On the force of vertical winds in the upper atmosphere: consequences for small biological particles

For many decades, vertical winds have been observed at high altitudes of the Earth’s atmosphere, in the mesosphere and thermosphere layers. These observations have been used with a simple one-dimensional model to make estimates of possible altitude climbs by biologically sized particles deeper into the thermosphere, in the rare occurrence where such a particle has been propelled to these altitudes. A particle transport mechanism is suggested from the literature on auroral arcs, indicating that an altitude of 120 km could be reached by a nanometre-sized particle, which is higher than the measured 77 km limit on the biosphere. Vertical wind observations in the upper mesophere and lower thermosphere are challenging to make and so we suggest that particles could reach altitudes greater than 120 km, depending on the magnitude of the vertical wind. Applications of the larger vertical winds in the upper atmosphere to astrobiology and climate science are explored.


Comments to the Author(s)
The authors present an interesting and provocative hypothesis invoking observed vertical winds in the mesosphere and thermosphere to explain the possible existence of Earth biological particles in the thermosphere. As the authors explain, the reported observed large vertical winds are not yet represented in existing general circulation models. Taking in consideration the observed vertical velocities and assuming their existence for relatively large horizontal distance and in time scales of minutes to hours, the authors use a simple 1d model that explains altitude climbs of biological particles with masses larger than air. The model is far from being perfect, but it shows that if large vertical velocities like those reported are indeed real, Earth's biological particles could climb to thermospheric altitudes and perhaps leave Earth's atmosphere. Since demonstrating the validity of observed vertical velocities by other groups is not the aim of the paper, and the work motivates different research areas, e.g., understanding of observed vertical velocities, improvements of general circulation models to include such observations, astrobiological implications, geo-engineering, campaigns aim to measure biological particles in the mesosphere and thermosphere, etc., I recommend the paper to be published after considering the recommendations below.

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Title. Since the vertical velocities are invoke to justify the presence of biological particles at thermospheric altitudes, I suggest to modify the title to indicate that, something like: "On the force of vertical winds in the upper atmosphere to explain the existence of biological sized particles in the thermosphere" or similar ... • Section 3 Motivation. Since "motivation" has been already included in the Introduction (Section 1), and some of the text is used to discuss the results, I suggest changing the section label to "Discussion" or similar. • In page 13, L18-20 (geoengineering). Is your warming analogous to the Ozone hole and chlorofluorocarbons?
• General comments to consider in different parts of the text. o Particles and biological particles are used in different parts of the text. I suggest to be explicit when it refers to biological particles. Recall that significant dust-sized particles are continuously being deposited in the upper atmosphere by meteoroids (e.g., Plane et al., 2015) o Lidars and Incoherent scatter radars have observed signatures of metallic ion layers at altitudes as high as 180-200 km descending. Although their existence is associated to meteoroids, the high altitude concentrations are not expected to be from direct deposition, instead they might be due to some kind of transport (vertical winds?, parallel to B electric fields, etc.). Recent observations can be found in Chu et al (2021) and references there in. The authors might want to include such observations in their discussion. o Variance of vertical velocities have been observed (and are expected) to increase with increasing altitudes. Recently Chau et al. (2021) have reported the existence of values as high as 60 m/s! These values are smaller than those at thermospheric heights, but they are extreme, more than 5 sigma larger than normal vertical velocities variability. Not only this constitutes additional evidence of high vertical velocities, but also it might extent the possibilities to non-aurora arc situations. o I understand that the current speculation is limited to high-latitudes and during auroralarc periods, which helps with the assumptions in the 1D model.

Review form: Referee 2
Is the manuscript an original and important contribution to its field? Good

Is the paper of sufficient general interest? Good
Is the overall quality of the paper suitable? Acceptable Can the paper be shortened without overall detriment to the main message? No Do you think some of the material would be more appropriate as an electronic appendix? No

Recommendation?
Accept with minor revision (please list in comments)

