Evaluation of methods for characterizing the fine particulate matter emissions from aircraft and other diffusion flame combustion aerosol sources.

The U. S. Environmental Protection Agency in collaboration with the U. S. Air Force Arnold Engineering Development Complex conducted the VAriable Response In Aircraft nvPM Testing (VARIAnT) 3 and 4 test campaigns to compare nonvolatile particulate matter (nvPM) emissions measurements from a variety of diffusion flame combustion aerosol sources (DFCASs), including a Cummins diesel engine, a diesel powered generator, two gas turbine start carts, a J85-GE-5 turbojet engine burning multiple fuels, and a Mini-CAST soot generator. The VARIAnT research program was devised to understand reported variability in the ARP6320A sampling system nvPM measurements. The VARIAnT research program has conducted four test campaigns to date with the VARIAnT 3 and 4 campaigns devoted to: (1) assessing the response of three different black carbon mass analyzers to particles of different size, morphology, and chemical composition; (2) characterizing the particles generated by 6 different combustion sources according to morphology, effective density, and chemical composition; and (3) assessing any significant difference between black carbon as determined by the 3 mass analyzers and the total PM determined via other techniques. Results from VARIAnT 3 and 4 campaigns revealed agreement of about 20% between the Micro-Soot Sensor, the Cavity Attenuated Phase Shift (CAPS PMSSA) monitor and the thermal-optical reference method for elemental carbon (EC) mass, independent of the calibration source used. For the LII-300, the measured mass concentrations in VARIAnT 3 fall within 18% and in VARIAnT 4 fall within 27% of the reference EC mass concentration when calibrated on a combustor rig in VARIAnT 3 and on an LGT-60 start cart in VARIAnT 4, respectively. It was also found that the three mass instrument types (MSS, CAPS PMSSA, and LII-300) can exhibit different BC to reference EC ratios depending on the emission source that appear to correlate to particle geometric mean mobility diameter, morphology, or some other parameter associated with particle geometric mean diameter (GMD) with the LII-300 showing a slightly stronger apparent trend with GMD. Systematic differences in LII-300 measured mass concentrations have been reduced by calibrating with a turbine combustion as a particle source (combustor or turbine engine). With respect to the particle size measurements, the sizing instruments (TSI SMPS, TSI EEPS, and Cambustion DMS 500) were found to be in general agreement in terms of size distributions and concentrations with some exceptions. Gravimetric measurements of the total aerosol mass produced by the various DFCAs differed from the reference EC, BC and integrated particle size distribution measured aerosol masses. The measurements of particle size distributions and single particle analysis performed using the miniSPLAT indicated the presence of larger particles (≳150 nm) having more compact morphologies, higher effective density, and a composition dominated by OC and containing ash. This increased large particle fraction is also associated with higher values of single scattering albedo measured by the CAPS PMSSA instrument and higher OC measurements. These measurements indicate gas turbine engine emissions can be a more heterogeneous mix of particle types beyond the original E-31 assumption that engine exit exhaust particles are mainly composed of black carbon.

Shortwave absorption by wildfire smoke dominated by dark brown carbon.

Wildfires emit large amounts of black carbon and light-absorbing organic carbon, known as brown carbon, into the atmosphere. These particles perturb Earth's radiation budget through absorption of incoming shortwave radiation. It is generally thought that brown carbon loses its absorptivity after emission in the atmosphere due to sunlight-driven photochemical bleaching. Consequently, the atmospheric warming effect exerted by brown carbon remains highly variable and poorly represented in climate models compared with that of the relatively nonreactive black carbon. Given that wildfires are predicted to increase globally in the coming decades, it is increasingly important to quantify these radiative impacts. Here we present measurements of ensemble-scale and particle-scale shortwave absorption in smoke plumes from wildfires in the western United States. We find that a type of dark brown carbon contributes three-quarters of the short visible light absorption and half of the long visible light absorption. This strongly absorbing organic aerosol species is water insoluble, resists daytime photobleaching and increases in absorptivity with night-time atmospheric processing. Our findings suggest that parameterizations of brown carbon in climate models need to be revised to improve the estimation of smoke aerosol radiative forcing and associated warming.

Glass Transition Temperatures of Organic Mixtures from Isoprene Epoxydiol-Derived Secondary Organic Aerosol.

