Mass-specific Absorption Cross Sections Research Articles

Specifying the light-absorbing properties of aerosol particles in fresh snow samples, collected at the Environmental Research Station Schneefernerhaus (UFS), Zugspitze

Abstract. Atmospheric aerosol particles like mineral dust, volcanic ash and combustion particles can reduce Earth's snow and ice albedo considerably even by very small amounts of deposited particle mass. In this study, a new laboratory method is applied to measure the spectral light absorption coefficient of airborne particles that are released from fresh snow samples by an efficient nebulizing system. Three-wavelength photoacoustic absorption spectroscopy is combined with refractory black carbon (BC) mass analysis to determine the snow mass-specific and BC mass-specific absorption cross sections. Fullerene soot in water suspensions are used for the characterization of the method and for the determination of the mass-specific absorption cross section of this BC reference material. The analysis of 31 snow samples collected after fresh snowfall events at a high-altitude Alpine research station reveals a significant discrepancy between the measured snow mass-specific absorption cross section and the cross section that is expected from the BC mass data, indicating that non-BC light-absorbing particles are present in the snow. Mineral dust and brown carbon (BrC) are identified as possible candidates for the non-BC particle mass based on the wavelength dependence of the measured absorption. For one sample this result is confirmed by environmental scanning electron microscopy and by single-particle fluorescence measurements, which both indicate a high fraction of biogenic and organic particle mass in the sample.

Variation of the optical properties of soot as a function of particle mass

Light absorption and scattering properties of combustion-generated soot particles are important to measurement methods and are used as the inputs for climate models. For decades, mass-specific absorption cross section (MAC) of soot has been assumed to be independent of its size and mass. Here we investigate the size-dependent optical properties of particles produced by a single source.Soot particles were produced by combustion of CH4, and CH4 - N2 mixtures, within a laminar inverted diffusion flame. Extinction and scattering coefficients of mass-classified particles were measured by a Cavity Attenuated Phase Shift Single Scattering Albedo Spectrometer. Primary particle diameter and morphology of particles were characterized by transmission electron microscopy. Graphitization level of the size-classified particles was also investigated by Raman spectroscopy.Here, for the first time we observed that MAC and graphitization level of the soot particles increase with the particle mass. MAC number increases from approximately 4.5 to 8.5 (m2g−1) with the particle mass increasing from 0.05 fg to 8 fg. The soot aggregate size is correlated with primary particle size, which may reflect the inhomogeneity of the combustion environment that could affect the optical properties of soot.

Direct In Situ Mass Specific Absorption Spectra of Biomass Burning Particles Generated from Smoldering Hard and Softwoods.

Particles from smoldering biomass burning (BB) represent a major source of carbonaceous aerosol in the terrestrial atmosphere. In this study, mass specific absorption spectra of laboratory-generated smoldering wood particles (SWP) from 3 hardwood and 3 softwood species were measured in situ. Absorption data spanning from λ = 500 to 840 nm were collected using a photoacoustic spectrometer coupled to a supercontinuum laser with a tunable wavelength and bandwidth filter. SWP were size- (electrical mobility) and mass-selected prior to optical characterization allowing data to be reported as mass-specific absorption cross sections (MAC). The median measured MAC at λ = 660 nm for smoldering oak particles was 1.1 (0.57/1.8) × 10-2 m2 g-1 spanning from 83 femtograms (fg) to 517 fg (500 nm ≤ mobility diameter ≤950 nm), MAC values in parentheses are the 16th and 84th percentiles of the measured data (i.e., 1σ). The collection of all six wood species (Oak, Hickory, Mesquite, Western redcedar, Baldcypress, and Blue spruce) had median MAC values ranging from 1.4 × 10-2 m2 g-1 to 7.9 × 10-2 m2 g-1 at λ = 550 nm with absorption Ångström exponents (AAE) between 3.5 and 6.2. Oak, Western redcedar, and Blue spruce possessed statistically similar (p > 0.05) spectra while the spectra of Hickory, Mesquite, and Baldcypress were distinct (p < 0.01) as calculated from a point-by-point analysis using the Wilcox rank-sum test.

