Real-Time Chemical Analysis of Organic Aerosols Using a Thermal Desorption Particle Beam Mass Spectrometer

Self Cite

148

201

The effects of molecular structure on the products and mechanisms of SOA formation from OH radical-initiated reactions of linear, branched, and cyclic alkanes in the presence of NO x were investigated in a series of environmental chamber experiments. SOA mass spectra were obtained in real time and off line using a thermal desorption particle beam mass spectrometer and used to identify reaction products. Real-time mass spectra were used to classify products according to their temporal behavior, and off-line temperature-programmed thermal desorption analysis of collected SOA was used to separate products by volatility prior to mass spectral analysis and to gain information on compound vapor pressures. A reaction mechanism that includes gas-and particle-phase reactions was developed that explains the formation of SOA products and is consistent with the various lines of mass spectral information. Results indicate that the SOA products formed from the reactions of linear, branched, and cyclic alkanes are similar, but differ in a few important ways. Proposed first-generation SOA products include alkyl nitrates, 1,4-hydroxynitrates, 1,4-hydroxycarbonyls, and dihydroxycarbonyls. The 1,4-hydroxycarbonyls and dihydroxycarbonyls rapidly isomerize in the particle phase to cyclic hemiacetals that then dehydrate to volatile dihydrofurans. This conversion process is catalyzed by HNO 3 formed in the chamber and is slowed by the presence of NH 3 . Volatile products can react further with OH radicals, forming multi-generation products containing various combinations of the same functional groups present in firstgeneration products. For linear and branched alkanes, the products are acyclic or monocyclic, whereas for cyclic alkanes they are acyclic, monocyclic, or bicyclic. Some of the products, especially those formed from ring-opening reactions of cyclic alkanes appear This material is based on work supported by the National Science Foundation under Grants ATM-0234586 and ATM-0650061. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation (NSF). We also thank Roger Atkinson for helpful discussions.Address correspondence to Paul J. Ziemann, Air Pollution Research Center, University of California, 900 University Ave., Riverside, CA, 92521 USA. E-mail: paul.ziemann@ucr.edu to be low volatility oligomers. The implications of the results for the formation of atmospheric SOA are discussed.

Section: Particle Composition Analysismentioning

confidence: 99%

Chemistry of Secondary Organic Aerosol Formation from OH Radical-Initiated Reactions of Linear, Branched, and Cyclic Alkanes in the Presence of NO_x

Lim

Ziemann

2009

Self Cite

148

201

“…However, when particulate matter is collected insuch a manner, thereis potential for lossof semivolatile compounds (compounds that partition appreciably to both the particle and gas phases under normal ambient conditions) that maycontributesignificantlytothe toxicological properties ofthe aerosol (Eatoughet al2003;Seagrave et al 2003). Sincethe particles are under vacuum for less than I ms in the ATOFMS, loss of semivolatile compounds is minimal, making the analysis of these species more feasi ble (Tobias 2000). Further problems are assoc iated with bulk measurements because chemical reactions may occur on the filter substrate between sample collection and analysis, which may change the toxicological properties of the particulate matter.…”

Section: Introductionmentioning

confidence: 99%

Characterization of an Ambient Coarse Particle Concentrator Used for Human Exposure Studies: Aerosol Size Distributions, Chemical Composition, and Concentration Enrichment

Moffet

Shields

Berntsen³

et al. 2004

The goals of the experiments described herein involve determining in real time the size, concentration enrichment, and chemical composition of coarse-mode (> 2.5 j.tm) and fine-mode « 2.5 j.tm) particles within the nonconcentrated and concentrated flows of a coa rse particle concentrator used for human exposure studies. The coarse particle concentrator was intended to concentrate ambient particles in the PM IO -2 . 5 size range before sending them into a human exposure chamber. The aerodynamic size and chemical composition of particles in the upstream and downstream flows of the concentrator were monitored with an aerosol time-of-f1ight mass spectrometer (ATOFMS) for fixed time intervals over the course of three days. Based on the ATOFMS results , it was found that there was no change in the composition of the ten major particle types observed in the upstream and downstream flows of the concentrator under normal operating conditions. Furthermore , no new particle types were detected downstream that were not detected upstream of the concentrator. A characterization of the aerosol chemical composition and its dependence on sampling conditions is also discussed. Aerosol size distributions were measured with three aerodynamic particle-sizing (APS) instruments All members of the Prather research group at UCSD were extremely helpful during every stage of this study. David Kosnikowski for TRC Environmental gave assistance in operating the coarse concentrator used in this study. The research presented is supported by the University of Rochester EPA PM Center under grant R827354. This report has been reviewed by the National Health and Environmental Effects Research Laboratory, United States Environmental Protection Agency, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Agency nor does mention of trade names or commercial products constitute endorsement or recommendation for use.Address correspondence to Kimberly A. Prather, Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-03 14, USA. E-mail: kprather@ucsd.edu sampling simulta neously from different regions of the concentrator. The APS size distributions were used to scale ATOFMS data and measure the ambient concentration factors for the coarse particle concentrator and the exposure chamber. The average concentration factor (ratio of inlet number concentration to the outlet number concentration) for the particle concentrator was 60 ± 17 for the 2.5-7.2 j.tm size range before dilution and transport to th e exposure chamber. It was obser ved that not only were coarse particles being concentrated but fine « 2.5 j.tm ) particles were being concentrated as well, with concentration factors ranging from 2-46 for aerodynamic particle sizes from 0.54-2.5 um,

“…Ziemann has accomplished compound separation and identification with his particle beam mass spectrometer through integrated, sample preconcentration followed by programmed thermal desorption (Tobias and Ziemann 1999;Tobias et al 2000). This provides separation of compounds by volatility.…”

Section: Introductionmentioning

confidence: 99%

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

Williams

Goldstein

Kreisberg

et al. 2006

238

267

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.