Q. Shi

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.

Chase Studies of Particulate Emissions from in-use New York City Vehicles

Emissions from motor vehicles are a significant source of fine particulate matter (PM) and gaseous pollutants in urban environments. Few studies have characterized both gaseous and PM emissions from individual in-use vehicles under real-world driving conditions. Here we describe chase vehicle studies in which on-road emissions from individual vehicles were measured in real time within seconds of their emission. This work uses an Aerodyne aerosol mass spectrometer (AMS) to provide size-resolved and chemically resolved characterization of the nonrefractory portion of the emitted PM; refractory materials such as elemental carbon (EC) were not measured in this study. The AMS, together with other gas-phase and particle instrumentation, was deployed on the Aerodyne Research Inc. (ARI) mobile laboratory, which was used to “chase” the target vehicles. Tailpipe emission indices of the targeted vehicles were obtained by referencing the measured nonrefractory particulate mass loading to the instantaneous CO2 measured simultaneously in the plume. During these studies, nonrefractory PM1.0 (NRPM1) emission indices for a representative fraction of the New York City Metropolitan Transit Authority (MTA) bus fleet were determined. Diesel bus emissions ranged from 0.10 g NRPM1/kg fuel to 0.23 g NRPM1/kg, depending on the type of engine used by the bus. The average NRPM1 emission index of diesel-powered buses using Continuously Regenerating Technology (CRT™) trap systems was 0.052 g NRPM1/kg fuel. Buses fueled by compressed natural gas (CNG) had an average emission index of 0.034 g NRPM1/kg Fuel. The mass spectra of the nonrefractory diesel aerosol components measured by the AMS were dominated by lubricating oil spectral signatures. Mass-weighted size distributions of the particles in fresh diesel exhaust plumes peak at vacuum aerodynamic diameters around 90 nm with a typical full width at half maximum of 60 nm.

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.

Kinetic model for reaction of ClONO2 with H2O and HCl and HOCl with HCl in sulfuric acid solutions

The heterogeneous reactions ClONO2 + H2O → HOCl + HNO3 (1), ClONO2 + HCl → Cl2 + HNO3 (2), and HOCl + HCl → Cl2 + H2O (3) on stratospheric aerosols convert ClONO2 and HCl to photo-labile species, producing reactive Cl and ClO which are responsible for catalyzing ozone destruction in the lower stratosphere. The extent of the resulting ozone loss mirrors the steep negative temperature dependence of these reactions, which strongly depend on the solubility of ClONO2, HCl, and HOCl, and on the activity of H2O. Predicting the effect of these heterogeneous processes throughout the stratosphere requires detailed modeling of liquid phase solubility, diffusion, and reaction kinetics. A series of recent experiments from a number of laboratories have refined measurements of liquid diffusion coefficients, HCl and HOCl solubilities, and the reactivity of ClONO2 + H2O, ClONO2 + HCl and HCl + HOCl on liquid films, droplets, and aerosols. On the basis of those measurements we present a phenomenological uptake model in which parameterizations of ClONO2, HCl, and HOCl heterogeneous kinetics appropriate for stratospheric H2SO4/H2O aerosols are addressed. In this model we suggest that under high acid concentration conditions both HOCl and ClONO2 are protonated before they react with HCl. Data for all three reactions in concentrated H2SO4 solution indicate an acid-catalyzed reaction channel, which had previously been inferred for ClONO2 hydrolysis. This updated parameterization is most significant at relatively high temperatures above 205 K which produce H2SO4 aerosols of >60 acid wt%, where the acid-catalyzed reaction channels dominate. The comparisons between our new formulation and other recent formulations are presented.

Gas-phase diffusion in droplet train measurements of uptake coefficients

Abstract Experiments were conducted to study the nature of gas-phase diffusive transport to a stream of fast moving droplets in a droplet train apparatus used for measuring gas uptake coefficients. The stream of droplets is produced by forcing liquid through a vibrating orifice. Experiments over a wide range of Knudsen numbers (Kn; 0.05–4.5), gas mixtures, and uptake coefficients (γ0; 0.01–1), show that gas-phase diffusive transport to a stream of fast moving droplets has the same functional dependence on the Knudsen number (Kn) as transport to a stationary droplet, except that the droplet diameter in the expression for Kn must be replaced by a factor measured to be 2.0 (±0.1) times the diameter of the droplet generating orifice. This factor has been measured for droplet forming orifices of diameters in the range 22– 70 μm producing droplets in the size range from ∼70 to 300 μm in diameter. That is, for a given orifice the effective Kn for a train of moving, closely spaced droplets produced at frequencies in the range ∼4– 60 kHz , depends only on the orifice diameter and not on the diameter of the droplets. Using this formulation, mass accommodation collision probabilities and liquid-phase solubilities and reaction kinetics have been measured for wide range of gases and liquids.

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.