The phase states and glass transition temperatures (Tg) of secondary organic aerosol (SOA) particles are important to resolve for understanding the formation, growth, and fate of SOA as well as their cloud formation properties. Currently, there is a limited understanding of how Tg changes with the composition of organic and inorganic components of atmospheric aerosol. Using broadband dielectric spectroscopy, we measured the Tg of organic mixtures containing isoprene epoxydiol (IEPOX)-derived SOA components, including 2-methyltetrols (2-MT), 2-methyltetrol-sulfate (2-MTS), and 3-methyltetrol-sulfate (3-MTS). The results demonstrate that the Tg of mixtures depends on their composition. The Kwei equation, a modified Gordon-Taylor equation with an added quadratic term and a fitting parameter representing strong intermolecular interactions, provides a good fit for the Tg-composition relationship of complex mixtures. By combining Raman spectroscopy with geometry optimization simulations obtained using density functional theory, we demonstrate that the non-linear deviation of Tg as a function of composition may be caused by changes in the extent of hydrogen bonding in the mixture.

Using the Black Carbon Particle Mixing State to Characterize the Lifecycle of Biomass Burning Aerosols.

The lifecycle of black carbon (BC)-containing particles from biomass burns is examined using aircraft and surface observations of the BC mixing state for plume ages from ∼15 min to 10 days. Because BC is nonvolatile and chemically inert, changes in the mixing state of BC-containing particles are driven solely by changes in particle coating, which is mainly secondary organic aerosol (SOA). The coating mass initially increases rapidly (kgrowth = 0.84 h-1), then remains relatively constant for 1-2 days as plume dilution no longer supports further growth, and then decreases slowly until only ∼30% of the maximum coating mass remains after 10 days (kloss = 0.011 h-1). The mass ratio of coating-to-core for a BC-containing particle with a 100 nm mass-equivalent diameter BC core reaches a maximum of ∼20 after a few hours and drops to ∼5 after 10 days of aging. The initial increase in coating mass can be used to determine SOA formation rates. The slow loss of coating material, not captured in global models, comprises the dominant fraction of the lifecycle of these particles. Coating-to-core mass ratios of BC particles in the stratosphere are much greater than those in the free troposphere indicating a different lifecycle.

Oxygenated Aromatic Compounds are Important Precursors of Secondary Organic Aerosol in Biomass-Burning Emissions.

Biomass burning is the largest combustion-related source of volatile organic compounds (VOCs) to the atmosphere. We describe the development of a state-of-the-science model to simulate the photochemical formation of secondary organic aerosol (SOA) from biomass-burning emissions observed in dry (RH <20%) environmental chamber experiments. The modeling is supported by (i) new oxidation chamber measurements, (ii) detailed concurrent measurements of SOA precursors in biomass-burning emissions, and (iii) development of SOA parameters for heterocyclic and oxygenated aromatic compounds based on historical chamber experiments. We find that oxygenated aromatic compounds, including phenols and methoxyphenols, account for slightly less than 60% of the SOA formed and help our model explain the variability in the organic aerosol mass (R2 = 0.68) and O/C (R2 = 0.69) enhancement ratios observed across 11 chamber experiments. Despite abundant emissions, heterocyclic compounds that included furans contribute to ∼20% of the total SOA. The use of pyrolysis-temperature-based or averaged emission profiles to represent SOA precursors, rather than those specific to each fire, provide similar results to within 20%. Our findings demonstrate the necessity of accounting for oxygenated aromatics from biomass-burning emissions and their SOA formation in chemical mechanisms.

Impact of Biofuel Blends on Black Carbon Emissions from a Gas Turbine Engine.

Presented here is an overview of non-volatile particulate matter (nvPM) emissions, i.e. "soot" as assessed by TEM analyses of samples collected after the exhaust of a J-85 turbojet fueled with Jet-A as well as with blends of Jet-A and Camelina biofuel. A unifying explanation is provided to illustrate the combustion dynamics of biofuel and Jet-A fuel. The variation of primary particle size, aggregate size and nanostructure are analyzed as a function of biofuel blend across a range of engine thrust levels. The postulate is based on where fuels start along the soot formation pathway. Increasing biofuel content lowers aromatic concentration while placing increasing dependence upon fuel pyrolysis reactions to form the requisite concentration of aromatics for particle inception and growth. The required "kinetic" time for pyrolysis reactions to produce benzene and multi-ring PAHs allows increased fuel-air mixing by turbulence, diluting the fuel-rich soot-forming regions, effectively lowering their equivalence ratio. With a lower precursor concentration, particle inception is slowed, the resulting concentration of primary particles is lowered and smaller aggregates were measured. The lower equivalence ratio also results in smaller primary particles because of the lower concentration of growth species.

