Optical Properties of Individual Tar Balls in the Free Troposphere.

Tar balls are brown carbonaceous particles that are highly viscous, spherical, amorphous, and light absorbing. They are believed to form in biomass burning smoke plumes during transport in the troposphere. Tar balls are also believed to have a significant impact on the Earth's radiative balance, but due to poorly characterized optical properties, this impact is highly uncertain. Here, we used two nighttime samples to investigate the chemical composition and optical properties of individual tar balls transported in the free troposphere to the Climate Observatory "Ottavio Vittori" on Mt. Cimone, Italy, using multimodal microspectroscopy. In our two samples, tar balls contributed 50% of carbonaceous particles by number. Of those tar balls, 16% were inhomogeneously mixed with other constituents. Using electron energy loss spectroscopy, we retrieved the complex refractive index (RI) for a wavelength range from 200 to 1200 nm for both inhomogeneously and homogeneously mixed tar balls. We found no significant difference in the average RI of inhomogeneously and homogeneously mixed tar balls (1.40-0.03i and 1.36-0.03i at 550 nm, respectively). Furthermore, we estimated the top of the atmosphere radiative forcing using the Santa Barbara DISORT Atmospheric Radiative Transfer model and found that a layer of only tar balls with an optical depth of 0.1 above vegetation would exert a positive radiative forcing ranging from 2.8 W m-2 (on a clear sky day) to 9.5 W m-2 (when clouds are below the aerosol layer). Understanding the optical properties of tar balls can help reduce uncertainties associated with the contribution of biomass-burning aerosol in current climate models.

Cloud condensation nuclei activity of internally mixed particle populations at a remote marine free troposphere site in the North Atlantic Ocean.

This study reports results from research conducted at the Observatory of Mount Pico (OMP), 2225 m above mean sea level on Pico Island in the Azores archipelago in June and July 2017. We investigated the chemical composition, mixing state, and cloud condensation nuclei (CCN) activities of long-range transported free tropospheric (FT) particles. FLEXible PARTicle Lagrangian particle dispersion model (FLEXPART) simulations reveal that most air masses that arrived at the OMP during the sampling period originated in North America and were highly aged (average plume age > 10 days). We probed size-resolved chemical composition, mixing state, and hygroscopicity parameter (κ) of individual particles using computer-controlled scanning electron microscopy with an energy-dispersive X-ray spectrometer (CCSEM-EDX). Based on the estimated individual particle mass from elemental composition, we calculated the mixing state index, χ. During our study, FT particle populations were internally mixed (χ of samples are between 53 % and 87 %), owing to the long atmospheric aging time. We used data from a miniature Cloud Condensation Nucleus Counter (miniCCNC) to derive the hygroscopicity parameter, κCCNC. Combining κCCNC and FLEXPART, we found that air masses recirculated above the North Atlantic Ocean with lower mean altitude had higher κCCNC due to the higher contribution of sea salt particles. We used CCSEM-EDX and phase state measurements to predict single-particle κ (κCCSEM-EDX) values, which overlap with the lower range of κCCNC measured below 0.15 % SS. Therefore, CCSEM-EDX measurements can be useful in predicting the lower bound of κ, which can be used in climate models to predict CCN activities, especially in remote locations where online CCN measurements are unavailable.

Black carbon aerosol number and mass concentration measurements by picosecond short-range elastic backscatter lidar.

Black carbon aerosol emissions are recognized as contributors to global warming and air pollution. There remains, however, a lack of techniques to remotely measure black carbon aerosol particles with high range and time resolution. This article presents a direct and contact-free remote technique to estimate the black carbon aerosol number and mass concentration at a few meters from the emission source. This is done using the Colibri instrument based on a novel technique, referred to here as Picosecond Short-Range Elastic Backscatter Lidar (PSR-EBL). To address the complexity of retrieving lidar products at short measurement ranges, we apply a forward inversion method featuring radiometric lidar calibration. Our method is based on an extension of a well-established light-scattering model, the Rayleigh-Debye-Gans for Fractal-Aggregates (RDG-FA) theory, which computes an analytical expression of lidar parameters. These parameters are the backscattering cross-sections and the lidar ratio for black carbon fractal aggregates. Using a small-scale Jet A-1 kerosene pool fire, we demonstrate the ability of the technique to quantify the aerosol number and mass concentration with centimetre range-resolution and millisecond time-resolution.

A filter-based Raman spectrometer for non-invasive imaging of atmospheric water vapor.

