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the Creative Commons Attribution 4.0 License.
| 11 Oct 2023
Characterization of refractory aerosol particles collected in the tropical UTLS within the Asian Tropopause Aerosol Layer (ATAL)
Martin Ebert
Ralf Weigel
Stephan Weinbruch
Lisa Schneider
Konrad Kandler
Stefan Lauterbach
Franziska Köllner
Felix Plöger
Gebhard Günther
Bärbel Vogel
Stephan Borrmann
Abstract. Aerosol particles with diameters larger than 40 nm were collected during the flight campaign StratoClim2017 within the Asian Tropopause Aerosol Layer (ATAL) of the 2017 Monsoon Anticyclone above the Indian subcontinent. A multi-impactor system was installed on board of the aircraft M-55 Geophysica, which was operated from Kathmandu, Nepal. The size and chemical composition of more than 5000 refractory particles/inclusions of 17 selected particle samples from 7 different flights were analyzed by use of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray microanalysis (EDX). Based on chemical composition and morphology, the refractory particles were assigned to the particle groups: extraterrestrial, silicates, Fe-rich, Al-rich, Hg-rich, other metals, C-rich, soot, Cl-rich, and Ca-rich.
Most abundant particle groups within the refractory particles are silicates and C-rich (nonvolatile organics). In samples taken above the tropopause extraterrestrial particles are becoming increasingly important with rising altitude. The most frequent particle sources for the small (maximum in size distribution DP-max = 120 nm) refractory particles carried into the ATAL are combustion processes at ground (burning of fossil fuels / biomass burning) and the agitation of soil material. The refractory particles in the ATAL represent only a very small fraction (< 2 % by number for particles > 40 nm) of the total aerosol particles which are dominated by species like ammonium, sulfate, nitrate, and volatile organics. During one flight additionally a large number of very small (DP-max = 25 nm) cinnabar particles (HgS) were detected. These particles are most likely generated directly on ground by coal combustion in Northeastern India or Southern China.
These findings show that coal burning is an important source for the entry of refractory particles and in particular mercury into the ATAL respectively in the upper troposphere/ lower stratosphere (UTLS) region.
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Martin Ebert et al.
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RC1: 'Comment on egusphere-2023-2245', Anonymous Referee #1, 26 Oct 2023
Review of Ebert et al. Characterization of refractory aerosol particles collected in the tropical UTLS within the Asian Tropopause Aerosol Layer (ATAL)
This manuscript describes the results of electron microscopy measurements of particles collected on impactor plates during Geophysica flights in the StratClim mission. The manuscript is logically organized. There are few such measurements in the lower stratosphere so this manuscript is a valuable addition to the literature.
I am not convinced by one of the main conclusions of the manuscript, that coal burning is a source for nanoparticles of HgS in the Asian tropopause aerosol layer (ATAL).
General: The manuscript very much needs a figure showing vertical profiles of either the absolute or relative concentrations of various aerosol types. Either potential temperature, distance from the tropopause, or ozone could be used as a vertical coordinate. This could be put in somewhere around line 400. Figure 1b could be deleted or moved to supplemental material to make room; it largely duplicates Figure 1a.
Lines 25-30: These conclusions are not supported; see below.
Line 71-73: This paragraph keeps saying it is about the UTLS but in fact it is about the lower stratosphere. In particular, transport of refractory particles is much more important in the upper troposphere. This paragraph would be OK if “UTLS” is just changed to “lower stratosphere”.
Line 69: I appreciate the references – impressive how wrong some modeling studies can go when not constrained by data. This is a reason for data such as the measurements in this manuscript.
Section 2: What I am not clear on after reading this is what will be measured in the situation that there is a refractory species dispersed in a particle rather than being originally present as a distinct inclusion. Will it be missed? One example is that I kept wondering about: where is all the potassium? A significant minority of particles in the ATAL will be from biomass burning. Those particles contain a percent or a few percent potassium. 1% of the volume of a 200 nm particle is 43 nm, much more than the 25 nm Hg particles. Would this be missed?
Lines 253-255: The extraterrestrial particles are an odd size: too big to be the residuals from evaporated sulfuric acid particles with meteoric material, too small to be cosmic spherules. That doesn’t mean the data are wrong but it needs some justification.
Lines 282-290. Where is all of the small soot? The number size distribution of black carbon in the upper troposphere peaks at about 100 nm (Schwarz et al., 2006) with very few as large as the 250 nm mean diameter quoted here.
Line 331. I don’t see any justification for assuming that Al-rich particles come from rocket exhaust rather than Al-rich minerals
Section 4.4. The arguments for a coal combustion source of the cinnabar particles just don’t mesh. That South Asia is a large source of Hg (lines 435-436 and 475-481) is true but is largely irrelevant to a source of HgS (cinnabar). Coal combustion produces HgCl2, not HgS (Sirivasta et al., 2006; Peng et al., 2021). Coal combustion is under oxidizing conditions, not the reducing conditions required for HgS (line 450). The references for HgS aren’t strong: Weinbruch et al. (line 440) found Hg particles but did not speciate them as HgS. Seigneur et al. (line 443) showed that, if present, HgS could adsorb to other particles but didn’t measure that adsorption. Nadvudvari et al. (line 481) did measure HgS, but in solid waste samples, not aerosol emissions.
