the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Apr 2025
Laboratory studies on the optical, physical, and chemical properties of fresh and aged biomass burning aerosols
Abstract. Atmospheric brown carbon (BrC) plays a significant role in global warming, yet the evolution of its optical properties during aging remains poorly understood, leading to substantial uncertainties in its climate effects. In this study, we investigate the aging process of BrC and its driving factors using laboratory-generated biomass burning emissions, including four types of straw and one type of wood. Upon OH oxidation, there exists a large increase in OA fraction after 2-day aging, followed by a minor increase during aging to 7 days. The particle growth is dominated by the change in OA content and thus shows a similar trend during aging. The mass absorption efficiency (MAE) of fresh BrC measured at 370 nm is 2.1–5.7 m2 g−1. A sharp decline in MAE is observed after 2-day aging, equally attributed to photobleaching and secondary organic aerosol (SOA) formation, while the subsequent slight decrease during further aging to 7 days is dominated by SOA formation. Although a negative correlation is observed between particle size and MAE, the reduction in MAE is mainly driven by the decline in the imaginary part (k) of BrC, with particle size playing a minor role. Combined with positive matrix factorization (PMF) analysis, the study reveals that oxygenated OA, characterized by higher O/C ratios but lower MAE, increases significantly with aging. In contrast, two hydrocarbon-like OA factors with lower O/C ratios and higher MAE, decrease over time. These results emphasize the importance of categorizing BrC based on its MAE and atmospheric behavior in climate models.
Competing interests: Some authors are members of the editorial board of journal
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Status: open (until 06 Jun 2025)
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RC1: 'Comment on egusphere-2025-1020', Anonymous Referee #2, 02 May 2025
reply
The authors have addressed all of my previous comments. I recommend the paper for publication in its current form.
Citation: https://doi.org/10.5194/egusphere-2025-1020-RC1 -
RC2: 'Comment on egusphere-2025-1020', Anonymous Referee #1, 02 May 2025
reply
The authors present here results of fast photochemical aging of particles and gases derived from combustion of various fuels, mainly straws. The measurements appear to be of good quality. Their work adds to the literature, but could make much stronger connections to the existing literature to allow for stronger conclusions. This is especially the case when it comes to thinking about and discussing other laboratory aging experiments in the literature; I find the author’s approach to be fairly selective and a bit unfocused. There are also quite a few statements and conclusions made that require more support or context if they are going to stand.
I also strongly encourage that the data underpinning this work be made available in an open, doi-referenced archive.
My detailed comments follow.
L44: Here, the authors state that positive forcing can reach “up to +0.05 W/m2.” Yet, on line 32 they state that the forcing may be up to +0.57 W/m2. It would be good to have some consistency.
L47: The choice of Huo et al. (2018) seems random. Why this study? Especially given that it focuses on HULIS.
L50: It would be helpful to clarify/distinguish between process-level studies and ambient studies. Park et al. is an ambient study, and thus divining relationships is more complicated than in process-level studies such as Saleh and Xie.
L61: I encourage the authors to clarify what “formation of nitroaromatic compounds” means more specifically. Is this SOA formation? Or are they talking about heterogeneous transformations. This matters to thinking about process. Moreover, the paper cited her is about photooxidation of naphthalene and SOA formation, while the previous sentence implies an importance of “nitration of existing OA.” These are not at all the same thing, leading to ambiguity and some contradictions in the authors’ presentation of the literature.
L63: Here, I find that the authors continue to mix and match studies without really focusing on what people did in a sufficiently careful manner. The cited Choudary study is specifically about photooxidation of collected biomass burning particles in aqueous solutions while the Schnitzler study is of suspended particles produced from smoldering in the lab and then the Fang study is also for suspended particles in a chamber. Are these comparable? Maybe? But the authors don’t give the context necessary for a reader to make the connections. I encourage the authors to be more precise throughout their comparisons between studies in the introduction. The way things are presented seems a bit random to me.
L65: The authors here allude to “only a few studies” but fail to cite any. Which studies?
L87: Please indicate which lights (wavelength) were used in the PAM. Also, more details of the operation are needed. The authors simply state that the lights were turned on for 40 minutes. However, the intensity was presumably varied during this 40 minute period, leading to changes in the OH exposure. How many conditions were considered? How long per condition? This needs to be in the main text, not the supplemental.
Table S1: Are these average conditions? If so, need to indicate. What is the reproducibility?
L103: The authors state “Actually, flaming and smoldering phases occur simultaneously
during a fire.” Why is this important to note? Some context would be helpful.L106: The lifetimes of NOx and SO2 are very, very different. Yet, the loss through the OFR is similar for both. How can this be justified? Moreover, the SO2 consumption increases with aging in many cases. This seems non-physical. What is the explanation?
