Computer modeling of smog chamber data: Progress in validation of a detailed mechanism for the photooxidation of propene and n‐butane in photochemical smog

Abstract

A detailed mechanism is presented far reactions occurring during irradiation of part-permillion concentrations of propene and/or n-butane and oxides of nitrogen in air. Data from an extensive series of well-characterized smog chamber experiments carried out in our 5800-liter evacuable chamber-solar simulator facility designed for providing data suitable for quantitative model validation were used to elucidate several unknown or uncertain kinetic parameters and details of the reaction mechanism.The mechanism was then tested against the data base from the smog chamber runs. In general, most calculated concentration-time profiles agreed with experiments to within the experimental uncertainties. Fits were usually attained to within f20% or better for ozone, NO, propene, and n-butane, to within -+30% or better for NOz, PAN, methyl ethyl ketone, 2-butyl nitrate, butyraldehyde, and (in runs not containing propene) methyl nitrate, to within --f50% or better for the minor products 1-butyl nitrate and propene oxide, and to within a factor of 2 for methyl nitrate in propene-containing runs. Propionaldehyde was consistently underpredicted in all runs; it is probably a chamber contaminant. For formaldehyde and acetaldehyde, the major products in both systems, fits to within -420% were often obtained, yet for a number of experiments, significantly greater discrepancies were observed, probably as a result of experimental and/or analytical problems.The good fits to experimental data were attained only after adjusting several rate constants or rate constant ratios related to uncertainties concerning chamber effects or the chemical mechanism. The largest uncertainty concerns the necessity to include in the mechanism a significant rate of radical input from unknown sources in the smog chamber. Other areas where fundamental kinetic and mechanistic data are most needed before a predictive, detailed propene + n-butane-NO,-air smog model can be completely validated concern other chamber effects, the 03 + propene mechanism, decomposition rates of substituted alkoxy radicals, CARTER E T AL.primary quantum yields for radical production as a function of wavelength for aldehyde and ketone photolyses, and the mechanisms and rates of reactions of peroxy radicals with NO and NOz.

Effects of ConstantversusDiurnally-Varying Light Intensity on Ozone Formation

“…runs 315-317); this effect is both qualitatively and quantitatively consistent with both computer modeling 9 and environmental chamber 10 studies. Since the total integrated light flux in the diurnal irradiations (equivalent to a constant light intensity of 65% of the maximum for 12 hr) corresponds closely to that in the 70% constant light intensity irradiations, the effect of constant vs. diurnal light intensity can be most readily evaluated by comparison of these sets of runs.…”

Reactions of primary and secondary butoxy radicals in oxygen at atmospheric pressure

“…They found that an intramolecular alkoxy radical isomerization (1,5 hydrogen shift) must be the main reaction for the n-pentoxy radical, since the ratio (K,,,,r,,,t,on/Kco,petlng processes) is about 100, (competing processes are assumed to be reaction with ambient levels of O2 and unimolecular decomposition). They showed that this rapid isomerization via a low-strain six-member ring intermediate is also the dominant process for n-butoxy (but of course not for s-butoxy) radicals [12,13].…”

Reply to “comments on ‘a smog chamber and modeling study of the gas phase NOx‐air photooxidation of toluene and the cresols’”

“…Our results [5] showed that unknown chamber radical sources not only exist, but are present long after any nitrous acid initially present in the chamber has declined to its photostationary state concentration. Moreover, the radical fluxes we observe are of the magnitude predicted by our earlier model calculations [2,3]. Thus, it is unfortunate that these authors did not see our communication [5] prior to submitting their comment [l] for publication.…”

“…We agree that this is indeed unlikely, and although our previous kinetic modeling studies [2,3] represented the radical input as a simple flux of OH radicals, this was intended solely as a formalism for modeling purposes, and certainly does not represent what we believe to be actually occurring. Rather the radicals are probably formed by the gas-phase photolysis of some unidentified precursor, which may be formed heterogeneously and/or homogeneously.…”