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We investigated the collection efficiency and effective ionization efficiency for secondary organic aerosol (SOA) particles made from α-pinene + O3 using the single-particle capabilities of the Aerosol Mass Spectrometer (AMS). The mean count-based collection efficiency (CEp) for SOA across these experiments is 0.30 (±0.04 S.D.), ranging from 0.25 to 0.40. The mean mass-based collection efficiency (CEm) is 0.49 (0.07 S.D.). This sub-unit collection efficiency and delayed vaporization is attributable to particle bounce in the vaporization region. Using the coupled optical and chemical detection of the light scattering single-particle (LSSP) module 5 of the AMS, we provide clear evidence that "delayed vaporization" is somewhat of a misnomer for these particles: SOA particles that appear within the chopper window do not vaporize at a slow rate; rather, they flash-vaporize, but often not on the initial impact with the vaporizer, and instead upon a subsequent impact with a hot surface in the vaporization region. We also find that the effective ionization efficiency (defined as ions per particle, IPP) decreases with delayed arrival time. CEp is not a function of particle size (for the mobility diameter range investigated, 170-460nm), but we did see a decrease in CEp with thermodenuder temperature, implying that oxidation state and/or volatility can affect CEp for SOA. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols impact the Earth's climate system directly by scattering and absorbing solar radiation, and it is important to characterise the aerosol optical properties in detail. This study reports the development and validation of an airborne dual-wavelength cavity-attenuated phase-shift (CAPS) single monitor, named A2S2 (Aerosol Absorption Spectral Sizer), based on the commercial CAPS single-scattering albedo monitor (CAPS-PM SSA ; Aerodyne), to simultaneously measure the aerosol optical scattering and extinction at both 450 and 630 nm wavelengths. Replaced pressure and temperature sensors and an additional flow control system were incorporated into the A2S2 for its utilisation on board research aircraft measuring within the troposphere. The evaluation of A2S2 characteristics was performed in the laboratory and included the investigation of the signal-to-noise ratio, validation of performance at various pressure levels, optical closure studies and intercomparing with the currently validated techniques. The chamber experiments show that the A2S2 can perform measurements at sample pressures as low as 550 hPa and at sample temperatures as high as 315 K. Based on the Allan analysis results, we have evaluated that the minimum detection limit of the measurements shows that the measurements have a limit accuracy of ∼ 2 Mm-1 at 450 nm and ∼ 1 Mm-1 at 630 nm for 1 Hz measurements of both scattering coefficients (σsca) and extinction coefficients (σext). The optical closure study with size-selected polystyrene latex (PSL) particles shows that the truncation error of the A2S2 is negligible for particles with particle volume diameter (Dp) < 200 nm, while, for the larger sub-micrometre particles, the measurement uncertainty of A2S2 increases but remains less than 20 %. The average factors to correct the truncation error are 1.13 and 1.05 for 450 and 630 nm, respectively. A simplified truncation correction, dependent on the scattering Ångström exponent (SAE), was developed to rectify truncation errors of the future A2S2 field measurement data. The σsca and σext measured by the A2S2 show good agreement with the concurrent measured results from the nephelometer and the CAPS particle extinction monitor (CAPS-PM ex). The absorption coefficient (σabs) derived through the extinction-minus-scattering (EMS) method by the A2S2 also corresponds with the results obtained from the aethalometer. The A2S2 was successfully deployed during an aircraft measurement campaign (Atmospheric ChemistRy Of the Suburban foreSt – ACROSS) conducted in the vicinity of Paris and the surrounding regions. The average SSA measured during the entire ACROSS flight campaign is 0.86 and 0.88 at 450 and 630 nm, respectively, suggesting that light-absorbing organic aerosols play a significant role. The average SAE and absorption Ångström exponent (AAE) varied due to measurements in various pollution conditions. The results presented in this study indicate that the A2S2 instrument is reliable for measuring aerosol σsca and σext at both blue and red wavelengths, and it stands as a viable substitute for future airborne evaluations of aerosol optical properties. [ABSTRACT FROM AUTHOR]

