Brent J. Williams

Observations of aminium salts in atmospheric nanoparticles and possible climatic implications

We present laboratory studies and field observations that explore the role of aminium salt formation in atmospheric nanoparticle growth. These measurements were performed using the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) and Ultrafine Hygroscopicity Tandem Differential Mobility Analyzers. Laboratory measurements of alkylammonium—carboxylate salt nanoparticles show that these particles exhibit lower volatilities and only slightly lower hygroscopicities than ammonium sulfate nanoparticles. TDCIMS measurements of these aminium salts showed that the protonated amines underwent minimal decomposition during analysis, with detection sensitivities comparable to those of organic and inorganic deprotonated acids. TDCIMS observations made of a new particle formation event in an urban site in Tecamac, Mexico, clearly indicate the presence of protonated amines in 8–10 nm diameter particles accounting for about 47% of detected positive ions; 13 nm particles were hygroscopic with an average 90% RH growth factor of 1.42. Observations of a new particle formation event in a remote forested site in Hyytiälä, Finland, show the presence of aminium ions with deprotonated organic acids; 23% of the detected positive ions during this event are attributed to aminium salts while 10 nm particles had an average 90% RH growth factor of 1.27. Similar TDCIMS observations during events in Atlanta and in the vicinity of Boulder, Colorado, show that aminium salts accounted for 10–35% of detected positive ions. We conclude that aminium salts contribute significantly to nanoparticle growth and must be accounted for in models to accurately predict the impact of new particle formation on climate.

An In-Situ Instrument for Speciated Organic Composition of Atmospheric Aerosols: Thermal Desorption Aerosol GC/MS-FID (TAG)

We introduce a new in-situ instrument, Thermal desorption Aerosol GC/MS-FID (TAG), capable of hourly measurements of speciated organic compounds in atmospheric aerosols. Aerosol samples are collected into a thermal desorption cell by means of humidification and inertial impaction. The sample is thermally desorbed and transferred with helium carrier gas into a gas chromatography (GC) column, with subsequent detection by both quadrupole mass spectrometer (MS) and a flame ionization detector (FID). The collection and analysis steps are automated, yielding around the clock speciation. This approach builds on the extensive body of knowledge available for quantification and source apportionment of organic aerosols from past research using filter-based GC/MS analyses, but it is the first instrument to achieve in-situ time resolved measurements for an essentially unlimited number of samples, making it possible to analyze changes in organic aerosol speciation over timescales ranging from hours to seasons.

Regional variation of organic functional groups in aerosol particles on four U.S. east coast platforms during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign

Submicron atmospheric aerosol samples were collected during the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 campaign on four platforms: Chebogue Point (Nova Scotia, Canada), Appledore Island (Maine), the CIRPAS Twin Otter over Ohio, and the NOAA R/V Ronald H. Brown in the Gulf of Maine. Saturated aliphatic C-C-H, unsaturated aliphatic C=C−H, aromatic C=C−H, organosulfur C-O-S, carbonyl C=O, and organic hydroxyl C-OH functional groups were measured by calibrated Fourier Transform Infrared (FTIR) spectroscopy at all four sampling platforms. The ratio of molar concentrations of carbonyl C=O to saturated aliphatic C-C-H groups was nearly constant at each sampling platform, with the Twin Otter samples having the lowest ratio at 0.1 and the three more coastal platforms having ratios of 0.4 and 0.5. Organic mass (OM) to organic carbon (OC) ratios follow similar trends for the four platforms, with the Twin Otter having the lowest ratio of 1.4 and the coastal platforms having slightly higher values typically between 1.5 and 1.6. Organosulfur compounds were occasionally observed. Collocated organic aerosol sampling with two Aerodyne aerosol mass spectrometers for OM, a Sunset Laboratory thermo-optical analysis instrument for OC, and an ion chromatography-particle into liquid sampler (IC-PILS) for speciated carboxylic acids provided comparable results for most of the project, tracking the time series of FTIR OM, OC, and carbonyl groups, respectively, and showing simultaneous peaks of similar magnitude during most of the project. The FTIR/IC-PILS comparison suggests that about 9% of the carbonyl groups found in submicron organic particles on the Twin Otter are typically associated with low molecular weight carboxylic acids.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

