Initial Aerosol Research Articles

Inverse modeling of aerosol dynamics: Condensational growth

The feasibility of inverse modeling a multicomponent, size‐resolved aerosol evolving by condensation/evaporation is investigated. The adjoint method is applied to the multicomponent aerosol dynamic equation in a box model (zero‐dimensional) framework. Both continuous and discrete formulations of the model (the forward equation) and the adjoint are considered. A test example is studied in which the initial aerosol size composition distribution and the pure component vapor concentrations (i.e., vapor pressures) are estimated on the basis of measurements of all species, or a subset of the species, and the entire size distribution, or a portion of the size distribution. It is found that the adjoint method can successfully retrieve the initial size distribution and the pure component vapor concentrations even when only a subset of the species or a portion of the size distribution is observed, although this success is shown to depend upon the form of the initial estimates, the nature of the observations, and the length of the assimilation period. The results presented here provide a basis for the inverse modeling of aerosols in three‐dimensional atmospheric chemical transport models.

A two-moment parameterization of aerosol nucleation and impaction scavenging for a warm cloud microphysics: description and results from a two-dimensional simulation

A new two-moment warm bulk scheme has been developed including explicitly nucleation and impaction scavenging of aerosol particles as well as all other microphysical processes. The scheme is built upon a quasispectral representation of the aerosol particle, cloud droplet and raindrop distributions. It predicts mixing ratios and number concentrations for each category. Each process is treated explicitly and independently to establish an analytic expression for each contribution for the time-dependant microphysical equations. The scheme has been tested in the dynamical framework of a two-dimensional kinematic model, developed for the Hawaiian Rainband Project (HaRP, 1990). In this frame, the scheme has performed reasonably well compared to the observations as well as to other similar parameterization schemes, and to the spectral model DESCAM. Sensitivity tests demonstrate the great sensitivity of the scheme to the initial aerosol spectrum characteristics. Moreover, they have also shown its capability to calculate nucleation and impaction scavenging and to follow the taken up particle mass in the cloud and raindrop spectra until the deposition on the ground by the rain. Therefore, the parameterization offers a possibility of treating the evolution of the liquid phase of the cloud together with the aerosol particle scavenging. However, due to the severe limitations of a two-dimensional kinematic model, the scheme needs to be further validated in a three-dimensional dynamical model.

Explicit simulations of aerosol physics in a cloud-resolving model: a sensitivity study based on an observed convective cloud

Abstract. The role of convection in introducing aerosols and promoting the formation of new particles to the upper troposphere has been examined using a cloud-resolving model coupled with an interactive explicit aerosol module. A baseline simulation suggests good agreement in the upper troposphere between modeled and observed results including concentrations of aerosols in different size ranges, mole fractions of key chemical species, and concentrations of ice particles. In addition, a set of 34 sensitivity simulations has been carried out to investigate the sensitivity of modeled results to the treatment of various aerosol physical and chemical processes in the model. The size distribution of aerosols is proved to be an important factor in determining the aerosols' fate within the convective cloud. Nucleation mode aerosols (here defined by 0≤d≤5.84 nm) are quickly transferred to the larger modes as they grow through coagulation of aerosols and condensation of H2SO4. Accumulation mode aerosols (here defined by d≥31.0 nm) are almost completely removed by nucleation (activation of cloud droplets) and impact scavenging. However, a substantial part (up to 10% of the boundary layer concentration) of the Aitken mode aerosol population (here defined by 5.84 nm≤d≤31.0 nm) reaches the top of the cloud and the free troposphere. These particles may continually survive in the upper troposphere, or over time form ice crystals, both that could impact on the atmospheric radiative budget. The sensitivity simulations performed indicate that critical processes in the model causing a substantial change in the upper tropospheric number concentration of Aitken mode aerosols are coagulation of aerosols, condensation of H2SO4, nucleation scavenging, nucleation of aerosols and the transfer of aerosol mass and number between different aerosol bins. In particular, for aerosols in the Aitken mode to grow to CCN size, coagulation of aerosols appears to be more important than condensation of H2SO4. Less important processes are dry deposition, impact scavenging and the initial vertical distribution and concentration of aerosols. It is interesting to note that in order to sustain a vigorous storm cloud, the supply of CCN must be continuous over a considerably long time period of the simulation. Hence, the treatment of the growth of particles is in general much more important than the initial aerosol concentration itself.

