the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Dec 2024
Retention During Freezing of Raindrops, Part II: Investigation of Ambient Organics from Beijing Urban Aerosol Samples
Abstract. The freezing of hydrometeors incurs certain water-soluble organic compounds dissolved in the supercooled cloud droplets to be released into the gas phase. This may lead to the vertical redistribution of substances that become available for new particle formation in the upper troposphere. Drop freezing experiments were performed on the Mainz Acoustic Levitator (M-AL) using aqueous extracts of ambient samples of Beijing urban aerosol. The retention coefficients of over 450 compounds were determined. Most nitroaromatics and organosulfates were fully retained along with the aliphatic amines (AA) and higher-order amines and amides while sulfides, lipids, aromatic hydrocarbons, and long chain compounds are among the most unretained and incidentally the fewest species observed. The findings here also indicate that NOx and SOx chemistry, particularly anthropogenically related, enhances the retention of the resulting secondary organic aerosols (SOA). A positive correlation between polarity and freezing retention along with a negative correlation with vapor pressure and freezing retention was observed. No sigmoidal relationship with effective Henry’s law constant was observed which differs with the parameterizations of riming retention presented in current literature, which is justified by the lower surface-to-volume ratio of the large drop size investigated.
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RC1: 'Comment on egusphere-2024-3940', Anonymous Referee #1, 16 Jan 2025
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The manuscript is based on extensive laboratory studies on the mechanism of retention/release of semi-volatile organic compounds upon freezing of raindrop-sized droplets generated from the aqueous extracts of Beijing wintertime urban aerosol. The whole set of experiment is carefully designed and executed, using state-of-the-art equipment including analytical techniques, and the statistical processing of analytical data is adequate meeting all standards of science. The objectives of the manuscript are clear, the hypotheses are valid and important, to explain a potential transport mechanism of semi-volatile organic compounds to the free troposphere. This is relevant in understanding new particle formation in the upper troposphere, which has serious implication on cloud formation and water-particle interactions in a changing climate hosting more water vapour in the atmosphere. The manuscript is comprehensive and well-written, so there is little to criticize expect typography (e.g. using ‘en dash’ characters instead of the ‘minus sign’ in all formulae.) However, the reviewer has a series of serious concerns about the relevance of the laboratory results for real-life atmosphere.
- In the study water-soluble organic compounds (WSOC) in the aqueous extract of wintertime urban aerosol serve as a proxy for real-life composition of droplets of convective clouds. However, large-scale convective cloud formation is not typical during the winter, furthermore, in winter frequent inversion and low mixing layer height prevent surface emissions to be transported to higher altitudes and participate in ice cloud formation. Whereas I agree that studying such a complex mixture may be more informative that of a few cherry-picked model species, to draw meaningful conclusions from the experiments it is important to elaborate on this issue in the manuscript.
- The laboratory experiment is designed to study the freezing of large (2 mm in diameter) raindrops. In real-life mixed phase clouds freezing of large supercooled droplets may occur via multiple mechanisms, such as riming, immersion or contact freezing. These processes are also active for much smaller cloud droplets, i.e. for those below or around the precipitation threshold. I wonder what the probability is for such a large (supercooled) raindrop to survive such effective freezing processes inside a vigorous mixed phase cloud high in the troposphere? I would guess it is very low, but it would be worth discussing anyway.
- In addition, the real-life freezing processes described above are superfast relative to the cooling process applied by the authors in their experiments, which lasts on average for 90 seconds. So again, the question arises how relevant the experimental parameters are for real-life cloud conditions in determining the gas-to-droplet partitioning of organic compounds?
- Another issue to be clarified is the application of the standing ultrasonic wave for levitating the droplets in the experimental setup. It is well-established that ultrasonic energy is an extremely effective way of mixing (see e.g. ultrasonic bath for extraction). So in terms of fluid dynamics, can we assume that in the laboratory experiments the droplets had remained thoroughly mixed until they froze up? If so, how it relates to fluid dynamics prevalent for droplets in convective clouds, in which mixing inside such large droplets may be way much less effective?
- The last issue is that in large droplets mass transfer to the atmosphere is strongly limited by the low surface area to mass ratio. Furthermore, freezing of the droplets starts from the outside because the enthalpy of fusion needs to be dissipated, then freezing forms a solid outer layer that hinders heat and prevents material transport to and from the interior of the droplet. I wonder if this mechanism is largely responsible for the nearly complete retainment of soluble species irrespective of the vast range of their physical and chemical properties?
To summarise my general comments it would be more than welcome if the authors addressed these points in their revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3940-RC1 -
RC2: 'Comment on egusphere-2024-3940', Anonymous Referee #2, 01 Mar 2025
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This study presents the retention of organics during the freezing of rain size droplets. The experiments were conducted on aqueous extracts of Beijing urban ambient aerosols using an acoustic levitator. The retention coefficients were determined by freezing the rain size droplets and then measured the remained substances in the frozen droplets. They showed near 1 of retention coefficients for the compounds found in the HRMS in (−)HESI mode, but more variation for the compounds found in (+)HESI mode. Correlations between the estimated chemical properties, such as henry’s low constant, and retention coefficients were discussed. The subject of this manuscript fits the scope of ACP. There are several issues need to be addressed before it can be considered for publication.
Major comments:
- The manuscript presented in the current form is more like a Measurement Report, as its atmospheric implication is not fully discussed or addressed. The only three nighttime samples also limits a broader atmospheric implication. If I understood correctly, the samples were combined before the freezing and determination of retention coefficient. Why not perform the experiments for individual samples, which one can exam the variation of retention for the same compounds?
- In the Abstract, the statement in Line 20-22 is overstated as the S- and N-containing compounds are not solely from the NOx and Sox chemistry. The statement in Line 24-26 is not supported by the results of non-sigmoidal relationship or the related discussion. The lower surface-to volume ratio of the large drop size investigated may be one of the reasons. The differences in the experimental techniques, as I understand, may be the main reason, the wind tunnel and levitation, which have totally different air dynamic environments and surrounding corresponding gas-phase organic concentrations that will affect the gas-liquid diffusion and partitioning, and thus the retention.
- The limitation of this technique or the limited number of sample size should be discussed. For example, as the authors indicate the implication for the convective clouds, will the values measured by this levitation techniques be likely underestimated for the convection system?
- The correlation of retention coefficients with chemical properties. The discussion on the results of these correlations should be carefully examined. For example, Line385-392, the R square values are less than 0.1 and the F-test shows no significance, why do the statements still say that they have correlations? Then, the related conclusions are not valid anymore.
Other comments:
- Line227-234, it is not clear what are those compound number means. How many compounds were used for and analysis? Does that include the 77 and 84 compounds which were selected for additional property calculation?
- Line 235-237, the sentence is confusing.
- Figure 2B, why there are such a large portion of the compounds have retention coefficients higher than 1.0?
- Line316, why a nonnormal distribution is expected or is a true distribution?
- Line393, in the sentence, the Figure 8 should be Figure 7?
- Line398 and Figure 8, can you comment on why the R square is so low?
- Line492 and 505, the authors mean “indicates” not “insulates”?
- Line505, lower potential to retain, why is likely to reach the upper atmosphere?
Citation: https://doi.org/10.5194/egusphere-2024-3940-RC2
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