Frank Drewnick

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.

A New Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS)—Instrument Description and First Field Deployment

We report the development and first field deployment of a new version of the Aerosol Mass Spectrometer (AMS), which is capable of measuring non-refractory aerosol mass concentrations, chemically speciated mass distributions and single particle information. The instrument was constructed by interfacing the well-characterized Aerodyne AMS vacuum system, particle focusing, sizing, and evaporation/ionization components, with a compact TOFWERK orthogonal acceleration reflectron time-of-flight mass spectrometer. In this time-of-flight aerosol mass spectrometer (TOF-AMS) aerosol particles are focused by an aerodynamic lens assembly as a narrow beam into the vacuum chamber. Non-refractory particle components flash-vaporize after impaction onto the vaporizer and are ionized by electron impact. The ions are continuously guided into the source region of the time-of-flight mass spectrometer, where ions are extracted into the TOF section at a repetition rate of 83.3 kHz. Each extraction generates a complete mass spectrum, which is processed by a fast (sampling rate 1 Gs/s) data acquisition board and a PC. Particle size information is obtained by chopping the particle beam followed by time-resolved detection of the particle evaporation events. Due to the capability of the time-of-flight mass spectrometer of measuring complete mass spectra for every extraction, complete single particle mass spectra can be collected. This mode provides quantitative information on single particle composition. The TOF-AMS allows a direct measurement of internal and external mixture of non-refractory particle components as well as sensitive ensemble average particle composition and chemically resolved size distribution measurements. Here we describe for the first time the TOF-AMS and its operation as well as results from its first field deployment during the PM 2.5 Technology Assessment and Characterization Study—New York (PMTACS-NY) Winter Intensive in January 2004 in Queens, New York. These results show the capability of the TOF-AMS to measure quantitative aerosol composition and chemically resolved size distributions of the ambient aerosol. In addition it is shown that the single particle information collected with the instrument gives direct information about internal and external mixture of particle components.

Chase Studies of Particulate Emissions from in-use New York City Vehicles

Emissions from motor vehicles are a significant source of fine particulate matter (PM) and gaseous pollutants in urban environments. Few studies have characterized both gaseous and PM emissions from individual in-use vehicles under real-world driving conditions. Here we describe chase vehicle studies in which on-road emissions from individual vehicles were measured in real time within seconds of their emission. This work uses an Aerodyne aerosol mass spectrometer (AMS) to provide size-resolved and chemically resolved characterization of the nonrefractory portion of the emitted PM; refractory materials such as elemental carbon (EC) were not measured in this study. The AMS, together with other gas-phase and particle instrumentation, was deployed on the Aerodyne Research Inc. (ARI) mobile laboratory, which was used to “chase” the target vehicles. Tailpipe emission indices of the targeted vehicles were obtained by referencing the measured nonrefractory particulate mass loading to the instantaneous CO2 measured simultaneously in the plume. During these studies, nonrefractory PM1.0 (NRPM1) emission indices for a representative fraction of the New York City Metropolitan Transit Authority (MTA) bus fleet were determined. Diesel bus emissions ranged from 0.10 g NRPM1/kg fuel to 0.23 g NRPM1/kg, depending on the type of engine used by the bus. The average NRPM1 emission index of diesel-powered buses using Continuously Regenerating Technology (CRT™) trap systems was 0.052 g NRPM1/kg fuel. Buses fueled by compressed natural gas (CNG) had an average emission index of 0.034 g NRPM1/kg Fuel. The mass spectra of the nonrefractory diesel aerosol components measured by the AMS were dominated by lubricating oil spectral signatures. Mass-weighted size distributions of the particles in fresh diesel exhaust plumes peak at vacuum aerodynamic diameters around 90 nm with a typical full width at half maximum of 60 nm.

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.

Measurement of Ambient Aerosol Composition during the PMTACS-NY 2001 using an Aerosol Mass Spectrometer. Part I: Mass Concentrations

Semicontinuous ambient aerosol composition measurements performed during the PMTACS-NY Summer 2001 field campaign in Queens/New York with an aerosol mass spectrometer (AMS, developed by Aerodyne Research Inc.) are described. The measurements include 10 min averages of the nonrefractory sulfate, nitrate, ammonium, chloride, and organic mass concentrations in the particle size range of 50 to approximately 1000 nm. Particle-bound water concentrations (i.e., aerosol liquid water content) were estimated from the mass spectral information and local meteorological data. Aggregate semicontinuous AMS mass measurements were compared with those from a TEOM mass monitor that was also deployed at the PMTACS-NY 2001 site. On average, the AMS observed 64% of the total particulate matter mass measured by the TEOM Monitor. Filter and additional semicontinuous particulate sulfate measurements performed simultaneously at the site suggest that the observed discrepancy in mass balance between the two instruments is attributable to a combination of large particles (≥1 μm) lost in the AMS inlet system and the refractory aerosol components not measured by the AMS. Measured diurnal patterns of sulfate, nitrate, organics, and total nonrefractory mass concentrations indicate that elevated PM levels measured during this campaign were due to regional transport as well as local production of particulate matter.