Repeated Aerosol Allergen Exposure Suppresses Inflammation in B-Cell-Deficient Mice with Established Allergic Asthma

Background: Repeated allergen administration is a well-established therapeutic strategy for desensitizing patients with allergic disease. Similarly, repeated inhalation of antigen by mice with established allergen-induced asthma suppresses allergic inflammation. The mechanisms underlying antigen-dependent suppression of allergic immune responses remain unknown. In previous studies, we found that repeated aerosol antigen challenges in sensitized mice reduced eosinophils while increasing plasma cells and antibody in the lungs. We sought to test whether plasma cells and antibody played a role in suppression of allergic disease. Methods: We primed wild-type and B-cell-deficient (µMT) mice with 25 µg ovalbumin (OVA) precipitated in alum on days 0 and 5, nebulized weekly with 1% OVA, 1 h, twice daily, for up to 6 weeks, and assessed lung inflammation, mucus hypersecretion, and IgE/IgG1. Results: Kinetic studies revealed that initial aerosol exposure induced high numbers of eosinophils, lymphocytes, and macrophages within lung infiltrates and increased mucus production in wild-type mice. After 3–4 weeks of antigen exposure, eosinophils diminished while lymphocytes, plasma cells, and macrophages and mucus hypersecretion increased. However, by 6 weeks, lung inflammation and mucus hypersecretion were dramatically reduced. In contrast, repeated aerosol challenges maintained OVA-specific IgG1 and IgE production. Repeated aerosol antigen challenges in µMT mice resulted in reduced lung inflammation and mucus hypersecretion and the development of smooth muscle hypertrophy of the pulmonary microvasculature. Conclusions: B cells and antibody do not appear to play a role in antigen-dependent suppression of allergic responses in mice.

The use of the pulse height analyser ultrafine condensation particle counter (PHA-UCPC) technique applied to sizing of nucleation mode particles of differing chemical composition

A modified white-light pulse-height analyser (PHA) TSI 3025 ultra-fine condensation particle counter (UCPC) is often used to provide fast response of aerosol size distributions between 3 and 10 nm since there is a monotonic link between initial aerosol size and nucleated droplet final size. The use of the PHA-UCPC for sizing nucleation mode particles depends on the droplet nucleation in the condenser chamber being somewhat independent of particle composition. Laboratory characterization of the PHA-UCPC for a range of chemical compositions, thought to be involved in atmospheric aerosol nucleation and growth, are presented here. Ammonium sulphate, pinic acid, cis-pinonic acid, malonic acid and an iodine oxide were studied and their PHA-UCPC calibration kernels are presented. It was found that all species possessed significantly different PHA responses. The results suggest that, unless the nano-particle chemical composition is known, then the PHA-UCPC cannot be used for measurements of aerosol size distributions. However, the PHA-UCPC, if used in parallel with mobility size distribution measurements, can help elucidate nano-particle chemical composition. Using the combination of mobility size distributions and the PHA-UCPC response during a nucleation and growth event over the boreal forest indicates that new particle formation, in this region, is driven by condensation of organic vapours.

Secondary organic aerosol formation from aromatic precursors. 1. Mechanisms for individual hydrocarbons.

Quantitative kinetic and physical phase partitioning models of secondary organic aerosol (SOA) formation resulting from the reactions of aromatic species were integrated into a mechanism for gas-phase reactions. Using the resulting model, analyses of the sensitivity of SOA formation to several parameters (e.g., VOC/NOx ratio, rate parameters) were performed. Results indicated that aerosol yield (SOA formed per amount of hydrocarbons reacted) depends on the extent of conversion of parent hydrocarbons, partitioning coefficient, initial aerosol mass concentration, and rate parameters. On the basis of the sensitivity studies, models for SOA yield were developed for 11 aromatic compounds. Comparison of the results from current SOA models to the results from this study suggests that mechanisms describing SOA formation from aromatic species must incorporate the reactions of reactive intermediates.

