James W. Morris

Ambient aerosol sampling using the Aerodyne Aerosol Mass Spectrometer

The Aerodyne Aerosol Mass Spectrometer (AMS) has been designed to measure size-resolved mass distributions and total mass loadings of volatile and semivolatile chemical species in/on submicron particles. This paper describes the application of this instrument to ambient aerosol sampling. The AMS uses an aerodynamic lens to focus the particles into a narrow beam, a roughened cartridge heater to vaporize them under high vacuum, and a quadrupole mass spectrometer to analyze the vaporized molecules. Particle size is measured via particle time-of-flight. The AMS is operated in two modes: (1) a continuous mass spectrum mode without size information; and (2) a size distribution measurement mode for selected m/z settings of the quadrupole. Single particles can also be detected and sized if they have enough mass of a chemical component. The AMS was deployed at a ground sampling site near downtown Atlanta during August 1999, as part of the Environmental Protection Agency/Southern Oxidant Study Particulate Matter “Supersite” experiment, and at a suburban location in the Boston area during September 1999. The major observed components of the aerosol at both sites were sulfate and organics with a minor fraction of nitrate, consistent with prior studies and colocated instruments. Different aerosol chemical components often had different size distributions and time evolutions. More than half of the sulfate mass was contained in 2% of the ambient particles in one of the sampling periods. Trends in mass concentrations of sulfate and nitrate measured with the AMS in Atlanta compare well with those measured with ion chromatography-based instruments. A marked diurnal cycle was observed for aerosol nitrate in Atlanta. A simple model fit is used to illustrate the integration of data from several chemical components measured by the AMS together with data from other particle instruments into a coherent representation of the ambient aerosol.

Numerical Characterization of Particle Beam Collimation: Part II Integrated Aerodynamic-Lens–Nozzle System

As a sequel to our previous effort on the modeling of particle motion through a single lens or nozzle, flows of gas–particle suspensions through an integrated aerodynamic-lens–nozzle inlet have been investigated numerically. It is found that the inlet transmission efficiency (ηt) is unity for particles of intermediate diameters (Dp ∼ 30–500 nm). The transmission efficiency gradually diminishes to ∼40% for large particles (Dp > 2500 nm) because of impact losses on the surface of the first lens. There is a catastrophic reduction of ηt to almost zero for very small particles (Dp ≤ 15 nm) because these particles faithfully follow the final gas expansion. We found that, for very small particles, particle transmission is mainly controlled by nozzle geometry and operating conditions. A lower upstream pressure or a small inlet can be used to improve transmission of very small particles, but at the expense of sampling rate, or vice versa. Brownian motion exacerbates the catastrophic reduction in ηt for small particles; we found that the overall particle transmission efficiency can be roughly calculated as the product of the aerodynamic and the purely Brownian efficiencies. For particles of intermediate diameters, Brownian motion is irrelevant, and the modeling results show that the transmission efficiency is mainly controlled by the lenses. Results for an isolated lens or nozzle are used to provide guidance for the design of alternative inlets. Several examples are given, in which it is shown that one can configure the inlet to preferentially sample large particles (with ηt > 50% for Dp = 50–2000 nm) or ultrafine particles (with ηt > 50% for Dp = 20–1000 nm). Some of the results have been compared with experimental data, and reasonable agreement has been demonstrated.

Kinetics of submicron oleic acid aerosols with ozone: A novel aerosol mass spectrometric technique

The reaction kinetics of submicron oleic (9-octadecanoic (Z)-) acid aerosols with ozone was studied using a novel aerosol mass spectrometric technique. In the apparatus a flow of size-selected aerosols is introduced into a flow reactor where the particles are exposed to a known density of ozone for a controlled period of time. The aerosol flow is then directed into an aerosol mass spectrometer for particle size and composition analyses. Data from these studies were used to: (a) quantitatively model the size-dependent kinetics process, (b) determine the aerosol size change due to uptake of ozone, (c) assess reaction stoichiometry, and (d) obtain qualitative information about the volatility of the reaction products. The reactive uptake probability for ozone on oleic acid particles obtained from modeling is 1.6 (±0.2) × 10^(−3) with an upper limit for the reacto-diffusive length of ∼10 nm. Atmospheric implications of the results are discussed.

Kinetics of the reactive uptake of ozone on oleic acid aerosols

Abstract Heterogeneous reactions involving particles and gas-phase species can alter important chemical and microphysical properties of aerosols, complicating modeling efforts to assess their effects on climate and human health. Organic aerosols are common in the troposphere; precursors include vegetation, the ocean surface, and various combustion processes. Ozone is an important oxidant in the troposphere (Finlayson-Pitts and Pitts, 1997), often in concentrations sufficient to cause adverse effects on human health and vegetation. Oleic acid is one of a group of organic species proposed as an important tracer species for use in source characterization of ambient aerosols (Rogge et al., 1991). However, the relative fraction of these species may change as the particle ages, and new product species are likely to be introduced. Little is known about the kinetics of organic species in atmospheric aerosols. Such knowledge is necessary for quantitative assessment of field studies as well as for use in climate models. With this motivation, we have chosen oleic acid (one of the simplest condensed phase organics with atmospheric relevance) as the first species to be used in a series of experiments aimed at investigating the dynamic evolution of the composition and size of organic particles in the presence of ozone. Our experimental setup includes an atmospheric pressure flow reactor, in which oleic acid particles of a pre-selected size are allowed to interact with ozone for a controlled time, coupled to a newly developed aerosol mass spectrometer (AMS), (Jayne et al., 2000), which monitors changes in the size distribution and composition of the aerosols. This AMS/flow reactor system permits a new approach to kinetic studies in that the depletion of the particle-phase reactant (rather than the gas-phase reactant) is monitored. In the present study we report the size dependent rate of reactive uptake of ozone by three different sizes of oleic acid particles. Depletion of oleic acid, appearance of product species, and the growth of particle size are all observed simultaneously. With interaction times from 2 to 11 seconds, a given run takes place at nearly constant ozone concentration. The figure below indicates the fraction of oleic acid remaining in a particle as a function of ozone exposure time. We are developing an uptake model which allows determination of the following fundamental physico-chemical parameters: Henrys Law solubility constant for ozone in oleic acid, H, the liquid phase diffusion coefficient for ozone in oleic acid, D, and the second order rate coefficient for ozone reacting with oleic acid, k2. The plotted curves show a preliminary fit to the size dependent reaction data, yielding the values shown at 293°K. These parameters allow calculation of the reactive collision probability, γ, which ranges from about 10−3 to 10−5 (decreasing with a decrease in oleic acid concentration). Our data also show a dependence in the uptake on the purity of the oleic particles, an increase in viscosity upon reaction, and an increase in particle diameter as the interaction time increases. These effects (including those of secondary reaction of products) are currently being modeled in order to assess aerosol phase oleic acid atmospheric lifetimes.