Radiative absorption enhancements by black carbon controlled by particle-to-particle heterogeneity in composition.

Black carbon (BC) absorbs solar radiation, leading to a strong but uncertain warming effect on climate. A key challenge in modeling and quantifying BC's radiative effect on climate is predicting enhancements in light absorption that result from internal mixing between BC and other aerosol components. Modeling and laboratory studies show that BC, when mixed with other aerosol components, absorbs more strongly than pure, uncoated BC; however, some ambient observations suggest more variable and weaker absorption enhancement. We show that the lower-than-expected enhancements in ambient measurements result from a combination of two factors. First, the often used spherical, concentric core-shell approximation generally overestimates the absorption by BC. Second, and more importantly, inadequate consideration of heterogeneity in particle-to-particle composition engenders substantial overestimation in absorption by the total particle population, with greater heterogeneity associated with larger model-measurement differences. We show that accounting for these two effects-variability in per-particle composition and deviations from the core-shell approximation-reconciles absorption enhancement predictions with laboratory and field observations and resolves the apparent discrepancy. Furthermore, our consistent model framework provides a path forward for improving predictions of BC's radiative effect on climate.

Spherical tarball particles form through rapid chemical and physical changes of organic matter in biomass-burning smoke.

Biomass burning (BB) emits enormous amounts of aerosol particles and gases into the atmosphere and thereby significantly influences regional air quality and global climate. A dominant particle type from BB is spherical organic aerosol particles commonly referred to as tarballs. Currently, tarballs can only be identified, using microscopy, from their uniquely spherical shapes following impaction onto a grid. Despite their abundance and potential significance for climate, many unanswered questions related to their formation, emission inventory, removal processes, and optical properties still remain. Here, we report analysis that supports tarball formation in which primary organic particles undergo chemical and physical processing within ∼3 h of emission. Transmission electron microscopy analysis reveals that the number fractions of tarballs and the ratios of N and O relative to K, the latter a conserved tracer, increase with particle age and that the more-spherical particles on the substrates had higher ratios of N and O relative to K. Scanning transmission X-ray spectrometry and electron energy loss spectrometry analyses show that these chemical changes are accompanied by the formation of organic compounds that contain nitrogen and carboxylic acid. The results imply that the chemical changes increase the particle sphericity on the substrates, which correlates with particle surface tension and viscosity, and contribute to tarball formation during aging in BB smoke. These findings will enable models to better partition tarball contributions to BB radiative forcing and, in so doing, better help constrain radiative forcing models of BB events.

The Cooling Rate- and Volatility-Dependent Glass-Forming Properties of Organic Aerosols Measured by Broadband Dielectric Spectroscopy.

Glass transitions of secondary organic aerosols (SOA) from liquid/semisolid to solid phase states have important implications for aerosol reactivity, growth, and cloud formation properties. In the present study, glass transition temperatures (Tg) of isoprene SOA components, including isoprene hydroxy hydroperoxide (ISOPOOH), isoprene-derived epoxydiols (IEPOX), 2-methyltetrols, and 2-methyltetrol sulfates, were measured at atmospherically relevant cooling rates (2-10 K/min) by thin film broadband dielectric spectroscopy. The results indicate that 2-methyltetrol sulfates have the highest glass transition temperature, while ISOPOOH has the lowest glass transition temperature. By varying the cooling rate of the same compound from 2 to 10 K/min, the Tg of these compounds increased by 4-5 K. This temperature difference leads to a height difference of 400-800 m in the atmosphere for the corresponding updraft induced cooling rates, assuming a hygroscopicity value (κ) of 0.1 and relative humidity less than 95%. The Tg of the organic compounds was found to be strongly correlated with volatility, and a semiempirical formula between glass transition temperatures and volatility was derived. The Gordon-Taylor equation was applied to calculate the effect of relative humidity (RH) and water content at five mixing ratios on the Tg of organic aerosols. The model shows that Tg could drop by 15-40 K as the RH changes from <5 to 90%, whereas the mixing ratio of water in the particle increases from 0 to 0.5. These results underscore the importance of chemical composition, updraft rates, and water content (RH) in determining the phase states and hygroscopic properties of organic particles.

Extensive Soot Compaction by Cloud Processing from Laboratory and Field Observations.