Numerical simulations of homogeneous freezing processes in the aerosol chamber AIDA

Abstract. The homogeneous freezing of supercooled H2SO4/H2O aerosols in an aerosol chamber is investigated with a microphysical box model using the activity parameterization of the nucleation rate by Koop et al. (2000). The simulations are constrained by measurements of pressure, temperature, total water mixing ratio, and the initial aerosol size distribution, described in a companion paper Möhler et al. (2003). Model results are compared to measurements conducted in the temperature range between 194 and 235 K, with cooling rates in the range between 0.5 and 2.6 K min-1, and at air pressures between 170 and 1000 hPa. The simulations focus on the time history of relative humidity with respect to ice, aerosol size distribution, partitioning of water between gas and particle phase, onset times of freezing, freezing threshold relative humidities, aerosol chemical composition at the onset of freezing, and the number of nucleated ice crystals. The latter four parameters can be inferred from the experiments, the former three aid in interpreting the measurements. Sensitivity studies are carried out to address the relative importance of uncertainties of basic quantities such as temperature, total H2O mixing ratio, aerosol size spectrum, and deposition coefficient of H2O molecules on ice. The ability of the numerical simulations to provide detailed explanations of the observations greatly increases confidence in attempts to model this process under real atmospheric conditions, for instance with regard to the formation of cirrus clouds or polar stratospheric ice clouds, provided that accurate temperature and humidity measurements are available.

Speciation of volatile organic compound emissions for regional air quality modeling of particulate matter and ozone

A new classification scheme for the speciation of organic compound emissions for use in air quality models is described. The scheme uses 81 organic compound classes to preserve both net gas‐phase reactivity and particulate matter (PM) formation potential. Chemical structure, vapor pressure, hydroxyl radical (OH) reactivity, freezing point/boiling point, and solubility data were used to create the 81 compound classes. Volatile, semivolatile, and nonvolatile organic compounds are included. The new classification scheme has been used in conjunction with the Canadian Emissions Processing System (CEPS) to process 1990 gas‐phase and particle‐phase organic compound emissions data for summer and winter conditions for a domain covering much of eastern North America. A simple postprocessing model was used to analyze the speciated organic emissions in terms of both gas‐phase reactivity and potential to form organic PM. Previously unresolved compound classes that may have a significant impact on ozone formation include biogenic high‐reactivity esters and internal C6–8 alkene‐alcohols and anthropogenic ethanol and propanol. Organic radical production associated with anthropogenic organic compound emissions may be 1 or more orders of magnitude more important than biogenic‐associated production in northern United States and Canadian cities, and a factor of 3 more important in southern U.S. cities. Previously unresolved organic compound classes such as low vapour pressure PAHs, anthropogenic diacids, dialkyl phthalates, and high carbon number alkanes may have a significant impact on organic particle formation. Primary organic particles (poorly characterized in national emissions databases) dominate total organic particle concentrations, followed by secondary formation and primary gas‐particle partitioning. The influence of the assumed initial aerosol water concentration on subsequent thermodynamic calculations suggests that hydrophobic and hydrophilic compounds may form external mixtures, and that separate treatment for these groups may be required in future air quality model simulations. The post‐processing model used here overestimates the organic particle formation relative to measurements, lacks the complexity of a regional air quality model, and is not intended as an alternative to the latter. Results from the post‐processing model do, however, provide guidance for the treatment of organic gases and particles in future air quality modeling work. Future air quality model simulations should attempt to speciate primary particulate organic compounds and include more detailed organic compound classes. Future emissions profile measurements should speciate gaseous high‐molecular‐mass organic compounds and primary organics emitted in particulate form (primary particle emissions are only available as a total particulate mass in currently available emissions data).