Atmospheric aerosols impact the Earth’s climate system directly by scattering and absorbing solar radiation, and it is important to characterise the aerosol optical properties in detail. This study reports the development and validation of an airborne dual-wavelength cavity-attenuated phase shift-single (CAPS) monitor, named A2S2 (Aerosol Absorption Spectral Sizer) based on the commercial CAPS single scattering albedo monitor (CAPS-PMSSA, Aerodyne), to simultaneously measure the aerosol optical scattering and extinction at both 450 nm and 630 nm wavelengths. New pressure and temperature sensors and an additional flow control system were incorporated into the A2S2 for its utilization onboard research aircraft measuring within the troposphere. The evaluation of A2S2 characteristics was performed in the laboratory and included the investigation of the signal-to-noise ratio, validation of performance at various pressure levels, optical-closure studies and intercomparing with the currently validated techniques. These laboratory characterisation experiments show that the A2S2 can perform measurements at sample pressures as low as 550 hPa and at sample temperatures as high as 315 K, with an uncertainty of 1 Mm-1 at 450 nm and 0.3 Mm-1 at 630 nm for 1 Hz measurements of both scattering coefficients (σsca) and extinction coefficients (σext). The optical-closure study with size-selected polystyrene latex (PSL) particles show that the truncation error of the A2S2 is negligible for particles with particle volume diameter (Dp) < 200 nm, while for the larger sub-micrometre particles, the measurement uncertainty of A2S2 increases but remains less than 20 %. The A2S2 shows good agreement with the validated instruments for the σsca and σext at 450 nm and 630 nm. The A2S2 was successfully deployed during an aircraft measurement campaign (ACROSS) conducted in the vicinity of Paris and the surrounding regions. The average SSA measured during the entire ACROSS flight campaign is 0.86 and 0.88 at 450 nm and 630 nm, respectively, while the Scattering Ångström Exponent (SAE) varies due to measurements in various pollution conditions. The A2S2 measured σsca results exhibit overall good agreement with the nephelometer results, and it successfully produced altitude profile results over the varied background conditions. The results presented in this study indicate that the A2S2 instrument is reliable for measuring aerosol σsca and σext at both blue and red wavelengths, and it is suitable to replace the nephelometer onboard for future aircraft campaigns. [ABSTRACT FROM AUTHOR]

The optical properties (light scattering and absorption) of soot particles depend on soot size and index of refraction, but also on the soot complex morphology and the internal mixing with other material at the single particle level. For example, freshly emitted (nascent) soot particles can interact with other materials in the atmosphere, materials that can condense on soot and coat it. This coating can affect the soot optical properties by refracting light, or by changing the soot aggregate structure. A common approach to studying the effect of coating on soot optical properties is to measure absorption and scattering values in ambient air and then measure them again after removing the coating using a thermodenuder. In this approach, it is assumed that: 1) Most of the coating material is removed; 2) charred organic coating does not add to the refractory carbon; 3) oxidation of soot is negligible; and 4) the pre-existing core soot structure is left unaltered despite potential oxidation of the core at elevated temperature. In this study, we investigate the validity of the last assumption, by studying the effect of thermodenuding on the structure of nascent soot. To this end, we analyze the morphological properties of laboratory generated nascent soot, before and after thermodenuding. Our investigation shows that there is only minor restructuring of nascent soot by thermodenuding. [ABSTRACT FROM AUTHOR]

Aerosol intensive optical properties, including Ångström exponents for aerosol light extinction (EAEs), scattering (SAEs) and absorption (AAEs) as well as and the single-scattering albedo (SSA), are indicators for aerosol size, chemical composition, radiative behaviour and particle sources. Derivation of these parameters requires the measurement of aerosol optical properties at multiple wavelengths, which usually involves the use of several instruments. Our study aims to quantify the uncertainties in the determination of these intensive properties using an optical closure approach. In our laboratory closure study, we measured the full set of optical properties for a range of light-absorbing particles with different properties externally mixed with ammonium sulfate to generate aerosols with controlled SSA values. The investigated absorbing particle types were fresh combustion soot emitted by an inverted flame soot generator (SOOT; fractal agglomerates), Aquadag (AQ; compact aggregates), Cabot black (BC; compact agglomerates) and an acrylic paint (magic black, shape unknown). The instruments used in this study were two cavity attenuated phase shift particle monitors for single-scattering albedo (CAPS PM SSA 's; λ=450 , 630 nm) for measuring light-extinction and light-scattering coefficients, one integrating nephelometer (λ=450 , 550, 700 nm) for light-scattering coefficients, and one tricolour absorption photometer (TAP; λ=467 , 528, 652 nm) for filter-based light-absorption coefficients. One key finding is that the coefficients of light absorption, scattering and extinction derived from combing the measurements of two independent instruments agree with measurements from single instruments; the slopes of regression lines are equal within reported uncertainties (i.e. closure is observed). Despite closure for measured absorption coefficients, we caution that the estimated uncertainties for absorption coefficients, propagated for the differential method (DM; absorption = extinction minus scattering), can exceed 100 % for atmospherically relevant SSA values (>0.9). This increasing estimated uncertainty with increasing SSA yields AAE values that may be too uncertain for measurements in the range of atmospheric aerosol loadings. We recommend using the DM for measuring AAE values when the SSA < 0.9. EAE- and SAE-derived values achieved closure during this study within stated uncertainties for extinction coefficients greater than 15 Mm -1. SSA values for 450 and 630 nm wavelengths internally agreed with each other within 10 % uncertainty for all instrument combinations and sampled aerosol types, which fulfils the defined goals for measurement uncertainty of 10 % proposed by Laj et al. (2020) for GCOS (Global Climate Observing System) applications. [ABSTRACT FROM AUTHOR]