Soot particles form during combustion of carbonaceous materials and impact climate and air quality. When freshly emitted, they are typically fractal-like aggregates. After atmospheric aging, they can act as cloud condensation nuclei, and water condensation or evaporation restructure them to more compact aggregates, affecting their optical, aerodynamic, and surface properties. Here we survey the morphology of ambient soot particles from various locations and different environmental and aging conditions. We used electron microscopy and show extensive soot compaction after cloud processing. We further performed laboratory experiments to simulate atmospheric cloud processing under controlled conditions. We find that soot particles sampled after evaporating the cloud droplets, are significantly more compact than freshly emitted and interstitial soot, confirming that cloud processing, not just exposure to high humidity, compacts soot. Our findings have implications for how the radiative, surface, and aerodynamic properties, and the fate of soot particles are represented in numerical models.

Aging Effects on Biomass Burning Aerosol Mass and Composition: A Critical Review of Field and Laboratory Studies.

Biomass burning is a major source of atmospheric particulate matter (PM) with impacts on health, climate, and air quality. The particles and vapors within biomass burning plumes undergo chemical and physical aging as they are transported downwind. Field measurements of the evolution of PM with plume age range from net decreases to net increases, with most showing little to no change. In contrast, laboratory studies tend to show significant mass increases on average. On the other hand, similar effects of aging on the average PM composition (e.g., oxygen-to-carbon ratio) are reported for lab and field studies. Currently, there is no consensus on the mechanisms that lead to these observed similarities and differences. This review summarizes available observations of aging-related biomass burning aerosol mass concentrations and composition markers, and discusses four broad hypotheses to explain variability within and between field and laboratory campaigns: (1) variability in emissions and chemistry, (2) differences in dilution/entrainment, (3) losses in chambers and lines, and (4) differences in the timing of the initial measurement, the baseline from which changes are estimated. We conclude with a concise set of research needs for advancing our understanding of the aging of biomass burning aerosol.

Open-path, closed-path, and reconstructed aerosol extinction at a rural site.

The Handix Scientific open-path cavity ringdown spectrometer (OPCRDS) was deployed during summer 2016 in Great Smoky Mountains National Park (GRSM). Extinction coefficients from the relatively new OPCRDS and from a more well-established extinction instrument agreed to within 7%. Aerosol hygroscopic growth (f(RH)) was calculated from the ratio of ambient extinction measured by the OPCRDS to dry extinction measured by a closed-path extinction monitor (Aerodyne's cavity-attenuated phase shift particulate matter extinction monitor [CAPS PMex]). Derived hygroscopicity (relative humidity [RH] < 95%) from this campaign agreed with data from 1995 at the same site and time of year, which is noteworthy given the decreasing trend for organics and sulfate in the eastern United States. However, maximum f(RH) values in 1995 were less than half as large as those recorded in 2016-possibly due to nephelometer truncation losses in 1995. Two hygroscopicity parameterizations were investigated using high-time-resolution OPCRDS+CAPS PMex data, and the κext model was more accurate than the gamma model. Data from the two ambient optical instruments, the OPCRDS and the open-path nephelometer, generally agreed; however, significant discrepancies between ambient scattering and extinction were observed, apparently driven by a combination of hygroscopic growth effects, which tend to increase nephelometer truncation losses and decrease sensitivity to the wavelength difference between the two instruments as a function of particle size. There was not a statistically significant difference in the mean reconstructed extinction values obtained from the original and the revised IMPROVE (Interagency Monitoring of Protected Visual Environments) equations. On average, IMPROVE reconstructed extinction was ~25% lower than extinction measured by the OPCRDS, which suggests that the IMPROVE equations and 24-hr aerosol data are moderately successful in estimating current haze levels at GRSM. However, this conclusion is limited by the coarse temporal resolution and the low dynamic range of the IMPROVE reconstructed extinction. IMPLICATIONS: Although light extinction, which is directly related to visibility, is not directly measured in U.S. National Parks, existing IMPROVE protocols can be used to accurately infer visibility for average humidity conditions, but during the large fraction of the year when humidity is above or below average, accuracy is reduced substantially. Furthermore, nephelometers, which are used to assess the accuracy of IMPROVE visibility estimates, may themselves be biased low when humidity is very high. Despite reductions in organic and sulfate particles since the 1990s, hygroscopicity, particles' affinity for water, appears unchanged, although this conclusion is weakened by the previously mentioned nephelometer limitations.

