Wind-Driven Emissions of Coarse Mode Particles in an Urban Environment

Petters, Markus D.; Pujiastuti, Tyas; Rasheeda Satheesh, Ajmal; Kasparoglu, Sabin; Sutherland, Bethany; Meskhidze, Nicholas

doi:https://doi.org/10.5194/egusphere-2023-951

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https://doi.org/10.5194/egusphere-2023-951

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13 Jun 2023

| 13 Jun 2023

Wind-Driven Emissions of Coarse Mode Particles in an Urban Environment

Markus D. Petters, Tyas Pujiastuti, Ajmal Rasheeda Satheesh, Sabin Kasparoglu, Bethany Sutherland, and Nicholas Meskhidze

Abstract. Quantifying surface-atmosphere exchange rates of particles is important for understanding the role of suspended particulate matter on radiative transfer, clouds, precipitation, and climate change. Emissions of coarse mode particles with a diameter greater than 0.5 µm provide giant cloud condensation nuclei and ice nuclei. These emissions are critical for understanding the evolution of cloud microphysical properties yet remain poorly understood. Here we introduce a new method that uses lidar retrievals of the elastic backscatter and Doppler velocity to obtain surface number emissions of particles with a diameter greater than 0.53 µm. The technique is applied to study particle number fluxes over a two-month period during the TRACER campaign at an urban site near Houston, TX, USA. We found that all the observed fluxes were positive (upwards) indicating particle emission from the surface. The fluxes followed a diurnal pattern and peaked near noon local time. Flux intensity varied through the two months with multi-day periods of strong fluxes and multi-day periods of weak fluxes. Emission particle number fluxes peaked near ~100 cm^-2 s^-1. The daily averaged emission fluxes correlated with friction velocity and were anticorrelated to surface relative humidity. The emission flux can be parameterized as F = 3000u*⁴ where u* is the friction velocity in m s^-1 and the emission flux F is in cm^-2 s^-1. The u* dependence is consistent with emission from wind-driven erosion. Estimated values for the mass flux are in the lower range of literature values from non-urban sites. These results demonstrate that urban environments may play an important role in supplying coarse mode particles to the boundary layer. We anticipate that quantification of these emissions will help constrain aerosol-cloud interaction models that use prognostic aerosol schemes.

Received: 09 May 2023 – Discussion started: 13 Jun 2023

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Markus D. Petters et al.

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RC1:
'Comment on egusphere-2023-951', Anonymous Referee #1, 21 Jul 2023
The manuscript deals with the vertical flux of accumulation-mode and coarse-mode particles from the surface into the atmosphere. The authors use coherent Doppler lidar signals (at 1548 nm wavelength) backscattered from heights around 105 m above the ground to retrieve particle backscatter coefficients and vertical wind velocity (heterodyne technique). In addition, particle number concentrations are measured at ground with an OPC.
The manuscript provides new insight into the vertical aerosol flux and thus should be published.
Here my questions and comments.
p2, line 31: How did you obtain the backscatter coefficient from the HALO Doppler lidar observations? I checked the paper of Chouza et al.,
Chouza, O. Reitebuch, S. Groß, S. Rahm, V. Freudenthaler, C. Toledano, and B. Weinzierl, Retrieval of aerosol backscatter and extinction from airborne coherent Doppler wind lidar measurements, Atmos. Meas. Tech., 8, 2909–2926, https://doi.org/10.5194/amt-8-2909-2015, 2015,

