Timothy B. Onasch

Evolution of Organic Aerosols in the Atmosphere

Framework for Change Organic aerosols make up 20 to 90% of the particulate mass of the troposphere and are important factors in both climate and human heath. However, their sources and removal pathways are very uncertain, and their atmospheric evolution is poorly characterized. Jimenez et al. (p. 1525; see the Perspective by Andreae) present an integrated framework of organic aerosol compositional evolution in the atmosphere, based on model results and field and laboratory data that simulate the dynamic aging behavior of organic aerosols. Particles become more oxidized, more hygroscopic, and less volatile with age, as they become oxygenated organic aerosols. These results should lead to better predictions of climate and air quality. Organic aerosols are not compositionally static, but they evolve dramatically within hours to days of their formation. Organic aerosol (OA) particles affect climate forcing and human health, but their sources and evolution remain poorly characterized. We present a unifying model framework describing the atmospheric evolution of OA that is constrained by high–time-resolution measurements of its composition, volatility, and oxidation state. OA and OA precursor gases evolve by becoming increasingly oxidized, less volatile, and more hygroscopic, leading to the formation of oxygenated organic aerosol (OOA), with concentrations comparable to those of sulfate aerosol throughout the Northern Hemisphere. Our model framework captures the dynamic aging behavior observed in both the atmosphere and laboratory: It can serve as a basis for improving parameterizations in regional and global models.

Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes

[1] Organic aerosol (OA) data acquired by the Aerosol Mass Spectrometer (AMS) in 37 field campaigns were deconvolved into hydrocarbon-like OA (HOA) and several types of oxygenated OA (OOA) components. HOA has been linked to primary combustion emissions (mainly from fossil fuel) and other primary sources such as meat cooking. OOA is ubiquitous in various atmospheric environments, on average accounting for 64%, 83% and 95% of the total OA in urban, urban downwind, and rural/remote sites, respectively. A case study analysis of a rural site shows that the OOA concentration is much greater than the advected HOA, indicating that HOA oxidation is not an important source of OOA, and that OOA increases are mainly due to SOA. Most global models lack an explicit representation of SOA which may lead to significant biases in the magnitude, spatial and temporal distributions of OA, and in aerosol hygroscopic properties.

Radiative absorption enhancements due to the mixing state of atmospheric black carbon

Dark Forcing Soot, or black carbon, is a ubiquitous atmospheric pollutant whose warming effect might be second only to carbon dioxide. When black carbon is emitted, it combines with other aerosols to form heterogeneous mixtures. Models have predicted that internal mixing of black carbon with other materials can double the amount of radiation absorbed. Cappa et al. (p. 1078) report that in situ measurements of the enhancement of radiation absorption by these mixed black carbon–containing particles in the atmosphere show a much smaller effect. Thus, many climate models may be overestimating the amount of warming caused by black carbon emissions. Direct measurements show that ambient atmospheric particulate black carbon absorbs less solar radiation than theory suggested. Atmospheric black carbon (BC) warms Earth’s climate, and its reduction has been targeted for near-term climate change mitigation. Models that include forcing by BC assume internal mixing with non-BC aerosol components that enhance BC absorption, often by a factor of ~2; such model estimates have yet to be clearly validated through atmospheric observations. Here, direct in situ measurements of BC absorption enhancements (Eabs) and mixing state are reported for two California regions. The observed Eabs is small—6% on average at 532 nm—and increases weakly with photochemical aging. The Eabs is less than predicted from observationally constrained theoretical calculations, suggesting that many climate models may overestimate warming by BC. These ambient observations stand in contrast to laboratory measurements that show substantial Eabs for BC are possible.

An Aerosol Chemical Speciation Monitor (ACSM) for Routine Monitoring of the Composition and Mass Concentrations of Ambient Aerosol

We present a new instrument, the Aerosol Chemical Speciation Monitor (ACSM), which routinely characterizes and monitors the mass and chemical composition of non-refractory submicron particulate matter in real time. Under ambient conditions, mass concentrations of particulate organics, sulfate, nitrate, ammonium, and chloride are obtained with a detection limit <0.2 μg/m3 for 30 min of signal averaging. The ACSM is built upon the same technology as the widely used Aerodyne Aerosol Mass Spectrometer (AMS), in which an aerodynamic particle focusing lens is combined with high vacuum thermal particle vaporization, electron impact ionization, and mass spectrometry. Modifications in the ACSM design, however, allow it to be smaller, lower cost, and simpler to operate than the AMS. The ACSM is also capable of routine stable operation for long periods of time (months). Results from a field measurement campaign in Queens, NY where the ACSM operated unattended and continuously for 8 weeks, are presented. ACSM data is analyzed with the same well-developed techniques that are used for the AMS. Trends in the ACSM mass concentrations observed during the Queens, NY study compare well with those from co-located instruments. Positive Matrix Factorization (PMF) of the ACSM organic aerosol spectra extracts two components: hydrocarbon-like organic aerosol (HOA) and oxygenated organic aerosol (OOA). The mass spectra and time trends of both components correlate well with PMF results obtained from a co-located high resolution time-of-flight AMS instrument.