A new instrument was designed and developed to map the spatial distribution of water vapor concentration in the atmosphere. The high spatial resolution, sensitivity, and accuracy of the instrument enable new studies of the role of turbulence on clouds and aerosols in small-scale laboratory environments. The instrument exploits Raman scattering in a multi-pass laser configuration by using a set of narrow bandpass filters and a pair of charge coupled device imaging cameras in the 90° scattering geometry. The absolute concentration of water vapor was inferred from measured ratios of H2O and N2 vibrational Raman transitions. We have measured the number densities of water molecules in the atmosphere as low as 3.5 × 1017 cm-3, with an accuracy better than 20% and as high as 7.0 × 1017 cm-3 during minutes long observations. These measurements were taken within an imaging region 6 cm in diameter, with a per-pixel resolution 2.60 mm wide by 0.16 mm tall and 1 mm deep.

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.

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.

Near-infrared incoherent broadband cavity enhanced absorption spectroscopy (NIR-IBBCEAS) for detection and quantification of natural gas components.

The principle of near-infrared incoherent broadband cavity enhanced absorption spectroscopy was employed to develop a novel instrument for detecting natural gas leaks as well as for testing the quality of natural gas mixtures. The instrument utilizes the absorption features of methane, butane, ethane, and propane in the wavelength region of 1100 nm to 1250 nm. The absorption cross-section spectrum in this region for methane was adopted from the HITRAN database, and those for the other three gases were measured in the laboratory. A singular-value decomposition (SVD) based analysis scheme was employed for quantifying methane, butane, ethane, and propane by performing a linear least-square fit. The developed instrument achieved a detection limit of 460 ppm, 141 ppm, 175 ppm and 173 ppm for methane, butane, ethane, and propane, respectively, with a measurement time of 1 second and a cavity length of 0.59 m. These detection limits are less than 1% of the Lower Explosive Limit (LEL) for each gas. The sensitivity can be further enhanced by changing the experimental parameters (such as cavity length, lamp power etc.) and using longer averaging intervals. The detection system is a low-cost and portable instrument suitable for performing field monitorings. The results obtained on the gas mixture emphasize the instrument's potential for deployment at industrial facilities dealing with natural gas, where potential leaks pose a threat to public safety.

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.

Soot superaggregates from flaming wildfires and their direct radiative forcing.

Wildfires contribute significantly to global soot emissions, yet their aerosol formation mechanisms and resulting particle properties are poorly understood and parameterized in climate models. The conventional view holds that soot is formed via the cluster-dilute aggregation mechanism in wildfires and emitted as aggregates with fractal dimension Df ≈ 1.8 mobility diameter Dm ≤ 1 µm, and aerodynamic diameter Da ≤ 300 nm. Here we report the ubiquitous presence of soot superaggregates (SAs) in the outflow from a major wildfire in India. SAs are porous, low-density aggregates of cluster-dilute aggregates with characteristic Df ≈ 2.6, Dm > 1 µm, and Da ≤ 300 nm that form via the cluster-dense aggregation mechanism. We present additional observations of soot SAs in wildfire smoke-laden air masses over Northern California, New Mexico, and Mexico City. At 550 nm wavelength, [corrected] we estimate that SAs contribute, per unit optical depth, up to 35% less atmospheric warming than freshly-emitted (D(f) ≈ 1.8) [corrected] aggregates, and ≈90% more warming than the volume-equivalent spherical soot particles simulated in climate models.

Beam characteristics of fiber-based supercontinuum light sources with mirror- and lens-based beam collimators.

Commercially available supercontinuum light sources that cover most of the solar spectrum are well suited for instrumentation, where a well-collimated beam with wide spectral coverage is needed. Typically, the optical power is emitted from a single-mode photonic-crystal fiber and the output can either be collimated using a proprietary, permanently integrated, lens-based collimator or with a customer-provided, off-axis parabolic mirror. Here, we evaluate both approaches and conclude that, superior beam quality and collimation over the whole spectral range can be obtained with an off-axis parabolic mirror, however at the price of a more complex and bulky system requiring additional user alignment.

Effect of traffic and driving characteristics on morphology of atmospheric soot particles at freeway on-ramps.

Vehicles represent a major source of soot in urban environments. Knowledge of the morphology and mixing of soot particles is fundamental to understand their potential health and climatic impacts. We investigate 5738 single particles collected at six different cloverleaf freeway on-ramps in Southern Michigan, using 2D images from scanning electron microscopy. Of those, 3364 particles are soot. We present an analysis of the morphological and mixing properties of those soot particles. The relative abundance of soot particles shows a positive association with traffic density (number of vehicles per minute). A classification of the mixing state of freshly emitted soot particles shows that most of them are bare (or thinly coated) (72%) and some are partly coated (22%). We find that the fractal dimension of soot particles (one of the most relevant morphological descriptors) varies from site to site, and increases with increasing vehicle specific power that represents the driving/engine load conditions, and with increasing percentage of vehicles older than 15 years. Our results suggest that driving conditions, and vehicle age and type have significant influence on the morphology of soot particles.