Interactions of mineral dust particles and clouds: Effects on precipitation and cloud optical properties

Numerical simulations were performed to investigate the effect of cloud‐processed mineral dust particles on the subsequent development of cloud and precipitation and possible effects on cloud optical properties. A two‐dimensional (2‐D) nonhydrostatic cloud model with detailed microphysics was used. The initial aerosol spectra used in the 2‐D model consisted of both background cloud condensation nuclei and mineral dust particles. These were taken from the results of three successive runs of a parcel model that simulates the interaction of dust and sulfate particles with cloud drops and trace gases and then evaporates the cloud drops. The results show that insoluble mineral dust particles become effective cloud condensation nuclei (CCN) after passing through a convective cloud. Their effectiveness as CCN increases because of a layer of sulfate that is formed on their surface as they are first captured by growing drops or ice crystals and then released as these hydrometeors evaporate. Upon entering subsequent clouds, these particles increase the concentration of the activated drops and widen the drop size distribution. The present work shows that in continental clouds the effect of cloud‐processed dust particles is to accelerate the formation of precipitation particles, although the amount of precipitation depends on the concentration of the large and giant CCN. In maritime clouds the addition of cloud‐processed aerosol and mineral dust particles has a minimal effect on precipitation because the cloud starts with many large particles already. The addition of more CCN to either maritime or continental clouds increases their optical depth, even for those cases in which the precipitation amount is increased.

Impact of aerosol size representation on modeling aerosol‐cloud interactions

We use a one‐dimensional version of a climate‐aerosol‐chemistry model with both modal and sectional aerosol size representations to evaluate the impact of aerosol size representation on modeling aerosol‐cloud interactions in shallow stratiform clouds observed during the second Aerosol Characterization Experiment. Both the modal (with prognostic aerosol number and mass or prognostic aerosol number, surface area and mass, referred to as the Modal‐NM and Modal‐NSM) and the sectional approaches (with 12 and 36 sections) predict total number and mass for interstitial and activated particles that are generally within several percent of references from a high‐resolution 108‐section approach. The modal approach with prognostic aerosol mass but diagnostic number (referred to as the Modal‐M) cannot accurately predict the total particle number and surface areas, with deviations from the references ranging from 7 to 161%. The particle size distributions are sensitive to size representations, with normalized absolute differences of up to 12% and 37% for the 36‐ and 12‐section approaches, and 30%, 39%, and 179% for the Modal‐NSM, Modal‐NM, and Modal‐M, respectively. For the Modal‐NSM and Modal‐NM, differences from the references are primarily due to the inherent assumptions and limitations of the modal approach. In particular, they cannot resolve the abrupt size transition between the interstitial and activated aerosol fractions. For the 12‐ and 36‐section approaches, differences are largely due to limitations of the parameterized activation for nonlognormal size distributions, plus the coarse resolution for the 12‐section case. Differences are larger both with higher aerosol (i.e., less complete activation) and higher SO2 concentrations (i.e., greater modification of the initial aerosol distribution).

Cirrus Parcel Model Comparison Project. Phase 1: The Critical Components to Simulate Cirrus Initiation Explicitly

Abstract The Cirrus Parcel Model Comparison Project, a project of the GCSS [Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies] Working Group on Cirrus Cloud Systems, involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. In Phase 1 of the project reported here, simulated cirrus cloud microphysical properties from seven models are compared for “warm” (−40°C) and “cold” (−60°C) cirrus, each subject to updrafts of 0.04, 0.2, and 1 m s−1. The models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins or the evolution of each individual particle is traced. Simulations are made including both homogeneous and heterogeneous ice nucleation mechanisms (all-mode simulations). A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. Heterogeneous nucleation is disabled for a second...

