Katrin Fuhrer

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.

Continuous measurements of hydrogen peroxide, formaldehyde, calcium and ammonium concentrations along the new grip ice core from summit, Central Greenland

A new deep core drilling operation started in 1990 in central Greenland and in 1992 reached the bottom at a depth of 3028 m.b. surface. Taking advantage of recent developments in the analytical technique of chemical trace species, continuous high resolution measurements of H20 2, HCHO, NH2 and Ca 2+ concentrations were performed directly on the ice core in the field. During the 1991 season all four components were measured simultaneously between 1300 m.b. surface and 2300 m.b. surface, corresponding to the time interval between 8000 and 38,000 years B.P. In this paper an overview of the results and our first interpretations in terms of climatic changes are given. Key word index: Polar ice cores, glacial interstadials, palaeo climatic changes, flow injection analysis.

Timescales for dust variability in the Greenland Ice Core Project (GRIP) ice core in the last 100,000 years

The calcium (representing dust) concentration record of the last 100,000 years from the Greenland Ice Core Project (GRIP) ice core shows a huge dynamic range (factor>100). The relationship between dust concentrations and temperature (represented by the oxygen isotope ratio) is not a simple one, as has often been assumed. A rapid alternation (factor of 5–10) between low concentrations during the Dansgaard-Oeschger interstadial periods and high levels in colder periods is superimposed on a long-term trend encompassing a further factor of 5–10. Within climate periods, there is only a very weak relationship between Ca concentration and temperature. Previous authors [Biscaye et al., 1997] have suggested that the most likely source for the increased dust is eastern Asia. For the first time, we consider each possible cause of both rapid and slow increases from source to deposition. We suggest that, to account for the size and rapidity of the fast changes, significantly higher wind speed in the source area is required, although changes in atmospheric residence time could also play a role. For the slower long-term variability, changes in transport speed or, possibly, route are probably also involved. Changes in the size of the source area could give some change on longer time periods. The probable importance of changes in source area wind speed, almost simultaneous with Greenland temperature changes, confirms that climatic parameters in high and low latitudes were strongly coupled through the atmosphere during glacial climatic changes. This adds to evidence that the atmospheric circulation system underwent almost instantaneous large-scale changes during the last glacial period.

High‐resolution ammonium ice core record covering a complete glacial‐interglacial cycle

High-resolution ammonium measurements were performed along the Greenland Ice Core Program (GRIP) deep ice core, covering a complete climatic cycle. No overall anthropogenic increase is observed over the last 300 years; however, springtime concentrations have roughly doubled since 1950. Biomass burning is estimated to be a major source for ammonia emissions for preindustrial times. It contributes between 10% to 40% to the total ammonium deposited on the central Greenland ice sheet during the Holocene. No correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern North America, which is considered to be the main source area for ammonium deposited on the central Greenland ice sheet. This suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core. If unchanged meteorological conditions are assumed for the Holocene, as indicated by the δ18O ice record, a decreasing biomass burning activity toward present time can be derived from the ammonium ice record. Soil and vegetation emissions are responsible for the ammonium background concentrations in the ice. The record therefore may be used to trace back the biomass history of the North American continent. A pronounced decreasing trend in background ammonium is found during the Holocene, reflecting decreasing temperature and therefore lower NH3 emissions in the source region. Variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength, which can be related to the extent of the Laurentide ice sheet. The data suggest that the Laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age.

A continuous analysis technique for trace species in ice cores.

A continuous melting technique, combined with continuous flow analysis, has been developed for in situ measurements of chemical trace species in ice cores. A crosssection of 1.8×1.8 cm 2 of the core is needed for the simultaneous analysis of at least four species. The subcore is melted continuously from one side, and only the inner, uncontaminated part of the melted sample is used for the analysis. The main advantage of this method as compared to conventional sampling and analysis procedures is given by a very high spatial resolution, combined with a significant reduction of sample handling work. The method can be applied for any species for which a sensitive continuous flow analysis method exists

Development and Characterization of an Aircraft Aerosol Time-of-Flight Mass Spectrometer

Vertical and horizontal profiles of atmospheric aerosols are necessary for understanding the impact of air pollution on regional and global climate. To gain further insight into the size-resolved chemistry of individual atmospheric particles, a smaller aerosol time-of-flight mass spectrometer (ATOFMS) with increased data acquisition capabilities was developed for aircraft-based studies. Compared to previous ATOFMS systems, the new instrument has a faster data acquisition rate with improved ion transmission and mass resolution, as well as reduced physical size and power consumption, all required advances for use in aircraft studies. In addition, real-time source apportionment software allows the immediate identification and classification of individual particles to guide sampling decisions while in the field. The aircraft (A)-ATOFMS was field-tested on the ground during the Study of Organic Aerosols in Riverside, CA (SOAR) and aboard an aircraft during the Ice in Clouds Experiment-Layer Clouds (ICE-L). Initial results from ICE-L represent the first reported aircraft-based single-particle dual-polarity mass spectrometry measurements and provide an increased understanding of particle mixing state as a function of altitude. Improved ion transmission allows for the first single-particle detection of species out to approximately m/z 2000, an important mass range for the detection of biological aerosols and oligomeric species. In addition, high time resolution measurements of single-particle mixing state are demonstrated and shown to be important for airborne studies where particle concentrations and chemistry vary rapidly.

