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.

Source type and mechanism information for the four aerosol samples gathered from the Caribbean, the Sea of Japan, and 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(CH_n)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), CO₃^2- (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).

ALSNews Vol. 254, June 29, 2005