Atmospheric behaviour of oil-shale combustion fly ash in a chamber study

There are huge world deposits of oil shale, however, little is known about the fate of atmospheric oil-shale combustion fly ash. In the present work, oil-shale combustion fly-ash aerosol was investigated under simulated daytime and nighttime conditions. Fly-ash particles collected from the Baltic Power Plant (Estonia) were injected directly to a 190 m 3 outdoor Teflon film chamber. The initial concentration of particles was in the range from 15 to 20 mg/m 3. Particle size distributions were monitored continuously by various optical and electrical devices. During the course of an experiment the particle phase was collected on filters, and the gas phase was collected using denuders. The initial aerosol mass concentration decreased quickly due to the deposition of larger particles. Since fine particles dominated the count distribution, the change in aerosol number concentration was less significiant than the mass concentration over time. Experimental data showed a bimodal particle size distribution with maximums at about 0.07 and 4 μm. SEM images of aerosol particles also provided particle shape and size distribution information. The respirable fraction of particles, which contributes most to the health effects of the aerosol, significantly increased during the experiment, being 25% by mass immediately after the injection of fly ash and achieving 65% at the end of the experiment. Results of CG/MS analysis confirm the presence of different polycyclic aromatic hydrocarbons (PAHs) in the particle phase of the aerosol. Some of the individual compounds included phenanthrene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, and benzo(a)pyrene. Several PAHs were found in the gas phase of the chamber after fly ash had aged for 2 h, indicating that PAHs desorbed from the particles over time.

Microstructural rearrangement of sodium chloride condensation aerosol particles on interaction with water vapor

The interaction of water vapor with submicron NaCl condensation aerosols has been investigated with a tandem differential mobility analyzer (TDMA) and compared to experiments with NaCl dispersion and PbS condensation aerosols. The mobility equivalent diameters of the NaCl condensation aerosol particles decreased by up to 50% when exposed to relative humidities (RH) below the deliquescence threshold. The extent of shrinking was determined by relative humidity, initial particle size (19–200 nm) and aerosol generation parameters. Above ∼75% RH hygroscopic growth was observed in agreement with Köhler theory. Dynamic shape factors for the NaCl condensation aerosol particles before and after humidification were calculated, indicating a transformation from sparse branched-chain to more compact agglomerates. The investigated PbS particles shrank much less but still significantly when exposed to water vapor, which is discussed in view of the microstructural rearrangement processes at work.

PROLONGED AIRWAY RETENTION OF INSOLUBLE PARTICLES IN CYSTIC FIBROSIS VERSUS PRIMARY CILIARY DYSKINESIA

Patients with cystic fibrosis (CF) and primary ciliary dyskinesia (PCD) have been shown to have impaired large airway clearance of radiolabelled particles as measured by external gamma camera scanning up to 6 hours post deposition. Recent studies suggest that 24-hour retention of particles may reflect some airway retention in addition to alveolar retention. In a retrospective study, we analyzed the relationship between the deposition pattern and 24-hour retention (Ret24 hr) of technetium 99 radiolabelled iron oxide (99Tc-Fe2O3) particles in 20 patients with CF, 12 patients with PCD, and 17 normal subjects. By gamma camera analysis, initial aerosol deposition was analyzed in terms of central peripheral (C/P) activity within the lungs. Gamma camera scanning was performed immediately following deposition and again at 24 hours to assess residual retention (Ret24hr) as a percent of initial deposition. C/P analysis was also performed on the 24-hour scan (C/P24). For all subjects, initial deposition pattern (C/P) was inversely related to lung function (forced expiratory volume in 1 second \\[FEV1]%pred vs. C/P, r =-.54). Ret24hr was also inversely related to initial deposition pattern for all subjects (Ret24hr vs. C/P ratio, r=-.42). Analysis of covariance showed that for a given C/P ratio, CF patients had significantly greater Ret24hr compared to normal subjects (9.8 +/- 2.8\\[SE]%). In addition, the CF patients had similar C/P24 as the normal subjects (1.35 +/- 0.40\\[SD] vs. 1.10 +/- 0.39, respectively). These results suggest that small airway clearance is compromised in CF patients compared to normal subjects. On the other hand, PCD patients had C/P24 similar to their initial deposition C/P ratios (2.78 +/- 1.72 vs. 2.45 +/- 0.87, respectively), significantly greater than 1.0, and significantly greater than CF or normal subjects, suggesting that PCD patients have prolonged particle retention associated with their large bronchial airways.

