Thermal Desorption Chemical Ionization Mass Spectrometer Research Articles

Composition of Ultrafine Particles in Urban Beijing: Measurement Using a Thermal Desorption Chemical Ionization Mass Spectrometer.

Ultrafine particles (UFPs) dominate the particle number population in the urban atmosphere and revealing their chemical composition is important. The thermal desorption chemical ionization mass spectrometer (TDCIMS) can semicontinuously measure UFP composition at the molecular level. We modified a TDCIMS and deployed it in urban Beijing. Radioactive materials in the TDCIMS for aerosol charging and chemical ionization were replaced by soft X-ray ionizers so that it can be operated in countries with tight regulations on radioactive materials. Protonated N-methyl-2-pyrrolidone ions were used as the positive reagent ion, which selectively detects ammonia and low-molecular weight-aliphatic amines and amides vaporized from the particle phase. With superoxide as the negative reagent ion, a wide range of inorganic and organic compounds were observed, including nitrate, sulfate, aliphatic acids with carbon numbers up to 18, and highly oxygenated CHO, CHON, and CHOS compounds. The latter two can be attributed to parent ions or the decomposition products of organonitrates and organosulfates/organosulfonates, respectively. Components from both primary emissions and secondary formation of UFPs were identified. Compared to the UFPs measured at forest and marine sites, those in urban Beijing contain more nitrogen-containing and sulfur-containing compounds. These observations illustrate unique features of the UFPs in the urban environment and provide insights into their origins.

Chemical composition of ultrafine aerosol particles in central Amazonia during the wet season

Abstract. Central Amazonia serves as an ideal location to study atmospheric particle formation, since it often represents nearly natural, pre-industrial conditions but can also experience periods of anthropogenic influence due to the presence of emissions from large metropolitan areas like Manaus, Brazil. Ultrafine (sub-100 nm diameter) particles are often observed in this region, although new particle formation events seldom occur near the ground despite being readily observed in other forested regions with similar emissions of volatile organic compounds (VOCs). This study focuses on identifying the chemical composition of ultrafine particles as a means of determining the chemical species and mechanisms that may be responsible for new particle formation and growth in the region. These measurements were performed during the wet season as part of the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) field campaign at a site located 70 km southwest of Manaus. A thermal desorption chemical ionization mass spectrometer (TDCIMS) characterized the most abundant compounds detected in ultrafine particles. Two time periods representing distinct influences on aerosol composition, which we label as “anthropogenic” and “background” periods, were studied as part of a larger 10 d period of analysis. Higher particle number concentrations were measured during the anthropogenic period, and modeled back-trajectory frequencies indicate transport of emissions from the Manaus metropolitan area. During the background period there were much lower number concentrations, and back-trajectory frequencies showed that air masses arrived at the site predominantly from the forested regions to the north and northeast. TDCIMS-measured constituents also show distinct differences between the two observational periods. Although bisulfate was detected in particles throughout the 10 d period, the anthropogenic period had higher levels of particulate bisulfate overall. Ammonium and trimethyl ammonium were positively correlated with bisulfate. The background period had distinct diurnal patterns of particulate cyanate and acetate, while oxalate remained relatively constant during the 10 d period. 3-Methylfuran, a thermal decomposition product of a particulate-phase isoprene epoxydiol (IEPOX), was the dominant species measured in the positive-ion mode. Principal component analysis (PCA) was performed on the TDCIMS-measured ion abundance and aerosol mass spectrometer (AMS) mass concentration data. Two different hierarchical clusters representing unique influences arise: one comprising ultrafine particulate acetate, hydrogen oxalate, cyanate, trimethyl ammonium and 3-methylfuran and another made up of ultrafine particulate bisulfate, chloride, ammonium and potassium. A third cluster separated AMS-measured species from the two TDCIMS-derived clusters, indicating different sources or processes in ultrafine aerosol particle formation compared to larger submicron-sized particles.

Unexpectedly acidic nanoparticles formed in dimethylamine–ammonia–sulfuric-acid nucleation experiments at CLOUD

Abstract. New particle formation driven by acid–base chemistry was initiated in the CLOUD chamber at CERN by introducing atmospherically relevant levels of gas-phase sulfuric acid and dimethylamine (DMA). Ammonia was also present in the chamber as a gas-phase contaminant from earlier experiments. The composition of particles with volume median diameters (VMDs) as small as 10 nm was measured by the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS). Particulate ammonium-to-dimethylaminium ratios were higher than the gas-phase ammonia-to-DMA ratios, suggesting preferential uptake of ammonia over DMA for the collected 10–30 nm VMD particles. This behavior is not consistent with present nanoparticle physicochemical models, which predict a higher dimethylaminium fraction when NH3 and DMA are present at similar gas-phase concentrations. Despite the presence in the gas phase of at least 100 times higher base concentrations than sulfuric acid, the recently formed particles always had measured base : acid ratios lower than 1 : 1. The lowest base fractions were found in particles below 15 nm VMD, with a strong size-dependent composition gradient. The reasons for the very acidic composition remain uncertain, but a plausible explanation is that the particles did not reach thermodynamic equilibrium with respect to the bases due to rapid heterogeneous conversion of SO2 to sulfate. These results indicate that sulfuric acid does not require stabilization by ammonium or dimethylaminium as acid–base pairs in particles as small as 10 nm.

