Introduction to Aerosol Dynamics and Chemistry

1. Introduction to Aerosol Dynamics and Chemistry Chao-Jung Chien Regional Modeling Center, Center for Environmental Research and Technology , UC-Riverside

2. Outline • Introduction • Aerosol dynamics • Physical processes leading to particle production/growth • Visibility • Aerosol chemistry • Chemical processes transforming particles • Secondary organic aerosol • CMAQ vs. IMPROVE (aerosol model evaluation)

3. Aerosol Definitions • Aerosols: relatively stable suspensions of solid or liquid particles. • Properties: • Size, chemical composition, hygroscopiscity, density, and shape. • 0.002-10 mm size range important with respect to atmospheric chemistry and physics.

4. Impacts of Aerosol • Direct and indirect radiative forcing (Global warming) • Health effects • Acid deposition • Regional Visibility • Chemical deposition budget for ecosystems All effects depend on Size, Massand Composition.

5. Reproduced from “Chemistry of the Upper and Lower Atmosphere”, Finlayson-Pitts and Pitts, 2000

6. Particle Size Distribution and National Ambient Air Quality Standards (NAAQs)

7. Types of Aerosols • Marine Aerosol • 100 ~ 300 #/cc; high coarse particle mass, but low number. • Coarse marine aerosol are composed of salt from evaporated spray droplets. • Fine aerosols formed from DMS reaction products. • Remote Continental Aerosol • 2000-10000 #/cc; produced naturally over land. • Dust, pollen, or oxidation products from ammonia and sulfates. • Background against anthropogenic emissions • Urban Aerosol • As high as 108 to 109 #/cc; • Coarse particles contain crustal elements (Fe, Si, etc.) • Fine particles mainly from combustion sources, or by gas to particle conversion involving reaction products of sulfates, nitrates, ammonium and organics.

8. Fates of Atmospheric Aerosol • Sources: • Direct emission (primary particles) • Physical and Chemical processes (secondary particles) • Sinks: • Wet (rain out), • dry deposition • Transformations: • Coagulation, • Condensation, • Cloud Processing

9. Aerosol Dynamics • Mathematical Representations of the Aerosol Size Distribution • Discrete Distribution (Sectional Approach) • uses discrete size bins • very expensive for good size resolution • used in CAMx • Continuous Distribution (Modal Approach) • uses moments of log normal distributions • CMAQ uses 3 modes: Aitken, Accumulation, Coarsemodes. • Three integral properties are included: • Total particle number concentration • Total surface area concentration • Total mass concentration of the individual chemical components

10. Particle Production • Primary Emissions: In CMAQ… • Assumed PM2.5 mostly (~0.999) in accumulation mode– need to be evaluated. • 90% of PM10 is estimated to be fugitive dust– assigned to coarse mode (as ASOIL), among which 70% is PM25. • Emission rates (E) for aerosol number:

11. Particle Production/Growth • Nucleation • Process by which a gas interacts and combines with droplets. • Can occur in the absence or presence of foreign material. • Homogeneous nucleation • Heterogeneous nucleation • The production rate of new particle mass (mg/m3/s): d3.5: diameter of the 3.5 nm particle r: density of the particle at ambient relative humidity

12. Particle Growth (Mode Changing) • Coagulation: the process by which small particles collide with and adhere to one another to form larger particles. • Condensation: a process by which molecules in the atmosphere collide and adhere to small particles. • Mode Merging by Renaming • Growing particles reassigned to a larger mode and averaged with the new mode. gas molecules condens. nuclei

13. Cloud Processing of Aerosols • Aitken mode respond to in-cloud scavenging. • Accumulation mode forms cloud condensation nuclei. • Aqueous oxidation of SO2 by oxidants in cloud producing dissolved sulfate. • New sulfate mass is added to accumulation mode.

14. Aerosol Dry Deposition • Deposition rate depends on particle size, surface resistance. • Deposition velocity governed by • Brownian particle diffusivity, and • Gravitational settling velocity • In CMAQ, species mass in each mode is deposited separately.

15. Typical Time Scales for Various Aerosol Fates Adapted from Pandis et al., 1995

16. Aerosol Light Scattering and Absorption • Visibility – the furthest distance one can see and identify an object in the atmosphere. • Light absorption • Scattering • Rayleigh scattering: D << l ( D £ 0.03 mm, 290 < l < 750 nm) • Mie scattering: D ~ l ( 0.03 < D < 10 mm ) • Extinction coefficient • Sum of the scattering and absorption coefficients.