Comments to the Author(s)
This article investigates the possibility of vertical transport of biologically-sized particles into the Earth's thermosphere (~90-600 km). This potential extension of the biosphere from the currently reported altitudes in the mesosphere (~ 80 km) to the thermosphere (~ 120 km) has implications for inter-planetary transport of biological samples through planetary escape of aerosols and for geoengineering through the transport and fate of aerials in the middle-and upper-atmosphere (i.e., (i.e stratosphere-mesosphere and thermosphere respectively). The article presents evidence for significant vertical winds in the mesosphere and thermosphere, develops a vertical transport model for aerosols, and considers the structure of the wind systems that support the vertical transport model.
The article presents a scenario of how biologically active material can be transported high in the atmosphere, and highlights observations that may not be widely appreciated in the middle and upper-atmosphere communities. However, I have concerns about the evidence for vertical winds as currently presented, and the fate and transport of this material in the light of our understanding of the transport of other aerosols in the mesosphere.
The vertical transport of aerosols in the atmosphere will be of interest to the middle atmosphere community particularly in the study of the fate and transport of meteoric material, the meteoric metal layers, and noctilucent (polar mesospheric clouds, PMCs). The discussion of upward transport addresses both seasonal changes in the circulation (that result in transport of water vapor into the upper mesosphere and the diabetic circulation with the formation of the cold summer mesopause, and the formation of PMCs) and changes over periods of weeks (associated with events such as sudden stratospheric warming events (Randall etc al., 2006;2009;Meraner and Schmidt, 2016) . Investigations have focused on the ablation of meteors, formation of meteoric dust and the coagulation and growth of dust particles and their subsequent downward transport (Rapp and Thomas, 2006;Plane 2012;Wilms, 2016). Studies of mesospheric dust, detected when the dust becomes charged and detected as polar mesospheric summer echoes to radars, focus on their role as nucleation sites for mesospheric ice particles and the formation of PMCs. It is not clear in this environment how small aerosol particles will be transported through the upper mesosphere into the thermosphere without serving as nucleation sites for aerosol growth and sedimentation. Can the authors address if these processes could significantly impact the transport of aerosols from the stratosphere into the thermosphere?
At times the discussion of the upward winds in the mesosphere and thermosphere implies that winds of 100 m/s can be found in the upper mesosphere. The wind measurements are primarily reported by interferometric measurements of the green (558 nm) and red (630 nm) lines in the thermosphere. The red line emissions come from the mid-thermosphere around 250 km, while the green line emissions come from altitudes near 130 km, the height of the emission can move in altitude based on the energy of the auroral precipitation (e.g., Billett et al., 2020). In the review by Smith (1998) the vertical winds on order of 100 m/s are associated with measurements in the mid-thermosphere from in-situ satellites and red-line observations. In their review, Larsen and Meriwether (2012) report upward winds of up to 50 m/s (180 km/h) in green line , radar measurements at 103 km (Oyama et al., 2005), and in red-line measurements (Sipler et al., 2012). However, vertical winds of 100 m/s (360 km/hr) and larger are reported in the midthermosphere, rather than the lower thermosphere, based on measurements of the red line. Can the authors provide a clearer presentation of the observational evidence for the vertical winds and the altitude regions where they are found?
The discussion of the vertical winds by   As regards the transport of dust on Mars, the study by Vandaele et al. (2019) does not report dust at 80 km, but shows that during dust storms, that can last over weeks, there is vertical transport of dust to lower altitudes. Absorption of radiation by the lofted dust and and subsequent heating results in changes in the general circulation that can transport water vapor up to 80 km. Can the authors clarify the results from Vabdaele et al., they are citing. Furthermore, the authors note that the lower gravity on Mars will facilitate upward transport. However, the atmosphere on Mars is much thinner than the Earths atmosphere. How will the combination of lower gravity (~ 38%) and lower air density (< 1%) on Mars yield vertical transport of aerosols? The reviewer(s) have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the reviewer(s)' comments and revise your manuscript. Please note that we have a strict upper limit of 28 pages for each paper. Please endeavour to incorporate any revisions while keeping the paper within journal limits. Please note that page charges are made on all papers longer than 20 pages. If you cannot pay these charges you must reduce your paper to 20 pages before submitting your revision. Your paper has been ESTIMATED to be 17 pages. We cannot proceed with typesetting your paper without your agreement to meet page charges in full should the paper exceed 20 pages when typeset. If you have any questions, please do get in touch.
