Aerosol Mixtures Research Articles

Optical properties of aerosol mixtures derived from sun-sky radiometry during SAMUM-2

The SAMUM-2 experiment took place in the Cape Verde is lands in January–February 2008. The colocated ground-based and airborne instruments allow the study of desert dust optical and microphysical properties in a closure experiment. The Meteorological Institute of the University of Munich deployed one sun-sky photometer and two tropospheric lidar systems. A travelling AERONET-Cimel sun-sky radiometer was also deployed. During the measurement period the aerosol scenario over Cape Verde mostly consisted of a dust layer below 2 km and a smoke-dust layer above 2–4 km a.s.l. The Saharan dust arrived at the site from the NE, whereas the smoke originated in the African equatorial region. This paper describes the main results of the Sun photometer observations, supported by lidar information. An analysis of the variations in the aerosol optical depth (AOD) in the range 340–1550 nm, the Ångström exponent, volume size distributions and single scattering albedo is presented. The aerosol mixtures are analysed by means of the fine mode fraction of the AOD provided by the sun-sky inversion data and the Spectral Deconvolution Algorithm. The mean AOD (500 nm) was 0.31, with associated low ångström exponent of 0.46. Several types of events were detected within the data set, with prevalence of dust or mixtures as characterized by the Ångstr¨om exponents of extinction and absorption and the fine mode fraction. Aerosol properties derived from sunphotometry were compared to in situ measurements of size distribution, effective radius and single scattering albedo.

Empirical evaluation of lung solubilities of airborne contamination at Harwell facilities

Lung solubility is the key parameter in determining intakes and doses from inhalation of airborne contamination. However, information on lung solubility can be difficult to acquire, particularly for the historical exposures that are of relevance to lifetime-dose reconstruction. In this study, an empirical approach has been made in which over 200 dose assessments, mainly for Pu and Am, from the period 1986 to 2005 were re-evaluated and the solubility mix required for the best fit to the data was determined. The average of these solubility mixtures for any building or facility can be used as the default solubility for retrospective dose assessments for that facility. Results are presented for a radiochemistry facility, a materials development facility and a waste-storage/handling building at Harwell. The latter two areas are characterised by aerosols that are predominantly insoluble (type S), whereas the radiochemistry facility has a heterogeneous mixture of insoluble and soluble aerosols. The implications of these results for dose reconstruction are discussed in the paper.

A versatile salt evaporation reactor system for SOFC operando studies on anode contamination and degradation with impedance spectroscopy

The dependence of the degradation kinetics in Ni–CGO (cerium–gadolinium oxide) solid oxide fuel cell (SOFC) anodes upon salt evaporation is demonstrated operando with a custom built versatile reactor system. The system is based on evaporation and subsequent condensation of low concentration salt vapor aerosol mixtures representative of salt vapors typically present in biomass gasification processes. Fast changes in the charge transfer and ohmic resistance are observed in the anodes fuelled with a gas mixture containing a high KCl vapor concentration. Rapid condensation of salt vapors into the porous anode and partial delamination of the anode from the electrolyte surface because of salt deposits inside the porous anode is observed. The flexibility to produce vapor–aerosol mixtures with different concentrations and particle size distributions is proved, and suitability of these aerosols for anode testing in long term fuel cell test is evaluated.

Understanding the aerosol information content in multi-spectral reflectance measurements using a synergetic retrieval algorithm

