Night-time NO emissions suppress large amounts of chlorine radical formation in Delhi
- 1Department of Environmental Science, Stockholm University, Stockholm, Sweden (sophie.haslett@aces.su.se)
- 2Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland
- 3Department of Civil Engineering, Indian Institute of Technology, Kanpur, India
- 4Geosciences division, Physical Research laboratory, Ahmedabad, India
- 5Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi, India
- 6Department of Atmospheric Sciences, University of Washington, Seattle, USA
- 7Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, Finland
Concentrations of particulate chloride can reach values over 100 µg m-3 during the winter in Delhi, which is among the highest levels recorded across the globe. In the presence of nitrogen pentoxide (N2O5), this chloride can form nitryl chloride (ClNO2), which photolyses in sunlight and releases the Cl radical. The Cl radical is an incredibly potent oxidant, reacting with some volatile organic compounds (VOCs) orders of magnitude faster than more common oxidants such as OH. Chlorine would therefore be expected to play a significant role in the oxidation of VOCs in Delhi.
We carried out intensive measurements of particle- and gas-phase physical and chemical properties during a field campaign in Delhi in early 2019. A suite of instruments was used, including a chemical ionisation mass spectrometer fitted with a filter inlet for aerosols and gases (FIGAERO-CIMS) to measure N2O5 and ClNO2. Despite N2O5 typically being considered a night-time compound, we in fact observed the highest concentrations in the mid-afternoon and almost none at all during the night. Further analysis indicated that the ubiquity of night-time NOx emissions in the city suppresses night-time production of N2O5. As a result of this unusual diurnal pattern, high concentrations of ClNO2 are unable to form overnight. The morning peak in ClNO2 and the subsequent release of chlorine radicals, while large compared with some other urban environments, is therefore much smaller than might have been expected given the high levels of particulate chloride.
In this presentation, I will discuss our observations and the impact of this unusual diurnal pattern on the atmospheric chemical profile. Impacts include a shift of even typically ‘night-time’ oxidation patterns to the day and a likely overall reduced oxidative capacity in the city’s atmosphere. Our results indicate that a reduction in chlorine emissions must be considered in tandem with NOx emission reductions in efforts to reduce Delhi’s pollution.
How to cite: Haslett, S., Kumar, V., Prevot, A., Slowik, J., Bell, D., Tripathi, S., Mishra, S., Singh, A., Rastoji, N., Ganguly, D., Thornton, J., Dällenbach, K., Yan, C., and Mohr, C.: Night-time NO emissions suppress large amounts of chlorine radical formation in Delhi, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16048, https://doi.org/10.5194/egusphere-egu21-16048, 2021.
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