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the Creative Commons Attribution 4.0 License.
| 13 Dec 2024
Secondary Organic Aerosol Formation from Nitrate Radical Oxidation of Styrene: Aerosol Yields, Chemical Composition, and Hydrolysis of Organic Nitrates
Abstract. Styrene is emitted from anthropogenic sources and biomass burning and is highly reactive towards atmospheric oxidants. While it has the highest nitrate radical (NO3) reactivity among aromatic hydrocarbons, the NO3 oxidation of styrene and formation mechanisms of secondary organic aerosols (SOA) have not been investigated. In this study, we conduct chamber experiments with styrene concentrations ranging from 9.5–155.2 ppb. The resulting SOA yields range from 14.0–22.1 % with the aerosol mass loadings of 5.9–147.6 µg/m³ after wall loss corrections. The chemical composition of SOA is characterized by online measurements, revealing that dimeric organic nitrates (ONs) constitute 90.9 % of the total signal of particle-phase products. C16H16N2O8 and C8H9NO4 are identified as the major particle-phase products, which constitute 88.3 % and 4.1 % of the measured signal, respectively. We propose formation mechanisms for the ON products, including the common RO2+RO2/HO2 pathway and other radical chain termination reactions such as RO+R and R+R. We also investigate the hydrolysis of particulate ONs. The hydrolysis lifetime for ONs is determined to be less than 30 minutes. This short hydrolysis lifetime can be attributed to the stabilization of the carbocation by delocalized π orbitals of the benzene-related skeleton of aromatic ONs. This work provides the first fundamental laboratory data to evaluate SOA production from styrene+NO3 chemistry. Additionally, the formation mechanisms of aromatic ONs are reported for the first time, highlighting that compounds previously identified as nitroaromatics in ambient field campaigns could also be attributed to aromatic ONs.
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RC1: 'Comment on egusphere-2024-3849', Anonymous Referee #1, 23 Jan 2025
This manuscript demonstrates the formation of SOA and particulate organic nitrates (ONs) from styrene + NO3 oxidation. The SOA yields of styrene were 4.5% to 16.1% with the aerosol mass loading of 1.9 to 107.4 ug m-3. The chemical composition of SOA was detected by aerosol mass spectrometer (AMS) and a high-resolution time-of flight chemical-ionization mass spectrometer coupled with the filter inlet for gases and aerosol (FIGAERO-CIMS). It is interesting that the C16H16N2O8 and C8H9NO4 were identified as the major particle-phase products, which constitute 88.3% and 4.1% of the measured signal. The main mechanisms for those formation products were RO2+RO2/HO2, RO+R and R+R. In addition, the hydrolysis of ONs was not more than 30 minutes.
This work is suitable to publish in Atmosphere Chemistry and Physics. However, before publishing in ACP, this manuscript needs major revisions to address my comments:
Comments:
- Line 24 and 52: Please change the ug/m3 into ug m-3 through the manuscript.
- Line 37-38. There are no keywords in ACP journal.
- Line 116-118. This manuscript has mentioned that they used a simple kinetic box model to optimize N2O5 production while minimizing O3 concentration. And I did not see the results on how you used the model optimize the N2O5 production. In addition, I am curious whether O3 was also injected into the chamber during the experiment. If yes, what is the concentration?
- Line 129-134. Did you calibrate size distribution and ionization efficiency of aerosol mass spectrometer in this manuscript? If yes, provide the value. What is collection efficiency of AMS do you use in this manuscript?
- The FIGAERO-CIMS heating program is not clear. E.g., how long did you do ramp and desorption, etc.? Please describe the heating procedure in manuscript. Line 142-143, the author used different sampling 1 to 5 L min-1 depending on the aerosol concentration. How long did you collect the aerosol on filters? In addition, Figure 4 and S5 show the concentration of major compounds from CIMS. However, the units of compounds are counts per seconds (cps). Those values are strongly dependent on the aerosol mass in the filters. It is better to use the ppt or ng m-3 units to remove the aerosol loading effect.
- Line 171-172. The author determined the density by using SMPS and AMS. They found the SOA density is 1.35 g cm-3. However, what is the uncertainty for this value? In this whole experiment, is the SOA density always 1.35 g cm-3? I do not believe it. In addition, the author used this density value to calculate all styrene experiments. The SOA density can be changed by chemical compositions. Therefore, the density should be variable in the series of styrene experiments.
- Line 172-174. The author used the SMPS and aerosol density to calculate the density. How do you separate the SOA and seed particle mass?
- Line 201-204. What are the decay rates for CxHy, CxHyO, CxHyOzN, CxHyOz, CxHyON? Please provide them.
- Figure 4 shows the time series of major compounds by FIGAERO-CIMS measurements. Which experiments did you plot this figure?