Aerosol intensive optical properties like the Ångström exponents for aerosol light extinction, scattering and absorption, or the single-scattering albedo are indicators for aerosol size distributions, chemical composition and radiative behaviour and contain also source information. The observation of these parameters requires the measurement of aerosol optical properties at multiple wavelengths which usually implies the use of several instruments. Our study aims to quantify the uncertainties of the determination of multiple-wavelengths intensive properties by an optical closure approach, using different test aerosols. In our laboratory closure study, we measured the full set of aerosol optical properties for a range of light-absorbing aerosols with different properties, mixed externally with ammonium sulphate to generate aerosols of controlled single-scattering albedo. The investigated aerosol types were: fresh combustion soot emitted by an inverted flame soot generator (SOOT, fractal aggregates), Aquadag (AQ, spherical shape), Cabot industrial soot (BC, compact clusters), and an acrylic paint (Magic Black, MB). One focus was on the validity of the Differential Method (DM: absorption = extinction minus scattering) for the determination of Ångström exponents for different particle loads and mixtures of light-absorbing aerosol with ammonium sulphate, in comparison to data obtained from single instruments. The instruments used in this study were two CAPS PMssa (Cavity Attenuated Phase Shift Single Scattering Albedo, ? = 450, 630 nm) for light extinction and scattering coefficients, one Integrating Nephelometer (? = 450, 550, 700 nm) for light scattering coefficient and one Tricolour Absorption Photometer (TAP, ? = 467, 528, 652 nm) for filter-based light absorption coefficient measurement. Our key finding is that the coefficients of light absorption sap, scattering ssp and extinction sep from the Differential Method agree with data from single reference instruments, and the slopes of regression lines equal unity within the precision error. We found, however, that the precision error for the DM exceeds 100% for sap values lower than 10-20 Mm-1 for atmospheric relevant single scattering albedo. This increasing uncertainty with decreasing sap yields an absorption Ångström exponent (AAE) that is too uncertain for measurements in the range of atmospheric aerosol loadings. We recommend using DM only for measuring AAE values for sap > 50 Mm-1. Ångström exponents for scattering and extinction are reliable for extinction coefficients from 20 up to 1000 Mm-1 and stay within 10% deviation from reference instruments, regardless of the chosen method. Single-scattering albedo (SSA) values for 450 nm and 630 nm wavelengths agree with values from the reference method ssp (NEPH)/sep (CAPS PMSSA) with less than 10% uncertainty for all instrument combinations and sampled aerosol types which fulfil the defined goals for measurement uncertainty of 10% proposed by Laj et al., 2020 for GCOS (Global Climate Observing System) applications. [ABSTRACT FROM AUTHOR]

