Analysis of the relationship between ambient levels of O 3, NO 2 and NO as a function of NO x in the UK

Abstract

Monitoring data from the UK Automatic Urban and Rural Network are used to investigate the relationships between ambient levels of ozone (O 3), nitric oxide (NO) and nitrogen dioxide (NO 2) as a function of NO x , for levels ranging from those typical of UK rural sites to those observed at polluted urban kerbside sites. Particular emphasis is placed on establishing how the level of ‘oxidant’, OX (taken to be the sum of O 3 and NO 2) varies with the level of NO x , and therefore to gain some insight into the atmospheric sources of OX, particularly at polluted urban locations. The analyses indicate that the level of OX at a given location is made up of NO x -independent and NO x -dependent contributions. The former is effectively a regional contribution which equates to the regional background O 3 level, whereas the latter is effectively a local contribution which correlates with the level of primary pollution. The local oxidant source has probable contributions from (i) direct NO 2 emissions, (ii) the thermal reaction of NO with O 2 at high NO x , and (iii) common-source emission of species which promote NO to NO 2 conversion. The final category may include nitrous acid (HONO), which appears to be emitted directly in vehicle exhaust, and is potentially photolysed to generate HO x radicals on a short timescale throughout the year at southern UK latitudes. The analyses also show that the local oxidant source has significant site-to-site variations, and possible reasons for these variations are discussed. Relationships between OX and NO x , based on annual mean data, and fitted functions describing the relative contributions to OX made by NO 2 and O 3, are used to define expressions which describe the likely variation of annual mean NO 2 as a function of NO x at 14 urban and suburban sites, and which can take account of possible changes in the regional background of O 3.