Incremental Aerosol Reactivity: Application to Aromatic and Biogenic Hydrocarbons

The concept of incremental aerosol reactivity is introduced, and the incremental aerosol reactivities of a number of important anthropogenic and biogenic hydrocarbons are investigated for four ambient scenarios. The incremental aerosol reactivity, defined as a change in the secondary organic aerosol mass produced (in μg m-3) per unit change of parent organic reacted (in ppb), is a measure of the aerosol-forming capability of a given parent organic in a prescribed mixture of other organic compounds. The base-case scenario is a mixture of both aromatic and biogenic organics. Reactivity values depend on the choice of the initial organic mixture, so cases are also examined in which all biogenic hydrocarbon concentrations are set to zero and all aromatic concentrations are set to zero. The influence of additional organic aerosol is also investigated. For the compounds studied, incremental aerosol reactivities range from 0.228 μg m-3 ppb-1 (m-xylene) to 10.352 μg m-3 ppb-1 (α-humulene) for the base case, from 0.234 μg m-3 ppb-1 (m-xylene) to 9.446 μg m-3 ppb-1 (α-humulene) for the base case in the presence of initial organic aerosol, from 0.133 μg m-3 ppb-1 (m-xylene) to 0.801 μg m-3 ppb-1 (diethylbenzene) for the zero-biogenic case, and from 0.456 μg m-3 ppb-1 (linalool) to 6.923 μg m-3 ppb-1 (α-humulene) for the zero-aromatic case. Using m-xylene as a basis, relative incremental aerosol reactivities range from 1.153 (low-yield aromatics) to 3.338 (diethylbenzene) for aromatics in the base case and from 1.349 (low-yield aromatics) to 6.032 (diethylbenzene) in the zero-biogenic case. Using α-pinene as a basis, relative incremental aerosol reactivities range from 0.569 (terpinolene) to 12.843 (α-humulene) for biogenics in the base case and from 0.882 (linalool) to 13.385 (α-humulene) in the zero-aromatic case.

Rapid Improvement of Peak Flow in Asthmatic Patients Treated With Parenteral Methylprednisolone in the Emergency Department: A Randomized Controlled Study

Study objective: Corticosteroids are thought to exert their physiologic effects in asthma over the course of several hours. In this study we tested the hypothesis that intravenous methylprednisolone improves airflow in a shorter time frame (2 hours) in adults with acute asthma. Methods: In a randomized, double-blind, placebo-controlled trial, 56 adult asthmatic patients with peak expiratory flow rates (PEFRs) less than 50% predicted after an initial albuterol aerosol treatment were studied. These patients were randomly assigned to treatment with either 125 mg of intravenous methylprednisolone or an equivalent volume of normal saline solution (placebo). Patients were also treated with identical schedules of nebulized ipratropium and albuterol. Patients were recruited from an emergency department at an urban academic medical center. The primary endpoints were changes in PEFR and in percent predicted PEFR over time. PEFRs were assessed at baseline and at 1 and 2 hours. Heart rate changes over time and the proportion of admissions in the 2 groups were also compared. Results: The increases in PEFR and percent predicted PEFR over time were both significantly greater in the methylprednisolone treatment group ( P =.002 and P =.005, respectively). The increases in geometric mean peak flow at 60 and 120 minutes were 79 and 96 L/min for the methylprednisolone group and 54 and 68 L/min for the placebo group. There was also a significantly different change in heart rates with time between the methylprednisolone and placebo groups ( P =.029), with the placebo group showing a moderate increase in heart rate over time. Although the proportion of patients admitted for status asthmaticus was less in the methylprednisolone treatment group (8/30) compared with the placebo group (10/26), this difference in proportions (–.118, 95% confidence interval –.363 to .127) was not significant. Conclusion: These data suggest that use of corticosteroids should be considered relatively early in the treatment of patients with acute asthma in whom initial bronchodilator therapy fails to produce an adequate response. [Lin RY, Pesola GR, Bakalchuk L, Heyl GT, Dow AM, Tenenbaum C, Curry A, Westfal RE: Rapid improvement of peak flow in asthmatic patients treated with parenteral methylprednisolone in the emergency department: A randomized controlled study. Ann Emerg Med May 1999;33:487-494.]

