Title: Twin Otter Aerosol Mass Spectrometer Data - Version: 2.0 Authors: Roya Bahreini* and Jose L. Jimenez** California Institute of Technology, MS 138-78 Departments of Environmental Science & Engineering and Chemical Engineering Pasadena CA 91125 (626) 395-4476 broya@caltech.edu / jose.jimenez@colorado.edu ** Now at Department of Chemistry and Biochemistry and CIRES, University of Colorado at Boulder, CO Richard C. Flagan and John H. Seinfeld California Institute of Technology, MS 210-41 Department of Chemical Engineering Pasadena CA 91125 (626) 395-4383 / (626) 395-4635 flagan@cheme.caltech.edu / seinfeld@caltech.edu John T. Jayne and Douglas R.Worsnop Aerodyne Research Inc. Billerica, MA 01821 (978) 663-9500 jayne@aerodyne.com / worsnop@aerodyne.com * Corresponding authors Data Set Overview: Airborne measurements of aerosol phase sulfate, nitrate, ammonium, and organics were made using an Aerodyne Aerosol Mass Spectrometer (AMS). The AMS operated successfully on 15 out of total of 19 research flights during the period of March 31-May 1, 2001 in an area that covered 127 E- 135 E and 32 N- 38 N on longitude and latitude. The paths of all the flights in which the AMS operated successfully are shown in Figure 1. Instrument Description: The Twin Otter has a main inlet that samples air 1 m in front of the airplane iso-kinetically and at large flow rate. Almost all the air sampled by this inlet is exhausted towards the back of the plane. The AMS draws a small sample from the center of the stream. In the AMS, air passes through a critical orifice and an aerodynamic lens based on the design of Liu et al. [Liu et al., 1995a; Liu et al., 1995b] and Zhang et al.[Zhang et al., 2002a; Zhang et al., 2002b]. The lens focuses the particles, in the aerodynamic diameter range of 40 nm- 1 ?m, into a narrow beam which then enters a differentially pumped vacuum chamber. The particle beam is modulated by a chopper wheel. Upon gas expansion into the vacuum chamber, particles acquire a size-dependent velocity. Particle aerodynamic size can be determined from the measured particle time of flight. The particle beam is directed onto a resistively heated surface (~500 (C) under high vacuum. Upon impaction, the volatile/semi-volatile aerosol components in/on the aerosols are vaporized. These vapors are then electron impact ionized (70 eV), and the positive ions formed are mass-analyzed with a quadrupole mass spectrometer [Jayne et al., 2000; Jimenez et al., 2002]. The AMS, as operated in this study, is not sensitive to refractory aerosol components, such as mineral dust, sea salt, and black carbon. However, it will detect the volatile and semi-volatile components internally mixed with a refractory aerosol particle. Data Collection and Processing: The AMS has two modes of operation. Size-resolved mass distributions of pre-selected ion signals are determined in the TOF (Time of Flight) mode. The MS (Mass Spectrum) mode provides an ensemble mass spectrum of all the volatile species present in/on the aerosols. For either mode, the raw TOF or MS signal can be converted into mass concentration. Sulfate mass concentration is estimated from MS mode signals of SO+(m/z=48 amu), SO2+(m/z=64 amu), SO3+(m/z=80 amu), HSO3+(m/z=81 amu), and H2SO4+(m/z=98 amu). The fact that all these fragments show the same time and size dependence indicates that there has not been significant interferences of other species at m/z of sulfate fragments. This lack of large interferences for sulfate fragments has been observed in other studies as well [Jimenez et al., 2002]. Nitrate concentration is based on MS mode signals of NO+ (m/z=30 amu) and NO2+ (m/z=46 amu). The calculated ammonium concentration is based on TOF mode signals at m/z = 16 amu, which is due to NH2+. Interference of O+ ions arising from air molecules at m/z = 16 is corrected for by removing its TOF signal at small apparent sizes (<100 nm). Other fragments of ammonium, namely NH4+ (m/z=18 amu), NH3+ (m/z=17 amu), and NH+ (m/z=15 amu), were not used in the analysis because of high background resulting from H2O+ (m/z=18 