Abstract. An information content analysis for multi-wavelength SYNergetic AErosol Retrieval algorithm SYNAER was performed to quantify the number of independent pieces of information that can be retrieved. In particular, the capability of SYNAER to discern various aerosol types is assessed. This information content depends on the aerosol optical depth, the surface albedo spectrum and the observation geometry. The theoretical analysis is performed for a large number of scenarios with various geometries and surface albedo spectra for ocean, soil and vegetation. When the surface albedo spectrum and its accuracy is known under cloud-free conditions, reflectance measurements used in SYNAER is able to provide for 2–4° of freedom that can be attributed to retrieval parameters: aerosol optical depth, aerosol type and surface albedo. The focus of this work is placed on an information content analysis with emphasis to the aerosol type classification. This analysis is applied to synthetic reflectance measurements for 40 predefined aerosol mixtures of different basic components, given by sea salt, mineral dust, biomass burning and diesel aerosols, water soluble and water insoluble aerosols. The range of aerosol parameters considered through the 40 mixtures covers the natural variability of tropospheric aerosols. After the information content analysis performed in Holzer-Popp et al. (2008) there was a necessity to compare derived degrees of freedom with retrieved aerosol optical depth for different aerosol types, which is the main focus of this paper. The principle component analysis was used to determine the correspondence between degrees of freedom for signal in the retrieval and derived aerosol types. The main results of the analysis indicate correspondence between the major groups of the aerosol types, which are: water soluble aerosol, soot, mineral dust and sea salt and degrees of freedom in the algorithm and show the ability of the SYNAER to discern between this aerosol types. The results of the work will be further used for the development of the promising methodology of the construction error covariance matrices in the assimilation system.

Climatological aspects of the optical properties of fine/coarse mode aerosol mixtures

Aerosol mixtures composed of coarse mode desert dust combined with fine mode combustion generated aerosols (from fossil fuel and biomass burning sources) were investigated at three locations that are in and/or downwind of major global aerosol emission source regions. Multiyear monitoring data at Aerosol Robotic Network sites in Beijing (central eastern China), Kanpur (Indo‐Gangetic Plain, northern India), and Ilorin (Nigeria, Sudanian zone of West Africa) were utilized to study the climatological characteristics of aerosol optical properties. Multiyear climatological averages of spectral single scattering albedo (SSA) versus fine mode fraction (FMF) of aerosol optical depth at 675 nm at all three sites exhibited relatively linear trends up to ∼50% FMF. This suggests the possibility that external linear mixing of both fine and coarse mode components (weighted by FMF) dominates the SSA variation, where the SSA of each component remains relatively constant for this range of FMF only. However, it is likely that a combination of other factors is also involved in determining the dynamics of SSA as a function of FMF, such as fine mode particles adhering to coarse mode dust. The spectral variation of the climatological averaged aerosol absorption optical depth (AAOD) was nearly linear in logarithmic coordinates over the wavelength range of 440–870 nm for both the Kanpur and Ilorin sites. However, at two sites in China (Beijing and Xianghe), a distinct nonlinearity in spectral AAOD in logarithmic space was observed, suggesting the possibility of anomalously strong absorption in coarse mode aerosols increasing the 870 nm AAOD.

A modern miasma hypothesis and back-to-school asthma exacerbations

A sudden increase in the rate of asthma exacerbations has been observed among young children in many countries 2-3 weeks after their return-to-school following the summer holidays. These exacerbations are frequently associated with human rhinovirus (hRV) infections, with possible interactions with allergen sensitisation, allergen exposure and medication use. It was originally proposed that the sudden increase resulted from new strains of respiratory viruses acquired during the holidays spreading rapidly on return to school. While there is compelling evidence implicating hRV in these exacerbations, recent observations on virus transmission, infection patterns and immune responses to both viruses and allergens have led us to propose an additional hypothesis for this increase in exacerbations. We propose that classrooms typically provide persistent exposure to a mixture of airborne viruses, viral proteins, endotoxin, community allergens and other human-derived aerosols - a modern miasma. During the preceding school term, this exposure establishes and maintains a level of immune tolerance and herd immunity, which then declines during the two-month holidays due to lack of such exposure, creating a transitory window of susceptibility to viral infections and asthma. The return to school re-establishes exposure to these aerosols resulting in an acceleration of exacerbations, until the tolerance and herd immunity are re-established. Thus, the peak in return-to-school asthma is more a function of a transitory increase in susceptibility due to a temporary lack of this complex exposure, than it is to novel, locally endemic strains of hRV.

