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Understanding Air Pollution Reactivity for Smog Formation

Learn about the mechanisms and factors influencing hydrocarbons reactivity for urban smog (ozone) formation. Explore the sinks of air pollutants, reactions, and useful techniques for calculating fluxes or lifetimes.

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Understanding Air Pollution Reactivity for Smog Formation

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  1. LECTURE 12 AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON

  2. HYDROCARBRONS REACTIVITYFOR URBAN SMOG (OZONE) FORMATION

  3. Faster rate constant implies more reactivity and more smog (O₃) produced. For detailed mechanism see “Development of Ozone Reactivity Scales for Volatile Organic Compounds” by W.P.L. Carter, EPA-91:epavoc, 1991. Rates increase with increasing number of C atoms, with branching, and with sites of instauration (double bonds). Emissions From Autos

  4. Abstraction by O atoms Example Ozone oxidation of alkenes Example: oxidation of propene (propylene). OH attack on alkanes

  5. SINKS OF AIR POLLUTANTS • RAINOUT/WASHOUT • Only for soluble gases and particles • Lifetime the same as that for water ~10 days • Lifetime increases with altitude • DRY DEPOSITION • Only for “sticky” or reactive gases and particles • Rate determined by atmospheric turbulence, chemical and physical properties of both the atmospheric species and the surface, i.e., bare soil, vegetation etc. • REACTIONS • Transformation to other species, usually by oxidation.

  6. continue….

  7. Useful technique for calculating fluxes or lifetimes. • When the atmosphere shows horizontal uniformity, production and loss reduce to a 1 D problem. • This holds when vertical gradients are much greater than horizontal gradients and when the species X is in steady state. • Let z be altitude (m), F flux (g m-2s-1), [X] concentration (g/m3), k’ the pseudo first order rate constant (s-1) for loss of X, t is lifetime of X.

  8. Example for fertilized soil NO emissions: • We want to know the emission rate. • We have the NO profile at night; this only works at night. • NO goes from 20 mg/m3 at the surface to essentially zero at 100 m with a scale height of 10 m. • The column content is therefore • 10m*20x10-6g m-3 = 2x10-4 g m-2 • We know ozone is roughly constant at 50 ppb, therefore at RTP the lifetime is ~100 s. More generally, you can integrate with [O3](z) and k(z). • If t is a constant then k’is a constant:

  9. Example for crop soil NO emissions, continued:

  10. Example: What is the lifetime of SO2 over the eastern US?

  11. The emission flux is monitored.

  12. Figure IIa SO2 Emissions (tons/day) 0-20 20-75 75-150 150-300 300-500 Locations of flights made with aircraft (shown with black airplanes). Location of power plants emitting SO2 shown in pink circles (size of circle represents size of emissions for July 13, 2002).

  13. Lifetime of SO2 over the eastern US. See Lee et al., (2011).

  14. 2006 2010 2014 • Column SO2 from Krotkov et al., ACPD 2016.

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