amu) and OH+ (m/z=17 amu) or low signal and interference by the CH3+ fragment of organics (at m/z=15 amu). Organics mass concentration is estimated from MS mode signals that are not due to air molecules, ammonium, sulfate, and nitrate, or other inorganics (Cl, Ca, etc). Figure 2 represents the major organic fragments and their contribution to the total organic mass [Bahreini et al., 2002]. In-flight TOF and MS data were recorded with 1-min time resolution. All concentrations are reported in ?g m-3 (at STP: 1 atm, 298 K). One-minute detection limits can be defined as three times the standard deviation of the 1-min signal during periods with very low signal. Table 1 summarizes the estimated detection limits during ACE-Asia [Bahreini et al., 2002]. Data Format: Data files are in space-delimited format. The files are: AMS_TO_Alt_LONG_LAT.prn Parameters Remarks Date UTC Time UTC time Altitude_m Altitude (m) Longitude Decimal Degree Latitude Decimal Degree AMS_SO4_NO3_Organics_MassConc.prn Parameters Remarks Date_EndTime UTC, Time stamp at end of 1-min averaging time CenterTime UTC, Time stamp at center of 1-min averaging time SO4_microg/m3 Mass concentration of sulfate (?g m-3, STP) NO3_microg/m3 Mass concentration of nitrate (?g m-3, STP) Org_microg/m3 Mass concentration of organics (?g m-3, STP) AMS_NH4_MassConc.prn Parameters Remarks Date_EndTime UTC, Time stamp at end of 1-min averaging time CenterTime UTC, Time stamp at center of 1-min averaging time NH4_microg/m3 Mass concentration of ammonium (?g m-3, STP) Data Remarks: At this time, only mass concentration data are provided. Negative concentration measurements are due mostly to instrumental noise when the ambient concentration of the species was very low. They have not been removed from the dataset so as to not introduce a positive bias in averages of our data for longer time periods. Ammonium concentration data is limited to the flights during which TOF signal for ammonium was recorded. Size distributions of sulfate, ammonium, organics, and nitrate will be provided at a later time. In addition, at this time, the data for the flights during which the AMS has had power interruptions is not submitted (Twin Otter RF6 and RF19). NOTE: A more complete readme file in Word format is included with the dataset when it is ordered References: Bahreini, R., J.L. Jimenez, R.C. Flagan, J.H. Seinfeld, J.T. Jayne, and D.R. Worsnop, Aircraft-based Aerosol Size and Composition Measurements during ACE-Asia using an Aerodyne Aerosol Mass Spectrometer, Submitted to J. Geophys. Res., Special ACE-Asia Issue, 2002. Jayne, J.T., D.C. Leard, X. Zhang, P. Davidovits, K.A. Smith, C.E. Kolb, and D. Worsnop, Development of and Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles, Aerosol Science and Technology, 33, 49-70, 2000. Jimenez, J.L., J.T. Jayne, Q. Shi, C.E. Kolb, D.R. Worsnop, I. Yourshaw, J.H. Seinfeld, R.C. Flagan, X. Zhang, K.A. Smith, J. Morris, and P. Davidovits, Ambient Aerosol Sampling with an Aerosol Mass Spectrometer, J. Geophys. Res., In Press, 2002. Liu, P., P.J. Ziemann, D.B. Kittelson, and P.H. McMurry, Generating Particle Beams of Controlled Dimensions and Divergence: I. Theory of Particle Motion in Aerodynamic Lenses and Nozzle Expansions, Aerosol Science and Technology, 22, 293-313, 1995a. Liu, P., P.J. Ziemann, D.B. Kittelson, and P.H. McMurry, Generating Particle Beams of Controlled Dimensions and Divergence: II. Experimental Evaluation of Particle Motion in Aerodynamic Lenses and Nozzle Expansions, Aerosol Science and Technology, 22, 314-324, 1995b. Zhang, X., K.A. Smith, D.R. Worsnop, J.L. Jimenez, J.T. Jayne, and C.E. Kolb, A Numerical Characterization of Particle Beam Collimation by an Aerodynamic Lens-Nozzle System. Part I: Individual Lens and Nozzle, Aerosol Science and Technology, 36, 617-631, 2002a. Zhang, X., K.A. Smith, D.R. Worsnop, J.L. Jimenez, J.T. Jayne, C.E. Kolb, J. Morris, and P. Davidovits, Numerical Characterization of Particle Beam Collimation by Aerodynamic Lens-Nozzle System. Part II: Whole Inlet, In preparation for Aerosol Science and Technology, 2002b.