Another big problem with the arguments for a coal combustion source of the HgS is that data in the manuscript don’t show the other expected species from coal combustion. For example, the Weinbruch et al. (line 440) reference found more than 10 times as many soot particles as Hg-containing particles in coal smoke. Yet the data in this manuscript show many times more Hg-containing particles than soot. It isn’t plausible that the Hg-containing particles are transported to the upper troposphere whereas the soot, which is fairly similar in size and, when freshly emitted, not very hygroscopic, is not transported
Gas-phase tracers don’t support a primary surface source of HgS. The manuscript doesn’t show CO, but from Figure 4 in Keun-Ok Lee et al. (2021) carbon monoxide during Stratoclim flight 8 was 40 to 50 ppbv during the samples 8.2, 8.4, 8.5, and 8.6 that have very high Hg particle concentrations. This moderately low CO is hardly what one would expect from rapid convective uplift from a highly polluted region.
That HgS requires reducing conditions (line 450) would greatly restrict the possible sources to some very special sources. It is hard to imagine some very unusual reducing source affecting all of the samples on flight 8. One thought is if the HgS could be formed from other Hg compounds in sulfate particles (expected near the tropopause) during the in-vacuum electron bombardment to remove the more volatile material.
Lines 510-515: The interpretation of Murphy et al. (2006) is incorrect. That reference describes two types of Hg-containing particles. Sulfate-organic particles with Hg were measured with mass spectrometry but the paper concludes the Hg is added in the upper troposphere. Smaller 10-20 nm particles were detected with electron microscopy but these nanoparticles were not attached to sulfate particles.
Keun-Ok Lee et al., ACP, 2021, 10.5194/acp-21-3255-2021
Peng et al., Mercury speciation and size-specific distribution in filterable and condensable particulate matter from coal combustion, Science Total Environ., 2021.
Srivastava et al., Control of mercury emissions from coal-fired electric utility boilers, ES&T, 2006
Citation: https://doi.org/10.5194/egusphere-2023-2245-RC1 -
RC2: 'Comment on egusphere-2023-2245', Anonymous Referee #2, 29 Oct 2023
The author used the SEM/EDX technique to understand the physicochemical properties of individual refractory particles collected at the tropopause. Due to the lack of measurements in the tropopause, this paper is worth publication. However, there are still many areas that need to be improved. Please see my comments below.
Major comments:
- I suggest adding 3D flight pattern plots and back trajectory plots for each flight. It is hard for me to understand your flights and the potential source of your samples. Moreover, I don't understand how you label your samples since they are not integers.
- There is a lot of information missing for your home-built instruments. For example, what is the size range of COPAS? Did you have a dryer and impactor or PM inlet in front of the aerosol inlet to remove particles larger than the upper limit? Did you do any lab evaluation of the performance of the MULTI-MINI impactor system? Are there any references for ERICA-LAMS? Please provide details about how it works. Without knowing that, it is hard to understand the difference between the two technologies. Also, is this bulk aerosol measurement or individual aerosol measurement technique?
- For section 3.1, I have several major comments:
- Are your results based on the EDX spectra of some portion of the particle or EDX mapping? If yes, how much % of the area of the particle was covered? Do you think that is representative since you might only get partial chemical information about the particle? Also, how do you determine particle size? Is this area equivalent diameter?
- I suggest adding representative SEM images for different particle types and EDX spectra.
- What are the parameters you used for each class? Is that based on the weight or elemental %? How did you develop your classification method? From literature or K-mean clustering? I suggest adding a table to show how you classify each particle.
- Most of your classes sound like different types of dust (extraterrestrial material, silicates, Ca-rich, Fe-rich, Al-rich). It is very difficult to validate your classification without SEM imaging and EDX spectra. Also, if your back trajectory plots show a significant contribution from the boundary layer, then I think your refractory particles are mostly dust.
- Why don’t you consider refractory organic aerosols (e.g., ELVOC) as refractory particles? These can be very important compositions in the tropopause aerosol population.
Minor comments
- L106-108, “The absolute …. (upper part).” I would suggest having a SI plot to show temperature and pressure, and another SI plot to show time series of temperature and pressure for each sample.
- L134-135, “In this study … 400 nm).” 400 nm and 40 nm is the 50% cut-off size, not the boundary of each stage.
- L136-137, “A purge flow … ” Is this at the ground level or UTLS? Did you add a dryer and a filter in front of the purge flow? Ambient air might introduce additional contamination.
- L161-162, “A major challenge … boundary layer.” In future studies, you can put a filter in front of the sampling inlet to verify and quantify the contaminations in the sampling line.
- L175-181, “First, all … excluded.” This part is unclear to me. I do not understand how you identified contamination since you did not provide any representative SEM image and EDX spectrum. These steel-like and large particles still might be real particles. Also, when you say exclude samples with too few particles, how many are you considering too few?
- L200-201, “Therefore, … bombardment.” How did you get the number of volatile particles? Based on your COPAS data?
- Hünig et al., 2022 and Dragoneas et al., 2022 are not in the references list.
- Section 4.2. This section is unclear to me what you are trying to discuss. I do not think you can get the absolute concentration of refractory particles unless you did comprehensive calibration of your sampling system, which includes collection efficiency of the impactor, particle loss in the line and impactor, density of each particle type, etc.
- L347-379, “Only a small … above the tropopause.” I am not convinced in this part. First, it is not clear how you define ground-emitted and extraterrestrial particles based on my previous comments. Your discussion about the source is also not convincing to me.
- L394-395, “The presence of … silicates.” This part is not clear to me. You should also be able to collect droplets and see Fe and Mg in the droplet’s residual. If you use a dryer, you should collect dry Fe and Mg particles after removing moisture.
Citation: https://doi.org/10.5194/egusphere-2023-2245-RC2
Martin Ebert et al.
Martin Ebert et al.
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