Analysis of sources and partitioning of oxidant in the UK—Part 1: the NOX-dependence of annual mean concentrations of nitrogen dioxide and ozone

A description of the NOX-dependence of annual mean concentrations of nitrogen dioxide (NO2) is an important component of empirical policy models used to investigate the impacts of NOX emissions control measures. Consideration of nitric oxide (NO), NO2 and ozone (O3) as a set of chemically coupled species, rather than NO and NO2 alone, is used to gain additional insights into the factors controlling ambient levels of NO2 (and O3), and how they vary with NOX concentration. Monitoring data from 66 urban roadside, urban centre, urban background, suburban, urban industrial and rural sites in the UK are used in this analysis. Annual mean [NO2] vs. [NOX] and [O3] vs. [NOX] relationships are defined by establishing, first, how the concentration of oxidantʹ ([OX], defined as [O3]+[NO2]) varies with [NOX] and, secondly, how the fractional contributions of NO2 and O3 to OX (i.e., [NO2]/[OX] and [O3]/[OX]) vary with [NOX]. This allows site-specific relationships describing the NOX-dependence of annual mean NO2 and O3 concentrations to be recommended for 56 non-rural sites. The results are also used to derive representative expressions for different regions of the UK, and these are compared with those currently employed in national empirical modelling activities to calculate future annual mean NO2 concentrations. Because the present methodology decouples factors which relate to chemical partitioning from those which relate to sources of oxidant, the expressions presented here have the flexibility to enable predictions that take account of modest future changes in regional oxidant (i.e. the background ozone level), or changes in local oxidant inputs (e.g., primary NO2 emissions) which might arise, for example, from modifications in vehicle emissions control technologies or vehicle fleet composition.