Climate effects of seasonally varying Biomass Burning emitted Carbonaceous Aerosols (BBCA)

Abstract. The climate impact of the seasonality of Biomass Burning emitted Carbonaceous Aerosols (BBCA) is studied using an aerosol-climate model coupled with a slab ocean model in a set of 60-year long simulations, driven by BBCA emission data with and without seasonal variation, respectively. The model run with seasonally varying emission of BBCA leads to an increase in the external mixture of carbonaceous aerosols as well as in the internal mixture of organic carbon and sulfate but a decrease in the internal mixture of black carbon and sulfate relative to those in the run with constant annual BBCA emissions, as a result of different strengths of source/sink processes. The differences in atmospheric direct radiative forcing (DRF) caused by BBCA seasonality are in phase with the differences in column concentrations of the external mixture of carbonaceous aerosols in space and time. In contrast, the differences in all-sky radiative forcing at the top of the atmosphere and at the earth's surface extend beyond the BBCA source regions due to climate feedback through cloud distribution and precipitation. The seasonality of biomass burning emissions uniquely affects the global distributions of convective clouds and precipitation, indicating that these emissions have an impact on atmospheric circulation. In addition, the climate response to the periodic climate forcing of BBCA is not limited to biomass burning seasons but dynamically extends into non-biomass burning seasons as well.

Burning rates of turbulent iso-octane aerosol mixtures in spherical flame explosions

Experimental studies of aerosol combustion under quiescent and turbulence conditions have been conducted to quantify the differences in the flame structure and burning rates between aerosol and gaseous mixtures. Turbulence was generated by variable speed fans to yield rms turbulence velocities between 0.5 and 4.0 m/s and this was uniform and isotropic. Homogeneously distributed and near monodispersed iso-octane-air aerosol clouds were generated using a thermodynamic condensation method. Spherically expanding flames, following central ignition, at near atmospheric pressures were employed to quantify the flame structure and propagation rate. The effects of the diameter of fine fuel droplets on flame propagation were investigated. It is suggested that the inertia of fuel droplets is an important cause of flame enhancement during early flame development. During later stages, cellular flame instability and the effective, gaseous phase, equivalence ratio becomes important. The latter effect leads has increases the flame speed of rich mixtures, but decreases that of lean ones. Droplet enhancement of burning velocity can be significant at low turbulence but is negligible at high turbulence.

Measuring the influence of aerosols and albedo on sky polarization

All-sky distributions of the polarized radiance are measured using an automated fish-eye camera system with a rotating polarizer. For a large range of aerosol and surface albedo situations, the influence on the degree of polarization and sky radiance is investigated. The range of aerosol optical depth and albedo is 0.05–0.5 and 0.1–0.75, respectively. For this range of parameters, a reduction of the degree of polarization from about 0.7 to 0.4 was observed. The analysis is done for 90° scattering angle in the principal plane under clear sky conditions for a broadband channel of 450±25nm and solar zenith angles between 55° and 60°. Radiative transfer calculations considering three different aerosol mixtures are performed and and agree with the measurements within the statistical error.

Atmospheric and Environmental Impacts of Volcanic Particulates

Volcanic emissions consist of a mixture of gases, aerosol, and silicate particles, which collectively span seven orders of magnitude in size. Airborne ash and sulfate aerosol in the lower atmosphere has short-lived atmospheric and climatic effects. Volcanic aerosol injected high into the stratosphere impacts atmospheric chemical cycles and the solar and terrestrial radiation budgets, and may influence climate over longer time-scales than aerosol particles in the lower atmosphere. Once at the surface, the impacts on local environments can be substantial through transport of halogens, trace metals, and metalloids, and subsequent leaching in aqueous solutions. Volcanic emissions may cause disruption to travel and aviation, and may damage surface infrastructure, potentially causing large economic losses.

