|Manuscript: Measurements to determine mixing state of black carbon emitted from the 2017/2018 California wildfires and urban Los Angeles (Ko et al.,)|
Referee Review of manuscript - Synopsis of Original Review
The manuscript presents measurements of refractory black carbon (rBC) mass and number loadings along with derived rBC mixing state properties (e.g., fraction of thickly coated rBC particles and coating thickness) for three events (air masses) that were encountered at a sampling station located Catalina Island (CA). The measurement campaign itself can best be described as a measurement opportunity where the authors report on the correlation of rBC mass/number loadings and derived mixing state properties with emission source and estimated plume age. Examples of findings include mixing state for sampled biomass burns (BB) and urban emissions (Los Angeles). This reviewer recommended that this manuscript undergo a major revision and be resubmitted due to profound reservations regarding three areas in this manuscript: (i) source attribution and estimated plume age; (ii) the research team’s use of the rBC number size distribution data; (iii) and the discussions about increasing rBC diameter with atmospheric aging.
Review of Revised Manuscript.
While the authors have addressed some issues, it is the opinion of this reviewer that the authors are misinterpreting their some of their findings as being due to aging, when the simpler explanation - dynamic mixing of wildfire emissions with urban emissions - can better reconcile the reported findings. As this reviewer noted in the original review, this work is highly-focused but it contains very useful observations on rBC mass loading and mixing state that are of value to the community and thus should be published. But it also remains the opinion of this reviewer that the manuscript has not made convincing arguments to support all their observations and that this deficiency needs to be addressed.
Source attribution and increasing rBC diameter with atmospheric aging. As with the first review, this Reviewer still has issues with the authors interpretation of the derived rBC mixing state with respect to contributions from emission source vs plume age. The heart of the issue is that a fixed ground site cannot conduct a Lagrangian (or even a pseudo-Lagrangian) sampling strategy. This inescapable limitation for ground sites is why aircraft operations are so important - aircraft put the ground site measurements in context. Unfortunately no such aircraft measurements were available during this Catalina Island measurement campaign. This absence greatly complicates the authors ability to interpret the collected data in a region that is characterized by a dynamic mixing of fresh urban emissions, very aged aerosols, and biomass burning. For example, the authors argue (page 27, line 630-633) that significant correlation between thicker coatings and larger rBC modes is due to contributions from both differing sources (urban vs. wildfire) and longer time scales. This reviewer is not convinced plume aging plays any role in what the authors observe. Instead, the far simpler explanation is that the changes observed with the rBC particles are driven solely by the mixing of emissions sources (urban vs wildfire) - wildfires generate larger diameter rBC particles that are more thickly coated that become mixed with smaller diameter, more thinly coated urban rBC particles. Period. Such a simplified explanation also reconciles the unpersuasive argument by the authors (Page 27 lines 613-625) that coagulation is partly responsible for observed increase in rBC CMD (count median diameter). As this Reviewer pointed out in the original review, homogeneous coagulation under the conditions characteristic of this measurement period would be over 600x slower than at 10^4 cc-1, readily relegating coagulation as being negligible. Further, to go from 93 nm to 112 nm (lines 615 - 619) would require a HUGH amount of coagulation. Bottom line: source emissions are driving observed changes in rBC microphysical properties. The authors need to reassess the data as being primarily driven by source and less about aging - especially given the absence of concomitant aircraft measurements coupled with the complexities of the measurement location (dynamic mix of urban, BB, and aged aerosols) that may not be captured by HYSPLIT.
Black carbon number size distributions. As highlighted in my original review, this reviewer has a major concern with using a CMD from a lognormal curve fit to rBC number size distributions for which there is no obvious peak in the actual number size distribution data. Small changes in distribution width could easily shift the CMD and, in turn, impact how the authors interpret the findings (e.g., lines 602 - 604; “..some periods in which we observed a relative increase in the MMDfit, but concurrent decrease in CMDfit.” As stated before, any discussion using derived CMDs for which there is not discernible peak in the actual data must be deemphasized. Also, on line 604, the authors cite 53 nm as the CMD, but according to their text they are referencing “L5” in figure 11, which shows the CMD = 59 nm.