17. Visibility • Deciview index, deciV • Extinction coefficient, • Mie theory extinction • Reconstructed extinction bext(1/m) = 3*fRH*([(NH4)2SO4]+[NH4NO3]) + 4*[OC] + 10*[LAC] + [SOIL] + 0.6*[CM]

18. Aerosol Chemistry • Chemical Composition • Reactions involved in particle formation and growth • Thermodynamic of aerosols • Aerosol liquid water content • Chemical Equilibrium • Chemical reactions • Secondary organic aerosol

19. Aerosol Composition • Inorganics: sulfate, nitrate, ammonium, crustal species, sea salt, hydrogen ions, and water • Organics Measured size distributions of aerosol sulfate, nitrate, ammonium, chloride, sodium, and hydrogen in Claremont, CA (Reproduced from “Atmospheric Chemistry and Physics”, Seinfeld and Pandis, 1998).

20. Organic Atmospheric Aerosols • Components • Elemental Carbon (EC) • Black carbon or graphitic carbon • Emitted during combustion processes • Organic Carbon (OC) • Primary OC, directly emitted by sources • Secondary OC, formed in-situ by condensation of low-volatility products of the photooxidation of hydrocarbons

21. Aerosol Liquid Water Content • ZSR method • Influenced by relative humidity and ionic ratio • Chemical equilibrium: sulfate, nitrate, ammonium NH4NO3(s) NH3(g) + HNO3(g) W: liquid water content (kg/m3) Mn: conc. of nth species (moles/m3) mn0: molality (moles/kg3) aw: water activity

22. Chemical Reactions involved in particle formation and growth • Reactions of gases to form low-vapor-pressure products followed by nucleation, condensation, or coagulation between particles. • Reactions of gases on the surfaces to form condensed-phase products • NaCl(s) + HNO3(g) HCl(g) + NaNO3(s) • Reactions within the aqueous phases in fogs, cloud, or aerosol particles. • SO2 oxidation to sulfate

23. Formation of Secondary Organic Aerosol (SOA) • Reactions of alkanes, alkenes, and aromatics with atmospheric oxidants (OH, O3, NO3 etc.) to form condensable multifunctional oxygenated products. • Reactions of biogenic hydrocarbons, e.g. terpenes, to produce biogenic organic aerosol species. • Production rates are passed from the photochemical component to aerosol component. Representations are limited and require improvement.

24. Gas/Particle Partitioning • Yield of SOA as a function of the amount of organic material “already” in the particle phase – Pankow, 1994 • Adsorption to particle surfaces • Mainly mineral particles • Absorption into aerosol organic matter • Important for SOCs Kp(m3/mg): partitioning constant TSP(mg/m3): total suspended particles conc. F, A(ng/m3): particulate & gaseous associatedconc. of compounds of interest

25. Gas/Particle Partitioning • Adsorption pLo (torr): vapor pressure of the pure compound Ns (sites/cm2): surface conc. of sorption sites atsp (m2/g): particle surface area Q1, Qv (kJ/mol): enthalpy of vaporization R: gas constant T (K): temperature

26. Gas/Particle Partitioning • Absorption fom: weight fraction of TSP in organic matter (om) phase MWom: mean molecular weight of the om phase z: activity coefficient

27. Gas-Particle Partitioning • Aerosol Yields Odum et al. 1996

28. Atmospheric Aerosol Chemistry

29. References • Binkowski, F. S., Science Algorithms of the EPA Models-3 Community Multiscale Air Quality (CMAQ) Modeling System, EPA/600/R-99/030, 1999. • Seinfeld, J. H. and Pandis, S. N., Atmospheric Chemistry and Physics – From Air Pollution to Climate Change, 1998. • Finlayson-Pitts, B. J. and Pitts, J. N. Jr., Chemistry of the Upper and Lower Atmosphere – Theory, Experiments, and Applications, 2000. • Lane, D. A., Gas and Particle Phase Measurements of Atmospheric Organic Compounds, 1999. • d’Almeida, G. A., Koepke, P., and Shettle, E. P., Atmospheric Aerosols – Global Climatology and Radiative Characteristics, 1991.

30. CMAQ vs. IMPROVE Evaluation of Model Performance