The inhibition of photochemical smog VII. Inhibition by diethylhydroxylamine at atmospheric concentrations

Laboratory prepared mixtures of hydrocarbon and NO in O 2 or air were irradiated in a cylindrical Pyrex reaction vessel using fluorescent lights, in 12-1 Pyrex bulbs using summer mid-day sunlight, or in 40 ft. 3 Tedlar bags using summer mid-day sunlight. The effect of added diethylhydroxylamine (DEHA) was tested by measuring the change in the removal rate of hydrocarbon and NO, by the onset of O 3 production, the maximum O 3 concentrations reached and the time at the maximum. The initial concentrations of the reactants were varied from hydrocarbon levels (expressed as equivalent C 2H 4 concentrations based on HO radical reactivity) of 0.42–10 ppm and NO concentrations of 0.12–2.0 ppm. The results show the following: 1. (1) In fresh reaction vessels, the addition of DEHA at an initial concentration ≈25% that of a reactive hydrocarbon markedly reduces the photooxidation rate of hydrocarbon-NO mixtures in O 2 at conditions corresponding to those in polluted urban atmospheres. The inhibition effect is not much influenced by the presence of SO 2, the concentration of NO or the concentration of hydrocarbon, as long as the DEHA to hydrocarbon concentration ratio is held fixed. 2. (2) At DEHA to reactive hydrocarbon concentration ratios less than ≈10%, the inhibition vanishes, and can even be replaced by enhancement of the photooxidation rate in the presence of DEHA. This is attributed to the fact that at low [DEHA]/[hydrocarbon], the inhibition effect is so small that it is offset by the oxidation of DEHA to CH 3CHO (+ C 2H 5NO 2) which adds to the hydrocarbon loading and can enhance photochemical smog formation. 3. (3) In highly seasoned vessels, the inhibiting effect of DEHA seen in fresh vessels (either Pyrex or Tedlar) vanishes and can even shift to enhancement of the oxidation. This occurs because aerosol produced in early runs coats the vessel walls and then either degasses to increase the hydrocarbon loading or adsorbs DEHA to reduce the DEHA concentration, or both. 4. (4) For atmospheric conditions, results in 12-1 Pyrex vessels indicate that the introduction of DEHA at ≈40% of the effective C 2H 4 concentration (based on HO radical reactivity) can delay the onset of O 3 production from 1 1 2 h on the absence of DEHA to > 6 h in the presence of DEHA. Thus even under the most severe urban atmospheric hydrocarbon loadings, initial DEHA concentrations < 0.3 ppm should be adequate to markedly inhibit the onset of photochemical smog. For most days, even in Los