Collection Efficiencies in an Aerodyne Aerosol Mass Spectrometer as a Function of Particle Phase for Laboratory Generated Aerosols

The Aerodyne Aerosol Mass Spectrometer (AMS) is a useful tool to study ambient particles. To be quantitative, the mass or (number) of particles detected by the AMS relative to the mass (or number) of particles sampled by the AMS, or the AMS collection efficiency (CE), must be known. Here we investigated the effect of particulate phase on AMS CE for ammonium nitrate, ammonium sulfate, mixed ammonium nitrate/ammonium sulfate, and ammonium sulfate particles coated with an organic liquid. Dry, solid ammonium sulfate particles were sampled with a CE of 24 ± 3%. Liquid droplets and solid particles that were thickly coated with a liquid organic were collected with a CE of 100%. Mixed phase particles, solid particles thinly coated with liquid organic, and metastable aqueous ammonium sulfate droplets had intermediate CEs. The higher CEs for liquid particles compared with solid particles were attributed to wet or coated particles tending to stick upon impact with the AMS vaporizer, while a significant fraction of solid particles bounced prior to vaporization/detection. The consistency of single particle signals indicated that the phase (and hence CE) of mixed component particles did not affect the AMS sensitivity to a particular chemical species once volatilization occurred. Particle phase might explain a significant fraction of the variable AMS CEs reported in the literature. For example, ambient particles that were liquid (e.g., composition dominated by ammonium nitrate or acidic sulfate) have been reported to be sampled with 100% CE. In contrast, most ambient particle measurements report CEs of < 100% (typically~ 50%).

Transmission Efficiency of an Aerodynamic Focusing Lens System: Comparison of Model Calculations and Laboratory Measurements for the Aerodyne Aerosol Mass Spectrometer

The size-dependent particle transmission efficiency of the aerodynamic lens system used in the Aerodyne Aerosol Mass Spectrometer (AMS) was investigated with computational fluid dynamics (CFD) calculations and experimental measurements. The CFD calculations revealed that the entire lens system, including the aerodynamic lens itself, the critical orifice which defines the operating lens pressure, and a valve assembly, needs to be considered. Previous calculations considered only the aerodynamic lens. The calculations also investigated the effect of operating the lens system at two different sampling pressures, 7.8 × 104 Pa (585 torr) and 1.0 × 105 Pa (760 torr). Experimental measurements of transmission efficiency were performed with size-selected diethyl hexyl sebacate (DEHS), NH4NO3, and NaNO3 particles on three different AMS instruments at two different ambient sampling pressures (7.8 × 104 Pa, 585 torr and 1.0 × 105 Pa, 760 torr). Comparisons of the measurements and the calculations show qualitative agreement, but there are significant deviations which are as yet unexplained. On the small size end (30 nm to 150 nm vacuum aerodynamic diameter), the measured transmission efficiency is lower than predicted. On the large size end (> 350 nm vacuum aerodynamic diameter) the measured transmission efficiency is greater than predicted at 7.8 × 104 Pa (585 torr) and in good agreement with the prediction at 1.0 × 105 Pa (760 torr).

An Inter-Comparison of Instruments Measuring Black Carbon Content of Soot Particles

Inter-comparison studies of well-characterized fractal soot particles were conducted using the following four instruments: Aerosol Mass Spectrometer-Scanning Mobility Particle Sizer (AMS-SMPS), Single Particle Soot Photometer (SP2), Multi-Angle Absorption Photometer (MAAP), and Photoacoustic Spectrometer (PAS). These instruments provided measurements of the refractory mass (AMS-SMPS), incandescent mass (SP2) and optically absorbing mass (MAAP and PAS). The particles studied were in the mobility diameter range from 150 nm to 460 nm and were generated by controlled flames with fuel equivalence ratios ranging between 2.3 and 3.5. The effect of organic coatings (oleic acid and anthracene) on the instrument measurements was determined. For uncoated soot particles, the mass measurements by the AMS-SMPS, SP2, and PAS instruments were in agreement to within 15%, while the MAAP measurement of optically-absorbing mass was higher by ∼ 50%. Thin organic coatings (∼ 10 nm) did not affect the instrument readings. A thicker (∼ 50 nm) oleic acid coating likewise did not affect the instrument readings. The thicker (∼60 nm) anthracene coating did not affect the readings provided by the AMS-SMPS or SP2 instruments but increased the reading of the MAAP instrument by ∼ 20% and the reading of the PAS by ∼ 65%. The response of each instrument to the different particle types is discussed in terms of particle morphology and coating material.

