Night-time NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi

Haslett, Sophie L.; Bell, David M.; Kumar, Varun; Slowik, Jay G.; Wang, Dongyu S.; Mishra, Suneeti; Rastogi, Neeraj; Singh, Atinderpal; Ganguly, Dilip; Thornton, Joel; Zheng, Feixue; Li, Yuanyuan; Nie, Wei; Liu, Yongchun; Ma, Wei; Yan, Chao; Kulmala, Markku; Daellenbach, Kaspar R.; Hadden, David; Baltensperger, Urs; Prevot, Andre S. H.; Tripathi, Sachchida N.; Mohr, Claudia

doi:https://doi.org/10.5194/egusphere-2023-497

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https://doi.org/10.5194/egusphere-2023-497

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28 Mar 2023

| 28 Mar 2023

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Night-time NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi

Sophie L. Haslett, David M. Bell, Varun Kumar, Jay G. Slowik, Dongyu S. Wang, Suneeti Mishra, Neeraj Rastogi, Atinderpal Singh, Dilip Ganguly, Joel Thornton, Feixue Zheng, Yuanyuan Li, Wei Nie, Yongchun Liu, Wei Ma, Chao Yan, Markku Kulmala, Kaspar R. Daellenbach, David Hadden, Urs Baltensperger, Andre S. H. Prevot, Sachchida N. Tripathi, and Claudia Mohr

Abstract. Atmospheric pollution in urban regions is highly influenced by oxidants due to their important role in the formation of secondary organic aerosol (SOA) and smog. These include the nitrate radical (NO₃), which is typically considered a night-time oxidant, and the chlorine radical (Cl), an extremely potent oxidant that can be released in the morning in chloride-rich environments as a result of nocturnal build-up of nitryl chloride (ClNO₂). Chloride makes up a higher percentage of particulate matter in Delhi than has been observed anywhere else in the world, which results in Cl having an unusually strong influence in this city. Here, we present observations and model results revealing that atmospheric chemistry in Delhi exhibits an unusual diel cycle, controlled by high concentrations of NO during the night. As a result of this, the formation of both NO₃ and dinitrogen pentoxide (N₂O₅), a precursor of ClNO₂ and thus Cl, are suppressed at night and increase to unusually high levels during the day. Our results indicate that a substantial reduction in night-time NO has the potential to increase both nocturnal oxidation via NO₃ and the production of Cl during the day.

How to cite. Haslett, S. L., Bell, D. M., Kumar, V., Slowik, J. G., Wang, D. S., Mishra, S., Rastogi, N., Singh, A., Ganguly, D., Thornton, J., Zheng, F., Li, Y., Nie, W., Liu, Y., Ma, W., Yan, C., Kulmala, M., Daellenbach, K. R., Hadden, D., Baltensperger, U., Prevot, A. S. H., Tripathi, S. N., and Mohr, C.: Night-time NO emissions strongly suppress chlorine and nitrate radical formation during the winter in Delhi, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-497, 2023.

Received: 18 Mar 2023 – Discussion started: 28 Mar 2023

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RC1:
'Comment on egusphere-2023-497', Anonymous Referee #1, 03 May 2023 reply
This manuscript presents an atmospheric chemistry story from 25 days of data (11 January – 5 February) from a FIGAERO-CIMS complemented with AMS, NOx, and O3 measurements in Delhi, India. The authors use these measurements with a 0-D chemical box model to understand N2O5 sources and sinks. The study finds that the high night-time NO depletes O3, NO3, and N2O5 (a precursor to ClNO2 and thus Cl). The authors finally suggest that decreasing NO emissions will result in NO3 becoming an important nocturnal oxidant during the night and Cl during the day. The manuscript provides interesting insights into the “unusual” atmospheric chemistry, especially the role of various atmospheric oxidants, in Delhi which may be relevant for other polluted cities in the Indo-Gangetic Plain where there is similar combination of large emissions (including NOx) and unfavorable (nighttime) meteorology.
Some comments:
The authors should be careful in the distinction between “emissions” and “levels”. These are not always interchangeable. As the authors themselves discuss in the excellent discussion in the last paragraph of page-9, boundary layer dynamics (unfavorable nighttime meteorology) plays an important role in Delhi. As such, it may not be simply the “nighttime NO emissions”, but the “nighttime NO levels” that are of relevant to the discussion. While this may seem trivial, this is extremely relevant from a policy perspective and specifically how the authors discuss policy implications. I suggest that the authors carefully go through the manuscript and check if the use of “emissions” and “levels” is correct and intentional.

Delhi has large seasonal variations in pollution loadings (including aerosol and gas) because of a combination of changing sources (especially heating in winter) and meteorology. As a result, a 25-day study from January (2019) should be careful in generalizing the loadings (including chloride) and chemistry for the entire year. I am not saying that the findings are not important, but that they should be put in context of the study period. For example, the authors could use previously published year-round data for aerosol composition and NOx (even if no FIGAERO-CIMS) to put the study period in context.

The authors should also include the study-period in the figure captions. For example, instead of “The diel cycles of key oxidants and oxidation products in Delhi” it should be “The diel cycles of key oxidants and oxidation products in Delhi during the study period” (even better if you include the study dates)

How are “night” and “day” defined for the analysis? I could not find this in the methods.

In Section 2.1, the name of the centre is incorrectly written. I believe the correct name of the centre is the same as the affiliation of one of the authors of the study. Also, are the authors sure that the distance from the major roadway is just 80m? Please also include which floor the measurements were conducted in.

In Section 2.3, line 151, the measurement location for meteorological data should be included.

In Figure 2, adding MLH from reanalysis dataset such as ERA5 or MERRA2 may provide interesting insights.

Thanks to the authors for writing an interesting atmospheric chemistry manuscript. I hope that the comments above help improve the article.

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Citation: https://doi.org/10.5194/egusphere-2023-497-RC1
RC2: 'Comment on egusphere-2023-497', Anonymous Referee #2, 17 May 2023 reply

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Citation: https://doi.org/10.5194/egusphere-2023-497-RC2

Sophie L. Haslett et al.

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Short summary

In Delhi, some aspects of daytime and night-time atmospheric chemistry are “flipped,” and parodoxically, vehicle emissions may be limiting other forms of particle production. This is because the night-time emissions of nitrogen oxide (NO) by traffic, as well as biomass burning, prevents some chemical processes that would otherwise create even more particles, and worsen the urban haze.


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