the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2022
Water enhances the formation of fragmentation products via the cross-reactions of RO2 and HO2 in the photooxidation of isoprene
Abstract. The photooxidation of isoprene contributes significantly to the peroxy radical (RO2) pool in the atmosphere. With a widespread decreasing trend of nitrogen oxides (NOx) emissions, the cross-reactions of isoprene-derived RO2 with hydroperoxy radicals (HO2) become increasingly important. Yet large uncertainties remain in the effect of water vapor on the products yields in these reactions. In the present study, we investigated the photooxidation of isoprene under 30 % relative humidity and 80 % RH in which the cross-reactions of RO2 and HO2 were dominated. The experiments were conducted with ozone photolysis as the hydroxy radical (OH) source. We found that in the first-generation reactions, the branching ratios for methacrolein (MACR) and methyl vinyl ketone (MVK) in the cross-reactions of β-isoprene hydroxy peroxy radicals (β-ISOPOO) and HO2 under 30 % RH and 80 % RH increased to approximately three times and five times of those under dry conditions owing to a water-induced change in the complexation patterns of β-ISOPOO and HO2. Based on the branching ratios achieved in this study, we estimated that the MACR and MVK emissions are enhanced by 4.7−12 and 18−34 Tg yr−1 while the β-isoprene hydroxy hydroperoxide (β-ISOPOOH) emission is suppressed by 39−78 Tg yr−1 on a global scale when considering the water effect. In the multi-generation reactions, the yields of formic acid (FA) and acetic acid (AA) with water vapor were raised by over fivefold than we expected, able to narrow the bias between the modeled and observed global FA productions by 20 %. Since β-ISOPOOH and MACR, as well as FA and AA, play pivotal roles in aerosol formation and growth, a better interpretation of their yields helps understand the fate of isoprene in the atmosphere and improve the effect of the simulations of isoprene-derived aerosol burdens and chemical compositions.
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RC1: 'Comment on acp-2022-444', Anonymous Referee #1, 22 Aug 2022
Xu et al. investigated the effects of water on the formation of fragmentation products in isoprene photooxidation with a series of oxidation flow reactor (OFR) experiments. They found that water enhanced MVK and MACR formation and proposed water-assisted mechanisms for the reactions of β-ISOPOO with HO2 to explain the observed fragmentation. I believe that the authors did the experiments carefully and reported a lot of useful details about them in the paper. I also think that the observations from the experiments are reliable. However, I do not agree with how the authors interpreted some key observations.
First, I find it highly implausible that HMHP formed via CH2OH (+O2) -> HOCH2OO (+HO2) -> HMHP in the gas phase. Theoretical calculations showed that CH2OH and O2 are too energetic for HOCH2OO to be stable. Even transient existence of HOCH2OO in this pathway will also end up with HCHO and HO2 in picoseconds (Dibble, 2002). While I agree that the formation of HMHP as a first-generation product likely involves some C1 fragment(s), I believe that condensed phase is needed for the ultrafast dissipation of energy excess of CH2OH+O2.
In the paper the authors have ruled out the reactor walls as this condensed phase. They suggested that isoprene-derived SOA may provide some aqueous phase volume. However, I do not think that isoprene-derived SOA would be enough. The SOA yield of isoprene is low even at equilibrium and without aerosol seed added a residence time of ~60 s is too short for SOA growth in OFR experiments (Palm et al., 2016).
The authors also reported much more formation of formic and acetic acids than explained by the mechanisms that the authors proposed. The strong production of FA and AA, together with the formation of HMHP as a first-generation product, lead me to think about a possible role that aqueous-phase chemistry could play in the experiments in this study.
I suspect that the movable sampling tube might have provided the aqueous phase needed. The Teflon lining might have adsorbed water (Huang et al., 2018) and its length and surface-to-volume ratio could be high enough to affect the experimental results.
I think that the authors should verify the possibility of aqueous-phase chemistry (not just in the sampling tube as I suspected) for the formation of HMHP, FA and AA. If they were not formed in the aqueous phase, more convincing mechanisms of their formation are needed for gas-phase water-assisted mechanisms for MVK and MACR formation to be more plausible.
Specific comments:
Line 31: further oxidation does not necessarily lead to carbon skeleton fragmentation.