Processes determining the relationship between aerosol number and non‐sea‐salt sulfate mass concentrations in the clean and perturbed marine boundary layer

An evaluation of the indirect radiative forcing by aerosols requires knowledge about aerosol number densities, and more particularly the number of particles that can be activated in clouds. In this study we present a data set relating the total number (NTOT) and the number of particles with dry diameter > 80 nm (N>80) to the aerosol volume and non‐sea‐salt (nss) SO4= mass (MSO4). The data refer to submicron aerosol and have been obtained in both clean and polluted conditions in the North Atlantic marine boundary layer (MBL). Over this whole range, the relationships of both NTOT and N>80 versus MSO4 are close to linear. Detailed aerosol dynamics modeling shows that dilution of the initial pollution aerosol by entrainment of free tropospheric (FT) aerosol is the major process determining these relationships. Entrainment further explains our observation that the contribution of nss‐sulfate (i.e., (NH4)x(SO4)y) to the dry MBL aerosol mass decreases from over 85% near the continent to 45–70% in more remote and clean conditions, as smaller contributions of sulfate to the FT aerosol mass have been observed. Finally, the linear relationships between aerosol number and MSO4 suggest that the observed nonlinear relation between the number of cloud droplets and MSO4 must be mainly ascribed to nonlinearities in the cloud activation process.

Gas-to-particle conversion in the atmosphere: II. Analytical models of nucleation bursts

Simple models are developed to describe the formation of particles from condensable vapours in different atmospheric circumstances. The models are designed for use in large scale global transport models, where sub-grid descriptions are required for such phenomena. We solve the evolution equation for the density of a condensable vapour. When the concentration of existing aerosol is low, nucleation can occur, but only in intermittent, isolated bursts. In the absence of an initial aerosol, two analytical expressions are obtained for the number of particles produced in such bursts, valid for high and low rates of vapour production, respectively. These results compare favourably with calculations made using a detailed numerical code, using the homogeneous nucleation of sulphuric acid/water droplets as an illustration. Then we consider barrierless nucleation, where clusters are always stable against evaporation, which is relevant to the production of ammonium sulphate particles in the atmosphere. We go on to consider conditions where existing aerosol can affect the production of particles, and also consider slower bursts where the time dependence of the vapour production rate, and not condensation on the nucleated aerosol, cuts off nucleation.

Dynamical properties of Mars water ice clouds and their interactions with atmospheric dust and radiation

A fully coupled zonally-averaged one-dimensional time marching model of the Martian atmosphere is discussed. The model incorporates interactively radiation transfer, microphysics and eddy transport of both mineral and volatile aerosols. It is shown that nonlinear feedback caused by water ice clouds may result in quick (with a timescale of several days) reconfiguration of dust vertical distribution and thermal profile. Given different initial conditions, aerosols distribution and temperatures may equilibrate to different steady states. This might give a clue to understanding of the interannual variability of the seasonal temperature trend on Mars, particularly near aphelion.

Extended monodisperse aerosol modelling for fusion power plant containments

It is found for conditions relative to fusion safety studies that there is a range of initial aerosol masses (up to ≈ 1000 kg) within which the aerosol particle growth dynamics are dominated by Brownian agglomeration. Above this range, the dominant growth mechanism is gravitational agglomeration. Previously, monodisperse aerosol modelling has been reported (W.E. Han, 19th Symposium on Fusion Teechnology, Lisbon, Portugal, 15–20 September, 1996) which enables the accurate prediction of aerosol behaviour up to initial masses of ≈ 1000 kg. Here monodisperse modelling is extended to deal with initial aerosol masses above 1000 kg, enabling the prediction of aerosol transport and the release to the environment in the gravitationally dominated agglomeration regime for hypothetical accidents involving mobilised radionuclides. It is shown that increasing the initial aerosol mass in the Brownian dominated agglomeration regime results in increased release to the environment. However, increases in initial mass in the gravitationally dominated regime soon lead to a maximum in release to the environment; as a result, further increases in initial mass produce smaller releases to the environment.