An evaluation of the performance and relative accuracy of a Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor (CAPS PM SSA ; Aerodyne Research, Inc.) was conducted in an optical-closure study with proven technologies: Cavity Attenuated Phase-Shift Particle Extinction Monitor (CAPS PM ex ; Aerodyne Research, Inc.), three-wavelength integrating nephelometer (TSI Model 3563) and three-wavelength filter-based Particle Soot Absorption Photometer (PSAP; Radiance Research Inc.). The evaluation was conducted by connecting the instruments to a controlled aerosol generation system and comparing the measured scattering, extinction and absorption coefficients measured by the CAPS PM SSA with the independent measurements. Three different particle types were used to generate aerosol samples with single-scattering albedos (SSAs) ranging from 0.4 to 1.0 at 630 nm wavelength. The CAPS PM SSA measurements compared well with the proven technologies. Extinction measurement comparisons exhibited a slope of the linear regression line for the full dataset between 1.05 and 1.01, an intercept below ±1.5×10-6 m -1 (±1.5 Mm -1), and a regression coefficient R2>0.99 , whereas scattering measurements had a slope between 0.90 and 1.04, an intercept of less than ±2.0×10-6 m -1 (2.0 Mm -1), and a coefficient R2>0.96. The derived CAPS PM SSA absorption compared well to the PSAP measurements for the small particle sizes and modest (0.4 to 0.6) SSA values tested, with a linear regression slope between 0.90 and 1.07, an intercept of ±3.0×10-6 m -1 (3.0 Mm -1), and a coefficient R2>0.99. For the SSA measurements, agreement was highest (regression slopes within 1 %) for SSA =1.0 particles at extinction levels of per tens of inverse megameters and above; however, as extinctions approach 0, small uncertainties in the baseline can introduce larger errors. SSA measurements for absorbing particles exhibited absolute differences up to 18 %, though it is not clear which measurement had the best relative accuracy. For a given particle type, the CAPS PM SSA instrument exhibited the lowest scatter around the average. This study demonstrates that the CAPS PM SSA is a robust and reliable instrument for the direct measurement of the scattering and extinction coefficients and thus SSA. This conclusion also holds for the indirect measurement of the absorption coefficient with the constraint that the relative accuracy of this particular determination degrades as the SSA and particle size increases. [ABSTRACT FROM AUTHOR]

An evaluation of the performance and accuracy of a Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor (CAPS PMssa, Aerodyne Res. Inc.) was conducted in an optical closure study with proven technologies: Cavity Attenuated Phase-Shift Particle Extinction Monitor (CAPS PMex, Aerodyne Res. Inc.); 3-wavelengh Integrating Nephelometer (TSI Model 3563); and 3-wavelength filter-based Particle Soot Absorption Photometer (PSAP, Radiance Research). The evaluation was conducted by connecting the instruments to a controlled aerosol generation system and comparing the measured scattering, extinction, and absorption coefficients measured by the CAPS PMssa with the independent measurements. Three different particle types were used to generate aerosol samples with single-scattering albedos (SSA) ranging from 0.4 to 1.0 at 630 nm wavelength. The CAPS PMssa measurements compared well with the proven technologies. Extinction measurement comparisons exhibited a slope of the linear regression line for the full data set of 0.96 (-0.02/+0.06), an intercept near zero, and a regression coefficient R² > 0.99; whereas, scattering measurements had a slope of 1.01 (-0.07/+0.06), an intercept of less than +/-2 x 10-6 m-1 (Mm-1), and a coefficient R² ∼ 1.0. The derived CAPS PMssa absorption compared well to the PSAP measurements at low levels (< 70 Mm-1) for the small particle sizes and modest (0.4 to 0.6) SSA values tested, with a linear regression slope of 1.0, an intercept of -4 Mm-1, and a coefficient R² = 0.97. Comparisons at higher particle loadings were compromised by loading effects on the PSAP filters. For the SSA measurements, agreement was highest (regression slopes within 1% ) for SSA = 1.0 particles, though the difference between the measured values increased to 9 % for extinction coefficients lower than 55 Mm-1. SSA measurements for absorbing particles exhibited absolute differences up to 18 %, though it is not clear which measurement had the lowest accuracy. For a given particle type, the CAPS PMssa instrument exhibited the lowest scatter around the average. This study demonstrates that the CAPS PMssa is a robust and reliable instrument for the direct measurement of the scattering and extinction coefficients and thus SSA. This conclusion also holds as well for the indirect measurement of the absorption coefficient with the constraint that the accuracy of this particular measurement degrades as the SSA and particle size increases. [ABSTRACT FROM AUTHOR]

Glass transitions from liquid to semi-solid and solid phase states have important implications for reactivity, growth, and cloud-forming (cloud condensation nuclei and ice nucleation) capabilities of secondary organic aerosols (SOAs). The small size and relatively low mass concentration of SOAs in the atmosphere make it difficult to measure atmospheric SOA glass transitions using conventional methods. To circumvent these difficulties, we have adapted a new technique for measuring glass-forming properties of atmospherically relevant organic aerosols. Aerosol particles to be studied are deposited in the form of a thin film onto an interdigitated electrode (IDE) using electrostatic precipitation. Dielectric spectroscopy provides dipole relaxation rates for organic aerosols as a function of temperature (373 to 233 K) that are used to calculate the glass transition temperatures for several cooling or heating rates. IDE-enabled broadband dielectric spectroscopy (BDS) was successfully used to measure the kinetically controlled glass transition temperatures of aerosols consisting of glycerol and four other compounds with selected cooling and heating rates. The glass transition results agree well with available literature data for these five compounds. The results indicate that the IDE-BDS method can provide accurate glass transition data for organic aerosols under atmospheric conditions. The BDS data obtained with the IDE-BDS technique can be used to characterize glass transitions for both simulated and ambient organic aerosols and to model their climate effects. [ABSTRACT FROM AUTHOR]

