ATMO 469b/569b, CHEE 469b/569bAir Pollution II: Aerosols

Chemical Composition: New Frontiers

Jan. 24, 2007

Dr. Song GAO

(songatmo@email.arizona.edu)

Typical Particle Diameters (m)

Photochemical aerosols

0.01-1

Tobacco smoke 0.25

Coal fly ash 1-50

Flour dust 15-20

Pollens 15-70

Human hair: ~25-100 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.

CH3

CH3

CH3

Precursor Hydrocarbon

Molecular Structure

Precursor Hydrocarbon

Molecular Structure

cyclopentene 1-methyl cyclopentene

cyclohexene 1-methyl cyclohexene

cycloheptene 3-methyl cyclohexene

cyclooctene α-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)0324n_f #1-20 RT: 0.01-0.34 AV: 20 NL: 3.71E4T: - p Full ms [ 70.00-800.00]

100 150 200 250 300 350 400 450 500 550 600

m/z

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Re

lative

Ab

un

da

nce

373.2

417.2

159.0

329.1145.0 461.2

96.9

272.9 505.4

259.0175.0243.1

194.9549.5 598.7431.2289.1 478.8336.8 520.2130.9

Oligomers m/z:

329, 373, 417, 461, 505…+ O3

Oligomers in SOA (Aromatics – Kalberer et al. 2004 Science paper)

Oligomers in SOA (Monoterpenes – Gao et al. 2004 J Phys. Chem. paper)

O

OH

OOH

OHO

O

OH

O

OHO

OH

200 300 400 500 600 700 800 900 1000 1100 1200

m/z

0

5

10

15

20

25

30

35

40

45

50

Rel

ativ

e A

bund

ance

185.0

198.9

356.7

342.8214.8

863.1

370.7328.8 864.1862.1384.6 648.7 1030.8626.8 1158.4560.6 784.8484.9 738.8 942.5

185: cis-pinic acid

171: norpinic acid

199: OH - pinonic acid

+ O3

Oligomers m/z: 329, 343, 357, 371, 385

Oligomers come from smaller oxidation products of initial hydrocarbons (Gao et al. (2004) ES&T paper)

160 180 200 220 240 260 280 300 320 340 360 380 400m/z

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Rel

ativ

e A

bu

nda

nce

170.9

185.0

357.0

338.9

313.1295.0 356.3170.2 256.7215.1 392.0357.6 375.7200.7 338.3281.0238.5

HO

OH

O

O OH

O

O -

171

313

185

357

- H2O

Gem-diol reaction two norpinonic acid

monomers:

O

OH

O

• 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, 1659 - 1662.

• “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), 10147 – 10164, doi: 10.1021/jp047466e, 2004.

• “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: 10.1021/es049125k, 2004.

• “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.