Aerosol mass spectrometry: Aerosol chemical and microphysical properties

1. Aerosol mass spectrometry: Aerosol chemical and microphysical properties Timothy B. Onasch, Douglas R. Worsnop, John T. Jayne, Manjula Canagaratna Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821 Dara Salcedo Universidad Autonoma del Estado de Morelos, Mexico Qi Zhang, Katja Dzepina, Jose L Jimenez Dept. of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309 R. Alfarra*, James Allan, Hugh Coe University of Manchester, UK * PSI, Germany April 29, 2005

2. Abstract Atmospheric aerosols play important roles in climatology, air quality, and human health. Assessment of these impacts requires detailed study of aerosol chemical and microphysical properties, including size-dependent chemistry and morphology. A recently developed Aerosol Mass Spectrometer (AMS) measures compositionally-resolved mass distributions and loadings of ambient aerosols in real-time. Sub-micron particles are aerodynamically focused into high vacuum and sized via particle time-of flight measurements. Flash vaporization, followed by 70 eV electron impact ionization mass spectrometry, provides universal and quantitative determination of mass loadings for the non-refractory components (that vaporize at 600-900 C). Mass spectra clearly separate inorganic (sulfate, nitrate, ammonium, chloride) and organic components, and provide insight into the chemical character of the organic matter. Beam width and light scattering probes measure shape and optical properties. Further aerosol characterization is achieved through comparisons with other gas and particulate instrumentation. For example, combining the mass distribution measurements with physical mobility size distribution measurements (measured via SMPS) provides detailed information of particle morphology and density. Aerosol measurements obtained in the field from ambient and source specific sampling will be presented. Analysis of aerosol mass, size, and composition trends obtained during these campaigns reveal diurnal variations in the photochemically driven formation of inorganic and oxidized organic aerosols. The chemical composition of these aerosols can be clearly distinguished from those observed during vehicular exhaust or biomass burning pollution events. In particular, algorithms have been developed for deconvolving the organic mass fraction into multiple mass spectral components (e.g. hydrocarbons, oxidized organics, etc.). These results, when combined with airmass backtrajectory calculations and ancillary gas phase measurements allow for the characterization of particle sources. Similarities and differences in aerosol size and chemical composition as well as aerosol formation and transformation at these widely varying sampling locations will be discussed.

3. Aerosols in the Atmosphere C.E. Kolb, Nature, 597-598, 6 Jun 2002

4. Outline Instrument – Aerosol Mass Spectrometer (AMS) Thermal vaporization and electron impact (EI) ionization Quantitative chemical speciation from mass spectrometry Organic mass spectrometry Hydrocarbon–based organic aerosol (HOA): Primary Combustion Oxygenated organic aerosol (OOA): Secondary Photochemistry Particle size distributions from time-of-flight analysis Morphology, mixing, and atmospheric aging Urban-to-rural measurements Aerosol characterization, processing, and climatology New mass spectrometric applications “Soft” vacuum ultraviolet (VUV) Photoionization High resolution time-of-flight aerosol mass spectrometry (ToF-AMS)

5. Aerosol Mass Spectrometer - Schematic Particle Inlet (1 atm) • Efficient transmission (30-1000 nm), aerodynamic sizing, linear mass signal • Non-refractory PM1.0 mass loadings and chemically-speciated mass distributions.

6. Flash Vaporization of Non-Refractory Components (NR) Electron Emitting Filament Focused Particle Beam e- Vaporizer 600 C R+ Electron Impact Ionization Positive Ion Mass Spectrometry AMS Particle Detection Process Universal and quantitative chemical analysis: 0.01mg/m3 sensitivity. Vaporization and analysis of most aerosol chemical constituents - with primary exception of crustal oxides and elemental carbon.

7. Group Molecule/Species Ion Fragments Mass Fragments Water H2O H2O+ , HO+ , O+18,17, 16 Ammonium NH3 NH3+, NH2+, NH+ 17, 16, 15 Nitrate HNO3 HNO3+, NO2+, NO+ 63, 46, 30 Sulfate H2SO4 H2SO4+, HSO3+, SO3+ 98, 81, 80 SO2+, SO+64, 48 Organic CnHmOy H2O+, CO+, CO2+ 18, 28, 44 (Oxygenated)H3C2O+, HCO2+, Cn’Hm+ 43, 45, ... Organic CnHm Cn’Hm’+27,29,41,43,55,57,69,71... (hydrocarbon) e- e- e- e- e- e- MS Signatures for Aerosol Species Identification color coded to match spectra Standard electron impact ionization @ 70 eV Easy to quantify: ca. NIST MS library Easy to separate inorganic and organic components Speciation of organic composition is challenging