Glass transitions from liquid to semi-solid and solid phase states have important implications for reactivity, growth, and cloud forming (cloud condensation nuclei and ice nucleation) capabilities of secondary organic aerosols (SOA). The small size and relatively low mass concentration of SOA in the atmosphere make it difficult to measure atmospheric SOA glass transitions using conventional methods. To circumvent these difficulties, we have adopted a new technique for measuring glass forming properties of atmospherically relevant organic aerosols. Aerosol particles to be studied are deposited in the form of a thin film onto an interdigitated electrode (IDE) using electrostatic precipitation. Dielectric spectroscopy provides dipole relaxation rates for organic aerosols as a function of temperature (373 to 233 K) that are used to calculate the glass transition temperatures for several cooling rates. IDE-enabled broadband dielectric spectroscopy (BDS) was successfully used to measure the kinetically controlled glass transition temperatures of glycerol and citric acid aerosols with selected cooling rates. The glass transition results agree well with available literature data for these two compounds. The results indicate that the IDE-BDS method can provide accurate glass transition data for organic aerosols under atmospheric conditions. The BDS data obtained with the IDE-BDS technique can be used to characterize glass transitions for both simulated and ambient organic aerosols and to model their climate effects. [ABSTRACT FROM AUTHOR]

Oxidation flow reactors that use low-pressure mercury lamps to produce hydroxyl (OH) radicals are an emerging technique for studying the oxidative aging of organic aerosols. Here, ozone (O3) is photolyzed at 254 nm to produce O(1D) radicals, which react with water vapor to produce OH. However, the need to use parts-per-million levels of O3) hinders the ability of oxidation flow reactors to simulate NOx-dependent secondary organic aerosol (SOA) formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NOx (NO + NO2) to nitric acid (HNO3), making it impossible to sustain NOx at levels that are sufficient to compete with hydroperoxy (HO2) radicals as a sink for organic peroxy (RO2) radicals. We developed a new method that is well suited to the characterization of NOx-dependent SOA formation pathways in oxidation flow reactors. NO and NO2 are produced via the reaction O(1D) + N2O → 2NO, followed by the reaction NO + O3 → NO2 + O2. Laboratory measurements coupled with photochemical model simulations suggest that O(1D) + N2O reactions can be used to systematically vary the relative branching ratio of RO2 + NO reactions relative to RO2 + HO2 and/or RO2 + RO2 reactions over a range of conditions relevant to atmospheric SOA formation. We demonstrate proof of concept using high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) measurements with nitrate (NO3-) reagent ion to detect gas-phase oxidation products of isoprene and -pinene previously observed in NOx-influenced environments and in laboratory chamber experiments. [ABSTRACT FROM AUTHOR]

Oxidation flow reactors that use low-pressure mercury lamps to produce hydroxyl (OH) radicals are an emerging technique for studying the oxidative aging of organic aerosols. Here, ozone (O3) is photolyzed at 254 nm to produce O(1D) radicals, which react with water vapor to produce OH. However, the need to use parts-per-million levels of O3 hinders the ability of oxidation flow reactors to simulate NOx-dependent SOA formation pathways. Simple addition of nitric oxide (NO) results in fast conversion of NOx (NO + NO2) to nitric acid (HNO3), making it impossible to sustain NO at levels that are sufficient to compete with hydroperoxy (HO2) radicals as a sink for organic peroxy (RO2) radicals. We developed a new method that is well suited to the characterization of NOx-dependent SOA formation pathways in oxidation flow reactors. NO and NO2 are produced via the reaction O(1D) + N2O→ 2NO, followed by the reaction NO + O3 → NO2+ O2. Laboratory measurements coupled with photochemical model simulations suggest that O(1D) + N2O reactions can be used to systematically vary the relative branching ratio of RO2 + NO reactions relative to RO2 + HO2 and/or RO2 + RO2 reactions over a range of conditions relevant to atmospheric SOA formation. We demonstrate proof of concept using high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) measurements with nitrate (NO3-) reagent ion to detect gas-phase oxidation products of isoprene and α-pinene previously observed in NOx-influenced environments and in laboratory chamber experiments. [ABSTRACT FROM AUTHOR]