Assessment of ClNO2 as a missing oxidant in the UK atmosphere

Lead Research Organisation: University of Leicester
Department Name: Chemistry


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

The nocturnal formation of nitryl chloride (ClNO2) via reaction of N2O5 on Cl- containing particles has been shown to be a very efficient mechanism to activate chlorine. Recent observations of high levels of ClNO2 (up to 1 ppb) in marine/coastal and continental polluted regions in USA and Germany strongly suggests 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.

The work in this proposal 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 N2O5. The first wide-scale measurements of N2O5 above the UK were recently 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.

Planned Impact

The project will have major scientific, commercial and policy impacts, as well as be of interest to the general public. We have identified the following ways to make sure that the project outcomes will be effectively disseminated:

a) Scientific - The project will provide much needed information regarding the presence and the role of ClNO2 and chlorine in the atmospheric chemical processes and, as such, the results will be of great interests to all researchers working in atmospheric sciences as well as to policy makers involved in air quality and climate issues. The data and the results would be of international significance and interest and will be widely disseminated through the scientific literature (via peer-reviewed high-impact journals) and during national and international conferences. Collaboration with internationally recognized scientists at NOAA will provide an additional - informal - venue to inform the scientific community of the results of this project as well as improve the cooperation between US and British research groups, with major benefit for future scientific progress. The project outcomes will also gain impact through the IGBP programs SOLAS and IGAC.

b) Commercial - Pathways to the commercial impact of the CIMS technology will be actively explored. The University of Leicester team have expertise through their technology demonstrator laboratory RAFT ( that has links to SME and national businesses interested in technology development and analytical services. A full technology impact and commercialisation impact assessment will be made as part of the project, with particular focus on potential technology "re-uses" in the commercial arena.

c) Policy - A major potential impact of the outcomes of this research is in the policy arena. There is a significant science-to-policy linkage in air quality/climate and there will be strong input into the Air Quality Expert Group (AQEG) at Department for Environment, Food and Rural Affairs (DEFRA). The project outcomes will help inform more accurate control regulation of emissions and a more effective air quality policy, with substantial benefits for public and environmental health. In this regard, a policy brief will be prepared and circulated to DEFRA and the EU.

d) General outreach on the work will be made through magazine articles, web articles, podcasts and social media (e.g. Twitter, Google+), as well as via the Universities of Leicester, York and UEA outreach programs.


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Description Nitryl Chloride is ubiquitous in the UK atmosphere. Its impact on oxidation budgets seems to be non-negligible. ClNO2 - clear seasonal cycle with a maxima in spring and winter
Sea-salt aerosol man source of particulate chloride
NO3 production rate limiting
Leading to NO2 or ozone limitation
Sweet spot for large concentrations
GEOS-Chem reproduce daily but not seasonal cycle
Resolution helps
Ozone bias
Can be produced in ship-plumes

ClNO2 formation and presence widespread
Wintertime oxidant?
Exploitation Route Impact on Policy
Sectors Environment,Government, Democracy and Justice

Description Atmospheric Chemistry and Air Pollution "Masterclass" 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Postgraduate students
Results and Impact Theme science "master class" for second year PGR students funded by CENTA doctoral training program. Event gave basis training in atmospheric chemistry to PGR students from across the NERC remit.
Year(s) Of Engagement Activity 2016,2017