Bed agglomeration in fluidized combustor fueled by wood and rice straw blends

Petrographic techniques have been used to examine bed materials from fluidized bed combustion experiments that utilized wood and rice straw fuel blends. The experiments were conducted using a laboratory-scale combustor with mullite sand beds, firing temperatures of 840 to 1030 °C, and run durations of 5.5 h. A narrow continuous zone borders virtually all bed particles. The highest concentrations of potassium are found in this surface zone that also is enriched in appreciable amounts of other elements. Thin discontinuous films of adhesive cement, formed preferentially on surfaces and contact areas between bed particles, ultimately led to bed agglomeration. The interfaces and the presence of gas bubbles in the cement suggest a bonding material with a high surface tension and a liquid state. The cement films originate by filling of irregularities on individual and partially agglomerated bed particle surfaces by accumulation of liquid droplets preferentially in areas sheltered from turbulence and mechanical interaction. The composition of the film suggests melting of locally accumulated dust or aerosol mixture of ash particles and mullite. The film only locally enlarged bed particles. Large straw ash particles appear to have mostly been passively incorporated into the adhesive melt without melting or reaction.

Evaluation of a detailed model of secondary organic aerosol formation from α-pinene against dark ozonolysis experiments

BOREAM, a detailed model for the gas-phase oxidation of α-pinene and its subsequent formation of Secondary Organic Aerosol (SOA), is tested against a large set of SOA yield measurements obtained in dark ozonolysis experiments. For the majority of experiments, modelled SOA yields are found to agree with measured yields to within a factor 2. However, the comparisons point to a general underestimation of modelled SOA yields at high temperatures (above 30 °C), reaching an order of magnitude or more in the worst cases, whereas modelled SOA yields are often overestimated at lower temperature (by a factor of about 2). Comparisons of results obtained using four different vapour pressure prediction methods indicate a strong sensitivity to the choice of the method, although the overestimated temperature dependence of the yields is found in all cases. Accounting for non-ideality of the aerosol mixture (based on an adapted UNIFAC method) has significant effects, especially at low yields. Our simulations show that the formation of oligomers through the gas-phase reactions of Stabilised Criegee Intermediates (SCI) with other molecular organic products could increase the SOA yield significantly only at very low relative humidity (below 1%). Further tests show that the agreement between model and measurements is improved when the ozonolysis mechanism includes additional production of non-volatile compounds.

A detailed aerosol mixing state model for investigating interactions between mixing state, semivolatile partitioning, and coagulation

Abstract. A new method for describing externally mixed particles, the Detailed Aerosol Mixing State (DAMS) representation, is presented in this study. This novel method classifies aerosols by both composition and size, using a user-specified mixing criterion to define boundaries between compositional populations. Interactions between aerosol mixing state, semivolatile partitioning, and coagulation are investigated with a Lagrangian box model that incorporates the DAMS approach. Model results predict that mixing state affects the amount and types of semivolatile organics that partition to available aerosol phases, causing external mixtures to produce a more size-varying composition than internal mixtures. Both coagulation and condensation contribute to the mixing of emitted particles, producing a collection of multiple compositionally distinct aerosol populations that exists somewhere between the extremes of a strictly external or internal mixture. The selection of mixing criteria has a significant impact on the size and type of individual populations that compose the modeled aerosol mixture. Computational demands for external mixture modeling are significant and can be controlled by limiting the number of aerosol populations used in the model.

Absorption Angstrom Exponent in AERONET and related data as an indicator of aerosol composition

