Comparative performance of a thermal denuder and a catalytic stripper in sampling laboratory and marine exhaust aerosols

The performance of a thermal denuder (thermodenuder—TD) and a fresh catalytic stripper (CS) was assessed by sampling laboratory aerosol, produced by different combinations of sulfuric acid, octacosane, and soot particles, and marine exhaust aerosol produced by a medium-speed marine engine using high sulfur fuels. The intention was to study the efficiency in separating non-volatile particles. No particles could be detected downstream of either device when challenged with neat octacosane particles at high concentration. Both laboratory and marine exhaust aerosol measurements showed that sub-23 nm semi-volatile particles are formed downstream of the thermodenuder when upstream sulfuric acid approached 100 ppbv. Charge measurements revealed that these are formed by re-nucleation rather than incomplete evaporation of upstream aerosol. Sufficient dilution to control upstream sulfates concentration and moderate TD operation temperature (250°C) are both required to eliminate their formation. Use of the CS following an evaporation tube seemed to eliminate the risk for particle re-nucleation, even at a ten-fold higher concentration of semi-volatiles than in case of the TD. Particles detected downstream of the CS due to incomplete evaporation of sulfuric acid and octacosane aerosol, did not exceed 0.01% of upstream concentration. Despite the superior performance of CS in separating non-volatile particles, the TD may still be useful in cases where increased sensitivity over the traditional evaporation tube method is needed and where high sulfur exhaust concentration may fast deplete the catalytic stripper adsorption capacity.

Copyright © 2018 American Association for Aerosol Research     

Investigation of vehicle exhaust sub-23 nm particle emissions

A solid particle number limit was applied to the European legislation for diesel vehicles in 2011. Extension to gasoline direct injection vehicles raised concerns because many studies found particles below the lower size limit of the method (23 nm). Here we investigated experimentally the feasibility of lowering this size. A nano condensation nucleus counter system (nCNC) (d_50% = 1.3 nm) was used in parallel with condensation particle counters (CPCs) (d_50% = 3 nm, 10 nm and 23 nm) at various sampling systems based on ejector or rotating disk diluters and having thermal pre-treatment systems consisting of evaporation tubes or catalytic strippers. An engine exhaust particle sizer (EEPS) measured the particle size distributions. Depending on the losses and thermal pre-treatment of the sampling system, differences of up to 150% could be seen on the final detected particle concentrations when including the particles smaller than 23 nm in diameter. A volatile artefact as particles with diameters below 10 nm was at times observed during the cold start measurements of a 2-stroke moped. The diesel vehicles equipped with the Diesel Particulate Filter (DPF) had a low solid sub-23 nm particles fraction (<20%), the gasoline with direct injection vehicles had higher (35–50%), the gasoline vehicles with port fuel injection and the two mopeds (two and four-stroke) had the majority of particles below 23 nm. The size distributions peaked at 60–80 nm for the DPF equipped vehicles, at 40–90 nm for the gasoline vehicles with a separate nucleation mode peak at approximately 10 nm sometimes. Mopeds peaked at sizes below 50 nm when their aerosol was thermally pre-treated.

© 2017 American Association for Aerosol Research     

Ambient aerosol composition by infrared spectroscopy and partial least-squares in the chemical speciation network: Organic carbon with functional group identification

The Fourier-transform infrared (FT-IR) spectra of ambient fine aerosols were used with partial least-squares (PLS) regression to accurately, inexpensively, and nondestructively predict organic carbon (OC) on polytetrafluoroethylene (PTFE) filters in the U.S. Environmental Protection Agencies' Chemical Speciation Network (CSN). Recently, a similar FT-IR method was used for OC determination in the rural United States Interagency Monitoring of PROtected Visual Environments network, with the present work extending the method to urban aerosols with low mass loadings. In the present study, FT-IR spectra were calibrated to collocated thermal/optical reflectance (TOR) OC measurements following numerical processing with a second derivative filter, backward Monte Carlo unimportant variable elimination, and a quadratic discriminant analysis-PLS vapor correction routine. After processing and vapor correcting spectra, the number of model components (latent variables) were reduced from thirty-five to three with only the first PLS component patently predicting OC. The two lesser components modeled PTFE and inorganic interference remaining in the spectra. A wavenumber ranking procedure— using both the variable importance in projection and bootstrapped confidence intervals— underscored the primacy of aliphatic C-H stretches and carbonyl vibrations for OC prediction. Aliphatic deformations, amines, organonitrate, carboxylate, and aromatic vibrations were also valuable for OC quantification. This study demonstrates that PLS models quantifying TOR OC are explicable in terms of organic functional group absorption after judiciously processing FT-IR spectra.

