1. Aerosols Dr. Martin Leach November 1, 2010

2. Atmospheric Aerosols Bibliography Seinfeld & Pandis, Atmospheric Chemistry and Physics, Chapt. 7-13 Finlayson-Pitts & Pitts, Chemistry of the Upper and Lower Atmosphere, Chapt. 9. Classic papers: Prospero et al. Rev. Geophys. Space Phys., 1607, 1983; Charlson et al. Nature 1987; Charlson et al., Science, 1992. Recent Papers: Ramanathan et al., Science, 2001; Andreae and Crutzen, Science, 1997; Dickerson et al., Science 1997; Jickells et al., Global Iron Connections Between Desert Dust, Ocean Biogeochemistry and Climate, Science, 308 67-71, 2005.

3. Aerosols: General Comments  Any solid, liquid (or mixture) in the atmosphere  Sources –Natural –Anthropogenic (urban, construction, agriculture) –Primary (introduced directly into the atmosphere) –Secondary (formed in the attmosphere)  Any solid, liquid (or mixture) in the atmosphere  Sources –Natural –Anthropogenic (urban, construction, agriculture) –Primary (introduced directly into the atmosphere) –Secondary (formed in the attmosphere)

4. Aerosol Effects  Climate  Weather  Visibility  Health Effects  Climate  Weather  Visibility  Health Effects Clouds?

5. Natural Sources and Estimates of Global Emissions of Atmospheric Aerosols SourceAmount-range (Tg yr -1 ) Amount -best estimate (Tg yr -1 ) Soil Dust1000-30001500 Sea Salt1000-100001300 Botanical Debris26-8050 Volcanoes4-1000030 Forest Fires3-15020 Gas conversion100-260180 Photochem40-20060 Total2200-240003100

6. Anthropogenic Sources of Aerosols SourceAmount Range (Tg yr -1 ) Best Estimate Direct Emission50-160120 Gas to particle260-460330 Photochemistry5-2510 Total320-640460 Reference: W.C. Hinds, Aerosol Technology, 2nd Edition, Wiley Interscience

7. Gas-to-particle conversion:  Certain gas phase reactions result in formation of low-vapor-pressure reaction products.  Because of their low vapor pressure, they exist at high supersaturations and can form particles.  Certain gas phase reactions result in formation of low-vapor-pressure reaction products.  Because of their low vapor pressure, they exist at high supersaturations and can form particles.

8. Natural Background Aerosol  Stratospheric –Major volcanic activity injects sulfur dioxide (SO 2 ) into the stratosphere –Gas to particle conversion, SO 2 into sulfuric acid (H 2 SO 4 )  Tropospheric –Vegetation, deserts and ocean –Primarily in the lowest few km  Stratospheric –Major volcanic activity injects sulfur dioxide (SO 2 ) into the stratosphere –Gas to particle conversion, SO 2 into sulfuric acid (H 2 SO 4 )  Tropospheric –Vegetation, deserts and ocean –Primarily in the lowest few km

9. Mount Pinatubo, 1991

10. Urban Aerosol  Dominated by anthropogenic sources  Three Modes –NucleiAitken –AccumulationLarge –CoarseGiant  Dominated by anthropogenic sources  Three Modes –NucleiAitken –AccumulationLarge –CoarseGiant What is meant by the size of an aerosol? What does a size distribution mean?

11. ORIGIN OF THE ATMOSPHERIC AEROSOL Soil dust Sea salt Aerosol:Size range: 0.001  m (molecular cluster) to 100  m (small raindrop) Environmental importance: health (respiration), visibility, radiative balance, cloud formation, heterogeneous reactions, delivery of nutrients…

12. AEROSOL NUCLEATION # molecules 1 2 3 4 GG cluster sizeCritical cluster size Surface tension effect Thermo driving force

13. Atmospheric Aerosols

14. Question?  Considering the Urban Aerosol, where are most of the particles? Where is the most mass?  How many 0.01  m particles are necessary to have the same mass as one 1  m particles?  Considering the Urban Aerosol, where are most of the particles? Where is the most mass?  How many 0.01  m particles are necessary to have the same mass as one 1  m particles?

