the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Sep 2023
New particle formation induced by anthropogenic-biogenic interactions in the southeastern Tibetan Plateau
Shiyi Lai
Ximeng Qi
Sijia Lou
Xuguang Chi
Liangduo Chen
Chong Liu
Yuliang Liu
Mengmeng Li
Tengyu Liu
Wei Nie
Veli-Matti Kerminen
Tuukka Petäjä
Markku Kulmala
Aijun Ding
Abstract. New particle formation (NPF) plays a crucial role in the atmospheric aerosol population and has significant implications on climate dynamics, particularly in climate-sensitive zone such as the Tibetan Plateau (TP). However, our understanding of NPF in the TP is still limited due to a lack of comprehensive measurements and verified model simulations. To fill this knowledge gap, we conducted an integrated study combining comprehensive field measurements and chemical transport modeling to investigate NPF events in the southeastern TP during the pre-monsoon season. NPF was observed to occur frequently on clear-sky days in the southeastern TP, contributing significantly to the cloud condensation nuclei (CCN) budget in this region. The observational evidence suggests that highly oxygenated organic molecules (HOMs) from monoterpene oxidation participate in the nucleation in southeastern TP. After updating the monoterpene oxidation chemistry and nucleation schemes in the meteorology-chemistry model, the model well reproduces observed NPF and reveals an extensive occurrence of NPF across the southeastern TP. The dominant nucleation mechanism is the synergistic nucleation of sulfuric acid, ammonia and HOMs, driven by the transport of anthropogenic precursors from South Asia and the presence of abundant biogenic gases. By investigating the vertical distribution of NPF, we find a significant influence of vertical transport in the southeastern TP. More specifically, strong nucleation near the surface leads to an intense formation of small particles, which are subsequently transported upward. These particles experience enhanced growth to larger sizes in the upper planetary boundary layer (PBL) due to favorable conditions such as lower temperatures and reduced condensation sink. As the PBL evolves, the particles in larger sizes are brought back to the ground, resulting in a pronounced increase in near-surface particle concentrations. This study highlights the important roles of anthropogenic-biogenic interactions and meteorological dynamics in NPF in the southeastern TP.
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Shiyi Lai et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1848', Anonymous Referee #1, 28 Oct 2023
General comments:
New particle formation (NPF) is essential for understanding aerosol dynamics, particularly in climate-sensitive regions like the Tibetan Plateau (TP). To address the knowledge gaps specific to NPF in the TP, the authors conducted a study that combined field measurements and modeling, revealing frequent NPF events in the southeastern TP. Lulang, characterized by relatively low anthropogenic emissions but high biogenic emissions and trace-level SO2 concentrations, along with high monoterpene concentrations, served as a pertinent study area.
To assess the influence of biogenic volatile organic compounds (BVOCs) and atmospheric transport on NPF in Lulang, a well-chosen set of regional simulations was performed using the updated WRF-Chem model during a typical NPF episode. For the most part, the conclusions were substantiated. The inclusion of organic NPF pathways helped bridge remaining gaps and yielded good agreement with observations, underscoring the vital role of organic compounds in nucleation. Crucially, oxygenated organic molecules resulting from monoterpene oxidation were identified as pivotal for NPF. The model updates successfully replicated observed NPF, highlighting the dominant nucleation mechanism involving sulfuric acid, ammonia, and highly oxygenated organic molecules. Vertical transport was found to have a substantial impact on NPF in the southeastern TP, emphasizing the significance of anthropogenic-biogenic interactions and meteorological dynamics. The analysis disclosed that nucleation primarily drives the production of near-ground particles below 10 nm range, while vertical mixing enhances particle concentration in larger size ranges near the ground. These findings underscore the importance of vertical mixing in redistributing particles within the southeastern TP.
In conclusion, this well-written and compelling paper provides valuable insights into the complex dynamics of NPF, focusing on the roles of different precursors and meteorological factors in the formation and growth of aerosol particles. However, I have some reservations about two aspects: the potential limitations of the 27 km grid resolution and the climatology of NPF events, specifically the model's ability to accurately represent the vertical transport of aerosols back into the boundary layer.
Specific comments:
Comment 1: Page 5, line 109-113 “(Fig. 1a). In general, the measurement site is characterized by extensive emissions of BVOCs, especially monoterpenes … (Fig. 1b).”
Please provide more details regarding the sources, data level, and resolution depicted in Figures 1a and 1b. The mention of a "nearby cottage" prompts the need for clarification: is this a single cottage, or does it represent a local distribution of cottages surrounding the measurement site? This is particularly crucial in assessing the direct emissions originating from these cottages.
Comment 2: Page 7, line 132-134 “Besides, on a typical NPF day, such as 29 April 2021, …”
Please explain the climatology/frequency of NPF days and describe the nature of a typical NPF day. As there have been no references to NPF events thus far, providing some context regarding the occurrence of NPF events at the study location would be helpful.
Comments 3: Page 7: line 146-147- “In this study, the simulation domain covers the southeastern TP, with a grid resolution of 27 km and 30 vertical layers… 100 hPa.”
