This project was funded by the Natural Environmental Research Council (NERC) with the grant references - NE/K004069/1 and NE/K004603/1. It was lead by Professor Paul Monks (University of Leeds) and Professor Mathew Evans (University of York) and ran from 2013-2016.

It asked the key question: Is nitryl chloride (ClNO2) a chlorine activation pathway and a strong chlorine (Cl) source that substantially affects tropospheric composition and oxidative capacity of the atmosphere in the UK?

The nocturnal formation of nitryl chloride (ClNO2) via reactions of N2O5 on Cl- containing particles has been shown to be a very efficient mechanism to activate chlorine. Observations of high levels of ClNO2 (up to 1 ppb) in marine/coastal and continental polluted regions in USA and Germany strongly suggested that ClNO2 chemistry is active on a much larger scale than previously thought, with very important consequences for air composition and quality, public and environmental health and global climate.

This work explores by way of concerted measurements and modelling whether, in the UK context, ClNO2 can release large concentrations of reactive chlorine (Cl) into the troposphere. The chemistry could be especially important for the United Kingdom as the UK is surrounded by the ocean, which provides a continuous source of sea-salt in the coastal areas and further inland. In addition, emissions from coal-fired power stations, spread of grit on roads during winter and usage of chlorinated compounds in swimming pools, sewage and water treatment plants, can provide significant sources of non sea-salt Cl.

The other atmospheric precursor to ClNO2 is nitrogen pentoxide (N2O5). The first wide-scale measurements of N2O5 above the UK were conducted from the FAAM aircraft during the NERC-funded RONOCO campaigns. The RONOCO flights found elevated N2O5 concentrations aloft (up to 1 ppbv), typically associated with atmospheric processing of NOx (nitrogen oxides) in pollution plumes from major UK cities, such as the London outflow over the English Channel/North Sea. The co-location of large sources of VOC, NOx and Cl- containing particles means that ClNO2 chemistry should be active in large parts of the UK. Since most of the population in the UK lives within ca. 100 km from the ocean and several large metropolitan areas (e.g., London, Glasgow, Liverpool) are located near the coast, this chemistry is likely to have a significant impact on the health and life quality of many people.

The overall objective of this project was to assess the importance of ClNO2 as a chlorine activation pathway and its presence as a strong chlorine source that could affect the composition and oxidative capacity of the atmosphere, with a particular focus on the UK.

This project addresses three major scientific questions:
a) What are the concentrations of nitryl chloride (ClNO2) in the UK and how ubiquitous is it?
b) Is ClNO2 a significant missing oxidant source in the UK oxidative budget that should be taken into account by regional and global models?
c) What is the impact of ClNO2 on air quality and the levels of greenhouse gases in the UK?

In order to achieve this overall goal the project was organised into five phases:
1. Acquire and optimise a Chemical Ionisation Mass Spectrometer (CIMS) instrument for the measurement of ClNO2.
2. Develop and deploy an accurate, field-deployable ClNO2 calibration system.
3. Make the first ambient measurements of ClNO2 in the UK. Quantify ambient ClNO2 (and its immediate precursors, N2O5 and particulate Cl--) under different conditions at a range of representative sites in the UK.
4. Analyse the results of the ambient measurements using a box-model and investigate the detailed chemical processes involved in the formation and destruction of ClNO2.
5. Scale up the analysis of the measurements to the regional and global scales and analyse the impact of ClNO2 and Cl chemistry on the atmospheric oxidative budget and on the concentrations and atmospheric residence times of pollutants and greenhouse gases, with particular focus on the implications for the UK air quality.

*****The datasets include measurements of ambient concentrations of Cl2, ClNO2, N2O5 and aerosol chloride.*****