Photolysis Constants Research Articles

UV absorption spectra of trans-2-pentenal, trans-2-hexenal and 2-methyl-2-pentenal

C5 and C6 unsaturated aldehydes are a category of BVOCs released by plants in response to their wounds and stress. In this work, the gas phase UV absorption spectra of trans-2-pentenal (t-2P), trans-2-hexenal (t-2H) and 2-methyl-2-pentenal (2M2P) are determined between 240 and 400 nm, at room temperature and at pressures (1–9) torr. The reported UV absorption cross sections are (cm2molecule−1): σt-2P, 330 = (4.86 ± 0.53) × 10−20, σt-2H, 330 = (5.43 ± 0.57) × 10−20 and σ2M2P, 320 = (6.62 ± 0.50) × 10−20. Their photolysis constants as well as their atmospheric lifetimes toward photolysis were then estimated.

Photolytic removal of DBPs by medium pressure UV in swimming pool water

Medium pressure UV is used for controlling the concentration of combined chlorine (chloramines) in many public swimming pools. Little is known about the fate of other disinfection by-products (DBPs) in UV treatment. Photolysis by medium pressure UV treatment was investigated for 12 DBPs reported to be found in swimming pool water: chloroform, bromodichloromethane, dibromochloromethane, bromoform, dichloroacetonitrile, bromochloroacetonitrile, dibromoacetronitrile, trichloroacetonitrile, trichloronitromethane, dichloropropanone, trichloropropanone, and chloral hydrate. First order photolysis constants ranged 26-fold from 0.020min−1 for chloroform to 0.523min−1 for trichloronitromethane. The rate constants generally increased with bromine substitution.Using the UV removal of combined chlorine as an actinometer, the rate constants were recalculated to actual treatment doses of UV applied in a swimming pool. In an investigated public pool the UV dose was equivalent to an applied electrical energy of 1.34kWhm−3d−1 and the UV dose required to removed 90% of trichloronitromethane was 0.4kWhm−3d−1, while 2.6kWhm−3d−1 was required for chloral hydrate and the bromine containing haloacetonitriles and trihalomethanes ranged from 0.6 to 3.1kWhm−3d−1. It was predicted thus that a beneficial side-effect of applying UV for removing combined chlorine from the pool water could be a significant removal of trichloronitromethane, chloral hydrate and the bromine containing haloacetonitriles and trihalomethanes.

Tests on combined projection/forward differencing integration for stiff photochemical family systems at long time step

Accurate estimates of final tracer concentrations guide adjustment of individual species levels so that forward integration of stiff families can proceed at time steps an order of magnitude larger than inherent loss constants. Maintenance of a unified concentration vector in the solver ensures mass conservation. The sequence was perfected by raising the step size upward from 100 s in a standard 25 family 45 species stratospheric box model with explicit Euler numerics. Projections were inserted as instabilities arose. Ultimately, a uniform Δ t of 1 h was achieved from 20–50 km at mid-latitudes. Results were compared over five equinoctial diurnal cycles with runs at 0.1 and 1 s steps in which every constituent other than the individual atoms, CHO and CH 3O, was handled separately. Agreement was better than 10 −2 for the major reservoirs in almost all cases, and was equally close for radicals between 6 a.m. and 6 p.m., provided that photolysis constants were time averaged. The few exceptions were due to experimentation with a linearized implicit concentration predictor, or to the approximations underlying family partitioning recipes. CPU timings extrapolated to a hypothetical GCM grid suggest that 3-D modeling will be possible at the level of chemical resolution in the programs.

Photochemical degradation of brominated dibenzo-p-dioxins and -furans in organic solvents

The photolytic behavior of halogenated dibenzo-p-dioxins and -furans in the organic solvents methanol and n-hexane was investigated and the corresponding quantum-yields and first order decay rates are reported. The photolysis constants in n-hexane increase with increasing number of bromine atoms in the dibenzo-p-dioxin or dibenzofuran skeleton, e.g. from 4.5×10 −4 s −1 for the monobrominated to 7.7×10 −3 s −1 for the octabrominated dioxin, and from 4.0×10 −3 s −1 for the dibrominated to 8.3×10 −2 s −1 for the heptabrominated furan. Photolysis in methanol is nearly six times slower than in n-hexane. The bromine compounds react an order of magnitude faster than the chlorine analogues. The results were used to extrapolate the photochemical fate to lipohilic environmental surfaces.

Determination of photolysis constants of some aliphatic amines in the mixed solvent 80% dimethylsulfoxide/20% water and assessment of titration conditions

The p K b values of some aliphatic amines in 80% dimethylsulfoxide—20% water are reported. The mixed solvent offers worse titration conditions than water for the determination of these bases. The only advantage of using this solvent for titration of bases is its excellent solvating ability.

The photochemistry of the nitrate radical and the kinetics of the nitrogen pentoxide-ozone system

The kinetics of the nitrogen pentoxide catalyzed decomposition of ozone were studied in the dark and with photolytic light absorbed by the NO/sub 3/ radical. Ultraviolet, visible, or infrared absorption cross sections were measured for N/sub 2/O/sub 5/, HNO/sub 3/, NO/sub 2/ and NO/sub 3/. The equilibrium constant for N/sub 2/O/sub 5/ reversible NO/sub 2/ + NO/sub 3/ was found to be (molecules cm/sup -3/, 298--329 K) K = (8.4 +- 1.8) x 10/sup 26/ exp(-(11180 +- 100)/T). The rate constants for several reactions were measured: (cm/sup 3/ molecule/sup -1/s/sup -1/) 2NO/sub 3/ ..-->.. 2NO/sub 2/ + O/sub 2/, g = (8.5 +- 2.8) x 10/sup -13/ exp(-(2450 +- 100)/T); O + N/sub 2/O/sub 5/ ..-->.. product, m less than or equal to 2 x 10/sup -14/; O + NO/sub 3/ ..-->.. O/sub 2/ + NO/sub 2/, n = (1.0 +- 0.4) x 10/sup -11/. By combining the equilibrium constant K with literature values of Ke and Kf, rate constants for reactions e and f were evaluated: NO/sub 2/ + NO/sub 3/ ..-->.. NO + O/sub 2/ + NO/sub 2/, e = (2.5 +- 0.5) x 10/sup -14/ exp(-(1230 +- 100)/T); NO + NO/sub 3/ ..-->.. 2NO/sub 2/, f =more » (1.9 +- 0.4) x 10/sup -11/ at 297 K. At 1 atm total pressure, the photolysis of NO/sub 3/ occurs with a primary quantum yield less than one in the red region of the spectrum. The solar photolysis constants at 298 K for the two paths of dissociation are consistent with the following values: NO/sub 3/ + h..nu.. ..-->.. NO + O/sub 2/, j/sub 1/ = 0.040 +- 0.02 s/sup -1/; NO/sub 3/ + h..nu.. ..-->.. NO/sub 2/ + O, j/sub 2/ = 0.099 +- 0.02 s/sup -1/. The average quantum yield for process two was approximately 0.77 for light with wavelengths between 470 and 610 nm, and that for process one was about 0.23 between 470 and 610 nm but only 0.07 in the strong absorption region between 610 and 670 nm. The photolysis constants may be larger at lower total pressures. Infrared absorption ascribed to NO/sub 3/ is due to N/sub 2/O/sub 5/; previously reported visible cross sections for NO/sub 3/ were found to be about a factor of 4 too low and are corrected here; previously reported ultraviolet cross sections for N/sub 2/O/sub 5/ are reversed slightly.« less