8. Urban organic aerosol is a mixture of hydrocarbon and oxygenated components 43 44 57

9. AMS Mass Distributions Urban SiteBi-modal Local Sources Remote SiteMono-modal Aged-Transported Accumulation mode ‘Traffic’ mode ACE ASIA Scotland SASUA-3 Allan, Alfarra et al. (U. Manchester)

10. Primary and Secondary Organic Mass Distributions Morning Rush Hour Accumulation mode • Small mode primary traffic organic PM observed in m/z 57 • Accumulation mode oxidized aerosol dominated by m/z 44

11. Hydrocarbon (primary) and oxidized (secondary) organics m/z = 44 CO2+ m/z = 43 C3H7+ m/z = 57 C4H9+ Hydrocarbon series Ultra fine traffic mode

12. MS (m/z) Tracers for Organic Components Zhang et al., Environmental Science and Technology, 2004 m/z 57 (mostly C4H9+): Hydrocarbon-based Organic Aerosol (HOA) m/z 44 (mostly CO2+): Oxygenated Organic Aerosol (OOA) Gas & EC data from E. Wittig & R. Subramanian (CMU)

13. Deconvolution of HOA and OOA Components Zhang et al., Environmental Science and Technology, 2004 Organic Mass Spectra Customized Iterative Principal Component Analysis Time series & mass spectra of HOA and OOA Organic MS Tracers

14. Primary vs. Oxidized Organics Zhang et al., Environmental Science and Technology, 2004.

15. Mexico City Map Measurements were obtained at several fixed sites in Mexico City (Xalostoc, Merced, Cenica, Pedregal, and Santa Ana) with differing influences of air pollution sources and meteorology.

16. CENICA - Intensive Ground Site • University of Colorado (Katja Dzepina, Alex Huffman, Dara Salcedo, Prof. Jose Jimenez) operated an AMS at CENICA for full study • Aerodyne stationed the mobile laboratory at CENICA overnight and during specific time periods for comparisons • Measurements were obtained at several fixed sites (Pedregal, La Merced, Santa Ana) in Mexico City during 2003. • Time trends for mass loadings of components of NR PM1.0

17. Aerosol mass loadings in Mexico City City center, near busy market Urban university wind direction Boundary site

18. Diurnal trends in Organic and Nitrate PM Organic Nitrate Refuse burn • Highest organic loadings during mornings in city center near daily market. • Evening refuse burn at Santa Ana. • Large secondary aerosol pollution event throughout city. • Evidence for transport of PM pollution from city center to edge in afternoon.

19. Primary combustion aerosol: emission, dilution and aging, and transport Merced – city center, near busy market CENICA - Urban university and residential Santa Ana – boundary site

20. Urban Aerosols – City Center and Down Wind • City center • Residential urban • Boundary site

21. Organic Aerosol Climatology: Northern Hemisphere Collaborators: U. Colorado, U. Manchester (UK), SUNY-Albany, NIES (Japan)

22. Apr 2004 Diurnal Cycles: Urban and Rural Locations Apr 2003 Jan 2004 RuralUrban - Winter Urban- Summer

23. HOA and OOA Mass Spectra • HOA similar to POA from fuel combustion • Rural OOA more oxidized; similar to fulvic acid (model compound of highly aged/processed environmental organics)

24. HOA & OOA Mass Conc.

25. New mass spectrometric applications • “Soft ionization” Switch between EI and Photoionization • High resolution time-of-flight aerosol mass spectrometry (ToF-AMS)

26. Higher Sensitivity True Single Particle Detection ToF-AMS Q-AMS Mixed (NH4)2SO4 and Organic Singe Particle • ToF-AMS  Large enhancement of signal to noise • Size resolved, complete mass spectra (0-400 amu)  “single particle analysis” Drewnick, Hings DeCarlo et al, 2005

27. High Resolution MS Albumen (protein) particles Diesel Exhaust OOA Marker m/z 44: dominated by CO2+ HOA Marker m/z 57: dominated by C4H9+

28. Summary • Aerosol Mass Spectrometry is a powerful, quantitative tool for measuring real-time chemically-speciated mass loadings • Combining particle mass spectrometry and sizing measurements provides information on particle chemical (composition, aging, source) and physical characteristics (density, morphology) • Observe a ‘traffic’ mode in urban regions that consists of fractal soot particles with condensed hydrocarbon-based organics • Secondary inorganic and organic aerosol growth • Primary, fractal, organic particles evolve down wind from direct traffic sources, becoming less fractal through dilution, reaction, and/or condensation • Processed/aged aerosol in the accumulation mode appear to be mainly internally mixed inorganics and oxidized organics Acknowledgements Silke Weimer, Ken Demerjian ; SUNY Albany (New York City Winter & Summer) Frank Drewnick; Max Plank Institute of Chemistry, Germany (New York City Summer) Akinori Takami, Takao Miyoshi, Shiro Hatakeyama; NIES, Japan (Fukue, Okinawa)