Morphology and mixing state of individual freshly emitted wildfire carbonaceous particles.

Biomass burning is one of the largest sources of carbonaceous aerosols in the atmosphere, significantly affecting earth's radiation budget and climate. Tar balls, abundant in biomass burning smoke, absorb sunlight and have highly variable optical properties, typically not accounted for in climate models. Here we analyse single biomass burning particles from the Las Conchas fire (New Mexico, 2011) using electron microscopy. We show that the relative abundance of tar balls (80%) is 10 times greater than soot particles (8%). We also report two distinct types of tar balls; one less oxidized than the other. Furthermore, the mixing of soot particles with other material affects their optical, chemical and physical properties. We quantify the morphology of soot particles and classify them into four categories: ~50% are embedded (heavily coated), ~34% are partly coated, ~12% have inclusions and~4% are bare. Inclusion of these observations should improve climate model performances.

The In-Plume Emission Test Stand: an instrument platform for the real-time characterization of fuel-based combustion emissions.

The In-Plume Emission Test Stand (IPETS) characterizes gaseous and particulate matter (PM) emissions from combustion sources in real time. Carbon dioxide (CO2), carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO2), and other gases are quantified with a closed-path Fourier transform infrared spectrometer (FTIR). Particle concentrations, chemical composition, and other particle properties are characterized with an electrical low-pressure impactor (ELPI), a light-scattering particle detector, an optical particle counter, and filter samples amenable to different laboratory analysis. IPETS measurements of fuel-based emission factors for a diesel generator are compared with those from a Mobile Emissions Laboratory (MEL). IPETS emission factors ranged from 0.3 to 11.8, 0.2 to 3.7, and 22.2 to 32.8 g/kg fuel for CO, NO2, and NO, respectively. IPETS PM emission factors ranged from 0.4 to 1.4, 0.3 to 1.8, 0.3 to 2.2, and 1 to 3.4 g/kg fuel for filter, photoacoustic, nephelometer, and impactor measurements, respectively. Observed linear regression statistics for IPETS versus MEL concentrations were as follows: CO slope = 1.1, r2 = 0.99; NO slope = 1.1, r2 = 0.92; and NO2 slope = 0.8, r2 = 0.96. IPETS versus MEL PM regression statistics were: filter slope = 1.3, r2 = 0.80; ELPI slope = 1.7, r2 = 0.87; light-scattering slope = 2.7, r2 = 0.92; and photoacoustic slope = 2.1, r2 = 0.91. Lower temperatures in the dilution air (approximately 25 degrees C for IPETS vs. approximately 50 degrees C for MEL) may result in greater condensation of semi-volatile compounds on existing particles, thereby explaining the 30% difference for filters. The other PM measurement devices are highly correlated with the filter, but their factory-default PM calibration factors do not represent the size and optical properties of diesel exhaust. They must be normalized to a simultaneous filter measurement.

A case study of real-world tailpipe emissions for school buses using a 20% biodiesel blend.

Numerous laboratory studies report carbon monoxide, hydrocarbon, and particulate matter emission reductions with a slight nitrogen oxides emission increase from engines operating with biodiesel and biodiesel blends as compared to using petroleum diesel. We conducted a field study on a fleet of school buses to evaluate the effects of biodiesel use on gaseous and particulate matter fuel-based emission factors under real-world conditions. The field experiment was carried out in two phases during winter 2004. In January (phase I), emissions from approximately 200 school buses operating on petroleum diesel were measured. Immediately after the end of the first phase measurement period, the buses were switched to a 20% biodiesel blend. Emission factors were measured again in March 2004 (phase II) and compared with the January emission factors. To measure gaseous emission factors we used a commercial gaseous remote sensor. Particulate matter emission factors were determined with a combination of the gaseous remote sensor, a Lidar (light detection and ranging), and transmissometer system developed at the Desert Research Institute of Reno, NV, U.S.A. Particulate matter emissions from school buses significantly increased (up to a factor of 1.8) after the switch from petroleum diesel to a 20% biodiesel blend. The fuel used during this campaign was provided by a local distributor and was independently analyzed at the end of the on-road experiment. The analysis found high concentrations of free glycerin and reduced flash points in the B 100 parent fuel. Both measures indicate improper separation and processing of the biodiesel product during production. The biodiesel fuels used in the school buses were not in compliance with the U.S.A. ASTM D6751 biodiesel standard that was finalized in December of 2001. The U.S.A. National Biodiesel Board has formed a voluntary National Biodiesel Accreditation Program for producers and marketers of biodiesel to ensure product quality and compliance with the ASTM standard. The results of our study underline the importance of the program since potential emission benefits from biodiesel may be reduced or even reversed without appropriate fuel quality control on real-world fuels.