Atmospheric amines and ammonia measured with a chemical ionization mass spectrometer (CIMS)

Abstract. We report measurements of ambient amines and ammonia with a fast response chemical ionization mass spectrometer (CIMS) in a southeastern US forest and a moderately polluted midwestern site during the summer. At the forest site, mostly C3-amines (from pptv to tens of pptv) and ammonia (up to 2 ppbv) were detected, and they both showed temperature dependencies. Aerosol-phase amines measured thermal-desorption chemical ionization mass spectrometer (TDCIMS) showed a higher mass fraction in the evening with cooler temperatures and lower in the afternoon with warmer temperatures, a trend opposite to the gas-phase amines. Concentrations of aerosol-phase primary amines measured with Fourier transform infrared spectroscopy (FTIR) from micron and submicron particles were 2 orders of magnitude higher than the gas-phase amines. These results indicate that gas to particle conversion is one of the major processes that control the ambient amine concentrations at this forest site. Temperature dependencies of C3-amines and ammonia also imply reversible processes of evaporation of these nitrogen-containing compounds from soil surfaces in daytime and deposition to soil surfaces at nighttime. During the transported biomass burning plume events, various amines (C1–C6) appeared at the pptv level, indicating that biomass burning is a substantial source of amines in the southeastern US. At the moderately polluted Kent site, there were higher concentrations of C1- to C6-amines (pptv to tens of pptv) and ammonia (up to 6 ppbv). C1- to C3-amines and ammonia were well correlated with the ambient temperature. C4- to C6-amines showed frequent spikes during the nighttime, suggesting that they were emitted from local sources. These abundant amines and ammonia may in part explain the frequent new particle formation events reported from Kent. Higher amine concentrations measured at the polluted site than at the rural forested site highlight the importance of constraining anthropogenic emission sources of amines.

Composition of 15–85 nm particles in marine air

Abstract. The chemical composition of 15–85 nm diameter particles was measured at Mace Head, Ireland, during May 2011 using the TDCIMS (thermal desorption chemical ionization mass spectrometer). Measurable levels of chloride, sodium, and sulfate were present in essentially all collected samples of these particles at this coastal Atlantic site. Acetaldehyde and benzoic acid were also frequently detected. Concomitant particle hygroscopicity observations usually showed a sea-salt mode and a lower hygroscopicity mode with growth factors near to that of ammonium sulfate. There were many periods lasting from hours to about 2 days during which the 10–60 nm particle number increased dramatically in polar oceanic air. These periods were correlated with the presence of benzoic acid in the particles and an increase in the number of lower hygroscopicity mode particles. Very small (< 10 nm) particles were also present, suggesting that new particle formation contributed to these nanoparticle enhancement events.

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.

Chemical composition of atmospheric nanoparticles formed from nucleation in Tecamac, Mexico: Evidence for an important role for organic species in nanoparticle growth

We report Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements of the composition of ambient 10–33 nm diameter particles formed from nucleation on 16 March 2006 in Tecamac, Mexico during the Megacity Initiative: Local and Global Research Observations (MILAGRO) field study. On this day, nucleated particles contained far more organics than sulfates: average ion molar ratios with measurement uncertainties for nitrate, organics and sulfur species were 6 ± 2%, 84 ± 5%, and 10 ± 1%, respectively. The measured organic species include nitrogen‐containing organic compounds, organic acids, and hydroxy organic acids. Independent calculations show that sulfuric acid condensation could have accounted for only 10 ± 2% of the growth that was observed on this day, which is consistent with the TDCIMS measurements of composition. It follows that organic compounds play a dominant role the high growth rates that were observed.