In situ measurements of gas/particle-phase transitions for atmospheric semivolatile organic compounds

An understanding of the gas/particle-phase partitioning of semivolatile compounds is critical in determining atmospheric aerosol formation processes and growth rates, which in turn affect global climate and human health. The Study of Organic Aerosol at Riverside 2005 campaign was performed to gain a better understanding of the factors responsible for aerosol formation and growth in Riverside, CA, a region with high concentrations of secondary organic aerosol formed through the phase transfer of low-volatility reaction products from the oxidation of precursor gases. We explore the ability of the thermal desorption aerosol gas chromatograph (TAG) to measure gas-to-particle-phase transitioning for several organic compound classes (polar and nonpolar) found in the ambient Riverside atmosphere by using in situ observations of several hundred semivolatile organic compounds. Here we compare TAG measurements to modeled partitioning of select semivolatile organic compounds. Although TAG was not designed to quantify the vapor phase of semivolatile organics, TAG measurements do distinguish when specific compounds are dominantly in the vapor phase, are dominantly in the particle phase, or have both phases present. Because the TAG data are both speciated and time-resolved, this distinction is sufficient to see the transition from vapor to particle phase as a function of carbon number and compound class. Laboratory studies typically measure the phase partitioning of semivolatile organic compounds by using pure compounds or simple mixtures, whereas hourly TAG phase partitioning measurements can be made in the complex mixture of thousands of polar/nonpolar and organic/inorganic compounds found in the atmosphere.

Quantification of Hourly Speciated Organic Compounds in Atmospheric Aerosols, Measured by an In-Situ Thermal Desorption Aerosol Gas Chromatograph (TAG)

The Thermal desorption Aerosol Gas chromatograph (TAG) is a recently developed instrument for the in-situ, hourly measurement of speciated organic compounds in atmospheric aerosols. This paper presents a method for the in-field calibration of this instrument, with the objective of providing quantitative concentrations for a large suite of low polarity organic compounds. A new collection and thermal desorption cell was developed that incorporates an injection port for in-situ calibrations with liquid standard mixtures. Two classes of injection standards, instrument tracking and auxiliary, provide the means to calibrate the instrument in the field for a wide range of compounds. A routinely injected tracking standard suite of compounds generates a time-dependent correction of detector drift through the course of a measurement study that accounts for the bulk of the change in response of the TAG instrument. Injection response data for the tracking standard is also used to measure instrument precision and limits of quantitation. Auxiliary standards extend the range of compounds calibrated through use of relative response factors. The accuracy of this in-situ calibration approach is assessed through comparisons of TAG analyzed reference filter punches to published NIST assay values. A subset of compound classes, alkanes and PAHs, are used to illustrate the method and provide a means of reducing an 11-day period of data collected in Riverside, CA during the fall of 2005.

A wind profiler trajectory tool for air quality transport applications

[1] Horizontal transport is a key factor in air pollution meteorology. In several recent air quality field campaigns, networks of wind profiling Doppler radars have been deployed to help characterize this important phenomenon. This paper describes a Lagrangian particle trajectory tool developed to take advantage of the hourly wind observations provided by these special profiler networks. The tool uses only the observed wind profiles to calculate trajectory positions and does not involve any model physics or parameterizations. An interpolation scheme is used to determine the wind speed and direction at any given location and altitude along the trajectory. Only the horizontal winds measured by the profilers are included because the type of profiling radars used in this study are unable to resolve synoptic-scale vertical motions. The trajectory tool is applied to a case study from the International Consortium for Research on Transport and Transformation air quality experiment conducted during the summer of 2004 (ICARTT-04). During this international field study, air chemistry observations were collected at Chebogue Point, a coastal station in southwestern Nova Scotia, and factor analysis was used to identify time periods when air pollution from the United States arrived at the site. The profiler trajectories are compared to trajectories produced from numerical model initialization fields. The profiler-based trajectories more accurately reflect changes in the synoptic weather pattern that occurred between operational upper air soundings, thereby providing a more accurate depiction of the horizontal transport responsible for air pollution arriving in Nova Scotia.