and realized that this retrieval is a rather difficult approach. One has to consider turbulence effects and that the heterodyne efficiency is obviously range-dependent. And such a description is missing in your manuscript. Please provide a proper description how you got the particle backscatter coefficient at 1.5 µm!
p3, line 1: D is what… ? Please write: …diameter D > 0.53 µm.
I would use D_low instead of D_lo.
p4, section 2.4.2.: Why do you present this lengthy and quite complicated section on Mie scattering, when you, at the end, use the empirical correlations between attenuated backscatter from lidar at 105 m and particle number concentration measured in situ at ground?
Please reduce this part as much as possible!
There are so many sources of uncertainty when applying Lorenz-Mie theory to large particles: a) large particles are usually non-spherical (they are irregularly shaped). Large particles are typical desert and soil dust particles and thus almost hydrophobic.
What about the shape effect on the lidar ratio? If you take, for example, a Mie code to compute lidar ratios for spherical coarse-mode dust particles, you will end up with lidar ratios of 15-20 sr. But the real-world dust lidar ratio (for irregularly-shaped dust particles) at the wavelengths of 1064 nm and probably also at 1548 nm is 60-70 sr (Haarig et al., 2022). Mattis et al. (GRL, 10.1029/2002GL014721, 2002) showed already 20 years ago that the lidar ratio at 532 nm for non-spherical dust particles is around 50 sr and not around 20 sr. All this means: if you use Mie-modeled backscatter values these values may be wrong by a factor of 3 and thus the derived vertical aerosol fluxes may be wrong by a factor of 3.
p17, line 26 to p18, line 14: I would leave out this paragraph on INPs. This is just a jump into a completely different story. And this discussion makes only sense if the vertical flux of aerosol particles is directly connected with cloud evolution at the top of a well-mixed boundary layer (with cloud top temperatures of -10°C, -20°C or even -30°C…. and all this in June-August). By the way, n_aer,0.5 is not explained in this paragraph.

Figure 3: What results do you get for almost hydrophobic dust particles and with the true dust lidar ratio of 60-70sr at 1548 nm?
Figure 4: Again the question arises: How did you obtain the backscatter values from the heterodyne Doppler lidar signals?
Figure 5: So, obviously you get even negative backscatter coefficients! Please comment on that!
Figure 8: red values indicate backscatter coefficients of 30 to 50 Mm-1 sr-1. If we multiply these values with a ‘real world’ dust lidar ratio, we end up with aerosol extinction coefficients of 1800 to 3000 Mm-1. These are extinction coefficients of liquid-water clouds. Thick aerosol layers may show extinction coefficients up to 500 Mm-1 at 532 nm and up to 200 Mm-1 at 1548 nm.
What went wrong here?
The backscatter values should be < 10 Mm-1 sr-1 at 1548 nm wavelength, if not < 1 Mm-1 sr-1 during your observations, to my opinion.
Figure 9: Again, time series of backscatter at 105 m height are shown. Please check AERONET 1640 nm AOD observations (Houston University) for June and July 2022 and compare these AERONET AODs with respective lidar-derived AODs when multiplying the backscatter coefficient (in km-1 sr-1) with 60 sr and with a boundary layer height of 1.5 km. Are the lidar-derived AODs close to the observed AERONET AODs for specific days? Yes or no… please mention this effort, i.e., the comparison with AERONET data, in the manuscript.
Figure 10: F, Fflc, FwS, Fdep should be explained in the figure or in the figure caption.
Figure 11: We need this figure for the most trustworthy backscatter values….
Figure 12: I would remove this figure plus the discussion, as mentioned already. This is not needed in this paper. You may consider it in a follow up paper on aerosol-cloud interaction.
The INP dependence on temperature is confusing. Usually, INP efficiency increases by an order of magnitude when temperature decreases by 5 K, at least for temperatures from -20°C to -35°C. This is by far not the case in Figure 12.
Citation: https://doi.org/10.5194/egusphere-2023-951-RC1
RC2:
'Comment on egusphere-2023-951', Anonymous Referee #2, 30 Oct 2023
This paper describes the use of surface-based Doppler lidar measurements to retrieve the surface number particulate emissions for aerosol particles larger than 0.53 micrometers in diameter. The technique is applied to Doppler lidar measurements acquired near Houston, TX. The paper describes this method and how it used surface-based optical particle counter measurements of aerosol size distribution to calibrate the Doppler lidar measurements of near-surface aerosol backscatter. The paper presents a new method for retrieving coarse mode surface number emissions. The paper is well written, and the figures are quite adequate for presenting the results and for illustrating the discussion. I recommend publication after the authors address the relatively minor questions and comments given below.
Page 1, Line 12. It would be helpful to indicate when the two-month period occurred.

Page 4, equation 1, I believe this expression assumes the lidar ratio does not vary with range; it would be helpful to indicate this in the text.