Chemical evolution of atmospheric organic carbon over multiple generations of oxidation.

The evolution of atmospheric organic carbon as it undergoes oxidation has a controlling influence on concentrations of key atmospheric species, including particulate matter, ozone and oxidants. However, full characterization of organic carbon over hours to days of atmospheric processing has been stymied by its extreme chemical complexity. Here we study the multigenerational oxidation of α-pinene in the laboratory, characterizing products with several state-of-the-art analytical techniques. Although quantification of some early generation products remains elusive, full carbon closure is achieved (within measurement uncertainty) by the end of the experiments. These results provide new insights into the effects of oxidation on organic carbon properties (volatility, oxidation state and reactivity) and the atmospheric lifecycle of organic carbon. Following an initial period characterized by functionalization reactions and particle growth, fragmentation reactions dominate, forming smaller species. After approximately one day of atmospheric aging, most carbon is sequestered in two long-lived reservoirs-volatile oxidized gases and low-volatility particulate matter.

Condensed-phase biogenic-anthropogenic interactions with implications for cold cloud formation.

Anthropogenic and biogenic gas emissions contribute to the formation of secondary organic aerosol (SOA). When present, soot particles from fossil fuel combustion can acquire a coating of SOA. We investigate SOA-soot biogenic-anthropogenic interactions and their impact on ice nucleation in relation to the particles' organic phase state. SOA particles were generated from the OH oxidation of naphthalene, α-pinene, longifolene, or isoprene, with or without the presence of sulfate or soot particles. Corresponding particle glass transition (Tg) and full deliquescence relative humidity (FDRH) were estimated using a numerical diffusion model. Longifolene SOA particles are solid-like and all biogenic SOA sulfate mixtures exhibit a core-shell configuration (i.e. a sulfate-rich core coated with SOA). Biogenic SOA with or without sulfate formed ice at conditions expected for homogeneous ice nucleation, in agreement with respective Tg and FDRH. α-pinene SOA coated soot particles nucleated ice above the homogeneous freezing temperature with soot acting as ice nuclei (IN). At lower temperatures the α-pinene SOA coating can be semisolid, inducing ice nucleation. Naphthalene SOA coated soot particles acted as ice nuclei above and below the homogeneous freezing limit, which can be explained by the presence of a highly viscous SOA phase. Our results suggest that biogenic SOA does not play a significant role in mixed-phase cloud formation and the presence of sulfate renders this even less likely. However, anthropogenic SOA may have an enhancing effect on cloud glaciation under mixed-phase and cirrus cloud conditions compared to biogenic SOA that dominate during pre-industrial times or in pristine areas.

Regional Influence of Aerosol Emissions from Wildfires Driven by Combustion Efficiency: Insights from the BBOP Campaign.

Wildfires are important contributors to atmospheric aerosols and a large source of emissions that impact regional air quality and global climate. In this study, the regional and nearfield influences of wildfire emissions on ambient aerosol concentration and chemical properties in the Pacific Northwest region of the United States were studied using real-time measurements from a fixed ground site located in Central Oregon at the Mt. Bachelor Observatory (∼2700 m a.s.l.) as well as near their sources using an aircraft. The regional characteristics of biomass burning aerosols were found to depend strongly on the modified combustion efficiency (MCE), an index of the combustion processes of a fire. Organic aerosol emissions had negative correlations with MCE, whereas the oxidation state of organic aerosol increased with MCE and plume aging. The relationships between the aerosol properties and MCE were consistent between fresh emissions (∼1 h old) and emissions sampled after atmospheric transport (6-45 h), suggesting that biomass burning organic aerosol concentration and chemical properties were strongly influenced by combustion processes at the source and conserved to a significant extent during regional transport. These results suggest that MCE can be a useful metric for describing aerosol properties of wildfire emissions and their impacts on regional air quality and global climate.

Enhanced light absorption by mixed source black and brown carbon particles in UK winter.

Black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC) play key roles in warming the atmosphere, but the magnitude of their effects remains highly uncertain. Theoretical modelling and laboratory experiments demonstrate that coatings on BC can enhance BC's light absorption, therefore many climate models simply assume enhanced BC absorption by a factor of ∼1.5. However, recent field observations show negligible absorption enhancement, implying models may overestimate BC's warming. Here we report direct evidence of substantial field-measured BC absorption enhancement, with the magnitude strongly depending on BC coating amount. Increases in BC coating result from a combination of changing sources and photochemical aging processes. When the influence of BrC is accounted for, observationally constrained model calculations of the BC absorption enhancement can be reconciled with the observations. We conclude that the influence of coatings on BC absorption should be treated as a source and regionally specific parameter in climate models.

