the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Nov 2024
Divergent changes in aerosol optical hygroscopicity and new particle formation induced by heatwaves
Abstract. As a crucial climate-forcing driver, the aerosol optical enhancement factor (f(RH)) is significantly modulated by the evolution of particle number size distribution (PNSD), e.g., during new particle formation (NPF). The mechanisms regulating aerosol optical hygroscopicity during different NPF events and non-event days, particularly those influenced by heatwaves due to global warming, remain poorly understood. In the extremely hot summer of 2022 in urban Chongqing of southwest China, simultaneous measurements of aerosol optical and hygroscopic properties, PNSD, and bulk chemical compositions were conducted. Two distinct types of NPF were identified: the ones with relatively polluted period (P1) and clean cases during heatwave-dominated period (P2). Heatwaves triggered NPF earlier and prolonged the subsequent growth, resulting in smaller aerosol effective radius (Reff) and lower growth rate. This agreed with the concurrently increased aerosol hemispheric backscattering fraction and scattering Ångström exponent. f(RH) was generally higher during NPF events in comparison to that for non-event cases in both periods. Heatwave-induced stronger photooxidation may intensify the formation of more hygroscopic secondary components, as well as the subsequent growth of pre-existing particles and newly formed ultrafine ones, thereby enhancing aerosol optical hygroscopicity especially during heatwave-influenced NPF events. The promoted f(RH) and lowered Reff could synergistically elevate the aerosol direct radiative forcing, specifically under persistent heatwave conditions. Further in-depth exploration on molecular-level characterizations and aerosol radiative impacts of both direct and indirect interactions during weather extremes (e.g., heatwaves) with the warming climate are recommended.
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RC1: 'Comment on egusphere-2024-3242', Anonymous Referee #2, 10 Dec 2024
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Review of Hao et al., 2024
General Comments
The authors present simultaneous measurements of particle number size distributions, aerosol optical and hygroscopic properties, and bulk chemical composition from urban Chongqing during the extremely hot summer of 2022 to investigate the characteristics of new particle formation (NPF) events for two distinct cases: polluted and clean-heatwave event. The authors claim that heatwave(s) may induce stronger photooxidation, enhancing hygroscopic growth and thereby aerosol direct radiative forcing. Overall, the manuscript is well-written. Although the objective of this study is intriguing, I find that a single heatwave event or unusually hot summer is not necessarily sufficient to support the findings and therefore speculation or tall statements must be avoided. I would like to recommend the publication of this study after the authors carefully address all the following concerns.
Specific comments
Fig. 1c shows the time evolution of (hourly?) temperature during the study period. Air temperature (RH) steadily increased (decreased) after 8 Aug. Surprisingly, the wind speed is slightly higher during period P2 (heatwave. Fig 2i), but heatwaves are usually associated with stagnant conditions. Nairn et al. (2015) calculated the excess heat factor to identify heatwave events. Could this be explored to determine the spatial extent of this particular heatwave event, using gridded temperature data if it is available for the region? Heatwaves are anomalous events characterized by extremely high surface air temperatures, typically lasting over a week. A mere surface air temperature threshold is not the best indicator of a regional heatwave event. This is indeed critical in the context of regional NPF events and the conclusions drawn from this study. The question is – Did the heatwave event trigger the NPF event, or did relatively cleaner conditions favour the NPF event or specific dynamical weather pattern favoured NPF (high-pressure system) or a combination of everything?
Please provide statistics of NPF events and non-events for both periods. A total of 23 days is divided into 4 categories and conclusions are drawn from a mere one heatwave event. How confidently can you say heatwave(s) promote NPF (based on your results alone)? How about NPF frequency from previous years during the same time period? I also suggest showing an averaged contour plot of particle number size distributions for all these four categories.
Air mass history plays also a critical role in new particle formation processes. Consider showing an airmass history analysis (source and altitude) using HYSPLIT or Flexpart or similar models. The wind direction during the P2 period appears to be persistently east-southeast.
How is aerosol optical enhancement factor related to particle diameter for both cases (RH<30% and RH=-85%)? You may include a figure in the supplementary if you feel relevant.
Page 16, Section 3.4: I don’t understand why Figure 4 (c & d) focuses on non-events. I suggest showing results in a similar fashion for all four categories in Fig 4c and 4d, and also in Fig 5. Lines 415-418: Are you referring to NPF and non-events during P2? the subsequent discussion appears to be for non-events during P2? Please update Figure 4 and 5, and revise this section thoroughly.
Why GR (<25nm, 25-100 and >100 nm) and FR are not reported and compared between the event types based on SMPS data.?
Technical comments
Abstract: “Heatwaves triggered NPF earlier” – please quantify. You may want to plot sunrise and sunset times in Fig. S5. Define NPF event end time and growth event end time somewhere in the text.
Consider an obvious abbreviation for event classification – relatively polluted period (P1) to be indicated as NPFpolluted and clean heatwave-induced to be indicated as NPFclean,HW
All figure captions should clearly mention what is being plotted, time resolution, time (local to UTC), etc and they should be self-explanatory.
There is an interesting recent paper by Garmash et al., 2024, the authors should consider citing and discussing it – DOI 10.1088/1748-9326/ad10d5
Particle size distribution measurement size range (and number of bins), and time resolution may be mentioned.
How was MLH obtained? All data and methods must be explicitly stated.
Page 4, line 127 -130: consider revising. The data/event sample is too small to draw implications for climate.
Page 5, Line 147-149: If I understand correctly, the authors deployed two nephelometers, one with a humidification unit and the other without. I would suggest giving explicit details of how the measurements were conducted.
Page 7, Line 212: chemical analysis results are plotted in Fig. S2, correct it.
Page 7, Line 220: “Fig.” S4? I can not find meteorological and air quality data. Or do you mean Fig.2? Please check all figures numbering and citations in the text.
Page 18, Lines: 443-445, consider revising the sentence starting “In this sense….” What pollution level are you referring to?
Page 18, Lines:445, Remove “Meanwhile”
Avoid unnecessary use of “pretty” (page 14, line 351), “relatively”, “meanwhile” , “In this sense” as above, etc. throughout the manuscript. Also Page 3, line 97 “there have been a great many studies” – looks unnecessary and no study is cited either. Simply say “Previous studies showed….” and cite relevant studies.
How was the aerosol effective radius calculated? Figures S4c1, c2, and c3 are unclear to me. Further, authors should show how the particle mode diameter behaved during P1 and P2 (averaged diurnal variation) for both event types and the condensation sink
There are several linguistic errors or issues with sentence phrasing. As I am not a native English speaker, I prefer not to correct them for the authors. I kindly urge authors to thoroughly proofread the manuscript to ensure clarity before submission. This will greatly enhance the readability and overall impact of the work.
ReferencesNairn J R and Fawcett R J B 2015 The excess heat factor: A metric for heatwave intensity and its use in classifying heatwave severity; Int. J. Environ. Res. Public Health 12(1) 227–253.
Citation: https://doi.org/10.5194/egusphere-2024-3242-RC1
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