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

Abstract

Recently Killus and Whitten [l] commented on our experimentall modeling study of the gas-phase NO, -air photooxidation of aromatics [2] to the effect that they did not believe that unknown radical sources exist in smog chambers, despite the fact that we [2,3] and others [4] find it necessary to include such a source in order to obtain agreement between model simulations and chamber data. They also suggested experiments designed to show if such radical sources exist and, if so, at what levels.We wish to point out that prior to the publication of their comment [l], this journal received (and subsequently published) a preliminary communication [5] in which we reported results of experiments very similar to those suggested by Killus and Whitten [l]. 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.Killus and Whitten [l] argue that it is unlikely that the chamber walls can serve as a source of hydroxyl or hydroperoxyl radicals. 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. Indeed, in the modeling study of Carter et al.[ 3 ] , possible photolabile precursors of these OH radicals and their necessary concentrations were considered [3, Table V], and it was concluded [3] that nitrous acid (HONO) or alkyl nitrites (RONO) were the likeliest candidates.

Correlation of the ozone formation rates with hydroxyl radical concentrations in the propylene-nitrogen oxide dry air system: effective ozone formation rate constant

“…For example, the simulation for run 1 predicts the maximum ozone formation rate at 120 min, whereas the experimental run gives the maximum rate at 80 min after the irradiation began. Although this deviation could be due to the presence of unknown radical sources as discussed by Carter et al (12,13), this problem was not pursued further in the present study since the following discussion on the relationship between (d[03]/dí)max and [OH]mai will not be affected by the delay of photooxidation.…”