Night-time NO emissions suppress large amounts of chlorine radical formation in Delhi

<p>Concentrations of particulate chloride can reach values over 100 µg m<sup>-3</sup> during the winter in Delhi, which is among the highest levels recorded across the globe. In the presence of nitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>), this chloride can form nitryl chloride (ClNO<sub>2</sub>), 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.</p><p>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 N<sub>2</sub>O<sub>5</sub> and ClNO<sub>2</sub>. Despite N<sub>2</sub>O<sub>5</sub> 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 NO<sub>x</sub> emissions in the city suppresses night-time production of N<sub>2</sub>O<sub>5</sub>. As a result of this unusual diurnal pattern, high concentrations of ClNO<sub>2</sub> 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.</p><p>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 NO<sub>x</sub> emission reductions in efforts to reduce Delhi’s pollution.</p>

N2O5 reactive uptake kinetics and chlorine activation on authentic biomass-burning aerosol

The reactive uptake kinetics of nitrogen pentoxide (N2O5) to authentic biomass-burning aerosol and the production of nitryl chloride (ClNO2) was determined using an entrained aerosol flow tube reactor.