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Composition and Reactions of Atmospheric Aerosol Particles Print

Microscopic aerosol particles in the atmosphere contain carbonaceous components from mineral dust and combustion emissions released from around the world. How long these tiny particles remain in the atmosphere can have a huge impact on the global climate. Measurements based on high-resolution scanning transmission x-ray images obtained at the ALS have revealed chemical reactions on and in atmospheric aerosol particles that caused particle growth while changing organic composition by 13 to 24% per day, an oxidation rate significantly slower than is currently used in atmospheric models. Since oxidation has a strong effect on particle lifetime in the atmosphere, these results will help climate scientists refine the computer models used to predict climate change.

Tiny Specks with Large Effects

Most people equate aerosols with hairspray and household cleaning products, but a large portion of these microscopic particles floating in the air actually originate from the incomplete burning of coal and oil and dust storms. According to mathematical models of the earth's climate, if the particles remain airborne long enough, they can contribute to both atmospheric heating and cooling. In effect, the longer they linger, the more time they have to wreak havoc on Earth's climate. By means of high-resolution x-ray microscopy of particles collected at several locations around the world, Maria et al. have determined that carbon-containing aerosol particles oxidize more slowly than earlier estimates indicated. Since oxidized aerosols fall relatively rapidly back to the earth in rain, the slower oxidation buys them more time to do their atmospheric damage. These longer-lived particles will increase the carbon-containing aerosol burden on climate models by up to 70 percent.

A large portion of the microscopic particles floating in the air originate from incomplete combustion of coal and oil and from dust storms. Once in the atmosphere, they can have either cooling or warming effects. Lighter-colored organic carbon particles cool regions of the planet by scattering sunlight back into space. Other aerosol particles composed of black carbon, or soot, warm the atmosphere by absorbing sunlight and heating the surrounding air. Impacts like these are why scientists want to know how long carbon-containing aerosols remain in the atmosphere.

Source type and mechanism information for the four aerosol samples gathered from the Caribbean, the Sea of Japan, and New Jersey.

One way to gauge an aerosol's ability to stay aloft is to determine its oxidation rate. Because oxidized aerosols absorb moisture and subsequently form clouds and fall as rain, the faster an aerosol particle oxidizes, the less time it spends in the atmosphere, and the less impact it has on the climate. Hoping to learn more about aerosol particle oxidation rates and confident that measurements of aerosol particle composition could reveal signatures of atmospheric chemical reactions such as oxidation, a multi-institutional collaboration based at the University of California, San Diego, examined a variety of carbon-containing aerosol particles that had undergone vastly different journeys.

Aerosol particles for scanning transmission x-ray microscopy (STXM) analysis were collected on silicon nitride substrates by impaction on the National Center for Atmospheric Research C-130Q aircraft over the Caribbean Sea in July 2000 and over the Sea of Japan in April 2001. The sources of the particles collected were determined to be Asian combustion and African mineral dust, respectively. Additional aerosol samples representing eastern U.S. combustion were collected on lacey-carbon transmission electron microscopy (TEM) grids in New Jersey in August 2003 on both clear and foggy days. In all, more than 120 particles were analyzed as part of eight samples from the Caribbean Sea, from eastern Asia, and from New Jersey.

The group carried out STXM measurements near the carbon absorption K edge at ALS Beamlines 7.0.1 and 5.3.2 in a helium-filled sample chamber maintained at 1 atm. X-ray transmission images, typically spanning an area of 64 mm2, were scanned sequentially at increasing photon energies in the range 279–305 eV to create a stack of images from which a spectrum could be extracted at each point. The jump in the carbon K-edge absorbance (namely the difference above and below the absorption edge), which is linearly related to the number of absorbing atoms, was used as a semi-quantitative measure of total carbon, with absorbance below the carbon edge quantifying total mass. In this way, maps of carbon and total mass were constructed.

Images, speciated maps of detectable regions, and representative spectra of measured organic and inorganic species for marine boundary layer particles near St. Croix in the Caribbean [Russell et al., Geophys. Res. Lett.29, 10.1029/2002GL014874 (2002)]. High-resolution soft x-ray images (a,e,i) show (a) an absorbance image at 300 eV for particles collected at 310 m and (e,i) images at 289.9 eV for particles at 30 m. Threshold contours (b,f,j) at the detection limits are drawn for the same three samples: R(C=C)R0 (purple) (285.0 ± 0.2 eV), R(C=O)R (cyan) (286.7 ± 0.2 eV), R(CHn)R0 (blue) (287.7 ± 0.7 eV), R(C=O)OH (green) (288.7 ± 0.3 eV), σ* transition for CNH (orange) (289.5 ± 0.1 eV), CO32- (yellow) (290.4 ± 0.2 eV), K+ (red) (294.6, 297.2 ± 0.2 eV), and Ca2+ (magenta) (347.9, 351.4 ± 0.4 eV). Enlarged maps (c,g,k) and spectra (d,h,l) for representative particles labeled (i–vi) illustrate detailed composition characteristics for each sample.

These measurements revealed surface- and volume-limited chemical reactions on and in atmospheric aerosol particles. The observed much slower oxidation rates mean that organic aerosols will reflect more radiation because of increased atmospheric lifetimes, producing a 70% increase of the organic aerosol burden. These changes in organic aerosol burden will change the radiation balance of the atmosphere, increasing cooling by as much as 47%, while also causing offsetting warming changes of up to 61%.

Two-dimensional spectrally resolved maps of organic composition for particles representative of four aerosol samples. The colors in the left column represent various ratios of total carbon to total mass; the colors in the right column represent various ratios of carbonyl carbon to total carbon.

Research conducted by S.F. Maria (SciTec Inc.); L.M. Russell (Scripps Institution of Oceanography, University of California, San Diego); M.K. Gilles (Berkeley Lab); and S.C.B. Myneni (ALS and Princeton University).

Research funding: National Science Foundation and James S. McDonnell Foundation. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences (BES).

Publication about this research: S.F. Maria, L.M. Russell, M.K. Gilles, and S.C.B. Myneni, "Organic aerosol growth mechanisms and their climate forcing implications," Science 306, 1921 (2004).