L111: DeCarlo et al. (2006) do not provide a description of the ACSM. Please select an appropriate reference.
L114: It is stated that “the collection efficiency (CE) value was 0.5 in this study.” How was this determined? The particles are, presumably, primarily organic and often OA has a CE closer to unity, although this can change with aging. Did the authors aim to quantify this value? Or is this simply an assumption that was not tested? The authors have size distributions and so should be able to test this, with an assumption of the particle density. Is the ratio between the AMS mass and the derived particle mass from the size-distribution measurements independent of all conditions?
L139: The authors should clarify that the MAE from Drinovec et al. is not the true MAC for BC at 880 nm. It is specific to the Aethelometer. The actual BC MAC is much smaller than the value given here. As stated, a reader is left thinking that this is the actual MAC for BC at 880 nm, which it is not.
L144: The authors use a few references to indicate that BC is the only absorbing component at 880 nm. However, this is not necessarily true. BrC may be very weakly absorbing, but if the concentration of BrC is >> than BC it can still be important. What matters is the ratio [BC]*MAC_BC/[BrC]*MAC_BrC. BrC can absolutely absorb at 880 nm, although such absorption is typically weak. I encourage the authors to clarify. (In this study, OA/BC is unlikely high enough to matter, but the point is still valid.)
L157: Why focus on 370 nm? Why not take a more holistic approach to the analysis and consider the BrC across the solar spectrum, or at least at the peak?
L162: I strongly encourage the authors to use words, rather than abbreviations here. I can’t keep track of what AB and SS are, for example. Apple Branch and Soybean Straw are much more descriptive. If there were only two to keep track of that would be perhaps okay, but here there are 6 fuel types and it is easy to get lost.
L165: The statement about how crop fertilization practices impact straw composition would be strengthened with a reference.
L165: The authors state “Higher MCE may also cause more emission of Cl−.” But, is any relationship observed here? Is this relevant to the current study?
L167: The authors state that “Upon the aging process, significant changes were found for aerosols with secondary sources, including SO42−, NO3- and OA.” However, the authors only present relative composition plots. This means that something could go down simply because something else went up more, not because it didn’t go up too. It might be useful to present (in the supplemental) some aspect of changes in absolute concentration changes.
L169: I find this argument very weak. The authors state that the higher enhancement for nitrate over sulfate could be “explained by a faster NO3− production rate and is also consistent with the higher NOx consumption rate compared to SO2.” A look at Fig. S3 indicates that, while this is technically true, the differences are small. And, moreover, inconsistent with expected OH-driven oxidation rates. Here, I feel that the authors are forwarding a narrative that is not supported by the data. I would welcome a more quantitative approach to this conclusion. Also, there is sufficient discussion of enhancement ratios that it seems appropriate that the table, or an equivalent figure, should be included in the main text. By having it in the supplemental it is too easy for details to be ignored. For example, with the NO3- production, the authors talk in broad terms but they do not ever discuss the fact that for WS the NO3- ER decreases a lot from A2 to A7 while for all other fuels it either remains essentially constant or increases slightly.
L173: It is not at all clear how “this could also explain the low ER value of CL-“ Detail is needed. Especially given that the literature generally supports the replacement of non-volatile (e.g. sea salt) chloride. A reference would at least help. But also some further quantitative discussion. Can displacement explain the changes?
Fig. 1b: I very strongly encourage removing the “slope” lines that presume to indicate some chemical process. These lines have many, many assumptions baked in. Specifically, they assume that the end member meets at H:C = 2 and O:C = 0. This is true only for very specific VOCs. The inclusion of lines is misleading in terms of process. There is no evidence that these lines indicate anything about process for the samples measured. Moreover, the discussion of how aging affects things on L181 does not make sense. The change needs to be considered relative to the starting condition. In this case, the H:C increases slightly while the O:C increases with aging. This is not a move from “a slope of -2 in fresh smoke to the region with a slope of -1 upon aging.” These slopes are not meaningful here. The relevant situation is that the H:C increased slightly while the O:C increased more, and thus corresponds to a “slope” of about +0.1. Interpreting this in terms of specific chemical groups cannot be done unless one knows the identity of the VOC precursors that lead to SOA formation. This discussion must be revised to focus on actual physical and chemical changes, not arbitrary lines.
Fig. S4: I find the “stack diagram” very difficult to interpret. It is extremely challenging to determine how the different aging conditions compare with each other from such a graph. Moreover, the % values seem wrong (too small) as they will not sum to unity.
L180: The ToF-ACSM does not allow for direct determination of H:C and O:C ratios. They must be inferred. The authors should state the assumption they made in translating the actual measurements to H:C and O:C.
Section 3.2 and PMF: Some justification for the choice of 3 factors should be provided. Why 3 (versus 2 or 4 or 5)?
