ATMO 469b/569b, CHEE 469b/569b Air Pollution II: Aerosols Chemical Composition: New Frontiers Jan. 24, 2007 Dr. Song GAO

Typical Particle Diameters (  m) Photochemical aerosols Tobacco smoke0.25 Coal fly ash1-50 Flour dust15-20 Pollens15-70 Human hair: ~  m

Aerosols: Basic Properties Size Number Microstructure Chemistry (composition & transformation)

Organic Compounds in Ambient Aerosols Highly complex ensemble - species with various functional groups, polarity, volatility, solubility. Speciation of organic aerosols is a formidable analytical task.

These 107 compounds together comprise < 12% of the total organic mass in fine aerosols in the SE United States. Commonly in ambient aerosols, only 10 ~ 15% of the total organic matter (OM, OC) can be resolved into individual compounds.

Atmospheric Aerosols: tiny solid or liquid particles suspended in the air Perturb the Earth’s radiation balance both directly and indirectly. Probably partially counteract greenhouse gases in climatic effects. Have the largest uncertainty of all climate forcings [ IPCC, 2001 ].

Aerosols: Largest Uncertainty in Climate Forcings

Global Scale: Volcanoes, Aerosols & Climate Large volcano eruptions provide dramatic evidence of the ability of aerosols to affect global climate.

Importance of Organic Compounds in Atmospheric Aerosols Comprise a substantial fraction of aerosol mass: -In polluted areas, 25 ~ 65% can be organic in nature. [ Wolff et al., 1991; Chow et al., 1994; Novakov et al., 1997 ] - In MBL, at least 10% of aerosol mass is organic [ ACE-1 ]. Often present in a single particle with inorganics. [ Murphy et al., 1998; Noble and Prather, 1996 ]

Potential Roles in Global Climate (I): light absorption Not only black carbon, but also certain organic carbon in aerosols, can absorb solar radiation: [ Pöschl 2003, 2005; Kirchstetter et al., 2004 ] “Light-absorbing brown or yellow carbon”?

Potential Roles in Global Climate (II): hygroscopic growth; CCN activation Classical Köhler theory: based on inorganic salts as the solutes. Some organics activate in accordance with classical Köhler theory, while others deviate from it. Organics can alter the microstructure of aerosol particles. ( Surfactants  kinetic limitations of CCN/IN activation ) Modified Köhler equations (considering organics) are needed to accurately describe atmospheric processes [ Laaksonen et al., 1997; Charlson et al., 2001 ].

Potential Roles in Human Health Epidemiological studies: fine aerosols are correlated with severe health effects, including enhanced mortality, cardiovascular, respiratory, and allergic diseases. Toxicological studies: model and real aerosols can cause pulmonary toxicity.

Experimental Studies – Smog Chamber Inject model hydrocarbons, ozone and/or other reactants (& light). Measure aerosol size distribution, number concentration. Collect aerosol samples on filters and analyze composition with chromatography and mass spectrometry.

Precursor Hydrocarbon Molecular Structure Precursor Hydrocarbon Molecular Structure cyclopentene1-methyl cyclopentene cyclohexene1-methyl cyclohexene cycloheptene3-methyl cyclohexene cycloocteneα-pinene Structures of Model Hydrocarbons

Multiple Analytical Techniques Employed Liquid Chromatography – Mass Spectrometry HP 1100 Series HPLC – single quadrupole MS (ESI source) Ion Trap Mass Spectrometry Finnigan LCQ ion trap MS (ESI source) Laser Desorption/Ionization Mass Spectrometry Voyager-DE PRO time-of-flight (TOF) MS (MALDI source) High-Resolution Mass Spectrometry Waters LCT Premier TOF MS (ESI source) JEOL JMS-600H double-focusing, magnetic sector MS (FAB ionization)

Cycloolefins can be oxidized to form a variety of compounds, some of which have low enough vapor pressures to condense into the aerosol phase. Secondary Organic Aerosol (SOA) Formation Oxidation reaction mechanism : example. Gao et al. (2004) J. Phys. Chem. paper.

Detection of Oligomers in SOA (Olefins) Oligomers m/z: 329, 373, 417, 461, 505… + O 3

Oligomers in SOA (Aromatics – Kalberer et al Science paper)

Oligomers in SOA (Monoterpenes – Gao et al J Phys. Chem. paper) m/z Relative Abundance : cis-pinic acid 171: norpinic acid 199: OH - pinonic acid + O 3 Oligomers m/z: 329, 343, 357, 371, 385

Oligomers come from smaller oxidation products of initial hydrocarbons (Gao et al. (2004) ES&T paper) m/z Relative Abundance H 2 O Gem-diol reaction two norpinonic acid monomers:

Particle phase acidity has an explicit effect on oligomer formation and SOA yield: Higher acidity  faster oligomer formation  larger oligomers  higher SOA yield. Oligomers can form from α-pinene ozonolysis with or without pre-existing seed: The organic acids produced from hydrocarbon oxidation itself can readily facilitate oligomer formation in SOA.

Summary Oligomers comprise a substantial fraction of secondary organic aerosols generated in chamber studies. Acid-catalyzed heterogeneous reactions are the proposed pathways to form these oligomers. As a consequence, some “semi-volatile” compounds can stay in the aerosol phase and increase the global aerosol burden.

Open Questions  Not yet clear whether oligomers are abundant in ambient aerosols.  Models to formulate the global burden and distribution of aerosols need to be re- evaluated to account for these new compounds and their formation pathways.  New analytical methods need to be developed to fully understand aerosol composition, esp. organic species.

References Seinfeld, J. H.; Pankow, J. F. Annual Rev. Phys. Chem. 2003, 54, 121 – 140. Iinuma, Y.; Böge, O.; Gnauk, T.; Herrmann, H. Atmos. Environ. 2004, 38, 761 – 773. Kalberer, M.; Paulsen, D.; Sax, M.; Steinbacher, M.; Dommen, J.; Prevot, A. S. H.; Fisseha, R.; Weingartner, E.; Frankevich, V.; Zenobi, R.; Baltensperger, U. Science 2004, 303, “Low-molecular-weight and oligomeric components in secondary organic aerosol from the ozonolysis of cycloalkenes and α-pinene”, Gao, S., Keywood, M., Ng, N. L., Surratt, J., Varutbangkul, V., Bahreini, R., Flagan, R. C., Seinfeld, J. H., J. Phys. Chem. A, 108 (46), – 10164, doi: /jp047466e, “Particle phase acidity and oligomer formation in secondary organic aerosol”, Gao, S., Ng, N. L., Keywood, M., Varutbangkul, V., Bahreini, R., Nenes, A., He, J., Yoo, K. Y., Beauchamp, J. L., Hodyss, R. P., Flagan, R. C., Seinfeld, J. H., Environ. Sci. Technol., dio: /es049125k, “Characterization of polar organic components in fine aerosols in the Southeastern United States”, Gao, S., J. Surratt, E. Knipping, E. Edgerton, M. Shahgholi and J. H. Seinfeld, J. Geophys. Res., 111 (D14): Art. No. D14314, 2006.