Ambient organic aerosols: Where they come from, and where they are going (some more slowly than others).

摘要

Although the inorganic component of ambient aerosols is relatively well understood, many questions remain concerning organic aerosols. One is whether some organic aerosols are kinetically limited from acting as cloud condensation nuclei (CCN). In the summer of 2006, we measured the rate of cloud droplet formation on atmospheric particles sampled at four sites across the United States. Overall, most ambient CCN grew at a rate similar to ammonium sulfate, but approximately one third grew more slowly. We made similar measurements in the summer of 2007 at a coastal California site. We observed persistent bimodal diameter spectra, and the slowly-growing mode contained 10-25% of all particles and had mass accommodation coefficients 10 -- 30 times smaller than that measured for ammonium sulfate.;While much aerosol hygroscopicity data is available at lower relative humidities (RH) and at supersaturations (RH> 100%), relatively little data is available at high RH (99.2--99.9%). We measured the size of droplets at high RH that had formed on particles composed of one of seven compounds with dry diameters between 0.1 and 0.5 mum. The hygroscopicity of the inorganic compounds was relatively constant with both RH and Dwet. In contrast, most organics, particularly sodium dodecyl sulfate (SDS), showed a slight to mild increase in hygroscopicity with Dwet. This suggests that surface partitioning tends to cancel out the increase in hygroscopicity due to surface tension reduction, increasingly so at low Dwet. SDS hygroscopicity did not decrease at lower RH, as would be expected if micelles formed. These results suggest that surface-activity parameters obtained from macroscopic solutions with organic solutes may be inappropriate for calculations of the hygroscopicity of micron-sized droplets.;Finally, we examined possible sources of ambient organic aerosols. A new analytical technique for aersol water-soluble organic carbon (WSOC) was applied to samples collected at four sites across the United States. It involves the separation of samples by size-exclusion chromatography, followed by the measurement of the 13C content. We observed a pattern of 13C depletion in molecules with larger hydrodynamic diameters in Tennessee, indicating that oligomerization reactions produce secondary aerosol where biogenic emissions such as terpenes are prevalent.