Preface to topical collection on new particle formation in Atlanta

[1] The Aerosol Nucleation and Real Time Characterization Experiment (ANARChE) took place in Atlanta, Georgia, during late July and through August 2002. The study was aimed at providing new insights into several discrepancies that arose from our previous observations on atmospheric new particle formation. These include (1) rates of new particle formation near the Earth’s surface are often orders of magnitude higher than are predicted by the binary theory of nucleation for sulfuric acid and water; (2) new particle formation rates often vary in proportion to the first or second power of sulfuric acid concentration, rather than the much stronger dependence predicted by nucleation theories; and (3) the growth rates of freshly nucleated particles are typically two to ten times faster than can be explained by the condensation of sulfuric acid and its associated water and ammonia. Established methods were used to measure sulfuric acid vapor concentration and particle size distributions down to 3 nm. Ammonia concentrations were measured with high time resolution by chemical ionization mass spectrometry to explore ammonia’s role in particle production. Two new instruments were developed to enable measurements aimed at explaining the above observations. These include the thermal desorption chemical ionization mass spectrometer (TDCIMS) for measuring the composition of nanoparticles down to 7 nm and the nanometer tandem differential mobility analyzer (Nano TDMA) for measuring the volatility and hygroscopicity of 4–10 nm particles. Measurements by the latter instruments allowed the primary growth species to be identified, and therefore a direct comparison could be made between precursor concentration and growth rates. [2] ANARChE measurements were carried out at the Jefferson Street site located about 4 km northwest of downtown Atlanta [Solomon et al., 2003]. We chose this site because continuous measurements of size distributions from August 1998 through August 2000 showed that new particle formation often occurs there, especially in late summer; that the concentrations of new sub-10 nm particles produced can be extremely high; and that new particle formation is associated with elevated levels of SO2 [Woo et al., 2001; McMurry and Woo, 2002]. Thus this study gave us the opportunity to investigate new particle formation in a polluted environment where SO2 is likely a major precursor. Also, measurements of meteorological parameters and gas and particle concentrations that are carried out routinely at the Jefferson Street site as a part of the SEARCH and ARIES programs were made available to us. [3] Our results show that the collision-controlled theory for nucleation in a chemically reacting system [McMurry, 1983] (also referred to as ‘‘kinetic’’ or ‘‘barrierless’’ nucleation) provides an upper limit to the number of particles produced. However, the number of particles observed is typically less than the number predicted by this theory, by factors that range from about ten to one thousand. We found some evidence for an association between ammonia and the number of particles produced, but ammonia apparently did not account for much of the observed variability. In addition, three separate observations (direct measurements of composition, measurements of nanoparticle hygroscopicity and volatility, and comparison of measured and predicted growth rates) support the conclusion that sub-40 nm particles that were recently formed by nucleation consisted primarily of sulfates neutralized to some extent by ammonia. However, growth rates of particles larger than 40 nm were about a factor of five greater than could be explained by the condensation of sulfuric acid. We did not measure the composition of nucleated particles as they grew past 40 nm during this study, but such measurements are clearly needed.

Hygroscopicity and volatility of 4–10 nm particles during summertime atmospheric nucleation events in urban Atlanta

Continuous measurements of hygroscopicity and volatility of atmospheric aerosol particles of 4–10 nm diameter were conducted with a nanometer tandem differential mobility analyzer (Nano TDMA) during the Aerosol Nucleation and Real‐time Characterization Experiment (ANARChE), which took place in Atlanta in July and August 2002. In the Nano TDMA measurements, particles were exposed to either a high humidity (∼90% RH) or an elevated temperature (∼100°C) downstream of the first differential mobility analyzer (DMA) and were then resized by the second DMA to determine the change in size due to water uptake or evaporation. There were several days when nucleation occurred and high concentrations of sub‐20 nm particles were observed, and during those events, particles of 4–10 nm diameter were very hygroscopic and nonvolatile. These observations, together with parallel Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements of sub‐20 nm particle composition, suggest that the particles were mostly composed of ammoniated sulfates. This finding strongly supports the hypothesis that the nucleation and subsequent growth of nanoparticles were driven by reactions involving sulfuric acid and ammonia during this study.

Chemical composition of atmospheric nanoparticles during nucleation events in Atlanta

We report the first direct, in situ measurements of the chemical composition of size‐segregated atmospheric nanoparticles in the 6–15 nm diameter range. These measurements were made of ambient aerosol directly following nucleation events in Atlanta, Georgia, during the 2002 Aerosol Nucleation and Real‐time Characterization Experiment (ANARChE). The recently developed Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) was used to make these measurements and featured a new inlet that delivers mass of charged and size‐segregated nanoparticles at sufficiently high rates to enable analysis at a typical time resolution of 10 min. Measurements in both the positive and negative ion spectra revealed that particles formed recently from nucleation events have enhanced concentrations of ammonium and sulfate and that to within the uncertainty of our measurements, ammonium sulfate could account for all of the sampled nanoparticle mass. No other compounds were detected in the particles during these events. Concurrent measurements of particle hygroscopicity and volatility, made using a Nanometer Tandem Differential Mobility Analyzer, support the conclusion that ammonium and sulfate are primary components of these newly formed particles.

Atmospheric Measurements of Sub-20 nm Diameter Particle Chemical Composition by Thermal Desorption Chemical Ionization Mass Spectrometry

We report the first online measurements of the chemical composition of atmospheric aerosol in the 6–20 nm diameter range. These measurements were performed using the recently developed Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS), and were made possible by recent sensitivity enhancements resulting from (a) the development of a unipolar charger optimized for high aerosol flow rates, and (b) an improved flow system in the TDCIMS sample inlet. Measurements of atmospheric aerosol in Boulder, CO revealed large concentration variations in most detected compounds. The most dominant observed compounds in the negative ion TDCIMS spectra were nitrate and sulfate, while in the positive ion spectra ammonium dominated all other observed compounds. Comparison with laboratory data suggests that particles are composed primarily of ammonium sulfate during times of relatively low ambient aerosol concentration.