Development of a new consumable-free thermal modulator for comprehensive two-dimensional gas chromatography.

A simple and cost-effective GC×GC modulator requiring no moving parts or consumables, hence suitable for field analysis and monitoring, was developed. The modulator was constructed from a specially designed Silcosteel® trapping capillary, installed outside the GC oven, and coated inside with polydimethylsiloxane (PDMS) stationary phase. Dual-stage modulation was accomplished by resistively heating alternate segments of the trap with a custom-designed capacitive discharge power supply. The performance of the proposed modulator was comparable to many GC×GC systems currently in use, with the injection band widths as low as 60 ms at half height. With proper selection of the stationary phase in the trap, the modulator can be used for the analysis of complex mixtures with volatility range spanning from n-C5 to n-C40.

Development of a volatility and polarity separator (VAPS) for volatility- and polarity-resolved organic aerosol measurement

ABSTRACT Discrepancies between modeled and measured atmospheric organic aerosol (OA) have highlighted the need for in situ instrumentation to better characterize the sources, formation mechanisms, and atmospheric evolution of ambient OA. We have developed the Volatility and Polarity Separator (VAPS) for hourly measurements of volatility- and polarity-resolved OA detected using high-resolution time-of-flight mass spectrometry (HR-ToF-MS). Here, atmospheric OA is inertially impacted onto a collection cell, material is transferred onto a short transfer line located inside a gas chromatography (GC) oven, the oven is heated to provide a first-dimension separation of volatility, then thermally pulsed through a short polar GC column for a second-dimension polarity separation, and finally detected by HR-ToF-MS. This novel instrument increases the mass throughput of ambient OA in comparison to traditional GC due to shorter transfer paths and passivated coatings. Molecular separation resolution is partially sacrificed for this increased mass recovery, but the high-resolution mass spectral data recovers information such as chemical classes and even some individual compounds along with elemental composition to determine aerosol oxidation states. Different techniques for interpreting and representing VAPS data are considered and its applicability to positive matrix factorization (PMF) analysis is demonstrated. Copyright

On-line chemical composition analysis of refillable electronic cigarette aerosol – measurement of nicotine and nicotyrine

INTRODUCTION Electronic cigarettes (e-cigs) generate aerosols that users inhale. Analyses of e-liquids inconsistently report nicotyrine, a nicotine analog that could impede nicotine metabolism, raising questions about nicotyrine formation. METHODS E-cig aerosols were analyzed on-line using a Thermal Desorption Aerosol Gas Chromatograph. Three e-liquids were tested: an unflavored solution in propylene glycol (PG); an unflavored solution in PG and vegetable glycerin (VG), and a flavored solution in PG and VG. A heating duration experiment determined the nicotyrine to nicotine ratio (NNR) in particle phase as a function of the duration of e-cig activation. An aging experiment determined the NNR in e-liquids and aerosols as a function of time since initial exposure to air and storage condition. RESULTS Nicotine and nicotyrine were quantified in all 3 e-liquids and aerosols. Duration of e-cig activation was inversely related to NNR (NNR = 0.04 with 3-s activation, 0.26 with 0.5 s). Aging influenced both e-liquid NNR and aerosol NNR. On average, the e-liquid NNR increased from 0.03 at 11 days after opening to 0.08 after 60 days. For similar heating durations, aerosol NNR increased from 0.05 at 11 days to 0.23 after 60 days. Storage conditions had little effect on NNR. CONCLUSIONS E-cig aerosols have variable nicotyrine quantities. Aerosol NNR depends on vaping technique and time elapsed since the e-liquid was exposed to air. It is hypothesized that aerosolized nicotyrine could facilitate nicotine absorption, inhibit the metabolism of nicotine, and reduce a users urge to smoke.