Atmospheric measurements of the physical evolution of aircraft exhaust plumes.

Drawing from a series of field measurement activities including the Alternative Aviation Fuels Experiments (AAFEX1 and AAFEX2), we present experimental measurements of particle number, size, and composition-resolved mass that describe the physical and chemical evolution of aircraft exhaust plumes on the time scale of 5 s to 2-3 min. As the plume ages, the particle number emission index initially increases by a factor of 10-50, due to gas-to-particle formation of a nucleation/growth mode, and then begins to fall with increased aging. Increasing the fuel sulfur content causes the initial increase to occur more rapidly. The contribution of the nucleation/growth mode to the overall particle number density is most pronounced at idle power and decreases with increasing engine power. Increasing fuel sulfur content, but not fuel aromatic content causes the nucleation/growth mode to dominate the particle number emissions at higher powers than for a fuel with "normal" sulfur and aromatic content. Particle size measurements indicate that the observed particle number emissions trends are due to continuing gas-to-particle conversion and coagulation growth of the nucleation/growth mode particles, processes which simultaneously increase particle mass and reduce particle number density. Measurements of nucleation/growth mode mass are consistent with the interpretation of particle number and size data and suggest that engine exit plane measurements may underestimate the total particle mass by as much as a factor of between 5 and 10.

Adaptation of a proton transfer reaction mass spectrometer instrument to employ NO+ as reagent ion for the detection of 1,3-butadiene in the ambient atmosphere.

A proton transfer reaction mass spectrometer (PTR-MS) instrument was adapted to employ NO+ as a chemical reagent ion without any hardware changes by switching the reagent ion source gas from water vapor to dry air. Ionization of dry air within the hollow cathode ion source generates a very intense source of NO+ with only a minor impurity of NO2+. The intensities of the primary NO+ reagent ion and the unwanted impurity NO2+ are controllable and dependent on the operational conditions of the hollow cathode ion source. Ion source tuning parameters are described, which maintain an intense source of NO+ while keeping the impurity NO2+ signal to less than 2% of the total reagent ion intensity. This method is applied to the detection of 1,3-butadiene. NO+ reacts efficiently with 1,3-butadiene via a charge exchange reaction to produce only the molecular ion, which is detected at m/z 54. Detection sensitivities of the order of 45 pptv for a 1-s measurement of 1,3-butadiene are demonstrated. We present the first real-time on-line sub parts per billion measurement of 1,3-butadiene in the ambient atmosphere. The only likely interference is from 1,2-butadiene. Concurrent measurements of benzene are provided and suggest that the vehicular emissions are the predominant source of 1,3-butadiene in a suburban Boston area monitoring location.

Location of terbium binding sites on acetylcholine receptor-enriched membranes.

Acetylcholine receptor-enriched membranes bind 45 terbium cations per receptor. The Tb(III) X-ray scattering factor changes by as much as 30% over a 50 eV range about the L3 absorption edge. We exploit these changes to modulate the contribution of these ions to the X-ray diffraction pattern of oriented receptor-enriched membranes by varying the incident X-ray energy. Difference Fourier analysis of the meridional diffraction amplitudes at two X-ray energies revealed six localized regions of Tb(III) density across the membrane. Most significant is the finding of 18 Tb(III) ions near the entrance and 11 ions near the exit of the ion channel as well as 4 or 5 Tb(III) ions localized in the channel itself. This evidence strongly suggests the presence of anionic carboxylate side-chains on the channel lining.

Anomalous x-ray scattering from terbium-labeled parvalbumin in solution.

We have used anomalous small-angle x-ray scattering as a structural probe for solutions of rabbit parvalbumin labeled with terbium. This technique makes use of the large changes in the terbium scattering factor that occur when the x-ray energy is tuned around an L3 absorption edge of this heavy-atom label. These changes in scattering result in changes in the small-angle scattering curve of the labeled protein as a whole, which can then be analyzed to derive structural information concerning the distribution of labels in the protein. Based on a Gaussian model for the protein electron density, the mean distance from the terbiums to the protein center of mass is determined to be 13.2 A and is consistent with crystallographic results. Our results demonstrate the usefulness of terbium as an anomalous scattering label and provide criteria to help establish anomalous scattering as a reliable structural technique for proteins in solution.