Long‐term changes in the acid and salt concentrations of the Greenland Ice Core Project ice core from electrical stratigraphy

Continuous electrical records covering a climatic cycle are presented for the Greenland Ice Core Project deep ice core from Greenland. Electrical conductivity measurement (ECM) measures the acid content of the ice, and the dielectric profile (DEP) responds to acid, ammonium, and chloride. All features seen can be explained by chemical changes in the ice, and there is no evidence so far for any major change in electrical response with depth or age of the ice. Both records are dominated by the acidity of the ice which varies strongly from acidic in warm periods to alkaline in cold periods, controlled by neutralization by alkaline dust (calcareous and other mineral dust). When Ca is low, the acidity (mainly nitric acid) has a fairly constant background level throughout the cycle, with slightly lower values in ice believed to be from the last interglacial. Ca has to rise only slightly to neutralize the available acidity, so that acidity is a highly nonlinear reflection of climate changes. If neutralization occurred in the aerosol (rather than in the ice), then the number of cloud condensation nuclei over parts of the northern hemisphere could have been reduced, leading to reduced cloud albedo. This nonlinear feedback may have some importance for modeling of climate change. When both acid and ammonium levels are low, the DEP signal can be used to give a rapid indication of chloride trends.

Electrical response of the Summit‐Greenland ice core to ammonium, sulphuric acid, and hydrochloric acid

Electrical and chemical analysis of the GRIP ice core from Summit in central Greenland confirms that the ECM current is controlled solely by acids in the ice, though there could be different responses for different acids. The dielectric conductivity is dependent on strong acid, on sea salt chloride, and also on ammonium concentrations in the ice. The response to NH4+ is similar to that of sea salt chloride, as they both conduct only at AC frequencies, but NH4+ is approximately twice as conductive per mole. The response to the strong acids shows results consistent with earlier work, with similar responses throughout the length of the core. It seems as if all the thousands of electrical peaks in the GRIP core may be explained by the response to just three chemical species: acidity, ammonium salts, and a third component which is probably chloride.

H2O2 in snow, air and open pore space in firn at Summit, Greenland

Measurements of H2O2 in firn gas down to a 1.7-m depth showed a consistent trend, with higher firn-gas concentrations generally associated with higher concentrations in the firn at the same depth. However, firn to firn-gas concentration ratios still exhibited a seasonal dependence, suggesting that for summer layers equilibrium has not yet been reached. The time to reach equilibrium between firn and firn gas is at least weeks. Snowfall and fog deposit several times more H2O2 than the surface snow will retain at equilibrium, supporting the idea that surface snow is a temporary reservoir for H2O2. Thus from an equilibrium standpoint, the snow-pack should be a source of atmospheric H2O2 in the summer as well as fall, resulting in higher daytime concentrations than would occur based on just atmospheric photochemical reactions. But firn-gas measurements reported here were generally near or lower than those in the atmosphere, suggesting that degassing is too slow to significantly influence atmospheric H2O2 levels.

A study of peptide-peptide interactions using MALDI ion mobility o-TOF and ESI mass spectrometry.

Matrix-assisted laser desorption ionization ion mobility coupled to orthogonal time-of-flight mass spectrometry (MALDI-IM-oTOF MS) is evaluated as a tool for studying non-covalent complex (NCX) formation between peptides. The NCX formed between dynorphin 1–7 and Mini Gastrin I is used as a model system for comparison to previous MALDI experiments (Woods, A. S.; Huestis, M. A. J. Am. Soc. Mass Spectrom.2001, 12, 88–96). The dynorphin 1–7/Mini Gastrin I complex is stable after more than a ms drift time through the He filled mobility cell. Furthermore, the effects of solution pH on NCX ion signal intensity is measured both by MALDI-IM-MS analysis and by nanoelectrospray mass spectrometry. When compared to the previous MALDI study this work shows that all three techniques give similar results. In addition, fragmentation can be observed from of the non-covalent complex parent ion that occurs prior to TOF mass analysis but after mobility separation, thus providing NCX composition information.