Design, modeling, optimization, and experimental tests of a particle beam width probe for the aerodyne aerosol mass spectrometer

Aerodynamic lens inlets have revolutionized aerosol mass spectrometry by allowing the introduction of a very narrow particle beam into a vacuum chamber for subsequent analysis. The real-time measurement of particle beam width after an aerodynamic lens is of interest for two reasons: (1) it allows a correction to be made to the measured particle concentration if the beam is so broad, due to poor focusing by non-spherical particles, that some particles miss the detection system; and (2) under constant lens pressure it can provide a surrogate particle non-sphericity measurement. For these reasons, a beam width probe (BWP) has been designed and implemented for the Aerodyne Aerosol Mass Spectrometer (AMS), although this approach is also applicable to other instruments that use aerodynamic lens inlets. The probe implemented here consists of a thin vertical wire that can be precisely positioned to partially block the particle beam at fixed horizontal locations in order to map out the width of the particle beam. A computer model was developed to optimize the BWP and interpret its experimental data. Model assumptions were found to be reasonably accurate for all laboratory-generated particle types to which the model was compared. Comparisons of particle beam width data from a number of publications are also shown here. Particle losses due to beam broadening are found to be minor for the AMS for both laboratory and ambient particles. The model was then used to optimize the choice of the BWP dimensions, and to guide its use during continuous operation. A wire diameter approximately 1.55 times larger than the beam width to be measured provides near optimal sensitivity toward both collection efficiency and surrogate non-sphericity information. Wire diameters of 0.62 mm and 0.44 mm (for the AMS “long” and “short” chambers, respectively) provide reasonable sensitivity over the expected range of particle beam widths, for both spherical and non-spherical particles. Three other alternative BWP geometries were also modeled and discussed.

Soot Particle Aerosol Mass Spectrometer: Development, Validation, and Initial Application

The Soot Particle Aerosol Mass Spectrometer (SP-AMS) was developed to measure the chemical and physical properties of particles containing refractory black carbon (rBC). The SP-AMS is an Aerodyne Aerosol Mass Spectrometer (AMS) equipped with an intracavity laser vaporizer (1064 nm) based on the Single Particle Soot Photometer (SP2) design, in addition to the resistively heated, tungsten vaporizer used in a standard AMS. The SP-AMS can be operated with the laser vaporizer alone, with both the laser and tungsten vaporizers, or with the tungsten vaporizer alone. When operating with only the laser vaporizer, the SP-AMS is selectively sensitive to laser-light absorbing particles, such as ambient rBC-containing particles as well as metal nanoparticles, and measures both the refractory and nonrefractory components. When operated with both vaporizers and modulating the laser on and off, the instrument measures the refractory components of absorbing particles and the nonrefractory particulate matter of all sampled particles. The SP-AMS design, mass spectral interpretation, calibration, and sensitivity are described. Instrument calibrations yield a sensitivity of greater than 140 carbon ions detected per picogram of rBC mass sampled, a 3σ detection limit of less than 0.1 μg·m−3 for 60 s averaging, and a mass-specific ionization efficiency relative to particulate nitrate of 0.2 ± 0.1. Sensitivities were found to vary depending upon laser-particle beam overlap. The utility of the instrument to characterize ambient rBC aerosol is demonstrated. Copyright 2012 American Association for Aerosol Research

Soot Particle Studies—Instrument Inter-Comparison—Project Overview

An inter-comparison study of instruments designed to measure the microphysical and optical properties of soot particles was completed. The following mass-based instruments were tested: Couette Centrifugal Particle Mass Analyzer (CPMA), Time-of-Flight Aerosol Mass Spectrometer—Scanning Mobility Particle Sizer (AMS-SMPS), Single Particle Soot Photometer (SP2), Soot Particle-Aerosol Mass Spectrometer (SP-AMS) and Photoelectric Aerosol Sensor (PAS2000CE). Optical instruments measured absorption (photoacoustic, interferometric, and filter-based), scattering (in situ), and extinction (light attenuation within an optical cavity). The study covered an experimental matrix consisting of 318 runs that systematically tested the performance of instruments across a range of parameters including: fuel equivalence ratio (1.8 ≤ φ ≤ 5), particle shape (mass-mobility exponent ( D fm ), 2.0 ≤ D fm ≤ 3.0), particle mobility size (30 ≤ d m ≤ 300 nm), black carbon mass (0.07 ≤ m BC ≤ 4.2 fg) and particle chemical composition. In selected runs, particles were coated with sulfuric acid or dioctyl sebacate (DOS) (0.5 ≤ Δ r ve ≤ 201 nm) where Δ r ve is the change in the volume equivalent radius due to the coating material. The effect of non-absorbing coatings on instrument response was determined. Changes in the morphology of fractal soot particles were monitored during coating and denuding processes and the effect of particle shape on instrument response was determined. The combination of optical and mass based measurements was used to determine the mass specific absorption coefficient for denuded soot particles. The single scattering albedo of the particles was also measured. An overview of the experiments and sample results are presented.