Final Report: Heterogeneous Chemistry of Sub-micron Aerosols Related to Tropospheric OxidantsEPA Grant Number: R825253
Title: Heterogeneous Chemistry of Sub-micron Aerosols Related to Tropospheric Oxidants
Investigators: Davidovits, Paul , Jayne, J. T. , Kolb, Charles E. , Worsnop, Douglas R.
Institution: Boston College
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999
Project Amount: $440,343
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:During the past decade, research in atmospheric chemistry has highlighted the critical role of atmospheric aerosols in trace gas chemical processing. In order that atmospheric models may accurately account for and predict environmental trends, quantitative laboratory measurements of gas-particle heterogeneous chemistry are needed to provide accurate data for determining rates of aerosol nucleation and growth. Of equal importance is a thorough understanding of the heterogeneous chemistry that takes place between these aerosol particles and key atmospheric trace gas constituents. Unfortunately, up till now adequate techniques to perform laboratory aerosol kinetics measurements which simulate both atmospheric aerosol size distributions and aerosol chemical compositions have not been available. The lack of such aerosol kinetics data has been a great hindrance in the development of atmospheric models that reliably include the effect of aerosol on chemistry and climate.
The purpose of the program funded by the U.S. Environmental Protection Agency (EPA) Office of Exploratory Research, Grant R825253 was to develop new laboratory instrumentation capable of studying important atmospheric heterogeneous processes involving aerosols and to apply these techniques to the study of relevant heterogeneous aerosols kinetics. A new apparatus has been built which combines an aerosol flow reactor with a chemical ionization mass spectrometer (CIMS) for gas phase species detection and a newly developed aerosol mass spectrometer (AMS) capable of measuring both the size and composition of submicron particles. The first set of heterogeneous aerosols kinetic studies has been completed.
Operational Principles of the Apparatus. The apparatus consists of four separate components: (1) an aerosol generation/conditioning device, (2) atmospheric pressure flow reactor for performing controlled aerosol kinetics, (3) an aerosol sampling mass spectrometer for determining aerosol size and chemical composition, and (4) a separate molecular beam sampling mass spectrometer for gas phase species detection. The schematic of the complete system is shown in Figure 1. Aerosols are generated and then injected into the flow reactor which contains the trace gas or gases of interest. The flow gas may also contain other gas phase species whose effect on the aerosol interaction is to be studied. Time dependent variations for both gas density and aerosol composition are monitored as a function of gas-particle interaction distance and aerosol size. At the exit end of the flow reactor the aerosol is sampled by the aerosol mass spectrometer where both aerodynamic size and composition are measured.
Figure 1. Gas -Particle heterogeneous kinetics apparatus. Gas phase species are detected by the chemical ionization mass spectrometer (CIMS) and particle size and composition detection is performed using the mass spectrometer (AMS).
The novel aspect of this device centers on the aerosol mass spectrometer, (AMS) which is discussed in greater detail below. The other components, while also important, are discussed in lesser detail.
1. Aerosol Generation. The aerosol generation system consists of a constant output collision atomizer (TSI, model 3076) for producing a polydisperse aerosol, a diffusion dryer, to dry the moist atomized aerosol, a differential mobility analyzer (TSI model 3071) and an optical particle counter (TSI, model 3010) to pre-size and count the aerosol before entering the flow reactor. As will be discussed below the AMS can also quantify aerosol size and number density.
2. Aerosol Flow Reactor. The aerosol flow reactor is designed to operate near atmospheric pressure in both laminar and turbulent flow regimes. The high pressure mode of operation allows submicron size aerosols to be effectively suspended in the flow gas. Depending on the nature of the study, either the aerosol or the trace gas is introduced into the flow reactor through a central moveable injector. Under atmospheric pressure, turbulent flow residence times from ~.05 to 2.0 seconds can be achieved for a 2.2 cm diameter, 200 cm length tube. Longer residence times (on the order of several seconds) can be achieved by operating the flow tube under slower, laminar flow conditions.
3. Aerosol Sampling Mass Spectrometer. The aerosol mass spectrometer (AMS) has been designed to simultaneously measure mass and number density distribution as a function of composition for micron and submicron sized particles containing volatile and/or semi-volatile components. The instrument employs an aerodynamic lens which forms a well defined particle beam (~1mm diameter), directed into a vacuum system. Aerodynamic particle diameter is determined via a particle time-of-flight (TOF) measurement with size resolution comparable to differential mobility analysis over the size range of 40 nm to 5 mm diameter range. The beam of particles is directed into a resistively heated vaporizer in the particle detection chamber where the volatile and semi-volatile components flash vaporize. The vaporized molecular constituents are ionized either by electron impact or with a pulsed eximer laser at 248 nm. The ions are extracted and analyzed using conventional quadrupole mass spectrometry.