Table 1: are the O3 concentrations reported in this table initial or final concentrations? They are below 1 ppm. With such low O3 concentrations and relatively high OH exposures reported here, I expect a substantial loss of O3 in the reactor by photochemistry.
Line 103: some hydroperoxides were reported to hydrolyze very rapidly (Qiu et al., 2019). The authors need to rule out this possible interference during sample collection or correct it.
Lines 203-204: to my knowledge, the interpretation of the MVK and MACR observed by Liu et al. (2013) has been subject to debate, with some believing that the observed MVK and MACR were artifacts during sampling.
Figure 4: the authors need to highlight the formation of tetroxide complexes in the scheme, which is an important point in the relevant discussions in the text.
Line 280: what are the uncertainties on the relative weights of these pathways taken from MCM? Some sensitivity simulations would be preferable.
Line S12: why kOH,ISO is used here with MACR and MVK as photochemical clock species?
Table S3: how was the UV flux at 254 nm obtained? I find it too small to generate OH exposure >1e11 molec cm-3 s with O3 < 1 ppm. What are the references for the quantum yields reported here? 0.05 for MACR seems to be too low given energetic 254 nm photons.
Technical correction:
Grotheer et al., 1985 cited in Line 254 is missing in the reference list.
References:
Dibble, T. S.: Mechanism and dynamics of the CH2OH+O2 reaction, Chem. Phys. Lett., 355, 193–200, 2002.
Huang, Y., Zhao, R., Charan, S. M., Kenseth, C. M., Zhang, X., and Seinfeld, J. H.: Unified Theory of Vapor–Wall Mass Transport in Teflon-Walled Environmental Chambers, Environ. Sci. Technol., 52, 2134–2142, 2018.
Liu, Y. J., Herdlinger-Blatt, I., McKinney, K. a., and Martin, S. T.: Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation, Atmos. Chem. Phys., 13, 5715–5730, 2013.
Palm, B. B., Campuzano-Jost, P., Ortega, A. M., Day, D. A., Kaser, L., Jud, W., Karl, T., Hansel, A., Hunter, J. F., Cross, E. S., Kroll, J. H., Peng, Z., Brune, W. H., and Jimenez, J. L.: In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor, Atmos. Chem. Phys., 16, 2943–2970, 2016.
Qiu, J., Ishizuka, S., Tonokura, K., Colussi, A. J., and Enami, S.: Water Dramatically Accelerates the Decomposition of α-Hydroxyalkyl-Hydroperoxides in Aerosol Particles, J. Phys. Chem. Lett., 10, 5748–5755, 2019.
Citation: https://doi.org/10.5194/acp-2022-444-RC1 -
AC1: 'Reply on RC1', Zhongming Chen, 18 Oct 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-444/acp-2022-444-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Zhongming Chen, 18 Oct 2022
-
RC2: 'Comment on acp-2022-444', Anonymous Referee #2, 02 Sep 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-444/acp-2022-444-RC2-supplement.pdf
-
AC2: 'Reply on RC2', Zhongming Chen, 18 Oct 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-444/acp-2022-444-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Zhongming Chen, 18 Oct 2022
Status: closed
-
RC1: 'Comment on acp-2022-444', Anonymous Referee #1, 22 Aug 2022
Xu et al. investigated the effects of water on the formation of fragmentation products in isoprene photooxidation with a series of oxidation flow reactor (OFR) experiments. They found that water enhanced MVK and MACR formation and proposed water-assisted mechanisms for the reactions of β-ISOPOO with HO2 to explain the observed fragmentation. I believe that the authors did the experiments carefully and reported a lot of useful details about them in the paper. I also think that the observations from the experiments are reliable. However, I do not agree with how the authors interpreted some key observations.
First, I find it highly implausible that HMHP formed via CH2OH (+O2) -> HOCH2OO (+HO2) -> HMHP in the gas phase. Theoretical calculations showed that CH2OH and O2 are too energetic for HOCH2OO to be stable. Even transient existence of HOCH2OO in this pathway will also end up with HCHO and HO2 in picoseconds (Dibble, 2002). While I agree that the formation of HMHP as a first-generation product likely involves some C1 fragment(s), I believe that condensed phase is needed for the ultrafast dissipation of energy excess of CH2OH+O2.