- 256-258. FIGAERO-CIMS employing iodide ions for chemical ionization can only measure the highly oxygenated organic compounds. The Sensitivity of Iodide-CIMS to compounds depends on their polarity and hydrogen bonding capability and is strongly influenced by molecular geometry and steric factors (Caldwell et al., 1989). Some organic nitrate with weak polarity and bonding could be completely insensitive by I-CIMS. In this manuscript, the organic nitrate molecular weight is 182.7-184.0. What is uncertainty or error for those values?
- Line: 267-268. Why do you think the non-nitrated organic species (C8H8O5) is from the hydrolysis of ONs?
- The C8H9NO4 have two peaks in the thermal desorption profile in figure S8. It indicates that it could have isomer on this molecule. However, the author only used the first peak (24.6°C) to calculate the volatility. Please classify it.
- Line 330-336. The C8H9NO4 and C8H7NO4 from the reference are nitroaromatic compounds. Aromatic nitro compounds are organic molecules that contain a nitro group (-NO2) attached to an aromatic ring. However, nitro group (-NO2) did not attach to the aromatic ring in this manuscript, as shown in Figure 6. Therefore, why do you think styrene hydroxy nitrate (C8H9NO4) and ketone nitrate from styrene-derived ONs accounted for nitroaromatic compounds?
- Table 1. The uncertainties in hydrocarbon concentration and aerosol mass loading were 1 standard deviation in this manuscript. However, this is not uncertainty. This is standard deviation. Uncertainty of aerosol mass from SMPS could be from 27%-31% (Buonanno et al., 2009). Please provide the uncertainty of hydrocarbon concentration and aerosol mass loading from your GC-FID and SMPS. And how do you calculate the uncertainty?
Reference:
Buonanno, G., Dell’Isola, M., Stabile, L., & Viola, A. (2009). Uncertainty Budget of the SMPS–APS System in the Measurement of PM1, PM2.5, and PM10. Aerosol Science and Technology, 43(11), 1130–1141. https://doi.org/10.1080/02786820903204078
Citation: https://doi.org/10.5194/egusphere-2024-3849-RC1 -
RC2: 'Comment on egusphere-2024-3849', Anonymous Referee #2, 29 Jan 2025
Comments
This study reports the formation of SOA from NO₃ radical oxidation of styrene through a series of environmental chamber experiments that were conducted under varying RH conditions. The SOA yields, chemical composition, and hydrolysis lifetimes of organic nitrates were characterized using a suite of complementary instruments. Overall, this study provides very useful information that can improve the current understanding of styrene SOA formation and hydrolysis of organic nitrates derived from aromatic VOC in aerosol water. I have some questions listed below for the authors’ consideration.
1) Lines 79-80: “The authors stated that “To our knowledge, there is no study on hydrolysis of ONs formed from oxidation of aromatic compounds.” This statement is not accurate, as there was study on the hydrolysis of organonitrate functional groups in SOA from the oxidation of 1,2,4-trimethylbenzene (TMB) (Liu et al. 2012). The authors also cite this reference in Lines 380-382. Please revise this statement.
Liu, S., Shilling, J. E., Song, C., Hiranuma, N., Zaveri, R. A., & Russell, L. M. (2012). Hydrolysis of organonitrate functional groups in aerosol particles. Aerosol Science and Technology, 46(12), 1359-1369.
https://doi.org/10.1080/02786826.2012.716175
2) Lines 109-110: Please elaborate on why the initial styrene concentrations of 9.5-155.2 ppb were used in chamber studies. Do they correspond to the known emission inventories, or for any other reasons?
3) Lines 219-221: Is it possible to use different analytical methods that do not require thermal desorption (e.g., LC/ESI-MS) to confirm that the presence of monomeric species are true SOA constituents, not a result from thermal decomposition of dimeric species?
4) Lines 261-268: Can the authors elaborate on how they quantitatively determine the hydrolysis lifetime to be less than 30 minutes and what parameters may influence this estimate? Does the hydrolysis lifetime refer to τ = 1/ k? I did not see any rate constants (k) calculated or discussed in the manuscript.
5) Lines 359-362 and 431-432: Please specify what parameters need to be calculated by density functional theory to support the proposed mechanisms. Given that the proposed mechanisms are based on molecular formulas, not confirmed molecular structures with functional group information, I would suggest clarifying what specific information should be obtained in further experimental studies to validate the proposed formation mechanisms (e.g., confirming the molecular structures and detecting the critical intermediates).
6) Lines 856-860 (Figure 5 a): In the time series data of the ratio of particulate ONs (pON) to total organic aerosols (OA) in Exp. 2 (RH<3%), Exp. 11 (RH~50%), and Exp. 12 (RH~70%), the signal for RH~50% drops more rapidly than the signal for RH~70%. Does this imply a faster hydrolysis rate? Can the authors provide some explanations for this observation?
7) Lines 862-869 (Figure 6): Can the authors label or clarify which products were measured and confirmed in the current study and which were proposed that need to be validated in further studies?
Citation: https://doi.org/10.5194/egusphere-2024-3849-RC2 - AC1: 'Comment on egusphere-2024-3849', Yuchen Wang, 04 Mar 2025
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