Enhanced organic mass fraction and decreased hygroscopicity of cloud condensation nuclei (CCN) during new particle formation events

In a forested near-urban location in central Germany, the CCN efficiency of particles smaller than 100 nm decreases significantly during periods of new particle formation. This results in an increase of average activation diameters, ranging from 5 to 8% at supersaturations of 0.33% and 0.74%, respectively. At the same time, the organic mass fraction in the sub-100-nm size range increases from approximately 2/3 to 3/4. This provides evidence that secondary organic aerosol (SOA) components are involved in the growth of new particles to larger sizes, and that the reduced CCN efficiency of small particles is caused by the low hygroscopicity of the condensing material. The observed dependence of particle hygroscopicity (k) on chemical composition can be parameterized as a function of organic and inorganic mass fractions (forg, finorg) determined by aerosol mass spectrometry: k = korg forg + kinorg finorg. The obtained value of korg ~ 0.1 is characteristic for SOA, and kinorg ~ 0.7 is consistent with the observed mix of ammonium, sulfate and nitrate ions. (Less)

Characterization of the South Atlantic marine boundary layer aerosol using an Aerodyne Aerosol Mass Spectrometer

Measurements of the submicron fraction of the atmospheric aerosol in the marine boundary layer were performed from January to March 2007 (Southern Hemisphere summer) onboard the French research vessel Marion Dufresnein the Southern Atlantic and Indian Ocean (20 S–60 S, 70 W–60 E). We used an Aerodyne HighResolution-Time-of-Flight AMS to characterize the chemical composition and to measure species-resolved size distributions of non-refractory aerosol components in the submicron range. Within the “standard” AMS compounds (ammonium, chloride, nitrate, sulfate, organics) “sulfate” is the dominant species in the marine boundary layer with concentrations ranging between 50 ng m −3 and 3μg m−3. Furthermore, what is seen as “sulfate” by the AMS is likely comprised mostly of sulfuric acid. Another sulfur containing species that is produced in marine environments is methanesulfonic acid (MSA). There have been previously measurements of MSA using an Aerodyne AMS. However, due to the use of an instrument equipped with a quadrupole detector with unit mass resolution it was not possible to physically separate MSA from other contributions to the same m/z. In order to identify MSA within the HR-ToF-AMS raw data and to extract mass concentrations for MSA from the field measurements the standard high-resolution MSA fragmentation patterns for the measurement conditions during the ship campaign (e.g. vaporizer temperature) needed to be determined. To identify characteristic air masses and their source regions backwards trajectories were used and averaged concentrations for AMS standard compounds were calculated for Correspondence to: S. R. Zorn (zorns@mpch-mainz.mpg.de) each air mass type. Sulfate mass size distributions were measured for these periods showing a distinct difference between oceanic air masses and those from African outflow. While the peak in the mass distribution was roughly at 250 nm (vacuum aerodynamic diameter) in marine air masses, it was shifted to 470 nm in African outflow air. Correlations between the mass concentrations of sulfate, organics and MSA show a narrow correlation for MSA with sulfate/sulfuric acid coming from the ocean, but not with continental sulfate.

Measurement of ambient aerosol composition during the PMTACS-NY 2001 using an aerosol mass spectrometer. Part II: Chemically speciated mass distributions

Ambient particulate mass distributions (10 min averages) for nitrate, sulfate, ammonium, and organic particles were obtained during the deployment of the aerosol mass spectrometer (AMS) in the PM2.5 Technology Assessment and Characterization Study–New York (PMTACS–NY) in Queens, New York in summer 2001. Nitrate and sulfate particles were found to be internally mixed and mainly represented by ammonium nitrate and ammonium sulfate. Their average mass distributions are monomodal, with mode diameters of 440 nm and 450 nm and distribution widths around 600 nm. The maximum of the ammonium mass distribution was found at 400 nm and its width was about 550 nm. The average mass distribution of organic particles was bimodal with maxima at 80 nm and 360 nm and widths of 80 nm and 700 nm. While most of these distributions did not exhibit any significant diurnal patterns, the relative intensity of the small particle mode of the organic particles (D p < 120 nm) was found to be most intense during rush-hour times, indicating that the small organic particle fraction is mostly traffic related. Short-time averages of the size distributions, measured for different species independently, showed the ability of the AMS to track the growth and evolution of chemically distinct particles.

Semicontinuous PM2.5 sulfate and nitrate measurements at an urban and a rural location in New York: PMTACS-NY summer 2001 and 2002 campaigns.

Abstract Several collocated semicontinuous instruments measuring particulate matter with particle sizes ≤2.5 μm (PM2.5) sulfate (SO4 22−) and nitrate (NO3 −) were intercompared during two intensive field campaigns as part of the PM2.5 Technology Assessment and Characterization Study. The summer 2001 urban campaign in Queens, NY, and the summer 2002 rural campaign in upstate New York (Whiteface Mountain) hosted an operation of an Aerosol Mass Spectrometer, Ambient Particulate Sulfate and Nitrate Monitors, a Continuous Ambient Sulfate Monitor, and a Particle-Into-Liquid Sampler with Ion Chromato-graphs (PILS-IC). These instruments provided near realtime particulate SO4 2− and NO3 − mass concentration data, allowing the study of particulate SO4 2−/NO3 − diurnal patterns and detection of short-term events. Typical particulate SO4 2− concentrations were comparable at both sites (ranging from 0 to 20 μg/m3), while ambient urban particulate NO3 − concentrations ranged from 0 to 11 μg/m3 and rural NO3 − concentration was typically less than 1 μg/m3. Results of the intercomparisons of the semicontinu-ous measurements are presented, as are results of the comparisons between the semicontinuous and time-integrated filter-based measurements. The comparisons at both sites, in most cases, indicated similar performance characteristics. In addition, charge balance calculations, based on major soluble ionic components of atmospheric aerosol from the PILS-IC and the filter measurements, indicated slightly acidic aerosol at both locations.