In the paper the authors have ruled out the reactor walls as this condensed phase. They suggested that isoprene-derived SOA may provide some aqueous phase volume. However, I do not think that isoprene-derived SOA would be enough. The SOA yield of isoprene is low even at equilibrium and without aerosol seed added a residence time of ~60 s is too short for SOA growth in OFR experiments (Palm et al., 2016).
The authors also reported much more formation of formic and acetic acids than explained by the mechanisms that the authors proposed. The strong production of FA and AA, together with the formation of HMHP as a first-generation product, lead me to think about a possible role that aqueous-phase chemistry could play in the experiments in this study.
I suspect that the movable sampling tube might have provided the aqueous phase needed. The Teflon lining might have adsorbed water (Huang et al., 2018) and its length and surface-to-volume ratio could be high enough to affect the experimental results.
I think that the authors should verify the possibility of aqueous-phase chemistry (not just in the sampling tube as I suspected) for the formation of HMHP, FA and AA. If they were not formed in the aqueous phase, more convincing mechanisms of their formation are needed for gas-phase water-assisted mechanisms for MVK and MACR formation to be more plausible.
Specific comments:
Line 31: further oxidation does not necessarily lead to carbon skeleton fragmentation.
Table 1: are the O3 concentrations reported in this table initial or final concentrations? They are below 1 ppm. With such low O3 concentrations and relatively high OH exposures reported here, I expect a substantial loss of O3 in the reactor by photochemistry.
Line 103: some hydroperoxides were reported to hydrolyze very rapidly (Qiu et al., 2019). The authors need to rule out this possible interference during sample collection or correct it.
Lines 203-204: to my knowledge, the interpretation of the MVK and MACR observed by Liu et al. (2013) has been subject to debate, with some believing that the observed MVK and MACR were artifacts during sampling.
Figure 4: the authors need to highlight the formation of tetroxide complexes in the scheme, which is an important point in the relevant discussions in the text.
Line 280: what are the uncertainties on the relative weights of these pathways taken from MCM? Some sensitivity simulations would be preferable.
Line S12: why kOH,ISO is used here with MACR and MVK as photochemical clock species?
Table S3: how was the UV flux at 254 nm obtained? I find it too small to generate OH exposure >1e11 molec cm-3 s with O3 < 1 ppm. What are the references for the quantum yields reported here? 0.05 for MACR seems to be too low given energetic 254 nm photons.
Technical correction:
Grotheer et al., 1985 cited in Line 254 is missing in the reference list.
References:
Dibble, T. S.: Mechanism and dynamics of the CH2OH+O2 reaction, Chem. Phys. Lett., 355, 193–200, 2002.
Huang, Y., Zhao, R., Charan, S. M., Kenseth, C. M., Zhang, X., and Seinfeld, J. H.: Unified Theory of Vapor–Wall Mass Transport in Teflon-Walled Environmental Chambers, Environ. Sci. Technol., 52, 2134–2142, 2018.
Liu, Y. J., Herdlinger-Blatt, I., McKinney, K. a., and Martin, S. T.: Production of methyl vinyl ketone and methacrolein via the hydroperoxyl pathway of isoprene oxidation, Atmos. Chem. Phys., 13, 5715–5730, 2013.
Palm, B. B., Campuzano-Jost, P., Ortega, A. M., Day, D. A., Kaser, L., Jud, W., Karl, T., Hansel, A., Hunter, J. F., Cross, E. S., Kroll, J. H., Peng, Z., Brune, W. H., and Jimenez, J. L.: In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor, Atmos. Chem. Phys., 16, 2943–2970, 2016.
Qiu, J., Ishizuka, S., Tonokura, K., Colussi, A. J., and Enami, S.: Water Dramatically Accelerates the Decomposition of α-Hydroxyalkyl-Hydroperoxides in Aerosol Particles, J. Phys. Chem. Lett., 10, 5748–5755, 2019.
Citation: https://doi.org/10.5194/acp-2022-444-RC1 -
AC1: 'Reply on RC1', Zhongming Chen, 18 Oct 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-444/acp-2022-444-AC1-supplement.pdf
-
AC1: 'Reply on RC1', Zhongming Chen, 18 Oct 2022
-
RC2: 'Comment on acp-2022-444', Anonymous Referee #2, 02 Sep 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-444/acp-2022-444-RC2-supplement.pdf
-
AC2: 'Reply on RC2', Zhongming Chen, 18 Oct 2022
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-444/acp-2022-444-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Zhongming Chen, 18 Oct 2022
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