Copyright © 2016 American Association for Aerosol Research     

Fractal scaling of soot packing density across five size decades

We experimentally map the scaling laws for packing density (θ_f, solid component volume fraction) of soot aggregates across five orders of magnitude of size (R_g/a, normalized radius of gyration by monomer radius). The θ_f ? R_g/a scaling relationship evolves through three successive regimes with distinct power-law exponents of ?1.20 ± 0.01, ?0.58 ± 0.06, and ?1.31 ± 0.14. The first cross-over agrees with the classical aerosol-to-gel transition theory. This agreement, however, breaks down at the second cross-over point, where a late-stage cluster-cluster aggregation of aerosol gels takes over.

© 2017 American Association for Aerosol Research     

Volatility characterization of nanoparticles from single and dual-fuel low temperature combustion in compression ignition engines

This work explores the volatility of particles produced from two diesel low temperature combustion (LTC) modes proposed for high-efficiency compression ignition engines. It also explores mechanisms of particulate formation and growth upon dilution in the near-tailpipe environment. The number distribution of exhaust particles from low- and mid-load dual-fuel reactivity controlled compression ignition (RCCI) and single-fuel premixed charge compression ignition (PPCI) modes were experimentally studied over a gradient of dilution temperature. Particle volatility of select particle diameters was investigated using volatility tandem differential mobility analysis (V-TDMA). Evaporation rates for exhaust particles were compared with V-TDMA results for candidate pure n-alkanes to identify species with similar volatility characteristics. The results show that LTC particles are mostly comprised of material with volatility similar to engine oil alkanes. V-TDMA results were used as inputs to an aerosol condensation and evaporation model to support the finding that smaller particles in the distribution are comprised of lower volatility material than large particles under primary dilution conditions. Although our results show that saturation levels are high enough to drive condensation of alkanes onto existing particles under the dilution conditions investigated, they are not high enough to allow homogeneous nucleation of these same compounds in the primary exhaust plume. Therefore, we conclude that observed particles from LTC operation must grow from low concentrations of highly nonvolatile compounds present in the exhaust.

Copyright © 2016 American Association for Aerosol Research     

Characterization of a new miniCAST with diffusion flame and premixed flame options: Generation of particles with high EC content in the size range 30 nm to 200 nm

Flame-generated soot from miniCAST burners is increasingly being used in academia and industry as engine exhaust soot surrogate for atmospheric studies and instrument calibration. Previous studies have found that elemental carbon (EC) content of miniCAST soot is proportional to the mean particle size. Here, the characterization of a prototype miniCAST generator (5201 Type BC), which was designed to decouple the soot composition from the particle size and produce soot particles with high EC and BC content in a large size range, is reported. This prototype may operate either in a diffusion-flame or a partially premixed-flame mode, an option that was not available in former models. It was confirmed that soot properties, such as EC content and Ångström absorption exponent (AAE), were linked to the overall flame composition. In particular, combustion under fuel-rich conditions provided particles with size coupled to the EC fraction and AAE, i.e. smaller particles exhibited a lower EC fraction and higher AAE. In contrast, with fuel-lean diffusion flames and especially with premixed flames under near overall stoichiometric conditions small particles (down to 30?nm) with high EC/TC ratios (>60%) and low AAE (≈1.4) could be generated even without any thermal after-treatment. This new source might thus serve in the future as a useful surrogate for engine exhaust emissions and help to improve calibration procedures of common aerosol instruments.

Copyright © 2018 The Author(s). Published with license by Taylor & Francis Group, LLC     

Chemistry of hydroperoxycarbonyls in secondary organic aerosol

Abstract

Highly oxidized multifunctional compounds (HOMs) formed through gas-phase reactions are thought to account for a significant fraction of the secondary organic aerosol (SOA) formed in low-nitric oxide (NO) environments. HOMs are known to be peroxide-rich and unstable in SOA, however, and their fate once they partition into particles is not well understood. In the study reported here, we identified particle-phase reactions and decomposition products for an α-alkoxy hydroperoxyaldehyde that served as a convenient model for HOMs, and also quantified rate and equilibrium constants for cyclic peroxyhemiacetal formation and the effects of particle acidity and relative humidity on reaction products and timescales for decomposition of peroxide-containing compounds. Sulfuric acid increased the rate of acetal formation and subsequent peroxide decomposition, but the effect was eliminated when aqueous seed particles were used in humid air, indicating that organic/aqueous phase separation can affect the ability of strong acids to catalyze these and other reactions in SOA. The results will be useful for understanding and predicting the atmospheric fate of organic peroxides and the effects of their particle-phase reactions on SOA composition.

