the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Oct 2021
Contribution of the gas-phase reaction between hydroxyl radical and sulfur dioxide to the sulfate aerosol over West Pacific
Abstract. Sulfate is among the major components of atmospheric aerosols or fine particulate matters. Aerosols loaded with sulfate result in low air quality, damage to ecosystems, and influences on climate change. Sulfate aerosols could originate from that directly emitted to the atmosphere and that produced by atmospheric physicochemical processes. The latter is generated from sulfur dioxide (SO2) via oxidation either in the gas phase reactions or in the aqueous phase. Several mechanisms of SO2 oxidation have been proposed, but the differentiation of the various mechanisms and identification of the sources remain challenging. To meet this need, a new method to estimate the contribution of the gas-phase reaction between hydroxyl radical (OH) and SO2 to the sulfate aerosol is proposed and investigated. Briefly, we consider the OH-reaction rates of the respective trace gases that compete for OH radicals with SO2 in the troposphere, and estimate the fraction of SO2-OH reaction in the total OH reactivity. Then the relationship between sulfate concentration and the SO2-OH reaction is analyzed statistically to investigate the sources of sulfate in aerosols. We test this method using the data from ground-based observations and aircraft measurements made during the Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales in Asia (EMeRGe-Asia) over the western Taiwan and West Pacific regions. Our results show that the estimated SO2-OH reactivity fraction is well-correlated with sulfate concentration. The sulfate production from SO2-OH reaction accounts for approximately 30 % of the total sulfate in aerosols collected at the surface and near-surface (altitude < 600 m) in our study area, comparable to the estimates from other model simulations. Within its assumptions and limitations, this new method provides a valuable approach to investigate the significance of SO2-OH reaction regionally and globally.
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Interactive discussion
Status: closed
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RC1: 'Comment on acp-2021-788', Anonymous Referee #1, 06 Jan 2022
I cannot find the novelty in this manuscript. The sulfate formation mechanisms in this study are not new, and even presented in textbooks. The conclusions, as stated by the authors, are also similar wtih the previous studies. Please revise the manuscript to meet the academic requirement in ACP.
Citation: https://doi.org/10.5194/acp-2021-788-RC1 -
RC2: 'Comment on acp-2021-788', Anonymous Referee #2, 12 Jan 2022
This paper investigates the contribution of the gas-phase oxidation of SO2 to the mass concentrations of sulfate aerosols over the West Pacific. The authors used airborne and surface measurement data obtained during the EMeRGe-Asia campaign. They proposed a new approach to derive the fraction of OH that reacted with SO2 (SO2 oxidation rate fraction) using the concentrations of trace gases and their rate constants with OH, and estimated the contribution of the gas-phase oxidation of SO2 to sulfate. The data presented in this paper are valuable, and the scope of this paper is appropriate for ACP. However, I do not recommend the publication of this paper in its present form because of the lack of robust conclusions.
(1) Assumptions and approximations
The key point of this study is the new approach to estimate the contribution of the gas-phase oxidation of SO2 to sulfate. My major concern is that there are too many assumptions and approximations, which affect the robustness of the estimates. Here are the assumptions and approximations to obtain Equation (13) and the major conclusions.
(i) The total OH reactivity of VOCs is proportional to the sum of the OH reactivity of CH4, CO, NOx, and SO2.
(ii) The scaling factor "t" for the VOC-OH reactions is ignored.
(iii) The O3 concentration is representative of the oxidizing capacity of the observed air masses.
(iv) The contribution of the CH4-OH reaction is ignored.
(v) NOy is composed of NOx and PAN in daytime.
(vi) The slope of sulfate versus SO2 oxidation rate fraction was due to the gas-phase reactions and the intercept was due to transport from other regions and/or aqueous-phase reactions.