A novel single-cavity three-wavelength photoacoustic spectrometer for atmospheric aerosol research

Abstract. The spectral light-absorbing behavior of carbonaceous aerosols varies depending on the chemical composition and structure of the particles. A new single-cavity three-wavelength photoacoustic spectrometer was developed and characterized for measuring absorption coefficients at three wavelengths across the visible spectral range. In laboratory studies, several types of soot with different organic content were generated by a diffusion flame burner and were investigated for changes in mass-specific absorption cross section (MAC) values, absorption and scattering Ångström exponents (αabs and αsca), and single scattering albedo (ω). By increasing the organic carbonaceous (OC) content of the aerosol from 50 to 90 % of the total carbonaceous mass, for 660 nm nearly no change of MAC was found with increasing OC content. In contrast, for 532 nm a significant increase, and for 445 nm a strong increase of MAC was found with increasing OC content of the aerosol. Depending on the OC content, the Ångström exponents of absorption and scattering as well as the single scattering albedo increased. These laboratory results were compared to a field study at a traffic-dominated urban site, which was also influenced by residential wood combustion. For this site a daily average value of αabs(445–660) of 1.9 was found.

Measured Wavelength-Dependent Absorption Enhancement of Internally Mixed Black Carbon with Absorbing and Nonabsorbing Materials.

Optical absorption spectra of laboratory generated aerosols consisting of black carbon (BC) internally mixed with nonabsorbing materials (ammonium sulfate, AS, and sodium chloride, NaCl) and BC with a weakly absorbing brown carbon surrogate derived from humic acid (HA) were measured across the visible to near-IR (550 to 840 nm). Spectra were measured in situ using a photoacoustic spectrometer and step-scanning a supercontinuum laser source with a tunable wavelength and bandwidth filter. BC had a mass-specific absorption cross section (MAC) of 7.89 ± 0.25 m(2) g(-1) at λ = 550 nm and an absorption Ångström exponent (AAE) of 1.03 ± 0.09 (2σ). For internally mixed BC, the ratio of BC mass to the total mass of the mixture was chosen as 0.13 to mimic particles observed in the terrestrial atmosphere. The manner in which BC mixed with each material was determined from transmission electron microscopy (TEM). AS/BC and HA/BC particles were fully internally mixed, and the BC was both internally and externally mixed for NaCl/BC particles. The AS/BC, NaCl/BC, and HA/BC particles had AAEs of 1.43 ± 0.05, 1.34 ± 0.06, and 1.91 ± 0.05, respectively. The observed absorption enhancement of mixed BC relative to the pure BC was wavelength dependent for AS/BC and decreased from 1.5 at λ = 550 nm with increasing wavelength while the NaCl/BC enhancement was essentially wavelength independent. For HA/BC, the enhancement ranged from 2 to 3 and was strongly wavelength dependent. Removal of the HA absorption contribution to enhancement revealed that the enhancement was ≈1.5 and independent of wavelength.

Measurement of Gas and Aerosol Phase Absorption Spectra across the Visible and Near-IR Using Supercontinuum Photoacoustic Spectroscopy

We demonstrate a method to measure the absorption spectra of gas and aerosol species across the visible and near-IR (500 to 840 nm) using a photoacoustic (PA) spectrometer and a pulsed supercontinuum laser source. Measurements of gas phase absorption spectra were demonstrated using H2O(g) as a function of relative humidity (RH). The measured absorption intensities and peak shapes were able to be quantified and compared to spectra calculated using the 2012 High Resolution Transmission (HITRAN2012) database. Size and mass selected nigrosin aerosol was used to measure absorption spectra across the visible and near-IR. Spectra were measured as a function of aerosol size/mass and show good agreement to Mie theory calculations. Lastly, we measured the broadband absorption spectrum of flame generated soot aerosol at 5% and 70% RH. For the high RH case, we are able to quantifiably separate the soot and water absorption contributions. For soot, we observe an enhancement in the mass specific absorption cross section ranging from 1.5 at 500 nm (p < 0.01) to 1.2 at 840 nm (p < 0.2) and a concomitant increase in the absorption Ångström exponent from 1.2 ± 0.4 (5% RH) to 1.6 ± 0.3 (70% RH).

