the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
22 Jan 2021
22 Jan 2021
SO2 and NH3 emissions enhance organosulfur compounds and fine particles formation from the photooxidation of a typical aromatic hydrocarbon
- 1Environment Research Institute, Shandong University, Qingdao, 266237, China
- 2Technology Center of Qingdao Customs, Qingdao, 266003, China
- 1Environment Research Institute, Shandong University, Qingdao, 266237, China
- 2Technology Center of Qingdao Customs, Qingdao, 266003, China
Abstract. Although atmospheric SO2 and NH3 levels can affect secondary aerosol formation, the influenced extent of their impact and their detailed driving mechanisms are not well understood. The focus of the present study is to examine the chemical compositions and formation mechanisms of secondary organic aerosols (SOA) from 1,2,4-trimethylbenzene (TMB) photooxidation influenced by SO2 and/or NH3. Here, we showed that SO2 emission could considerably enhance aerosol particle formation due to SO2-induced sulfates generation and acid-catalyzed heterogeneous reaction. Orbitrap mass spectrometry (MS) measurements revealed the generation of not only typical TMB products but also hitherto unidentified organosulfates (OSs) in SO2-added experiments. The OSs designated as unknown origin in earlier field measurements were also detected in TMB SOA, indicating that atmospheric OSs might be also originated from TMB photooxidation. For NH3-involved experiments, results demonstrated a positive correlation between NH3 levels and particle volume as well as number concentrations. The effects of NH3 on SOA composition was slight under SO2-free conditions but stronger in the presence of SO2. A series of multifunctional products with carbonyl, alcohols, and nitrate functional groups were tentatively characterized in NH3-involved experiments based on infrared spectra and HRMS analysis. Plausible formation pathways were proposed for detected products in the particle-phase. The volatility distributions of products, estimated using parameterization methods, suggested that the detected products gradually condense onto the nucleation particles to contribute to aerosol formation and growth. Our results suggest that strict control of SO2 and NH3 emissions might remarkably reduce organosulfates and secondary aerosol burden in the atmosphere. Updating the aromatic oxidation mechanism in models could result in more accurate treatment of particles formation for urban regions with considerable SO2, NH3, and aromatics emissions.
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Zhaomin Yang et al.
Status: open (until 19 Mar 2021)
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RC1: 'Comment on acp-2021-61', Anonymous Referee #1, 07 Feb 2021
reply
This smog chamber study investigated the effects of NH3, SO2, and NOx on SOA formation under UV irradiation from 1,2,4-trimethylbenzene (TMB) over a period of 5-6 hours. The smog chamber was monitored in real time using a scanning mobility particle sizer; gas analyzers for SO2, NOx, NH3, and O3; and gas chromatography-flame ionization for TMB. Offline analysis used infrared spectrophotometry, ion chromatography, and liquid chromatography-mass spectrometry. The study found that the presence of NH3 and SO2 both individually and synergetically increase SOA yield, and that SO2 speeds nucleation, possibly through uptake onto H2SO4 surfaces. New organosulfates were identified and reaction schemes and structures were proposed for some; some organosulfates had molecular weights consistent with TMB-derived aerosol components found in the atmosphere. NH3 was found to react to form organic nitrogen compounds in the aerosol phase, but only in the presence of SO2, attributed to formation of ammonium sulfate. Aerosol components had a wide range of volatility almost nine orders of magnitude, as predicted by elemental composition from mass spectrometry.
This study is a good match for the scope and aims of ACP, and it does an excellent job conveying its own novelty even if the reader is not well versed in aromatic aerosol chemistry. It represents a necessary contribution to the aerosol research community's understanding of SO2 and NH3 dynamics, and has implications for reactions of a large number of aromatic VOCs as well as human health in the findings regarding enhanced UFP fraction.
Major Comments
This article is solid, if not concise, and all sections contribute to the main point as a cohesive whole. The introduction, methods section, and discussion of limitations are excellent, and while there is far more description of results than there is analysis of results I believe that both the analysis and discussion of atmospheric implications are sufficient. All of my suggestions for improvement are minor, relating to clarity and readability.
Minor Comments
line 13: Consider a brief explanation of the atmospheric relevance of TMB in the abstract.line 69-70: "Equivocally not originated from biogenic VOCs" is a little unclear. Possibly change to "unidentified OSs with C2-C25 skeletons that may not have originated from biogenic VOCs" or similar.
line 254-55: Make it clear whether ammonium sulfate particles were introduced during experiments or as an independent wall loss experiment. Is this dependent on humidity or hygroscopicity of the particles? Not VERY important, but may be a limitation worth discussing.
line 284: Were these from two single experiments? If so, include experiment numbers. Otherwise I want to know the standard deviations for these values.
line 291-95: Before noting that UFP are more harmful to human health, you may want to make it explicit in the text analysis of figure 1 that increased SO2 concentration increases the fraction of UFP in the aerosol size distribution.