The key oxidants responsible for the aqueous-phase reactions (H2O2 at all pH and O3 at higher pH) are expected to show positive correlations with the oxidizing power in the air masses, and therefore the slope may not be exclusively due to the gas-phase reactions. The positive correlations of sulfate versus SO2 oxidation fraction may represent that air masses containing high SO2 yielded high sulfate. Unfortunately, potential uncertainties due to these critical assumptions and approximations are not discussed, and therefore the major conclusions are not convincing.(2) The normalization by O3
The r2 values in Figure 1 increased by the O3 normalization method. However, the rationale for the O3 normalization is not well described. The ratio of [SO2]/[O3] would be largely affected by the background O3, and the interpretation would not be straightforward. The normalization is generally done by inert tracers such as CO after subtracting the background.(3) Introduction
The introduction section is poorly written. I would recommend that the authors should thoroughly review the current understanding of the budget of sulfate aerosols in the troposphere rather than describing the well-known reaction mechanisms of SO2 (both the gas-phase and aqueous-phase).Citation: https://doi.org/10.5194/acp-2021-788-RC2
Interactive discussion
Status: closed
-
RC1: 'Comment on acp-2021-788', Anonymous Referee #1, 06 Jan 2022
I cannot find the novelty in this manuscript. The sulfate formation mechanisms in this study are not new, and even presented in textbooks. The conclusions, as stated by the authors, are also similar wtih the previous studies. Please revise the manuscript to meet the academic requirement in ACP.
Citation: https://doi.org/10.5194/acp-2021-788-RC1 -
RC2: 'Comment on acp-2021-788', Anonymous Referee #2, 12 Jan 2022
This paper investigates the contribution of the gas-phase oxidation of SO2 to the mass concentrations of sulfate aerosols over the West Pacific. The authors used airborne and surface measurement data obtained during the EMeRGe-Asia campaign. They proposed a new approach to derive the fraction of OH that reacted with SO2 (SO2 oxidation rate fraction) using the concentrations of trace gases and their rate constants with OH, and estimated the contribution of the gas-phase oxidation of SO2 to sulfate. The data presented in this paper are valuable, and the scope of this paper is appropriate for ACP. However, I do not recommend the publication of this paper in its present form because of the lack of robust conclusions.
(1) Assumptions and approximations
The key point of this study is the new approach to estimate the contribution of the gas-phase oxidation of SO2 to sulfate. My major concern is that there are too many assumptions and approximations, which affect the robustness of the estimates. Here are the assumptions and approximations to obtain Equation (13) and the major conclusions.
(i) The total OH reactivity of VOCs is proportional to the sum of the OH reactivity of CH4, CO, NOx, and SO2.
(ii) The scaling factor "t" for the VOC-OH reactions is ignored.
(iii) The O3 concentration is representative of the oxidizing capacity of the observed air masses.
(iv) The contribution of the CH4-OH reaction is ignored.
(v) NOy is composed of NOx and PAN in daytime.
(vi) The slope of sulfate versus SO2 oxidation rate fraction was due to the gas-phase reactions and the intercept was due to transport from other regions and/or aqueous-phase reactions.
The key oxidants responsible for the aqueous-phase reactions (H2O2 at all pH and O3 at higher pH) are expected to show positive correlations with the oxidizing power in the air masses, and therefore the slope may not be exclusively due to the gas-phase reactions. The positive correlations of sulfate versus SO2 oxidation fraction may represent that air masses containing high SO2 yielded high sulfate. Unfortunately, potential uncertainties due to these critical assumptions and approximations are not discussed, and therefore the major conclusions are not convincing.(2) The normalization by O3
The r2 values in Figure 1 increased by the O3 normalization method. However, the rationale for the O3 normalization is not well described. The ratio of [SO2]/[O3] would be largely affected by the background O3, and the interpretation would not be straightforward. The normalization is generally done by inert tracers such as CO after subtracting the background.(3) Introduction
The introduction section is poorly written. I would recommend that the authors should thoroughly review the current understanding of the budget of sulfate aerosols in the troposphere rather than describing the well-known reaction mechanisms of SO2 (both the gas-phase and aqueous-phase).Citation: https://doi.org/10.5194/acp-2021-788-RC2
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Yu-Wen Chen
Yi-Chun Chen
Charles C.-K. Chou
Hui-Ming Hung
Shih-Yu Chang
Lisa Eirenschmalz
Michael Lichtenstern
Helmut Ziereis
Hans Schlager
Greta Stratmann
Katharina Kaiser
Johannes Schneider
Stephan Borrmann
Florian Obersteiner
Eric Förster
Andreas Zahn
Wei-Nai Chen
Po-Hsiung Lin
Shuenn-Chin Chang
Maria Dolores Andrés Hernández
Pao-Kuan Wang
John P. Burrows
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