15. Urban Aerosol Size Distribution

16. Nuclei Mode (<0.1  m)  Consist of: –Direct combustion particles emitted –Particles formed by gas-to-particle conversion  Usually found near sources of combustion (e.g. highways!)  Due to their high number concentration: –Coagulate rapidly. –End up in accumulation mode –Relatively short lifetime  Consist of: –Direct combustion particles emitted –Particles formed by gas-to-particle conversion  Usually found near sources of combustion (e.g. highways!)  Due to their high number concentration: –Coagulate rapidly. –End up in accumulation mode –Relatively short lifetime Aitken Particles

17. Accumulation Mode (0.1 μm < particle size < 2.5 μm)  Includes combustion particles, smog particles, and coagulated nuclei-mode particles. (Smog particles are formed in the atmosphere by photochemical reactions)  Particles in this mode are small but they coagulate too slowly to reach the coarse-particle mode. –they have a relatively long lifetime in the atmosphere –they account for most of the visibility effects of atmospheric aerosols.  The nuclei and accumulation modes together constitute “fine” particles.  Includes combustion particles, smog particles, and coagulated nuclei-mode particles. (Smog particles are formed in the atmosphere by photochemical reactions)  Particles in this mode are small but they coagulate too slowly to reach the coarse-particle mode. –they have a relatively long lifetime in the atmosphere –they account for most of the visibility effects of atmospheric aerosols.  The nuclei and accumulation modes together constitute “fine” particles. Large Particles

18. Coarse-particle mode (particle size > 2.5 μm)  Consist of –Windblown dust, large salt particles from sea spray, –Mechanically generated anthropogenic particles such as those from agriculture and surface mining.  Due to their large size –Readily settle out or impact on surface, –Lifetime in the atmosphere is only a few hours.  Consist of –Windblown dust, large salt particles from sea spray, –Mechanically generated anthropogenic particles such as those from agriculture and surface mining.  Due to their large size –Readily settle out or impact on surface, –Lifetime in the atmosphere is only a few hours. Giant Particles

19. Dynamic Processes of Atmospheric Aerosol  Formation –Gas to particle conversion –Photochemical processes  Growth –Coagulation, condensation, evaporation  Removal –Settling –Deposition –Rainout, washout  Formation –Gas to particle conversion –Photochemical processes  Growth –Coagulation, condensation, evaporation  Removal –Settling –Deposition –Rainout, washout

20. Global Effects of Aerosols  Global Cooling –Direct effect –Indirect effect  Ozone depletion –Polar stratospheric clouds (PSC) –Surfaces of PSC act to catalyze Cl compounds to atomic Cl  Global Cooling –Direct effect –Indirect effect  Ozone depletion –Polar stratospheric clouds (PSC) –Surfaces of PSC act to catalyze Cl compounds to atomic Cl

21. What is the mean diameter of the particles?"  The answer to this question changes with your point of view.  What size particles carry the most mass? (Biogeochemical cycles)  What size particles cover the largest surface area? (visibility)  What is the size of the most abundant particles? (cloud microphysics)  The answer to this question changes with your point of view.  What size particles carry the most mass? (Biogeochemical cycles)  What size particles cover the largest surface area? (visibility)  What is the size of the most abundant particles? (cloud microphysics)

22. Aerosol Distributions Number  cloud formation Surface  visibility Volume  mass Mass & Number  human health Number  cloud formation Surface  visibility Volume  mass Mass & Number  human health

23. Number distribution function  The number of particles with diameter between D p and D p + dD p in a cm 3 f n (D p ) dD p (particles cm -3 /  m)  The total number of particles, N: N =  f n (D p ) dD p (particles cm -3 )  The number of particles with diameter between D p and D p + dD p in a cm 3 f n (D p ) dD p (particles cm -3 /  m)  The total number of particles, N: N =  f n (D p ) dD p (particles cm -3 )

24. Surface Area Distribution Function  The surface area of particles in a size range per cm 3 of air f s (D p )dD p =  D p 2 f n (D p ) (  m 2  m -1 cm -3 )  The total surface area of the particles, S, is given by the integral over all diameters: S =  f s (D p ) dD p =   D p 2 f n (D p ) dD p (  m 2 cm -3 )   The surface area of particles in a size range per cm 3 of air f s (D p )dD p =  D p 2 f n (D p ) (  m 2  m -1 cm -3 )  The total surface area of the particles, S, is given by the integral over all diameters: S =  f s (D p ) dD p =   D p 2 f n (D p ) dD p (  m 2 cm -3 ) 