Other specific issues:
Line 9: presumably the authors mean “composition” when referring to “…coating properties”. Please clarify.
Line 235: Can the authors give a citation for the assertion that the probability density function of lag-time values is often bimodal? As the partitioning of thickly and thinly coated rBC particles is very sensitive to the cut off value (in the present manuscript the authors use 1.8 usec ).
Line 239: only DBC > 170 nm; this and discussion later of this (Line 258) should go earlier in Section 2.2, which discusses instrumentation
Line 257: on line 239 the authors use 170 nm; here they use 180 nm? Please try to remain consistent with ranges and sizes if at all possible.
Figure 4: the figure legend refers to rBC count median diameter (CMD) but the Y-axis on trace “C” is labeled rBC Count Mean Diameter. Please correct the graph axis label.
Line 346: They state Section 3.4 in the figure caption but it is really Section 3.5.
Fig. 5c: A pedantic comment: mass and number concentrations on opposite axes from figure 4.
Lines 359-389: a lot of time discussing previous fBC ratios after stating (line 246) that they couldn’t be compared across different studies.
Figure 6: It is difficult to discern, but this reviewer is concerned that in trace 6B (? as there is not A, B, C label on figure) but it seems as if the lag-time data maybe exhibiting behavior characteristic of particle coincidence as at the positive and negative lag-times seem evenly distributed about zero lag-time. What does the actual full lag-time distribution look like? That is, please redo figure 7 to contain both positive and negative lag-times.
Fig. 7: This reviewer’s concern is that the authors are confounding things here – the different rBC core values come from different sources (each with a wide spread); thus when you plot them you get a difference, but they’re arguing it’s a causal relation (lines 437-439).
Line 457: Not necessarily true: thickly-coated particles could fragment.
Line 475: This reviewer would like to see Figure S9 (CT_BC vs f_BC) in the main manuscript rather than in supplemental. Also, how does this slope compare to figure 3 in Subramanian et al., (ACP 10, 2010)?
Line 500: Since only rBC particle diameters between 200 and 250 nm are included in the table should be stated in the text as well.
Fig. 9: Are these only for 200-250 nm particles, as for the previous figure? If so, state in text and caption.
Fig. 9: The fact that there are an appreciable number of negative coating thicknesses (e.g., L2, L4, L5, L6, L7) should be discussed somewhere. Why/how/what confidence does this leave for the others? What does a negative coating thickness mean? How do you get a negative coating?
Line 535: This reviewer would not expect much diurnal variability in the MBL height over the ocean at Catalina, and even if so, it would be AT MOST probably a factor of 2 – not nearly enough to explain the order of magnitude difference (line 533). The authors should be able to get measurements of this for Catalina or nearby locations from radiosondes, lidar, etc. to show what typical changes are near the coast.
Line 539: They stated already on lines 365, 385, 510 that BB has thicker coating that FF.
Line 570: This goes in the section that introduced the SP2 way above this.
Line 578: “it is reasonable” … “assuming” – lots of guesses. Not always reasonable, especially if you have a mixture of BB and FF, you wouldn’t expect a single lognormal. Also, at some point you run into individual spherules that cuts off your lower end. The issue is how robust any such fit is the how usable the CMD is as discussed above. The authors are encouraged to restrict their discussions to MMD, where the experimental data exhibit a peak, and discuss these values - which the authors do: 594-598 - and remove discussions about CMD (especially for those conditions when the fit CMD is not support by actual data at the derived peak).
Line 607: This reviewer is not convinced of the author’s logic: that MMD is expected to change more than CMD.
Line 611: Is there the possibility of smaller, undetected modes? Also, why assume only a single lognormal? As the authors show in their manuscript (figures 15 and 16), evidence is present for two populations in some of the LEO periods.