OOA: Further discussion of the relatively large contribution of OOA with zero aging seems needed. Certainly a factor is just a mathematical construct and not necessarily an actual thing. However, the authors discuss OOA as resulting from SOA formation. But if OOA equates to SOA formation, why does it make up to 40% of the OA with zero aging? Some consistency in the discussion would be welcome.
L218: Please clarify how a smaller particle distribution spread (sigma) means that the particles have a relatively uniform morphology or density. What does density have to do with the spread? Similarly morphology?
L222: Often in OFR’s there is substantial nucleation, given the fast oxidation conditions. The authors present maximum diameter and spread results, but do not provide any information on what the actual size distributions look like (are they, for example, log normal) or whether there is nucleation.
L229: Further explanation of the statement “On the other hand, more mass is required to increase one unit particle size as the particle grows. This may also explain the observed decease in σ as particles shift to larger size along with aging (Fig. 3b).” Please clarify how the former explains the latter. Is this just noting the well-known narrowing phenomenon that occurs during growth experiments?
L235: The authors state that “Under OH exposure, the AAE decreases due to a reduction in BrC light absorption.” It is not clear to me that this is a robust conclusion based on the data presented in the graphs. The bars overlap, certainly with the error bars shown. A statistical analysis seems needed to back up this statement.
L238: I think that it would be great if the authors clarify what they mean by “photobleaching” as it relates to process. Do the authors really mean “degradation through heterogeneous oxidation?” Or are the authors indicating some effect of directly photolysis? I generally think of photobleaching as referring to the direct influence of photon absorption, and I think that this is generally accepted. If the authors mean the latter (direct degradation via photons) then I cannot agree with the interpretation put forward. The photon flux and time spent in the OFR is likely insufficient to have any substantial direct impact. Instead, any changes that occur are likely a result of OH-dominated heterogeneous oxidation, coupled with changes driven by formation of SOA that has different properties than the primary OA.
L239: I am not convinced that the ER differences at A2 and A7 for brown carbon are actually (statistically) different from each other, and therefore the idea that there is first a photodegradation process but at later times some photoenhancement is suspect. I believe a statistical analysis is needed.
L243: There is no physical basis for an exponential relationship between AAE and BrC fraction. What is the purpose of presenting an exponential relationship? To the extent that it can be assumed that BC and BrC equal two end members the relationship should be linear. This should be placed in a physical framework.
L250: Any time that MAE is mentioned the wavelength should be included. I encourage simply using a subscript 365nm.
L255: The authors state “For OA at A-2, the reduction in MAE compared to the fresh smoke is dominated comparably by both the changes in babs, BrC and OA mass.” This needs clarification. How is it known that it is dominated “comparably” by both these things? And then how do the authors know that at A7 “the reduction in MAE is mainly driven by the increasing in SOA with weak absorption.” This is simply stated but has not been demonstrated.
L264: Please clarify how ER’s are calculated for each OA factor. It is not readily apparent.
L271: I appreciate the authors considering the relationship between MAE and size, but it’s not clear that this makes sense when considered separately from the evolving particle composition that comes with particle growth. Moreover, the authors should likely use the mass-weighted diameter, not the number-weighted diameter, when considering any MAE-size relationship, since absorption is driven by mass. Really, it seems like what the authors want to do is an optical closure study where they use the absorption and size distribution measurements to derive the value of k. They can then consider whether k is changing. The imaginary component of the refractive index is a conserved physical property, unlike the MAE. As it stands, I do not think that the authors can confidently make the conclusions they do in this paragraph. Lastly, the linear fit line on Fig. 5d should be removed. It has no physical basis and there is no reason to believe that it will prove broadly applicable.
Fig. 5b: Which laboratory and field observations are included here? It is not at all clear, and therefore not reproducible. Are these from Table S4 and S5? If so, how have the authors addressed the issue of wavelength differences? And which values are selected from this long list? Just those with BBOA in the name? Again, it is not clear, which makes the comparison less meaningful.
Fig. S5: Please report the R2 values associated with the fits. Also, please justify why it makes sense to merge the aged and non aged samples into the same dataset when it comes to a linear fit given that the authors have argued that the composition has changed. And, what is the dashed oval? The authors favored data? Were some of the points excluded? If so, why, and how is this justified? (I don’t believe that it is justified.)
L291: The idea that the derived slope of 8 is “within the range (~0.63-22)” is not especially meaningful. This is a HUGE range. Also, why are only these two studies cited? These seem randomly selected from the broad literature. The comparison to the literature here, and really throughout, is fairly weak in the sense that it often seems extremely selective.
Particle growth? Nucleation?
Particle size relationship with MAE?
Citation: https://doi.org/10.5194/egusphere-2025-1020-RC2
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