Abstract. Recent results from diverse air, ground, and laboratory studies using both radiometric and in situ techniques show that the fractions of black carbon, organic matter, and mineral dust in atmospheric aerosols determine the wavelength dependence of absorption (often expressed as Absorption Angstrom Exponent, or AAE). Taken together, these results hold promise of improving information on aerosol composition from remote measurements. The main purpose of this paper is to show that AAE values for an Aerosol Robotic Network (AERONET) set of retrievals from Sun-sky measurements describing full aerosol vertical columns are also strongly correlated with aerosol composition or type. In particular, we find AAE values near 1 (the theoretical value for black carbon) for AERONET-measured aerosol columns dominated by urban-industrial aerosol, larger AAE values for biomass burning aerosols, and the largest AAE values for Sahara dust aerosols. These AERONET results are consistent with results from other, very different, techniques, including solar flux-aerosol optical depth (AOD) analyses and airborne in situ analyses examined in this paper, as well as many other previous results. Ambiguities in aerosol composition or mixtures thereof, resulting from intermediate AAE values, can be reduced via cluster analyses that supplement AAE with other variables, for example Extinction Angstrom Exponent (EAE), which is an indicator of particle size. Together with previous results, these results strengthen prospects for determining aerosol composition from space, for example using the Glory Aerosol Polarimetry Sensor (APS), which seeks to provide retrievals of multiwavelength single-scattering albedo (SSA) and aerosol optical depth (and therefore aerosol absorption optical depth (AAOD) and AAE), as well as shape and other aerosol properties. Multidimensional cluster analyses promise additional information content, for example by using the Ozone Monitoring Instrument (OMI) to add AAOD in the near ultraviolet and CALIPSO aerosol layer heights to reduce height-absorption ambiguity.

Organic Aerosol Speciation: Intercomparison of Thermal Desorption Aerosol GC/MS (TAG) and Filter-Based Techniques

We compared measurements of organic molecular markers made using a novel Thermal Desorption Aerosol Gas Chromatograph/Mass Spectrometer (TAG) with two offline filter-based methods: solvent extraction GC/MS (SE-GC/MS) and thermal desorption GC/MS (TD-GC/MS). TAG is designed for automated, high time-resolved measurements of organic aerosol speciation. Laboratory and field measurements were performed to compare TAG and filter performance for n-alkanes, n-alkanoic acids, cholesterol, PAHs, and hopanes. Laboratory measurements of model organic aerosol mixtures of known composition were made in the Carnegie Mellon University smog chamber, and field measurements were made in downtown Pittsburgh. There was excellent agreement between techniques for hopanes and several PAHs, which are important markers for motor vehicle emissions. Agreement was also strong for moderately polar and nonpolar species in the high-concentration smog chamber experiments. Poorer agreement between filter and TAG observations was obtained for n-alkanes in ambient measurements. To further investigate the differences in n-alkane performance between these methods, potential matrix effects and internal consistency within the TAG and filter ambient air data sets were examined. We spiked a subset of ambient TAG samples with deuterated internal standards to examine potential matrix effects. Under typical conditions, there was little bias in recovery of deuterated standards. At extreme ambient aerosol levels, however, there were large biases in relative recovery, indicating matrix effects may be important under those conditions. Similar results were observed in laboratory experiments with engine lubricating oil. Applying internal standards in the TAG system would help track and correct for matrix effects influencing compound recovery.

Interaction of internally mixed aerosols with light

Atmospheric aerosols scatter and absorb solar radiation leading to variable effects on Earth's radiative balance. Aerosols individually comprising mixtures of different components ("internally mixed") interact differently with light than mixtures of aerosols, each comprising a different single component ("externally mixed"), even if the relative fractions of the different components are equal. In climate models, the optical properties of internally mixed aerosols are generally calculated by using electromagnetic "mixing rules", which average the refractive indices of the individual components in different proportions, or by using coated-sphere Mie scattering codes, which solve the full light scattering problem assuming that the components are divided into two distinct layers. Because these calculation approaches are in common use, it is important to validate them experimentally. In this article, we present a broad perspective on the optical properties of internally mixed aerosols based on a series of laboratory experiments and theoretical calculations. The optical properties of homogenously mixed aerosols comprised of non-absorbing and weakly absorbing compounds, and of coated aerosols comprised of strongly absorbing, non-absorbing, and weakly absorbing compounds in different combinations are measured using pulsed and continuous wave cavity ring down aerosol spectrometry (CRD-AS). The success of electromagnetic mixing rules and Mie scattering codes in reproducing the measured aerosol extinction values is discussed.