Copyright © 2018 American Association for Aerosol Research     

Hydroxyl radical formation and soluble trace metal content in particulate matter from renewable diesel and ultra low sulfur diesel in at-sea operations of a research vessel

Reactive oxygen species, including hydroxyl radicals generated by particles, play a role in both aerosol aging and PM2.5 mediated health effects. We assess the impacts of switching marine vessels from conventional diesel to renewable fuel on the ability of particles to generate hydroxyl radical when extracted in a simulated lung lining fluid or in water at pH 3.5, for samples of engine emissions from a research vessel when operating on ultra-low sulfur diesel (ULSD) and hydrogenation-derived renewable diesel (HDRD). Samples were collected during dedicated cruises in 2014 and 2015, including aged samples collected by re-intercepting the ship plume. After normalizing to particle mass, particles generated from HDRD combustion had slightly to significantly (5–50%) higher OH generation activity than those from ULSD, a difference that was statistically significant for some permutations of year/fuel/engine speed. Water soluble trace metal concentrations and fuel metal concentrations were similar, and compared to urban Los Angeles samples lower in soluble iron and manganese, but similar for most other trace metals. Because PM mass emissions were higher for HDRD, normalizing to fuel increased this difference. Freshly emitted PM had lower activity than the “plume chase” samples, and samples collected on the ship had lower activity than the urban reference. The differences in OH production correlated reasonably well with redox-active transition metals, most strongly with soluble manganese, with roles for vanadium and likely copper and iron. The results also suggest that atmospheric processing of fresh combustion particles rapidly increases metal solubility, which in turn increases OH production.

Copyright © 2017 American Association for Aerosol Research     

Application of a FIGAERO ToF CIMS for on-line characterization of real-world fresh and aged particle emissions from buses

On-line chemical characterization of real-world particle emissions from 13 transit buses was performed using a chemical ionization mass spectrometer (CIMS) equipped with a filter inlet for gases and aerosols (FIGAERO). In addition to the fresh emissions the emissions were artificially aged using a potential aerosol mass reactor (Go:PAM). The buses studied were running on different fuel types (diesel, compressed natural gas, and rapeseed methyl ester) and exhaust after-treatment systems (selective catalytic reduction (SCR), exhaust gas recirculation (EGR), and a three-way catalyst). When evaluating emissions from passing exhaust plumes using the FIGAERO ToF-CIMS, two technical features were highlighted from this work, the use of high mass calibrants and the factor enhancement method to be able to filter important compounds from mass spectra including hundreds of species. Here, acetate was used as the reagent ion to enable detection of highly oxygenated species in the exhaust particle emissions with potential high toxicity and/or secondary organic aerosol formation (SOA) potential. The acetate ionization scheme accounted for 4% to 46% of the total emitted particulate mass through identification of 61 species in the spectra. For aged emission the various fuel types provided overlapping species that could explain up to 19% of the aged emissions. This is hypothesized to come from the oxidation of engine lubrication oil, thus a common source for various fuels which was further supported by laboratory measurements. Specific markers from the SCR technology, such as urea oxidation products and further byproducts from hydrolysis were identified and attributed to reactions of isocyanic acid.

Copyright © 2019 The Author(s). Published with license by Taylor &; Francis Group, LLC     

Identification and quantification of oxidized organic aerosol compounds using derivatization,liquid chromatography,and chemical ionization mass spectrometry

A systematic approach for identifying and quantifying molecular components of complex organic aerosol mixtures is presented. The approach combines methods developed previously for derivatizing carbonyl, hydroxyl, carboxyl, and ester functional groups, which are commonly present in oxidized organic aerosol, with liquid chromatography, UV detection, and chemical ionization-ion trap mass spectrometry. The original derivatization-spectrophotometric methods were modified for compatibility with liquid chromatography and then evaluated by analyzing a variety of standard compounds that contain one or more functional groups. Detection limits for carbonyl, hydroxyl, carboxyl, and ester analysis are approximately 0.003, 0.02, 0.01, and 1 nmole, respectively. Mass spectral analysis of derivatives using isobutane and ammonia as reagent gases for chemical ionization can be used to determine compound molecular weight, and characteristic fragmentation patterns provide structural information for use in compound identification. The methods will be useful for analyzing the chemical composition of secondary organic aerosol (SOA) formed in laboratory studies to obtain information needed to develop quantitative reaction mechanisms that can be incorporated into atmospheric models to better predict the formation, composition, and fate of SOA.

Copyright © 2017 American Association for Aerosol Research