25. Volume Distribution Function  The Volume distribution function can be defined f v (D p ) dD p = {  /6} D p 3 f n (D p ) (  m 3  m -1 cm -3 )  So the total volume occupied can be written V =  f v (D p ) dD p =   /6 D p 3 f n (D p ) dD p (  m 3 cm -3 )  The Volume distribution function can be defined f v (D p ) dD p = {  /6} D p 3 f n (D p ) (  m 3  m -1 cm -3 )  So the total volume occupied can be written V =  f v (D p ) dD p =   /6 D p 3 f n (D p ) dD p (  m 3 cm -3 )

26. Log Normal  Distributions based on log D p can be defined n(log D p )dlogD p is the number of particles in one cm 3 with diameter from D p to D p + log D p. The total number is: N =  n(log D p ) d(logD p ) (particles cm -3 ) n (log D p ) = {dN} / {N dlogD p } n s (log D p ) = {dS} / {S dlogD p } n v (log D p ) = {dV} / {V dlogD p } This is the common notation for expressing the variation in particle number, surface area or volume with the log of the diameter.  Distributions based on log D p can be defined n(log D p )dlogD p is the number of particles in one cm 3 with diameter from D p to D p + log D p. The total number is: N =  n(log D p ) d(logD p ) (particles cm -3 ) n (log D p ) = {dN} / {N dlogD p } n s (log D p ) = {dS} / {S dlogD p } n v (log D p ) = {dV} / {V dlogD p } This is the common notation for expressing the variation in particle number, surface area or volume with the log of the diameter.

27. 27 Aerosol particle size distribution

28. Distributions which look like Gaussian distributions (“normal” distributions) when plotted with a logarithmic x-axis are called lognormal This size distribution has 2 lognormal modes

29. TYPICAL U.S. AEROSOL SIZE DISTRIBUTIONS Fresh urban Aged urban rural remote Warneck [1999]

30. SAMPLE AEROSOL SIZE DISTRIBUTION (MARINE AIR) Sea salt Sulfate (natural)

31. COMPOSITION OF PM2.5 (NARSTO PM ASSESSMENT)

32. Aerosols: Visibility Washington, DC

33. Light Extinction  I/I = e (-b  X) I0I0 I absorption scattering XX Intensity Extinction Coefficientb (in a few more slides)

34. EPA REGIONAL HAZE RULE: FEDERAL CLASS I AREAS TO RETURN TO “NATURAL” VISIBILITY LEVELS BY 2064   Acadia National Park clean day moderately polluted day http://www.hazecam.net/ …will require essentially total elimination of anthropogenic aerosols!

35. Radiation and fine particles Seinfeld and Pandis, 1998

36. Atmospheric Visibility Atmospheric Visibility (absorption & scattering) 1.Residual 2.Scattered away 3.Scattered into 4.Airlight 1.Residual 2.Scattered away 3.Scattered into 4.Airlight

37. b ext = b gas + b particles b ext = b abs + b scatt b abs (gases) = Beer's Law absorption b scatt (gases) = Rayleigh Scattering b abs (particles) = Usually < 10% of extinction b scatt (particles) = Mie Scattering = (b sp ) Extinction Coefficient

38. Visibility  The ultimate limit in a very clean atmosphere is Rayleigh scattering  Mie scattering usually dominates.  The range of b sp is 10 -5 m -1 to 10 -3 m -1.  The ultimate limit in a very clean atmosphere is Rayleigh scattering  Mie scattering usually dominates.  The range of b sp is 10 -5 m -1 to 10 -3 m -1.

39. Single scattering albedo   is a measure of the fraction of aerosol extinction caused by scattering:  = b sp /(b sp + b ap )   is a measure of the fraction of aerosol extinction caused by scattering:  = b sp /(b sp + b ap )

40. Optical Properties of Small Particles m = n + ik m = complex index of refraction n = scattering (real part) k = absorption (imaginary part) m = n + ik m = complex index of refraction n = scattering (real part) k = absorption (imaginary part) The real part of the index of refraction is only a weak function of wavelength, while the imaginary part, ik, depends strongly on wavelength.