line 323: I would be interested in more detail about the mechanics of H2SO4 as a "condensed surface for key compounds." I take this to mean reactive uptake or heterogeneous reactions of VOCs with H2SO4. However, you don't justify this assertion; I might recommend citing Wang et al. 2010 (doi.org/10.1021/es9036868) and/or Zhang et al. 2019 (doi.org/10.1021/acsearthspacechem.9b00209) for VOC heterogeneous reaction with sulfuric acid surfaces.
line 329-30: As neither this work nor that of Julin et al. attempts to explain the "nonlinear dynamics of aerosol populations," rather than attributing your results to these dynamics it might be more effective to say something like "The nonlinear response of the mean particle diameter to SO2 initial concentration is similar to results found by Julin et al. (2018) in a modelling study."
line 336: Is this the mean particle diameter after 30 minutes? Be explicit. You defined initial growth rate as average for 0-30 minutes, but did not define a timeframe for mean diameter.
line 383-84: If you believe you have not sufficiently justified the necessity of chemical composition studies, I recommend you do so in the introduction instead.
line 589: Figure 9 may belong in the supplement, because it has only a brief mention in the text that is limited to the observation that positive mode MS spectra skew toward lower m/z's.
line 594, 600: The contents of Figure 10 are also not discussed in the text, so it could be moved to the supplement as well.
line 509-10: use comparable measures to compare the SO2-involved and SO2 free conditions. I would recommend including the multiplicative factor of the change for both.
line 524-26: The sentence "Initial growth rate... as shown in Fig. S6" is redundant, and to the next sentence "In SO2-free experiments... initial growth rate of aerosol particles" I would add something like "compared to the SO2-involved condition."
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RC2: 'Comment on acp-2021-61', Anonymous Referee #2, 23 Feb 2021
reply
General comments
The authors have studied SOA formation from photo-oxidation of 1,2,4-trimethylbenzene under different concentrations of NOx, SO2 and NH3 in an indoor smog chamber. The reactants and the particle generation and growth were monitored using a series of standard instrumentations. The chemical functional groups were characterized by ATR-FTIR in each experiment and inorganic constituents were analyzed using ion chromatography. The molecular level information is provided using UPLC-HRMS and dd-MS2 scans. Ten new organosulfates are identified in the presence of SO2, including 3 in which the origin was previously unknown and some previously reported to be originated from biogenic precursors. Formation mechanisms for 8 of the newly identified organosulfates are proposed based on previous literature. Their results indicate that SO2 is a key parameter for ultrafine particle formation. A synergistic effect of NH3 and SO2 in particle formation is also shown, indicating the importance of reducing both SO2 and NH3 emissions to improve lowering PM. Their results also suggest that ammonium sulfate form by the reaction of NH3 with H2SO4 facilitate aerosol formation and growth through condensation of organic vapors.
This article advances the current knowledge of aerosol formation from photo-oxidation of a typical aromatic hydrocarbon in the presence of NOx, SO2 and NH3, therefore is of interest to the scientific community of ACP. The manuscript is well written and organized. The experiments are well executed, methods are explained adequately, results are discussed thoroughly, and conclusions are well supported. I have some minor comments to improve the quality of this manuscript that are listed below. I recommend accepting this manuscript for publication in ACP with minor revisions.
Specific comments
Line 13 – Indicate the major emission sources of TMB.
Line 45 – Define ‘certain regions.
Line 54-56 – Add reference/s.
Line 98-99 – Briefly indicate what MCM is and add a link to the version.
Line 210 – Indicate how the mass calibration was performed.
Line 200 – Indicate the mass resolution in full MS, top N in dd-MS2, isolation width (mass window) etc
Line 277 – Add initial growth rates to Table 1 or SI.
Line 277-280 – Can the authors elaborate the reasons for the observed non-linear response of the particle diameter with initial SO2 concentrations?
Figure 4 – Indicate what error bars represent.
Figure 5 – Indicate the experiment numbers relevant to a) to f)
Lines 346-348 – Add detailed information of the common products observed in SO2 free and SO2 involved experiments to the SI.
Table S3 – Indicate clearly whether this table shows the compounds that are detected in both SO2 free and SO2 involved experiments with NH3 or the products only formed from the experiments involved both NH3 and SO2.
Tables S2 and S3 – Add UPLC retention times.
Technical corrections
Line 105 – Add ‘in the atmosphere’ to the end of “Given the ubiquity of SO2, NH3, and TMB…
Line 200 – It is better to write it as data-dependent MS/MS (dd-MS2) scans
Line 206 – Add B after 3%.
Figure 6 – Label the red structures as OS-226, OS-228…etc. (Authors may replace the chemical formula with their abbreviated names as the structures are shown.)
Figure 10 – Match the color of the TMB on the figure with carbon number (should be light blue as it has 9 carbons)
Zhaomin Yang et al.
Zhaomin Yang et al.
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