What uncertainties are linked to the coarser model resolution? Can a grid resolution of 27 km adequately capture certain cloud structures, sub-regional aerosol and cloud variations, and boundary layer dynamics?
Comment 4: Page 9, Table 1- The authors mention the MB and RMSE over the entire study period. It's important to understand whether the model captures the diurnal variability of meteorological factors, aerosols, and trace gas components. This is especially critical for accurately reproducing NPF characteristics. While the table describes the comparison at Lulang, and the average values seem promising, is it possible to compare/validate the regional distribution of some of these parameters?
Comment 5: Page 12, Figure 3- Some of the NPF days showed morning peaks in number concentrations, which were associated with wood burning in a nearby residential cottage. Is this a regular practice? If so, residential burning near the measurement site could regularly influence NPF formation. For instance, on April 30th, it appears that NPF growth is influenced by the introduction of particles from wood burning. Is this phenomenon very local or more regional? If this practice is very localized, can the results be considered as specific to this region and not applicable to a larger area such as southern TP?
Comment 6: Page 12, Figure 3- Please elaborate on the start times and end times of all the NPF events.
Comment 7: Page 15, line 316-318- “The potential role of transport is also suggested by Fig. 3a, which … especially on 29 April.”
Would this also suggest that there has been cluster formation somewhere else, transported, and that the growth is rather observed at the Lulang site?
Comment 8: Page 17, line 348-350- “The typical “banana” shape particle number size distribution showing nucleation and subsequent … a regional scale.”
This description could probably be added in either the measurement or the results section when describing the characteristics of NPF.
Comment 9: Page 16, Figure 5- It appears that on the 28th of April, there are preexisting aerosols (30 -90 nm) present in comparison to the observations. This raises the question of whether the emission inventories used in the model are consistent with the observations, a task that is challenging to maintain. Consequently, it would be good to consider what kind of uncertainties might arise from this misrepresentation, if any, in the formation of NPF.
Comment 10: Page 17, Figure 6. The model results indicate that the nucleation and growth of aerosol particles extended widely over southeastern TP. It is interesting to note that nucleation events are not observed in the northeastern part of India (below southern TP), despite its high impact from biogenic sources, ammonia from agricultural activities, and anthropogenic sources. What are the other underlying conditions that favor nucleation events more in the southern TP region? Can this be explained by the higher OH radical concentration, which is a key oxidant for forming H2SO4, observed in the southeastern TP?
Comment 11: Page 19, line 374-377- “In contrast, the OH radical concentration, which is a key oxidant for forming…ozone background over the TP”.
The OH radical concentrations are also high over the Brahmaputra valley region and Bangladesh (Figure 7b), which are also the regions showing higher aerosols. How does the mechanism of OH formation differ over these regions from southern TP.
Comment 12: Page 19, line 391-393- “The vertical simulation of zonal averaged wind vector and NH3 concentrations over 94–96 °E … TP.”
This is where the coarse resolution of the model could result in the misrepresentation of vertical velocities, which is always difficult to simulate. The Himalayan ranges have complex topography. How do you think this would impact the long-range transportation of pollutants from South Asia? For example, in Figure 8b, the transfer of pollutants across higher elevations could require both local and synoptic influences in terms of advection and vertical mixing. What do the vertical velocities look like in the model simulation?
Comment 13: Page 20, Figure 8- To understand the long-range transportation of aerosols, it would be helpful to visualize the spatial distribution of wind vectors at around 3000m.
Comment 14: Page 21, Figure 9: The overall contribution looks very informative. However, is it possible to examine the resultant diurnal evolution of the number of aerosol number concentrations measured vs. modeled, like Figure 5c but in terms of diurnal evolution?
Comment 15: Page 24, line 460-462- “The lower PM2.5 concentration at the boundary layer top suggests a lower condensation sink … particles (Fig. S1b).
Is the vertical distribution of PM2.5 simulated at Lulang, or is it the spatial average?
Comment 16: Page 24, Figure 11b-c- The vertical mixing follows a diurnal cycle and responds to the diurnal variation of temperature, and the PBL. The lower boundary layer traps more pollutants closer to the surface, as observed in the supplementary figure. The PBL plays a significant role in the diurnal pattern of aerosol loading. So, how is this process unique concerning NPF formation? Is vertical mixing influenced by other factors such as convection or precipitation over the study location?
Comment 17: Page 25, line 490-491- “On clear-sky days, a high frequency of NPF exceeding 60% was observed.”
The frequency of NPF occurrence is not clear in the current study. The measurements were conducted during the period from April 4 to May 24, 2021. Please provide details regarding the number of occurrences, start time, end time, etc. If the study is based solely on selected days, please include this clarification in the measurement section.
Citation: https://doi.org/10.5194/egusphere-2023-1848-RC1 - RC2: 'Comment on egusphere-2023-1848', James Brean, 31 Oct 2023
Shiyi Lai et al.
Shiyi Lai et al.
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