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Dark-phase experiments between isoprene and 0 3 are discussed. UNC outdoor chamber experiments have shown that in high-concentration systems of isoprene and 0 3 (5 ppm C and 1 ppm) approximately 75% of the reacted carbon can be observed in the product formation of HCHO, CO, methacrolein, methylvinylketone, methylglyoxal, acetaldehyde, and propylene.Mechanisms were developed which gave reasonable fits to dark-phase chamber experiments of MACR, MVK, isoprene, and 03. Experimental data and modeling results were used to generate 0 3 rates of attack on MVK and MACR. An isoprene-03 rate of 1.67 X ppm-l-rnin-l was used and is consistent with other rates reported in the literature. Dark isoprene-03 systems appear to form homogeneously nucleated aerosol. Most of these particles appear and remain a t diameters well below the optical cutoff region (0.3-0.5 Fm), as opposed to the particles from similar a-pinene-03 systems, which also form at smaller sizes hut then grow into the optical size range (>0.5 Fm). Lower concentrations of a-pinene and O3 (0.2 ppm C and 0.12 ppm) still generated substantial aerosol, hut by comparison, rapid CN nucleation was not observed during a similar side-by-side system of isoprene and 03.

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