the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jan 2025
Significant contributions of biomass burning to PM2.5-bound aromatic compounds: insights from field observations and quantum chemical calculations
Abstract. Polycyclic aromatic hydrocarbons (PAHs), oxygenated PAHs (OPAHs), and nitrated phenols (NPs) are essential aromatic compounds that significantly affect both climate and human health. However, their sources and formation mechanisms, particularly for NPs, remain poorly understood. This study determined the concentration profiles and the main formation mechanisms of these substance classes in PM2.5 from Dongying, based on field observations and quantum chemical calculations. The daily concentrations of ∑13PAHs during heating were more than twice higher compared to those before the heating period. Benzo(b)fluoranthene was identified as the primary PAHs species. The average concentration of ∑8OPAHs reached 351 ng m-3, with significantly increased concentrations observed during the heating season, and 1-Naphthaldehyde (1-NapA) emerged as the most prevalent OPAH species. Concentrations of ∑9NPs increased approximately 1.2 times during the heating, with 4-methyl-5-nitrocatechol (4M5NC) having the highest concentration. Positive matrix factorization analysis identified biomass burning to be the primary source of these aromatic compounds, particularly for PAHs. Density functional theory calculations further revealed that phenol and nitrobenzene are two main primary precursors for 4-nitrophenol, with phenol showing lower reaction barriers, and P-Cresol was identified as the primary precursor for the formation of 4M5NC. This study provides the first detailed investigation of the sources and formation mechanisms of aromatic compounds in the atmosphere of petrochemical cities in the Yellow River Delta, which may provide fundamental insights and important guidance for reducing emissions of aromatic compounds in similar atmospheric environments.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2024-3678', Anonymous Referee #1, 21 Jan 2025
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RC2: 'Comment on egusphere-2024-3678', Anonymous Referee #2, 31 Jan 2025
In this study by Ren et al., the authors report measurements of polycyclic aromatic hydrocarbons (PAHs), oxidized PAHs (OPAHs) and nitrophenols (NP) at an industrial city in China. By comparing the concentration before and during the heating period and using PMF analysis, the authors conclude that major sources of these compounds at this location are biomass burning and coal combustion, with a minor contribution from secondary formation. The authors also perform quantum chemical calculations to identify the major reaction pathways for the formation of 4-nitrophenol and 4-methyl-5-nitrocresol. The manuscript is clear and well written, and within the scope of the journal. Some minor revisions are included below.
General Comments
Methods:
- It is never actually stated how the PAHs, OPAHs and NPs were analyzed.
- Details are also needed about how PMF was run and why the four-factor solution was selected.
- What were the limits of detection for the different compounds.
- Line 131-144: I would recommend converting this to a table and including the chemical structures. I would also consider including the vapor pressures of the different compounds if these have been reported in previous literature (see comment below).
Results
- In the introduction, it is discussed that PAHs are semi-volatile. Can the authors comment on the implications of this on the measured PAH concentrations? Are the differences seen between the heating period and the before heating period driven only by differences in emissions between the two periods, or is it possible that gas-particle partitioning also plays a role.
- Line 320-323 states that emissions from coal combustion increases during the heating period, however, line 350 states and figure 6 shows that there is little change in the contribution from coal combustion. Please clarify this contradiction.
- I’m assuming that 4-nitrophenol and 4M5NC were selected for the detailed quantum chemical analysis as they showed the highest contributions. Clarifying if this was the case would help motivate this section.
- At line 411, the authors state that only the addition at the para- position was considered for mechanisms 1 and 2, why was the ortho- position not considered? In connection with this, 2-nitrophenol concentrations are not reported. Is this because it was not detectable by the method used, or because the concentrations were below the limits of detection. If the quantum calculations show that the mechanism greatly favors the para- substitution, this would support that.
- Similarly, in mechanism 3, is it possible for the nitro group to add first forming an isomer?
- K+ and Levoglucosan are both used as tracers for biomass burning, however, only a weak correlation between the two is seen. Additionally, levoglucosan shows a strong relationship with PAHs while K+ shows a better relationship with OPAHs. Can the authors comment on these differences? Are K+ and levoglucosan emitted by different types of burning? Does this provide additional insight into the emissions of PAHs and OPAHs?
- Figure 1: SOR and NOR are not discussed in the text. Either add a description in the text, or remove it from the figure.
- Figure 2: What is the "Other" category? I don't think this is discussed at all in the text, but it does contribute 40% of aerosol mass. Is this a measurement between measured PM2.5 and the sum of all the speciated components?
- Figure 5: Are the differences between before heating and heating period statistically significant? For example, the OPAH concentration was described as "substantially higher" (line 253) during the heating period, but looking at figure 5, the values appear similar.
- Figure 6: Please make the top panels larger,
- Table 2: Please include the units
Minor comments:
- How are the “before heating” and “heating” periods defined and how is cutoff of Nov 14 selected. It would also help to show that cutoff in figure 1, similarly to how it is shown in figure 4.
- Line 121: There is some ambiguity in the methodology. Are the three 15-minute treatments three separate aliquots that are then recombined?
Typographical comments:
Line 46: a word is missing
Line 64: not sure what “carried out” means. Should this be rephrased as “emitted”?
Line 119: Does “membrane” mean “filter”?