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We welcome submissions of images for possible use on the cover of Proceedings A. Images should be square in dimension and please ensure that you obtain all relevant copyright permissions before submitting the image to us. If you would like to submit an image for consideration please send your image to proceedingsa@royalsociety.org The authors present an interesting and provocative hypothesis invoking observed vertical winds in the mesosphere and thermosphere to explain the possible existence of Earth biological particles in the thermosphere. As the authors explain, the reported observed large vertical winds are not yet represented in existing general circulation models. Taking in consideration the observed vertical velocities and assuming their existence for relatively large horizontal distance and in time scales of minutes to hours, the authors use a simple 1d model that explains altitude climbs of biological particles with masses larger than air. The model is far from being perfect, but it shows that if large vertical velocities like those reported are indeed real, Earth's biological particles could climb to thermospheric altitudes and perhaps leave Earth's atmosphere. Since demonstrating the validity of observed vertical velocities by other groups is not the aim of the paper, and the work motivates different research areas, e.g., understanding of observed vertical velocities, improvements of general circulation models to include such observations, astrobiological implications, geo-engineering, campaigns aim to measure biological particles in the mesosphere and thermosphere, etc., I recommend the paper to be published after considering the recommendations below.
• Title. Since the vertical velocities are invoke to justify the presence of biological particles at thermospheric altitudes, I suggest to modify the title to indicate that, something like: "On the force of vertical winds in the upper atmosphere to explain the existence of biological sized particles in the thermosphere" or similar ... • Section 3 Motivation. Since "motivation" has been already included in the Introduction (Section 1), and some of the text is used to discuss the results, I suggest changing the section label to "Discussion" or similar.
• General comments to consider in different parts of the text. o Particles and biological particles are used in different parts of the text. I suggest to be explicit when it refers to biological particles. Recall that significant dust-sized particles are continuously being deposited in the upper atmosphere by meteoroids (e.g., Plane et al., 2015) o Lidars and Incoherent scatter radars have observed signatures of metallic ion layers at altitudes as high as 180-200 km descending. Although their existence is associated to meteoroids, the high altitude concentrations are not expected to be from direct deposition, instead they might be due to some kind of transport (vertical winds?, parallel to B electric fields, etc. This article investigates the possibility of vertical transport of biologically-sized particles into the Earth's thermosphere (~90-600 km). This potential extension of the biosphere from the currently reported altitudes in the mesosphere (~ 80 km) to the thermosphere (~ 120 km) has implications for inter-planetary transport of biological samples through planetary escape of aerosols and for geoengineering through the transport and fate of aerials in the middle-and upper-atmosphere (i.e., (i.e stratosphere-mesosphere and thermosphere respectively). The article presents evidence for significant vertical winds in the mesosphere and thermosphere, develops a vertical transport model for aerosols, and considers the structure of the wind systems that support the vertical transport model.
The article presents a scenario of how biologically active material can be transported high in the atmosphere, and highlights observations that may not be widely appreciated in the middle and upper-atmosphere communities. However, I have concerns about the evidence for vertical winds as currently presented, and the fate and transport of this material in the light of our understanding of the transport of other aerosols in the mesosphere.
The vertical transport of aerosols in the atmosphere will be of interest to the middle atmosphere community particularly in the study of the fate and transport of meteoric material, the meteoric metal layers, and noctilucent (polar mesospheric clouds, PMCs). The discussion of upward transport addresses both seasonal changes in the circulation (that result in transport of water vapor into the upper mesosphere and the diabetic circulation with the formation of the cold summer mesopause, and the formation of PMCs) and changes over periods of weeks (associated with events such as sudden stratospheric warming events (Randall etc al., 2006;2009;Meraner and Schmidt, 2016) . Investigations have focused on the ablation of meteors, formation of meteoric dust and the coagulation and growth of dust particles and their subsequent downward transport (Rapp and Thomas, 2006;Plane 2012;Wilms, 2016). Studies of mesospheric dust, detected when the dust becomes charged and detected as polar mesospheric summer echoes to radars, focus on their role as nucleation sites for mesospheric ice particles and the formation of PMCs. It is not clear in this environment how small aerosol particles will be transported through the upper mesosphere into the thermosphere without serving as nucleation sites for aerosol growth and sedimentation. Can the authors address if these processes could significantly impact the transport of aerosols from the stratosphere into the thermosphere?