources (K-feldspar and illite) as well as a significant organic component. The INP activity observed in this study correlates well with agricultural soil INP activities from this and other regions of the world, suggesting that the observed INP spectra might be typical of many arable landscapes. These results demonstrate the strong influence of regional continental landscapes, emitting INPs of types that are not yet well represented in global models
- âŠ