Page 4, line 18. Here S represents lidar ratio. Later, in equation 15, S represents saturation ratio. It would be helpful if an alternative method to represent lidar ratio and/or saturation ratio was used to avoid confusion.

Page 4, line 2. What absorbing gases are present at the laser wavelength? I would assume absorbing gases would typically not be a factor.

Page 5, line 6. The authors note that the Mie solution for the relationships shown on this page assumes that the particles are spherical. However, given that coarse mode dust particles are often nonspherical, can the authors comment on the applicability of this analysis, especially since they later admit that the uncertainties associated with the optical model are too large to relate observed aerosol backscatter to particle number concentration? Also, given that these uncertainties are too large, and the later analyses rely on empirical correlations with surface OPC measurements, what is the point of this Mie analysis?

Page 5, line 6. Regarding the particle nonsphericity, the ARM measurements also included measurements of aerosol backscatter and depolarization by a Micropulse Lidar (MPL). Could these measurements of depolarization be used to provide some indication of the prevalence of nonspherical particles?

Page 5. This analysis also relies on the assumption of the refractive index of the particles, which depends on particle composition. Typically ARM also measures particle composition at the surface in these ARM AMF deployments. Were there no measurements of coarse mode particle composition available?

Page 7, line 11. The analyses that depend on RH use RH determined from interpolation from radiosondes. How often were the radiosondes launched?

Page 8, line 24. I think the sentence should say that the spectrum for backscatter is flat for frequencies above 0.035 Hz; the spectrum for vertical velocity does not look flat for frequencies above 0.05 Hz so I don’t follow the discussion at the top of page 9.

Page 11, line 18. If interpolated radiosondes were used to provide the RH at z=105 m, why weren’t these interpolated radiosondes also used to provide the temperature at z=105 m instead of using the surface temperature measurements?

Page 14, line 28. Figure 9 shows some backscatter values exceed 10 (Mm-sr)^-1. Assuming lidar ratios around 50 sr leads to extinction values around 500 Mm^-1 which are very large; too large for soil dust or biological activity. Were there local sources of dust nearby? The DOE ARM TRACER campaign field report (https://www.arm.gov/publications/programdocs/doe-sc-arm-23-038.pdf) states that several ARM and guest instruments and NASA GMAO models indicated that Saharan dust was observed one or more occasions. The report indicates that one of the events occurred on 17-18 July which coincides with the peak in aerosol backscatter shown in Figure 9 and so can possibly explain such large values. Can the authors please comment on the presence and impact of such aerosols on the derived number concentrations and fluxes?

Page 17. The last part of the discussion deals with role of coarse mode particles on INP. Page 18,lines 5-6 mention how the estimates of INP impacted by such near surface coarse mode particles are sensitive to many factors. Given such (large) uncertainties and the lack of validating INP data, I suggest that this discussion of INP be omitted.
Citation: https://doi.org/10.5194/egusphere-2023-951-RC2
AC1: 'Comment on egusphere-2023-951', Markus Petters, 21 Nov 2023

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-951/egusphere-2023-951-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2023-951-AC1

Status: closed

RC1:
'Comment on egusphere-2023-951', Anonymous Referee #1, 21 Jul 2023
The manuscript deals with the vertical flux of accumulation-mode and coarse-mode particles from the surface into the atmosphere. The authors use coherent Doppler lidar signals (at 1548 nm wavelength) backscattered from heights around 105 m above the ground to retrieve particle backscatter coefficients and vertical wind velocity (heterodyne technique). In addition, particle number concentrations are measured at ground with an OPC.
The manuscript provides new insight into the vertical aerosol flux and thus should be published.
Here my questions and comments.
p2, line 31: How did you obtain the backscatter coefficient from the HALO Doppler lidar observations? I checked the paper of Chouza et al.,
Chouza, O. Reitebuch, S. Groß, S. Rahm, V. Freudenthaler, C. Toledano, and B. Weinzierl, Retrieval of aerosol backscatter and extinction from airborne coherent Doppler wind lidar measurements, Atmos. Meas. Tech., 8, 2909–2926, https://doi.org/10.5194/amt-8-2909-2015, 2015,