41. Refractive indicies of aerosol particles at = 589 nm m = n + ik Substancenk Water1.33310 -8 Ice1.30910 -8 NaCl1.5440 H 2 SO 4 1.4260 NH 4 HSO 4 1.4730 (NH 4 ) 2 SO41.5210 SiO 2 1.550 Black Carbon (soot)1.960.66 Mineral dust~1.53~0.006

42. The scattering cross section is the product of the mass loading, and the surface area per unit mass; note the ln of 0.02 is about -3.9, thus Visibility ≈ 3.9(b sp ) -1 b sp = S  m Where b sp is the scattering coefficient in units of m -1 m is the mass loading in units of g m -3 S is the surface area per unit mass in units of m 2 g -1 For sulfate particles, S is about 3.2 m 2 g -1 where the humidity is less than about 70%; for other materials it can be greater. Visibility = 3.9/(3.2 m) = 1.2 /(m) Scattering Cross Section

43. Example: Visibility improvement during the 2003 North American Blackout Normal conditions over Eastern US during an air pollution episode: b sp ≈ 120 Mm -1 = 1.2 x 10 -4 m -1 at 550 nm b ap = 0.8 x 10 -5 m -1 b ext = 1.28 x 10 -4 m -1 Visual Range ≈ 3.9/b ext = 30 km During blackout b sp = 40 Mm -1 = 0.4 x 10 -4 m -1 b ap = 1.2 x 10 -5 m -1 b ext = 0.52 x 10 -4 m -1 Visual Range = 3.9/b ext = 75 km

44. Example: Visibility improvement during the 2003 North American Blackout Single scattering albedo, , normal = 1.20/1.28 = 0.94 Blackout = 0.4/0.52 = 0.77 With the sulfate from power plants missing, and the soot from diesel engines remaining the visual range is up, but the single scattering albedo is down. Ozone production inhibited. See: Marufu et al., Geophys Res. Lett., 2004.

45. Extinction Coefficient as a PM2.5 Surrogate PM 2.5 = 7.6  g/m 3 PM 2.5 = 21.7  g/m 3 PM 2.5 = 65.3  g/m 3 Glacier National Park images are adapted from Malm, An Introduction to Visibility (1999) http://webcam.srs.fs.fed.us/intropdf.htm

46. ANNUAL MEAN PARTICULATE MATTER (PM) CONCENTRATIONS AT U.S. SITES, 1995-2000 NARSTO PM Assessment, 2003 PM10 (particles > 10  m)PM2.5 (particles > 2.5  m) Red circles indicate violations of national air quality standard: 50  g m -3 for PM10 15  g m -3 for PM2.5

47. AEROSOL OPTICAL DEPTH (GLOBAL MODEL) Annual mean

48. AEROSOL OBSERVATIONS FROM SPACE Biomass fire haze in central America (4/30/03) Fire locations in red Modis.gsfc.nasa.gov

49. BLACK CARBON EMISSIONS Chin et al. [2000] DIESEL DOMESTIC COAL BURNING BIOMASS BURNING

50. RADIATIVE FORCING OF CLIMATE, 1750- PRESENT “Kyoto also failed to address two major pollutants that have an impact on warming: black soot and tropospheric ozone. Both are proven health hazards. Reducing both would not only address climate change, but also dramatically improve people's health.” (George W. Bush, June 11 2001 Rose Garden speech) IPCC [2001]

51. ASIAN DUST INFLUENCE IN UNITED STATES Dust observations from U.S. IMPROVE network April 16, 2001 Asian dust in western U.S. April 22, 2001 Asian dust in southeastern U.S. Glen Canyon, AZ Clear day April 16, 2001: Asian dust! 0 2 4 6 8  g m -3

52. LONGITUDE ALTITUDE (km) 100E150E150W 100W TRANSPACIFIC TRANSPORT OF ASIAN DUST PLUMES Subsidence over western U.S. Source region (inner Asia) Asian plumes over Pacific GEOS-CHEM Longitude cross-section at 40N, 16 April, 2001 0 5 10 ASIA UNITED STATES T.D. Fairlie, Harvard