Calculations of the mass absorption cross sections for carbonaceous nanoparticles modeling soot

In this paper we use an atomistic model to calculate the mass specific absorption cross section coefficient (MAC) of carbonaceous particles of nanometer size. The carbonaceous particles are built numerically to reproduce most of the structural characteristics of typical primary nanoparticles that are agglomerated in soot emitted in the Troposphere from combustion sources. Our model is based on the knowledge of the atomic positions and polarizabilities inside the primary nanoparticles and is used to study the influence of these atomistic characteristics on the optical properties of these nanoparticles. The results indicate that the atomistic composition of the soot primary nanoparticles may have a sufficiently strong impact on the mass specific absorption cross section coefficient curves to allow detection of differences between nanoparticles by using UV–visible spectroscopic measurements, in a well-suited wavelength range, i.e., typically between 200 and 350nm. In a more general way, our calculations show that MAC values as well as differences between MAC curves corresponding to different primary nanoparticles may strongly vary with wavelength. As a consequence, measurements at a given wavelength only are certainly not representative of the absorption properties of these nanoparticles and thus should be considered with caution. Moreover, our approach clearly shows significant differences with classical macroscopic electromagnetic theory when calculating the optical properties of realistic primary soot nanoparticles that, in fact, cannot be considered as homogeneous spherical particles due to the presence of defects in their atomistic structure.

Dependence of Soot Optical Properties on Particle Morphology: Measurements and Model Comparisons

We report the first mass-specific absorption and extinction cross sections for size- and mass-selected laboratory-generated soot aerosol. Measurement biases associated with aerosols possessing multiple charges were eliminated using mass selection to isolate singly charged particles for a specified electrical mobility diameter. Aerosol absorption and extinction coefficients were measured using photoacoustic and cavity ring-down spectroscopy techniques, respectively, for lacey and compacted soot morphologies. The measurements show that the mass-specific absorption cross sections are proportional to particle mass and independent of morphology, with values between 5.7 and 6 m(2) g(-1). Mass-specific extinction cross sections were morphology dependent and ranged between 12 and 16 m(2) g(-1) for the lacey and compact morphologies, respectively. The resulting single-scattering albedos ranged from 0.5 to 0.6. Results are also compared to theoretical calculations of light absorption and scattering from simulated particle agglomerates. The observed absorption is relatively well modeled, with minimum differences between the calculated and measured mass absorption cross sections ranging from ∼ 5% (lacey soot) to 14% (compact soot). The model, however, was unable to satisfactorily reproduce the measured extinction, underestimating the single-scattering albedo for both particle morphologies. These discrepancies between calculations and measurements underscore the need for validation and refinement of existing models of light scattering and absorption by soot agglomerates.

Relationship between Oxidation Level and Optical Properties of Secondary Organic Aerosol

Brown carbon (BrC), which may include secondary organic aerosol (SOA), can be a significant climate-forcing agent via its optical absorption properties. However, the overall contribution of SOA to BrC remains poorly understood. Here, correlations between oxidation level and optical properties of SOA are examined. SOA was generated in a flow reactor in the absence of NOx by OH oxidation of gas-phase precursors used as surrogates for anthropogenic (naphthalene, tricyclo[5.2.1.0(2,6)]decane), biomass burning (guaiacol), and biogenic (α-pinene) emissions. SOA chemical composition was characterized with a time-of-flight aerosol mass spectrometer. SOA mass-specific absorption cross sections (MAC) and refractive indices were calculated from real-time cavity ring-down photoacoustic spectrometry measurements at 405 and 532 nm and from UV-vis spectrometry measurements of methanol extracts of filter-collected particles (300 to 600 nm). At 405 nm, SOA MAC values and imaginary refractive indices increased with increasing oxidation level and decreased with increasing wavelength, leading to negligible absorption at 532 nm. Real refractive indices of SOA decreased with increasing oxidation level. Comparison with literature studies suggests that under typical polluted conditions the effect of NOx on SOA absorption is small. SOA may contribute significantly to atmospheric BrC, with the magnitude dependent on both precursor type and oxidation level.

Optical properties and mineralogical composition of different Saharan mineral dust samples: a laboratory study

Abstract. In aerosol chamber experiments optical properties of resuspended mineral dust samples of defined size distributions were measured. Extinction coefficients (bext) and mass specific extinction cross sections (σext) were determined for Saharan dust samples from different locations. The results for σext were not very sensitive to the type of dust and varied at λ=550 nm between 3.3±0.4 m2 g−1 and 3.7±0.4 m2 g−1. The absorption coefficients (babs) and mass specific absorption cross sections (σabs) were determined with a novel multi-wavelength photo-acoustic absorption spectrometer (PAS). The single scattering albedo was close to 1 (0.98 to 0.99) at 532 nm and 1064 nm, but significantly lower (0.63 to 0.76) at 266 nm. Additionally the chemical and mineralogical composition of the dust samples were analysed with special regard to the iron oxide phases hematite and goethite. At λ=266 nm the mineral dust sample without any detectable iron oxides showed a significantly higher SSA compared to the sample with a hematite content of 0.6 wt-%.