Line 156: typo of theory
Table 2: typo of levoglucosan
Fig 4: The pie charts are blurry
Citation: https://doi.org/10.5194/egusphere-2024-3678-RC2 -
RC3: 'Comment on egusphere-2024-3678', Anonymous Referee #3, 18 Feb 2025
In this work, the authors measured PAHs, oxygenated PAHs, and nitrophenols in a Chinese city and studied their sources. By analyzing their time series and performing PMF, the authors separated the contributions by residential biomass and coal burning and secondary formation by oxidation. Quantum chemical calculations were also performed to probe the mechanism of oxidation. Overall the manuscript is well written and easy to follow. The measurements are interesting and provide some insights into a group of atmospherically important compounds. I have a couple of major comments and I would recommend publication after considering my suggestions.
I find that the quantum calculations are unnecessary and provide no new insights. I do not see how they are linked to the atmospheric observations. The calculations shows the free energies of the different pathways and, at best, provide qualitative information about relative importance. Even then, they have nothing to do with the atmospheric observations. I suggest dropping the whole section 3.4. They may fit better with an experimental investigation, rather than field observations.
In the analysis, the compounds are lumped into the major groups of PAHs, OPAHs and NPs. I am curious if there are more specific differences within each group. For example, there is a lot of analysis that can be done on PAH ratios, some of which are source specific. Furthermore, within NPs, some phenols are specific to biomass burning (the methoxyphenols), whereas simple phenols may come from more sources. Deeper analysis into specific compounds would be really useful.
Line 21 and throughout: the term “nitrated phenols” is used, but the analytes are really nitrophenols.
Line 46: missing “of” in “presence benzene ring”
Line 47: aromatic compounds are not really resistant to decomposition. OH addition to a benzene ring occurs at similar rates as OH abstraction from an alkane.
Line 53: PAHs are not just semivolatile. They span a wide range of volatilities, from volatile (naphthalene) to non volatile (BaP).
Line 64: “OPAHs is carried out...” many errors in this sentence
Line 67: “nitro” is incomplete. PAHs can react with NO3 and NO2
Line 112: it seems like a filter is rectangular, but a diameter (with 2 different numbers) is reported. Perhaps it is not a diameter?
Method Lines 124 – 126: It seems like BSTFA derivatization is used. The main objective for BSTFA is to convert acidic hydrogen (e.g. -OH, -COOH) to trimethylsilyl derivatives for GC analysis. However, other than NPs, none of the other analytes really need BSTFA. Were all the samples derivatized, or just some for NP analysis?
Line 129: Curious about the choice of tridecane as internal standard. It is quite different from the analytes, because it is nonpolar and contains no aromatic groups.
Method: since quartz filters are used, what are the procedures to quantify the positive and negative artifacts for semivolatile organic compounds?
Line 174 – 175: it may be useful to report the blank values (e.g. <1 ng/m3), or better yet, limits of detection and quantification
Line 179 – 180: it might be best to expand the term “heating”. Maybe “residential heating”? Why is Nov 14/15 the cutoff for before and during heating? Was there an official government policy? Or was this determined based on observed SO2 (as detailed later in the paragraph)?
Line 201: Why was 1.6 chosen as the OM/OC ratio? Is there a reference?
Table 1 shows N = 74. Presumably this means 74 days? So are the measurements reported in this table daily averages?
Table 1: what does NA under NO mean? Below detection limit?
Figure 2: Interesting to see that “others” represent about 40% of the PM2.5 mass. That seems rather high to me. Is the mass fraction of metals expected to be high?
Figure 3: I am not sure how the trends show a linear relationship. There seems to be a considerable amount of scatter around the linear regression.
Line 243: what are the +/- values reported here? Are they standard deviations? It is incorrect to report an uncertainty larger than the measurement itself, as there is zero probability that a concentration can be negative. A standard deviation is useful as an uncertainty only when a measurement follows a normal distribution, and atmospheric measurements do not. It is preferable to report the range of measurements within a percentile range (e.g. 5th to 95th percentile).
Line 249: replace “accounting” with “accounting for”
Line 254 – 255: is the difference statistically significant?
Line 264: I suggest replacing “foreign sites” with “sites outside of China”. Furthermore, the measurements cited are all in Europe.
Lines 272 – 274: again, is the difference statistically significant?
For Figure 5, panels d, e and f, are the whiskers also representing 25th and 75th percentile?
Line 291: again, not really “international”. Maybe just “previous measurements in other regions”.
Line 315: Why not use K instead of Cl for biomass burning?
Line 321 – 323: The evidence for the difference between coal burning and biomass burning is not clear. Why does the author argue biomass burning is important throughout the campaign, while coal burning is increasing? From which measurements can one get to this conclusion?
Line 350: here the authors argue that the second source factor is coal combustion, and its contribution stays constant throughout the campaign. This is contradictory to statement made in Lines 321 - 323. Some clarification here is needed.
Line 421: what about NO3 in the nighttime? I would expect the dominant sink of aromatic compounds in the night time to be reaction with NO3. Oxidation by NO2 is generally rare, with the exception of heterogeneous oxidation of PAHs by NO2 (to form nitro PAHs).
Line 434: same comment. When OH levels are low at night, NO3 should be high enough to be the dominant sink. A simple rate calculation should show the difference.
The conclusions are too brief and do not quite fit the ACP guidelines.
Having 3 corresponding authors seems entirely unnecessary.
Citation: https://doi.org/10.5194/egusphere-2024-3678-RC3
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