On-road vehicle particulate matter and gaseous emission distributions in Las Vegas, Nevada, compared with other areas.

During the spring and summer of 2000, 2001, and 2002, gaseous and particulate matter (PM) fuel-based emission factors for approximately 150,000 low-tailpipe, individual vehicles in the Las Vegas, NV, area were measured via on-road remote sensing. For the gaseous pollutants (carbon monoxide, hydrocarbons, and nitrogen oxide), a commercial vehicle emissions remote sensing system (VERSS) was used. The PM emissions were determined using a Lidar-based VERSS. Emission distributions and their shapes were analyzed and compared with previous studies. The large skewness of the distributions is evident for both gaseous pollutants and PM and has important implications for emission reduction policies, because the majority of emissions are attributed to a small fraction of vehicles. Results of this Las Vegas study and studies at other geographical locations were compared. The gaseous pollutants were found to be close to those measured by VERSS in other U.S. cities. The PM emission factors for spark ignition and diesel vehicles are in the range of previous tunnel and dynamometer studies.

Remote sensing of PM, NO, CO and HC emission factors for on-road gasoline and diesel engine vehicles in Las Vegas, NV.

A novel light detection and ranging-based remote sensing system was assembled and used to measure mass particulate matter (PM) emissions per unit of fuel burned from in-use on-road vehicles. A commercially available remote sensing system was concurrently used to measure emissions of carbon monoxide (CO), nitrogen oxide (NO) and hydrocarbons (HC). The two systems were used to measure 61,207 gasoline and 1180 diesel powered vehicle emissions in Las Vegas, NV from 4/4/2000 to 5/16/2002. Emission factors were related to vehicle age, weight class and fuel type by matching license IDs to the state registration data. Measurements of vehicle speed and acceleration permitted the analysis of emission factors by vehicle specific power (VSP). Average emission factors were calculated for light-duty (<3863 kg [8500 lbs]) gasoline vehicles (LDGV), light-duty diesel vehicles (LDDV), heavy-duty (>3863 kg [8500 lbs]) gasoline vehicles (HDGV) and heavy-duty diesel vehicles (HDDV). LDDV and HDDV emitted approximately 25 times more PM per mass of fuel than LDGV and HDGV. Sufficient numbers of LDGV were measured to relate VSP with CO, HC and NO emissions. No relationship was observed between PM emissions and VSP. PM emission factors from LDGV increased with vehicle age. Fuel-based emission factors measured by remote sensing were compared with MOBILE6 and PART5 emissions model factors. Good agreement was observed for HC emission factors for vehicles less than 20 years old. MOBILE6 CO emission factors were approximately 2 times greater than measured CO emission factors for vehicles less than 13 years old. Measured NO emission factors were approximately 50% greater than MOBILE6 factors for vehicles 7-15 years old but in good agreement for vehicles less than 7 years old. Measured PM emission factors showed a clear increase with vehicle age, however, PART5 uses only a single PM emission factor for LDGV less than 18 years old. The PM emission factors for the fleet of LDGV, HDGV, LDDV and HDDV were 0.06, 0.05, 1.6 and 1.5 g/kg, respectively.

On-road measurement of automotive particle emissions by ultraviolet lidar and transmissometer: instrument.

A novel vehicle emissions remote sensing system (VERSS) for the on-road measurement of fuel-based particulate matter (PM) emission factors is described. This system utilizes two complementary PM channels using an ultraviolet Lidar and transmissometer for the measurement of PM mass column content behind a passing vehicle. Ratioing the PM mass column content with the carbon mass column content, simultaneously measured with infrared absorption, yields the fuel-based PM mass emission factor. The transmissometer directly yields PM extinction coefficients without calibration, while the Lidar measurement of PM backscatter coefficients is calibrated through laboratory measurements of gases with well-known backscatter coefficients. The PM mass column content is calculated from these extinction and backscatter coefficients with the help of mass backscatter and extinction efficiencies obtained from theoretical calculations. This novel VERSS has been used extensively in a major air quality study, and example data are presented.