The reaction between HO2 and RO2 radicals represents an important chemical sink for HO x radicals in the atmosphere. On the basis of a few product yield studies, these reactions are believed to form hydroperoxides almost exclusively (R8a), although several different reaction channels may be thermodynamically accessible (R8a−d):  RO2 + HO2 → ROOH + O2 (R8a); RO2 + HO2 → ROH + O3 (R8b); RO2 + HO2 → RO + OH + O2 (R8c); and RO2 + HO2 → R‘CHO + H2O + O2 (R8d). Branching ratios for reaction R8 were measured for three organic peroxy radicals:  ethyl peroxy (C2H5O2), acetyl peroxy (CH3C(O)O2), and acetonyl peroxy (CH3C(O)CH2O2) radicals. Product yields were measured using a combination of long-path Fourier transform infrared spectroscopy and high-performance liquid chromatography with fluorescence detection. Measured branching ratios for the reaction of the three organic peroxy radicals with HO2 are as follows:  ethyl peroxy, Y 8a - ETH > 0.93 ± 0.10, Y 8b - ETH = Y 8c - ETH = Y 8d - ETH = 0; acetyl peroxy, Y 8a - ACT = 0.40 ± 0.16, Y 8b - ACT = 0.20 ± 0.08, Y 8c - ACT = 0.40 ± 0.16, and Y 8d - ACT = 0; acetonyl peroxy, Y 8a - ACN = 0.33 ± 0.10, Y 8b - ACN = Y 8d - ACN = 0, and Y 8c - ACN = 0.67 ± 0.20. The atmospheric implications of these branching ratios are discussed.

The reactions of ozone with alkenes have been shown recently to lead to the direct production of OH radicals in quantities that vary from 7 to 100% depending on the structure of the alkene. OH radicals are the most important oxidizing species in the lower atmosphere, and the OH−alkene reaction is a large source of new OH radicals, important in urban and rural air during both day and night. Evidence for OH formation comes both from low-pressure direct measurements and from tracer experiments at high pressure. With the goal of measuring OH formation yields with good precision, a small-ratio relative rate technique was developed. This method uses small amounts of fast-reacting aromatics and aliphatic ethers to trace OH formation yields. Here, we report OH formation yields for a series of terminal alkenes reacting with ozone. Measured OH yields were 0.29 ± 0.05, 0.24 ± 0.05, 0.18 ± 0.04, and 0.10 ± 0.03 for 1-butene, 1-pentene, 1-hexene, and 1-octene, respectively. For the methyl-substituted terminal alkenes methyl propene and 2-methyl-1-butene, OH yields were 0.72 ± 0.12 and 0.67 ± 0.12, respectively. The results are discussed both in terms of their atmospheric implications and the relationship between structure and OH formation.

The mass loadings of quinones and their ability to generate reactive oxygen species (ROS) were investigated in total suspended particulate samples collected in Fresno, CA, over a 12-month period. Particles were collected on Teflon filters and were analyzed for the presence of 12 quinones containing one to four aromatic rings by gas chromatography with mass spectrometry. Measured levels are generally greater than mass loadings reported at other locations. The mass loadings were highest during winter months and were strongly anticorrelated with temperature. ROS generation was investigated by measuring the rate of hydrogen peroxide production from the reaction of laboratory standards and ambient samples with dithiothreitol (DTT). ROS generation from ambient samples shows a strong positive correlation with the mass loadings of the three most reactive quinones and may account for all of the ROS formed in the DTT test.

Ozone−alkene reactions form vibrationally excited Criegee intermediates (of the form R1R2COO), some of which, once thermalized, are thought to react with SO2, H2O, NO x , aldehydes, and alcohols. Several studies using relative rate techniques or ab initio calculations have resulted in estimates for the rate coefficients of reactions of the thermalized biradicals. The ranges of measured and estimated rate coefficients span 2−6 orders of magnitude, depending on the reaction partner. Using an atmospheric pressure flow reactor, we have made the first absolute rate coefficient determinations for reactions of a thermalized Criegee intermediate, measuring rates for unimolecular decomposition and reaction with acetaldehyde. For the thermalized CH3CHOO formed in trans-2-butene ozonolysis, values for k dec = 76 s-1 and k ald = 1.0 × 10-12 cm3 molecule-1 s-1, accurate to within a factor of 3 and 6, respectively, were obtained.