and realized that this retrieval is a rather difficult approach. One has to consider turbulence effects and that the heterodyne efficiency is obviously range-dependent. And such a description is missing in your manuscript. Please provide a proper description how you got the particle backscatter coefficient at 1.5 µm!
p3, line 1: D is what… ? Please write: …diameter D > 0.53 µm.
I would use D_low instead of D_lo.
p4, section 2.4.2.: Why do you present this lengthy and quite complicated section on Mie scattering, when you, at the end, use the empirical correlations between attenuated backscatter from lidar at 105 m and particle number concentration measured in situ at ground?
Please reduce this part as much as possible!
There are so many sources of uncertainty when applying Lorenz-Mie theory to large particles: a) large particles are usually non-spherical (they are irregularly shaped). Large particles are typical desert and soil dust particles and thus almost hydrophobic.
What about the shape effect on the lidar ratio? If you take, for example, a Mie code to compute lidar ratios for spherical coarse-mode dust particles, you will end up with lidar ratios of 15-20 sr. But the real-world dust lidar ratio (for irregularly-shaped dust particles) at the wavelengths of 1064 nm and probably also at 1548 nm is 60-70 sr (Haarig et al., 2022). Mattis et al. (GRL, 10.1029/2002GL014721, 2002) showed already 20 years ago that the lidar ratio at 532 nm for non-spherical dust particles is around 50 sr and not around 20 sr. All this means: if you use Mie-modeled backscatter values these values may be wrong by a factor of 3 and thus the derived vertical aerosol fluxes may be wrong by a factor of 3.
p17, line 26 to p18, line 14: I would leave out this paragraph on INPs. This is just a jump into a completely different story. And this discussion makes only sense if the vertical flux of aerosol particles is directly connected with cloud evolution at the top of a well-mixed boundary layer (with cloud top temperatures of -10°C, -20°C or even -30°C…. and all this in June-August). By the way, n_aer,0.5 is not explained in this paragraph.

Figure 3: What results do you get for almost hydrophobic dust particles and with the true dust lidar ratio of 60-70sr at 1548 nm?
Figure 4: Again the question arises: How did you obtain the backscatter values from the heterodyne Doppler lidar signals?
Figure 5: So, obviously you get even negative backscatter coefficients! Please comment on that!
Figure 8: red values indicate backscatter coefficients of 30 to 50 Mm-1 sr-1. If we multiply these values with a ‘real world’ dust lidar ratio, we end up with aerosol extinction coefficients of 1800 to 3000 Mm-1. These are extinction coefficients of liquid-water clouds. Thick aerosol layers may show extinction coefficients up to 500 Mm-1 at 532 nm and up to 200 Mm-1 at 1548 nm.
What went wrong here?
The backscatter values should be < 10 Mm-1 sr-1 at 1548 nm wavelength, if not < 1 Mm-1 sr-1 during your observations, to my opinion.
Figure 9: Again, time series of backscatter at 105 m height are shown. Please check AERONET 1640 nm AOD observations (Houston University) for June and July 2022 and compare these AERONET AODs with respective lidar-derived AODs when multiplying the backscatter coefficient (in km-1 sr-1) with 60 sr and with a boundary layer height of 1.5 km. Are the lidar-derived AODs close to the observed AERONET AODs for specific days? Yes or no… please mention this effort, i.e., the comparison with AERONET data, in the manuscript.
Figure 10: F, Fflc, FwS, Fdep should be explained in the figure or in the figure caption.
Figure 11: We need this figure for the most trustworthy backscatter values….
Figure 12: I would remove this figure plus the discussion, as mentioned already. This is not needed in this paper. You may consider it in a follow up paper on aerosol-cloud interaction.
The INP dependence on temperature is confusing. Usually, INP efficiency increases by an order of magnitude when temperature decreases by 5 K, at least for temperatures from -20°C to -35°C. This is by far not the case in Figure 12.
Citation: https://doi.org/10.5194/egusphere-2023-951-RC1
RC2:
'Comment on egusphere-2023-951', Anonymous Referee #2, 30 Oct 2023
This paper describes the use of surface-based Doppler lidar measurements to retrieve the surface number particulate emissions for aerosol particles larger than 0.53 micrometers in diameter. The technique is applied to Doppler lidar measurements acquired near Houston, TX. The paper describes this method and how it used surface-based optical particle counter measurements of aerosol size distribution to calibrate the Doppler lidar measurements of near-surface aerosol backscatter. The paper presents a new method for retrieving coarse mode surface number emissions. The paper is well written, and the figures are quite adequate for presenting the results and for illustrating the discussion. I recommend publication after the authors address the relatively minor questions and comments given below.
Page 1, Line 12. It would be helpful to indicate when the two-month period occurred.