Line 627: The opening sentence “The dominant factors that influence rBC core size (i.e., emission source type and aging) also influence rBC mixing state.” in this paragraph is very misleading. It is VERY unlikely that atmospheric aging explains the increase in rBC mode; rather it is the increasing contribution from BB emissions which contain both larger diameter rBC particles that are also more thickly coated. Please reword this sentence.
Line 628: Please state that the data shown in Figure 12 is for all campaigns (it is currently stated in the caption only).
Line 629: give r^2 rather than r.
Line 633: As discussed above and what this reviewer considers a major concern of this manuscript, how can aging affect rBC core size? The only mechanism that aging can contribute to the rBC core size change is coagulation. But the concentrations during the sampling periods are much too low to make coagulation important or even a contributing factor. Furthermore, the authors are confounding things. Longer aging timescales “are associated with increases …” Incorrect. Long aging timescales correspond to LARGER (NOT increases in) rBC diameter – one is a difference (larger than) and one has a temporal aspect to it, which cannot be inferred from a measurement from a single location. This Reviewer maintains that larger BC diameters come from fires farther away – with potentially different fuel sources. Thus, the larger diameters could result from different fires, with age having nothing to do with it.
Fig. 12: This is not so surprising: they have a number of different sources, each with a spread of values, which, in the limit of pure FF and BB plumes will give FF having lower rBC core diameter and lower CT_BC, and BB having the larger. Then there are essentially two points, with a positive slope. The authors are encouraged to colormap the points into BB and FF data which will likely show this.
Fig. 13a: Somewhat of an increase in coating with core diameter, which makes sense (for the same reason as in the previous comment) – not the same history; Fig. 13b and 13c have a decrease in coating thickness with increasing DBC. It should also be noted that the conclusion of increasing coating thickness with rBC diameter is derived from very weak signals (E, F, G, and H). What are the error bars associated with the derived coating thicknesses?
Fig. 16: To this Reviewer, this figure makes a robust argument that source is what is dominating the rBC mixing state properties and NOT atmospheric processing (i.e., aging).
Line 708: The authors assume that all their rBC is either urban or BB, and implicitly seem to assume that the urban is LA basin. However, for their first period the winds were from the west and concentrations were very low. How about ship emissions? There’s a rather large shipping route near there, and the rBC from that could look very different from LA basin. Also, if it’s quite aged, it could be from San Diego, or Tijuana, which might have a very different mix of diesel/auto/industry than LA, therefore restricting comparisons between their first period and when they know it’s from LA basin might not be justifiable.
Line 710: The authors write “In addition to emissions source type, atmospheric aging also appears to have an observable effect on the mixing state.” As discussed above, absence a Lagrangian measurement, such a conclusion is not justifiable. Instead, and as highlighted in this review, the likely cause for observed differences is simply differing sources getting mixed together.
Line 710-711: There is not Table 3.
Lines 728-730: In the absence of Lagrangian measurements, the authors cannot conclude this. Could be (likely is) different sources.
Line 743: they’re citing an AGU presentation
Line 747 (at end of paragraph): I wrote “much verbage” – lots of words but they didn’t say much in this paragraph
Line 754: “Assuming …” is FF influence BC necessarily urban? Baja California/ship emissions/etc. are possibilities that aren’t urban but that are BB.
Line 764: urban BC coagulation – as highlighted several times here and in my original review, the authors can estimate this since they have concentration data. But I think concentrations are too low for this to be a major contributor. Also, the authors assume that period 1 is either urban or BB, and there are other options.
Line 800: Are there results from HIPPO (or whatever the latest one was) on aged BC? I think there is something out from the NOAA (Murphy’s) group, but they didn’t cite it.
Page 37: Table 3 missing.
Line 836: any CO measurements?
Line 842: Again, the authors assume only urban (and specifically LA basin urban) or BB as the only choices.
Line 867: The authors assume period 1 is aged urban particles – what about ship exhaust?