53. Aerosols in the Atmosphere: Abundance and size  Aerosol concentration is highly variable in space and time. Concentrations are usually highest near the ground and near sources.  A concentration of 10 5 cm -3 is typical of polluted air near the ground, but values may range from 2 orders of magnitude higher in very polluted regions to several lower in very clean air.  Radii range from ~ 10 -7 cm for the for small ions to more than 10 µm (10 -3 cm) for the largest salt and dust particles.  Small ions play almost no role in atmospheric condensation because of the very high supersaturations required for condensation.  The largest particles, however, are only able to remain airborne for a limited time  Aerosol concentration is highly variable in space and time. Concentrations are usually highest near the ground and near sources.  A concentration of 10 5 cm -3 is typical of polluted air near the ground, but values may range from 2 orders of magnitude higher in very polluted regions to several lower in very clean air.  Radii range from ~ 10 -7 cm for the for small ions to more than 10 µm (10 -3 cm) for the largest salt and dust particles.  Small ions play almost no role in atmospheric condensation because of the very high supersaturations required for condensation.  The largest particles, however, are only able to remain airborne for a limited time

54. Summary:Origins of Atmospheric Aerosols 1.Condensation and sublimation of of vapors and the formation of smokes in natural and man-made combustion. 2.Reactions between trace gases in the atmosphere through the action of heat, radiation, or humidity. 3.The mechanical disruption and dispersal of matter at the earth’s surface, either as sea spray over the oceans, or as mineral dusts over the continents. 4.Coagulation of nuclei which tends to produce larger particles of mixed constitution 1.Condensation and sublimation of of vapors and the formation of smokes in natural and man-made combustion. 2.Reactions between trace gases in the atmosphere through the action of heat, radiation, or humidity. 3.The mechanical disruption and dispersal of matter at the earth’s surface, either as sea spray over the oceans, or as mineral dusts over the continents. 4.Coagulation of nuclei which tends to produce larger particles of mixed constitution

55. Cloud Condensation Nuclei - CCN  Comprises a small fraction of the total aerosol population  Sea salt is the predominant constituent of CCN with D > 1µm  For 0.1 µm < D < 1 µm, the main component is thought to be sulfate, which may be present as sulfuric acid, ammonium sulfate, or from phytoplankton produced dimethylsulfide (see Charlson et al., Nature, 326, 655-661).  Comprises a small fraction of the total aerosol population  Sea salt is the predominant constituent of CCN with D > 1µm  For 0.1 µm < D < 1 µm, the main component is thought to be sulfate, which may be present as sulfuric acid, ammonium sulfate, or from phytoplankton produced dimethylsulfide (see Charlson et al., Nature, 326, 655-661).

56. 56 INDOEX, 1999 INDOEX: Indian Ocean Experiment

57. 57 Mean Aerosol Optical Depth over INDOEX region from Dec 2001 to May 2003 from MODIS (Ramanathan & Ramana, Environ. Managers, Dec. 2003). + RV Ronald Brown INDOEX

58. 58 INDOEX From Ramanathan 2001 0 to 3 km layer

59. 59 NOAA R/V Ronald Brown

60. 60 Air Flow During INDOEX 1999

61. 61 Field data showing the high variability of aerosol light absorption coefficient with latitude and longitude, measured by NOAA/PMEL scientists aboard the NOAA Research Vessel Ron Brown during the Aerosols 99 and INDOEX (Indian Ocean Experiment) cruises. The aerosol light absorption coefficient is presented in all figures in units of Mm -1. Measurements are made at a wavelength of 550nm. (Courtesy of P.Quinn and T. Bates, NOAA/PMEL.)

62. Summary of Aerosol Physics  How big are atmospheric particles depends on which effect interests you. –CCN – number (r < 0.1  m) –Radiative transfer & health – surface area (0.1 < r < 1.0  m) –Biogeochemical cycles – mass (r > 0.5  m).  Composition varies with size.  Single scattering albedo and visibility  How big are atmospheric particles depends on which effect interests you. –CCN – number (r < 0.1  m) –Radiative transfer & health – surface area (0.1 < r < 1.0  m) –Biogeochemical cycles – mass (r > 0.5  m).  Composition varies with size.  Single scattering albedo and visibility