The quantum yields for CO and CH(4), formed in the photolysis of about 200 μmol mol(-1) of acetaldehyde in air at atmospheric pressure, were determined at 15 wavelengths in the range: 250-330 nm. The quantum yields for CO(2) were determined at nine wavelengths in the range: 250-315 nm. The products are mainly assigned to three primary processes: I) CH(3)CHO*→CH(3)+HCO, II) CH(3)CHO*→CH(4)+CO, III) CH(3)CHO*→CH(3)CO+H, with dissociation occurring from the initially populated S(1) singlet state of acetaldehyde. The pressure dependence of the quantum yields at wavelengths of 270, 304.4, and 313 nm exhibits a Stern-Volmer behavior in all cases. Collisional quenching is shown to deactivate the singlet state S(1) by pressure-induced intersystem crossing to the neighboring triplet state, which is subsequently rapidly quenched by oxygen. Quenching coefficients as a function of excitation energy are obtained by comparison with literature data; the wavelength dependence of individual primary quantum yields is also derived. CO(2) quantum yields at 304.4 and 313 nm were found to increase with rising CH(3)CHO concentrations due to secondary reactions. The reaction mechanism responsible for this effect was explored by means of computer simulations.

The room-temperature photodecomposition of acetone diluted with synthetic air was studied at nine wavelengths in the spectral region 250-330 nm. The quantum yields for the products CO 2 and CO indicated that it was not possible to suppress secondary reactions sufficiently, even with acetone/air mixing ratios as low as 150 ppmv, to derive from these data primary acetone photodissociation quantum yields. The behavior of CO 2 and CO formation nevertheless provides some insight into the mechanism of acetone photodecomposition. When small amounts of NO 2 are added to acetone/air mixtures, peroxyacetyl nitrate (PAN) is formed. Quantum yields for PAN are reported. They are better suited to represent primary quantum yields for acetone photodissociation, because PAN is a direct indicator for the formation of acetyl radicals. The data were combined with absorption cross-sections for acetone measured at wavelengths up to 360 nm to calculate photodissociation coefficients applicable to the ground4evel atmosphere at 40 ° northern latitude. Comparison with the rates for the reaction of acetone with OH radicals shows that both processes contribute almost equally to the total acetone losses in the lower atmosphere. The resulting atmospheric life time at 40 ° northern latitude is 32 days, on average. This value must be considered an upper limit, since it does not take into account acetone losses due to the reaction of excited triplet acetone with oxygen.

Two procedures for the calibration of an electron capture detector (ECD) for peroxyacetyl nitrate (PAN) are discussed. One is based on the first-order decay rate of the the PAN mixing ratio in conditioned glass storage vessels. The other method makes use of the photochemical generation of PAN in mixtures of acetone and NO z in air. For this purpose a Penray Hg lamp was inserted into a glass vessel filled with 1 atmosphere of air containing 10 ppm NO2 and 1% acetone. After 3 min of irradiation, the average PAN mixing ratio formed was 8.87 _+0.25 ppmv as determined in six separate runs.

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