This analysis approach provides a quantitative measure of compositionally resolved particle mass loading and also has the capability to detect and count single particles when the quadrupole mass spectrometer is tuned to a representative mass. The important advantage of this instrument is its ability to measure aerosol mass distributions with very high size resolution on the time scale of seconds. The instrument is coupled to a fast data acquisition and data analysis computer which sets the quadrupole mass and measures the ion signal as a function of particle TOF. Data are collected and processed in real-time. The operation of the AMS is described in detail by Jayne et al. (2000)
4. Gas Phase Species Detection. The mass spectrometer for gas phase species detection is a molecular beam sampling quadrupole system which can be operated using either conventional electron bombardment (EI), or high pressure chemical ionization (CI). In the EI mode this system has detection limit in the 100 ppb range. However, using our chemical ionization source, detection sensitivity on the order of 10 partspertrillion (2x108 cm-3) has been observed in our lab for SO2 sampled from one atmosphere of nitrogen gas. This part of the apparatus is being constructed and is scheduled to be completed in August 2000.
The capabilities of the apparatus have been demonstrated and three kinetics studies have been performed as described.
Aerosol Kinetic Studies. The use of this apparatus as a quantitative instrument requires knowledge of: (1) the density of the gas phase species of interest in the reaction zone; (2) the number of aerosol particles/cc in the reaction zone (particle loading), and (3) the area of the aerosol particles. The species gas phase density is determined by calibrating and monitoring the carrier + trace gas flow into the reaction zone using the mass spectrometer either in the EI or CE mode. The aerosol number density is determined by sampling the flow into a condensation optical particle counter (TSI, model 3010). The area of the aerosol is established by the differential mobility analyzer - TOF combination. We have performed the following three kinetic studies.
1. Uptake of Phenanthrene Vapor By Dioctyl Phathalate (DOP) Aerosols. For this experiment we used dioctyl phathalate (DOP) to form the aerosol. Primary organic aerosol is well modeled by DOP. The aerosol was pre-sized using a differential mobility analyzer (DMA) prior to injection into the aerosol flow tube. Phenanthrene, which is one of the EPA listed hazardous air pollutants, was then vaporized and admitted into the flow reactor where it was observed to be taken up by the DOP seed aerosol.
This DOP-phenanthrene experiment demonstrates aerosol growth. The data provide basic, previously unavailable, information about interactions of a polycyclic aromatic hydrocarbon (PAH) gas phase species with organic aerosol matter. The data yielded a value for the uptake coefficient (g) of phenanthrene on DOP of about 10-3.
2. Interaction of Asphalt-Released PAH Compounds With Ammonium Sulfate Aerosol. In this experiment, ammonium sulfate aerosol, 250 nm in diameter, was passed over asphalt. The results are shown in Figure 2. First, the asphalt is held at room temperature (middle trace B) and then it is heated to 1250C. (This is in the range of temperatures at which asphalt is deposited on road surfaces.) The PAH compounds released from the heated asphalt are deposited on the aerosol which is then vaporized and analyzed in the aerosol mass spectrometer. The PAH compounds deposited on the aerosol are clearly detected in the aerosol mass spectrometer (bottom trace C). We note that in the absence of the aerosol, the PAH vapor released by the asphalt is not detected by the aerosol mass spectrometer because if not deposited on a particle, very little of the species enters that region. Quantitative calibration of the data is in progress.
Figure 2. Interaction of asphalt vapor with 250 nm (NH4)2SO4 aerosol. (A) AMS signal: back ground (B) AMS signal: aerosol + asphalt at 23oC (C) AMS signal: aerosol + asphalt at 125oC
3. Reaction of Oleic Acid Aerosols With Ozone. In this experiment oleic acid was introduced into the flow tube in the presence of ozone. Oleic acid decay curves for several ozone concentrations and particle sizes have been obtained. Modeling of the results indicates that reaction ozone with oleic acid aerosol (which is a liquid at room temperature) occurs primarily in a thin outer shell of depth 10-20 nm. Products diffuse from the reaction zone into the core of the particle. Thus, rapid consumption of ozone depends on relatively fast transport of reactant within the particle, which is the case for liquid droplets. For solid particles, the diffusion of both reactants is slower by orders of magnitude; thus, the overall uptake for solid particles must be much slower.
Increases in viscosity may also decrease the uptake rate. Stearic acid is a solid at room temperature, because it is more efficient at packing than the bent-chain oleic acid. We expect that it will increase the viscosity of the liquid particles and reduce the diffusion rate. Preliminary results show that a 10% admixture of stearic acid reduces the overall uptake rate by a factor of 2. This surprisingly strong effect is being investigated in detail.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
|Other project views:||All 16 publications||3 publications in selected types||All 3 journal articles|
||Jayne JT, Leard DC, Zhang XF, Davidovits P, Smith KA, Kolb CE, Worsnop DR. Development of an aerosol mass spectrometer for size and composition analysis of submicron particles. Aerosol Science and Technology 2000;33(1-2):49-70.||
||Shi Q, Davidovits P, Jayne JT, Worsnop DR, Kolb CE. Uptake of gas-phase ammonia. 1. Uptake by aqueous surfaces as a function of pH. Journal of Physical Chemistry A 1999;103(44):8812-8823.||
||Worsnop DR, Shi Q, Jayne JT, Kolb CE, Swartz E, Davidovits P. Gas-phase diffusion in droplet train measurements of uptake coefficients. Journal of Aerosol Science 2001;32(7):877-891.||