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Reply to the ‘Comment on “Investigations on HONO formation from photolysis of adsorbed HNO3 on quartz glass surfaces”’ by M. N. Sullivan, L. T. Chu and L. Zhu, Phys. Chem. Chem. Phys., 2018, 20, DOI: 10.1039/C8CP04497J

Sebastian Laufs and Jörg Kleffmann*
Physikalische und Theoretische Chemie/Fakultät für Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, 42097 Wuppertal, Germany. E-mail: kleffman@uni-wuppertal.de; Fax: +49 202 439 2505; Tel: +49 202 439 3534

Received 26th September 2018 , Accepted 21st November 2018

First published on 30th November 2018


In their comment to our recent paper about low HONO and NO2 formation by photolysis of adsorbed HNO3 Sullivan et al. confirmed their former results of HNO3 adsorption on silica under dry conditions using a quartz crystal microbalance. The authors concluded that the differences between their results and our conclusions are caused by the different experimental conditions, i.e. adsorption under very dry conditions compared to our experiments at 50% r.h. While we agree that adsorption of the highly water soluble HNO3 will strongly depend on humidity, there is still the conflict in the photolysis frequency of adsorbed HNO3 under atmospheric conditions to which the authors referred in their previous publications (see their atmospheric implication sections) and to which also our paper refers. If their results on both the adsorption cross sections of HNO3 (two to three orders of magnitude larger compared to the gas phase) and the quantum yield for NO2 formation (close to unity) are applicable under conditions prevailing in the atmosphere, then the photolytic lifetime of HNO3 on surfaces would be only ∼5 min for atmospheric solar flux (0° SZA), which is highly unlikely.


In our previous paper1 we studied the photolysis of HNO3 adsorbed on quartz and Pyrex surfaces under conditions close to the atmosphere (actinic flux, HNO3 concentration, and humidity) and observed very low values of J(HNO3 → NO2) and low secondary heterogeneous formation of HONO, which were based on well quantified levels of adsorbed HNO3 (ion chromatography analysis) and gas phase HONO and NO2 concentrations (LOPAP techniques). Since the spectral actinic flux was also quantified in our study by a calibrated spectroradiometer, we used the absorption cross sections of adsorbed HNO3 published by the group of L. Zhu2,3 to calculate an average quantum yield of adsorbed HNO3 for NO2 formation of ϕ(HNO3 → NO2) = (3.4 ± 2.6) × 10−4, see eqn (4).1 This value is orders of magnitude lower compared to the studies of Zhu et al.4 and Abida et al.5 and the present comment by Sullivan et al.6 In these studies, quantum yields near unity were observed in excimer laser photolysis experiments at 308 and 351 nm, which cover the same spectral range as we used in our study. If both quantities are applied, the photolytic lifetime of HNO3 adsorbed on silica surfaces would be only ∼5 min for atmospheric spectral actinic fluxes (0° SZA), which is highly unlikely. Accordingly, we tried to explain this conflicting data either by overestimated absorption cross sections (by underestimated levels of adsorbed HNO32,3) and/or overestimated quantum yields (by using excimer laser photolysis experiments4,5). In our previous paper1 we left both explanations open, but finally used the high cross sections to calculate our low quantum yield, which rely on the measured adsorption isotherms.2,3

In their comment, Sullivan et al.6 now conclusively showed by quartz crystal microbalance measurements that these adsorption isotherms are valid, resulting in monolayer coverage (∼1014 cm−2) of adsorbed HNO3 under completely dry conditions at ca. 16 mTorr HNO3 gas phase partial pressure (∼20 ppm mixing ratio at 1 atm), while we obtained the same amount in the lower ppb range at 50% r.h. Although some of the differences may still be caused by the very different saturation times applied (2 min in Sullivan et al.;6 overnight in our study1), the explanation by Sullivan et al.6 is indeed reasonable, since HNO3 adsorption strongly increases with increasing humidity. It is worth mentioning that we observed in former experiments7 that low ppb levels of HNO3 easily pass through PFA (perfluoroalkoxy alkane) and Teflon tubes in very dry conditions, while it takes ∼2 h until HNO3 levels reach 90% of final values when passing through a 2 m long PFA tube (4 mm i.d.) at 1 l min−1 and 50% r.h. This is explained by the extremely high solubility of HNO3 in the adsorbed water layer at higher humidity. In addition, with respect to the comment by Sullivan et al.,6 it is not surprising that HNO3 does not form a physical monolayer on the surface at 1014 HNO3 cm−2 in the presence of several layers of adsorbed water at 50% r.h., where it will be present as a mixed HNO3/H2O phase. Since we quantified the amount of adsorbed HNO3 in our previous study (see e.g. Fig. 5),1 the used term “monolayer” clearly refers to a formal monolayer ignoring the unknown amount of co-adsorbed water. The same holds for the four (formal) monolayers of adsorbed H2O at 50% r.h. mentioned in Sullivan et al.,6 which is also strictly not correct (see island adsorption of H2O8).

However, besides this somewhat academic discussion, for the main question, i.e. the photolysis frequency of HNO3 and the resulting NO2/HONO formation, it is completely unimportant whether the 1014 HNO3 cm−2 are adsorbed as a pure (hypothetical) monolayer or in a mixed HNO3/H2O phase (reality). For both situations the UV light irradiates all HNO3 molecules. The photolysis frequency is by definition independent of the amount of HNO3 (first order kinetics) and can be simply calculated from the ratio of formed products divided by the amount of adsorbed HNO3 (see e.g. eqn (2)1), which were both directly measured in our study.

Since we performed our experiments under typical atmospheric conditions with respect to HNO3 concentration, humidity and actinic flux, we are still convinced that our atmospheric implications are correct, i.e. at typical atmospheric humidity either the published high absorption cross sections of adsorbed HNO3 or the near unity quantum yields for its photolysis are not valid. In contrast, we cannot draw this conclusion for completely dry conditions under which the former studies2–6 were performed. However, these studies should then not be extrapolated to the atmosphere (e.g. for the discussion on potential “renoxification” etc.), as the authors have done in their former publications.2–4,9 Accordingly, photolysis of HNO3 adsorbed on non-reactive substrates – which is the topic of all papers cited above1–6,9 – will be of minor importance in the atmosphere. However, as we already discussed in our previous paper,1 the situation may change for HNO3 adsorbed on complex substrates, e.g. on photosensitizer surfaces like humic acids – but that is a different story.

Conflicts of interest

There are no conflicts to declare.

References

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