the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jan 2025
On the presence of high nitrite (NO2-) in coarse particles at Mt. Qomolangma
Abstract. Atmospheric reactive nitrogen cycling is crucial for maintaining the atmospheric oxidation capacity of background atmosphere on the Tibetan Plateau , with nitrous acid (HONO) and particulate nitrite (NO2-) as important intermediates. During an eleven-day field campaign at the Base Camp of Mt. Qomolangma in spring of 2022, we observed significant enrichment of NO2- in total suspended particulate (TSP) with a mean concentration of 375 ± 386 ng m-3, while NO2- was absent in fine particles (PM2.5). The comparison revealed that NO2- predominately exists in coarse particles. Local surface soil at the sampling site also exhibited high levels of NO2-, with δ15N value similar to NO2- in TSP. This similarity suggests that wind-blown soil is probably the primary source of NO2- in TSP, accounting for the background levels. While concentration changes of water-soluble inorganic ions in TSP and PM2.5 in response to shifts in air mass back-trajectories imply that atmospheric pollutants transported from South Asia may further elevate the NO2-, the specific mechanisms of long-range transport resulting in NO2- accumulation in TSP rather than PM2.5 remain unknown and need to be investigated. Our results reveal an overlooked source of atmospheric NO2-, i.e., soil NO2-, and highlight in remote regions such as Tibet where other sources are limited, wind-blown soil may serve as an important source of atmospheric NO2-. Once lofted into the atmosphere, NO2- may readily participate in atmospheric reactive nitrogen cycling through gas-particle partitioning or photolysis, leading to the production of HONO, OH and NO and thereby influencing oxidation chemistry.
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RC1: 'Comment on egusphere-2024-4165', Anonymous Referee #1, 22 Feb 2025
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Understanding the chemistry of reactive nitrogen in pristine environments, such as the Tibetan Plateau, is crucial for advancing our knowledge of atmospheric chemistry. This study undertook a challenging field campaign, collecting PM₂.₅, total particle, and soil samples to investigate the unusual enrichment of nitrite in coarse particles and its potential sources. While the effort in sample collection and extensive analysis is commendable, the presentation of the data in this paper lacks clarity, and the interpretation of the results is, in my opinion, not entirely sound.
After reviewing this manuscript, I remain unconvinced by the authors’ arguments, as significant gaps in explanation persist. Given the high publication standards of Atmospheric Chemistry and Physics (ACP), I believe the current manuscript does not meet the criteria for publication without substantial revisions.
Here are my major concerns:
1. Uncertainties in ion concentrations of PM₂.₅ and total particles.
The comparison of ion concentrations between TSP and PM₂.₅ is particularly interesting. It clearly shows that nitrite ions are present only in TSP, while PM₂.₅ contains none. However, what stands out is that despite PM₂.₅ being a subset of TSP, some PM₂.₅ samples occasionally show higher ion concentrations (in µg/m³) than their corresponding TSP samples. For instance, the April 30 PM₂.₅ sample appears to contain more nitrate than the TSP sample.
I suspect this discrepancy may stem from analytical uncertainties in ion concentration measurements or uncertainties in the blank corrections, but this issue has not been addressed in the manuscript. Understanding these uncertainties is crucial, especially since the ion composition of coarse aerosols is determined by subtracting two similar measurements.
I also suggest that the authors directly present the ion concentrations of coarse aerosols. While this may sometimes result in negative values, it would provide a clearer picture of the uncertainties associated with ion concentration measurements.
2. Missing evidence: mass balance in coarse particles.
I would like to see a little bit more discussion in section 4.3 when the authors attempted to attribute the observed nitrite to lofted dust, maybe as simple as mass balance calculations. For example, if the observed nitrite is indeed coming from soil, to get ~1 ug/m3 of nitrite from soil, how much soil do you need in the air giving average soil nitrite concentration of < 100 ng/g? About 10 g/m3. Then, does the observed TSP concentration support your hypothesis?
Similarly, nitrate/nitrite ratio in soil also do not fully support authors’ hypothesis, the authors argue that it is likely nitrate/nitrite distributed in particles of different sizes but there is no evidence supporting this, nor is there any previous work mentioned such effect. Therefore, I am not fully convinced by the existing evidence that soil is the main source of particle nitrite.
3. Isotopic results do not seem to support authors’ argument.
The isotopic results also do not support the authors’ argument. The O17 signal in soil samples range from 1.4‰ to 7.3‰ but in TSP the O17 is 0-1‰, a clear discrepancy. δ18O also are significant different – 2‰ to 18‰ in the soil but lots of negative values in the TSP. I do not think this can be simply explained by isotope exchange with water because such isotope exchange always occurs: we need more evidence to believe that exchange never happens when the particles are on the surface, then once it was lifted into the air, within hours (typical lifetime of coarse particles) the exchange suddenly occurred.
4. More discussion needed for air mass from different regions.
The back-trajectory analysis clearly shows that the field campaign sampled air from two distinct regions, with corresponding differences in ion concentrations and d15N values. However, the discussion of these differences is too simplistic. It would be beneficial for the authors to separately analyze how the aerosols from each period differ and to explore in greater depth how nitrite concentrations and isotopic compositions varied between them, as the differences are quite significant.
Additionally, while the authors suggest that long-range transport is unimportant for the nitrite budget, their argument in the final section contradicts this claim. To strengthen the manuscript, the discussion should remain consistent and logically cohesive.
5. Implications to AOC is weak.
Since the source of nitrite remains unclear, the discussion on how lofted soil influences atmospheric chemistry is weak. The authors provide no data on atmospheric dust concentrations and do not address the transport potential of soil particles, which is likely limited due to their larger size. As a result, the final section lacks convincing supporting evidence.
Citation: https://doi.org/10.5194/egusphere-2024-4165-RC1
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