紫外到可见波段外混合气溶胶的传输特性

紫外到可见波段外混合气溶胶的传输特性

Thermodynamic consistency of Raoult’s Law and Henry’s Law approaches for multiphase organic aerosol partitioning

Partitioning of semivolatile organic compounds between gas, organic aerosol, and aqueous aerosol phases has been described in atmospheric models using Raoult’s Law and/or Henry’s Law, with activity coefficients accounting for nonideal behavior in the aerosol solutions. Raoult’s Law and Henry’s Law are thermodynamically consistent with each other as long as the parameters defining their different reference states are accurately known. Unfortunately, saturation vapor pressures, Henry’s law constants, and activity coefficients for organic aerosol compounds must typically be estimated. As a result, thermodynamic inconsistencies can arise when using Raoult’s and Henry’s Law approaches together due to errors in estimation methods. A test of predicted partitioning parameters for representative semivolatile organics suggests overall errors of at least an order of magnitude. Box model simulations with a simplified partitioning scenario demonstrate that these estimation errors can significantly alter partitioning for many compounds and, more importantly, that thermodynamic inconsistencies will lead to even greater errors than those due solely to uncertain parameters. To avoid these errors, a common reference state should be used to define equilibrium among all phases, improved estimation methods and measurements should continue to be pursued, and alternative reference states that better represent typical organic aerosol mixtures should be explored.

Aerosols and their influence on radiation partitioning and savanna productivity in northern Australia

Aerosols have been shown to affect the quantity and quality of solar radiation on the Earth’s surface. Savanna regions are subject to frequent burning and release of aerosols that may impact on radiation components and possibly vegetation productivity in this region. Therefore, in this study, we have analyzed the optical properties of aerosols (aerosol optical depth (AOD) and Angstrom coefficient) from the Atmospheric Radiation Measurement site in Darwin for the periods from April 2002 to June 2005 as measured by a multifilter rotating shadowband radiometer. The influence of aerosols and their effect on surface shortwave incoming solar radiation and savanna productivity were examined for the dry season using sky radiation collection of radiometers and eddy covariance measurements from the Howard Springs flux site. Results indicated that aerosol concentrations in the region were relatively low compared to other savanna regions with the maximum monthly average AOD over the period being the greatest in October (0.29 ± 0.003 standard error at 500 nm). The highest monthly average Angstrom exponent was also found in October (1.38 ± 0.008). The relatively low aerosol concentration in this region can be attributed to the mixture of smoke aerosols with humidity haze and local circulations. Over a range of AODs from 0.1 to 0.4, we found a modest increase in the fraction of diffuse radiation to total radiation from 11% to 21%. This small increase in diffuse fraction did not affect the carbon flux significantly. However, because the current range of AOD in the region is relatively low, the region could be sensitive to increases in aerosols and diffuse fraction in the future.

Investigations of the March 2006 African dust storm using ground‐based column‐integrated high spectral resolution infrared (8–13 μm) and visible aerosol optical thickness measurements: 2. Mineral aerosol mixture analyses

The mineral aerosol mixture composition for the March 2006 Saharan dust storm is assessed in this paper on the basis of the analysis of visible to near‐infrared (VIS‐NIR) and infrared (IR) aerosol optical thickness (AOT) spectra obtained during the Portable Infrared Aerosol Transmission Experiment (PIRATE). The AOT spectra from 8 to 13 μm were determined using column‐integrated solar transmission measurements using a Fourier transform infrared spectrometer. To determine the mineralogy and mixture composition of the dust, we determined the expected mineralogy of dust from the Algerian source region in a dust storm environment. Then we computed the modeled VIS‐IR AOT spectra using Mie theory for external and internal mixtures. We compared the modeled VIS‐NIR AOT spectra and derived index of refraction and single‐scattering albedo with the measured values from AERONET and compared the modeled IR AOT spectra with the values from our IR measurements. The fit between the measured and modeled values was best when we used an extinction resonance correction to the Mie theory results to better account for the exact wavelengths and shapes of some of the AOT peaks for mineral particles. The mineralogy and mixture composition of the best dust model includes external mixtures, internal mixtures, and mineralogy dominated by quartz, illite, and calcite. The modeled mean radius was determined, and several modes were computed in agreement with AERONET results.