Inkjet-printed gold nanoparticle surfaces for the detection of low molecular weight biomolecules by laser desorption/ionization mass spectrometry.

Effective detection of low molecular weight compounds in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) is often hindered by matrix interferences in the low m/z region of the mass spectrum. Here, we show that monolayer-protected gold nanoparticles (AuNPs) can serve as alternate matrices for the very sensitive detection of low molecular weight compounds such as amino acids. Amino acids can be detected at low fmol levels with minimal interferences by properly choosing the AuNP deposition method, density, size, and monolayer surface chemistry. By inkjet-printing AuNPs at various densities, we find that AuNP clusters are essential for obtaining the greatest sensitivity. Graphical Abstract ᅟ.

Ethylene glycol emissions from on-road vehicles.

Ethylene glycol (HOCH2CH2OH), used as engine coolant for most on-road vehicles, is an intermediate volatility organic compound (IVOC) with a high Henry's law coefficient. We present measurements of ethylene glycol (EG) vapor in the Caldecott Tunnel near San Francisco, using a proton transfer reaction mass spectrometer (PTR-MS). Ethylene glycol was detected at mass-to-charge ratio 45, usually interpreted as solely coming from acetaldehyde. EG concentrations in bore 1 of the Caldecott Tunnel, which has a 4% uphill grade, were characterized by infrequent (approximately once per day) events with concentrations exceeding 10 times the average concentration, likely from vehicles with malfunctioning engine coolant systems. Limited measurements in tunnels near Houston and Boston are not conclusive regarding the presence of EG in sampled air. Previous PTR-MS measurements in urban areas may have overestimated acetaldehyde concentrations at times due to this interference by ethylene glycol. Estimates of EG emission rates from the Caldecott Tunnel data are unrealistically high, suggesting that the Caldecott data are not representative of emissions on a national or global scale. EG emissions are potentially important because they can lead to the formation of secondary organic aerosol following oxidation in the atmospheric aqueous phase.

Chemical compositions of black carbon particle cores and coatings via soot particle aerosol mass spectrometry with photoionization and electron ionization.

Black carbon is an important constituent of atmospheric aerosol particle matter (PM) with significant effects on the global radiation budget and on human health. The soot particle aerosol mass spectrometer (SP-AMS) has been developed and deployed for real-time ambient measurements of refractory carbon particles. In the SP-AMS, black carbon or metallic particles are vaporized through absorption of 1064 nm light from a CW Nd:YAG laser. This scheme allows for continuous "soft" vaporization of both core and coating materials. The main focus of this work is to characterize the extent to which this vaporization scheme provides enhanced chemical composition information about aerosol particles. This information is difficult to extract from standard SP-AMS mass spectra because they are complicated by extensive fragmentation from the harsh 70 eV EI ionization scheme that is typically used in these instruments. Thus, in this work synchotron-generated vacuum ultraviolet (VUV) light in the 8-14 eV range is used to measure VUV-SP-AMS spectra with minimal fragmentation. VUV-SP-AMS spectra of commercially available carbon black, fullerene black, and laboratory generated flame soots were obtained. Small carbon cluster cations (C(+)-C5(+)) were found to dominate the VUV-SP-AMS spectra of all the samples, indicating that the corresponding neutral clusters are key products of the SP vaporization process. Intercomparisons of carbon cluster ratios observed in VUV-SP-AMS and SP-AMS spectra are used to confirm spectral features that could be used to distinguish between different types of refractory carbon particles. VUV-SP-AMS spectra of oxidized organic species adsorbed on absorbing cores are also examined and found to display less thermally induced decomposition and fragmentation than spectra obtained with thermal vaporization at 200 °C (the minimum temperature needed to quantitatively vaporize ambient oxidized organic aerosol with a continuously heated surface). The particle cores tested in these studies include black carbon, silver, gold, and platinum nanoparticles. These results demonstrate that SP vaporization is capable of providing enhanced organic chemical composition information for a wide range of organic coating materials and IR absorbing particle cores. The potential of using this technique to study organic species of interest in seeded laboratory chamber or flow reactor studies is discussed.

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.