What are Aerosols? Suspension of very small solid particles or liquid droplets Radii typically in the range of 10nm to

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1 What are Aerosols? Suspension of very small solid particles or liquid droplets Radii typically in the range of 10nm to 10µm Concentrations decrease exponentially with height N(z) = N(0)exp(-z/H) Long-lived and may be advected over long distances

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4 α = 2πr λ

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8 Aerosol Concentrations Clean Continental Air = 3000 particles/cm 3 Polluted Continental Air = 50,000/cm 3 Urban Air = 160,000/cm 3 Desert Air = 2300/cm 3 Clean Marine Air = 1500/cm 3 Arctic Air = 6600/cm 3 Antarctic Plateau = 43/cm 3

9 Why are Aerosols Important? Aerosols are considered a climate forcing, which alters the radiation balance of the Earth direct radiative forcing: the scattering of solar radiation and the absorption/emission of terrestrial radiation indirect radiative forcing, mainly by effects of aerosols on cloud properties one of the greatest sources of uncertainty in interpretation of climate change of the past century and in projection of future climate change

10 The effect of Aerosols on clouds is uncertain More aerosol => smaller cloud droplets => higher albedo => net cooling r ~ N or r/r = -( N/N)/3 (Twomey el al., 1994) aerosols may create clouds where none existed before, because they act as cloud condensation nuclei Soot aerosols absorb radiation and may warm atmosphere

11 Measuring Aerosol Concentrations The Problem local sources, the high spatial/temporal variability of aerosols, and the complexity of aerosol properties cause aerosols to be too undersampled to really understand their distribution Large regional sources (e.g. sea salt, dust) hard to measure generation depends on wind, RH, soil moisture

12 SAGE II, III SME Robock (1983)

13 Ground Based Sensors Raman Lidar Aerosol Observation System

14 Raman Lidar Measures aerosol extinction and backscattering independently

15 Laser sends a pulse of light into the atmosphere and a telescope measures the intensity scattered back By measuring the scattering and attenuation experienced by the incident pulse of light, one can investigate the properties of the scatterers (concentration of gaseous species, aerosol distribution and optical properties, cloud height) located in the atmosphere

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17 Aerosol Observation System Measures aerosol absorption and scattering coefficients as a function of the particle size and radiation wavelength. Also measures particle number concentration, size distribution, hygroscopic growth, and inorganic chemical composition. Includes a Nephelometer and Absorption Photometer A nephelometer measures aerosol light scattering An absorption photometer measures the light absorption coefficient

18 Nephelometer detects scattering properties by measuring light scattered by the aerosol and subtracting light scattered by the gas, the walls of the instrument and the background noise in the detector b ext = b scat + b abs = ln(i/i 0 )/x

19 Absorption Photometer based on the integrating plate technique in which the change in optical transmission of a filter caused by particle deposition on the filter is related to the light absorption coefficient of the deposited particles using Beers Law.

20 Satellite Based Remote Sensing of Aerosols SAGE AVHRR TOMS

21 Stratospheric Aerosol Gas Experiment (SAGE) Uses solar occultations to measure the attenuation of solar radiation Profiles of aerosol extinction are provided at 9 wavelengths from the surface to 40km. Also measures optical depth, stratospheric ozone and water vapor

22 One month of SAGE II coverage Hampton Mauna Loa

23 Solar Occultations SAGE instrument points toward the sun and measures its intensity As the spacecraft sets below the horizon, the sun's intensity will be attenuated due to aerosols and gases in the atmosphere that scatter and absorb sunlight

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25 Advanced Very High Resolution Radiometer cross-track scanning system instantaneous field-of-view (IFOV) of each sensor is approximately 1.4 milliradians scanning rate of the AVHRR is six scans per second

26 Advanced Very High Resolution Radiometer (AVHRR) measures emitted and reflected radiation in five channels (bands) of the electromagnetic spectrum: blue (0.58 to 0.68 micrometer) used for daytime cloud and surface mapping; near-infrared (0.725 to 1.1 micrometer) band used for surface water delineation, and vegetation cover mapping; mid-infrared (3.55 to 3.93 micrometer) band used for sea surface temperature, and nighttime cloud mapping; thermal infrared (10.5 to 11.5 micrometer) band used for surface temperature,and day/night cloud mapping; thermal infrared (11.5 to 12.5 micrometer) band used for surface temperature mapping (Kidwell, 1991).

27 Stowe et al. (1997)

28 Total Ozone Mapping Spectrometer (TOMS) Initially used to detect total column ozone also detect SO 2, sulfate aerosols in the stratosphere, and UV absorbing aerosols (smoke, dust) over land and ocean Makes 35 measurements every 8 seconds each covering kilometers wide on the ground. Close to 200,000 daily measurements cover almost every spot on the Earth except for areas near the poles TOMS uses the ratio of the upwelling radiance (or spectral contrast) between the 340 nm and 380 nm channels (I340/I380)

29 Aerosol index a measure of how much the wavelength dependence of backscattered UV radiation from an atmosphere containing aerosols (Mie scattering, Rayleigh scattering, and absorption) differs from that of a pure molecular atmosphere (pure Rayleigh scattering). Quantitatively, the aerosol index AI is defined to be AI = -100log 10 [I meas /I calc ] Under most conditions, the AI is positive for absorbing aerosols and negative for non-absorbing aerosols (pure scattering).

30 TOMS Aerosol Movie

31 Mt Pinatubo Volcanic Eruption June 12-15, 1991 Resulting pumice, ash and chemicals (sulfuric acid) spread worldwide, affecting weather and climate for nearly 5 years Ash deposits surrounded the volcano for 20km, destroying most of the forests Aerosol layer circled the Earth in 21 days and covered 42% in two months Net global cooling of 0.5C for a period of 2-4 years after eruption

32 Pinatubo June 12, 1991 Three days before major eruption of June 15, 1991

33 These two photos show the Earth s limb at sunset before and after the Mt. Pinatubo eruption. The first view (STS41D-32-14) shows a relatively clear atmosphere, taken August 30,1984. Astronauts were looking at the profiles of high thunderstorms topping out at the tropopause at sunset; different atmospheric layers absorbed the last rays of light from the sun as the spacecraft moved eastward. The same type of photograph (STS ) was taken August 8, 1991, less than two months after the Pinatubo eruption. Two dark layers of aerosols make distinct boundaries in the atmosphere. The estimated altitude of aerosol layers in this view is 20 to 25 km.

34 Bataan Casadevall et al. (1996)

35 After Pinatubo, Clark Air Force Base 25 km from volcano

36 After Pinatubo, Cubi Point Naval Air Station, 40 km from volcano U.S. Navy photograph by R. L. Rieger

37 After Pinatubo, Subic Bay Naval Base 35 km from volcano

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