Page 4, equation 1, I believe this expression assumes the lidar ratio does not vary with range; it would be helpful to indicate this in the text.

Page 4, line 18. Here S represents lidar ratio. Later, in equation 15, S represents saturation ratio. It would be helpful if an alternative method to represent lidar ratio and/or saturation ratio was used to avoid confusion.

Page 4, line 2. What absorbing gases are present at the laser wavelength? I would assume absorbing gases would typically not be a factor.

Page 5, line 6. The authors note that the Mie solution for the relationships shown on this page assumes that the particles are spherical. However, given that coarse mode dust particles are often nonspherical, can the authors comment on the applicability of this analysis, especially since they later admit that the uncertainties associated with the optical model are too large to relate observed aerosol backscatter to particle number concentration? Also, given that these uncertainties are too large, and the later analyses rely on empirical correlations with surface OPC measurements, what is the point of this Mie analysis?

Page 5, line 6. Regarding the particle nonsphericity, the ARM measurements also included measurements of aerosol backscatter and depolarization by a Micropulse Lidar (MPL). Could these measurements of depolarization be used to provide some indication of the prevalence of nonspherical particles?

Page 5. This analysis also relies on the assumption of the refractive index of the particles, which depends on particle composition. Typically ARM also measures particle composition at the surface in these ARM AMF deployments. Were there no measurements of coarse mode particle composition available?

Page 7, line 11. The analyses that depend on RH use RH determined from interpolation from radiosondes. How often were the radiosondes launched?

Page 8, line 24. I think the sentence should say that the spectrum for backscatter is flat for frequencies above 0.035 Hz; the spectrum for vertical velocity does not look flat for frequencies above 0.05 Hz so I don’t follow the discussion at the top of page 9.

Page 11, line 18. If interpolated radiosondes were used to provide the RH at z=105 m, why weren’t these interpolated radiosondes also used to provide the temperature at z=105 m instead of using the surface temperature measurements?

Page 14, line 28. Figure 9 shows some backscatter values exceed 10 (Mm-sr)^-1. Assuming lidar ratios around 50 sr leads to extinction values around 500 Mm^-1 which are very large; too large for soil dust or biological activity. Were there local sources of dust nearby? The DOE ARM TRACER campaign field report (https://www.arm.gov/publications/programdocs/doe-sc-arm-23-038.pdf) states that several ARM and guest instruments and NASA GMAO models indicated that Saharan dust was observed one or more occasions. The report indicates that one of the events occurred on 17-18 July which coincides with the peak in aerosol backscatter shown in Figure 9 and so can possibly explain such large values. Can the authors please comment on the presence and impact of such aerosols on the derived number concentrations and fluxes?

Page 17. The last part of the discussion deals with role of coarse mode particles on INP. Page 18,lines 5-6 mention how the estimates of INP impacted by such near surface coarse mode particles are sensitive to many factors. Given such (large) uncertainties and the lack of validating INP data, I suggest that this discussion of INP be omitted.
Citation: https://doi.org/10.5194/egusphere-2023-951-RC2
AC1: 'Comment on egusphere-2023-951', Markus Petters, 21 Nov 2023

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-951/egusphere-2023-951-AC1-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2023-951-AC1

Markus D. Petters et al.

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https://doi.org/10.5194/egusphere-2023-951-supplement

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Short summary

This work introduces a new method that uses remote sensing techniques to obtain surface number emissions of particles with a diameter greater than 500 nm. The technique was applied to study particle emissions at an urban site near Houston, TX, USA. The emissions followed a diurnal pattern and peaked near noon local time. The daily averaged emissions correlated with wind speed. The source is likely due to wind-driven erosion of material situated on asphalted and other hard surfaces.


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