The Air Quality Strategy contains objectives for ambient concentrations of the pollutant nitrogen dioxide (NO2) as follows:
- 200µg/m3 as an hourly mean, not to be exceeded more than 18 times per calendar year.
- 40µg/m3 as an annual mean, for protection of human health.
Of the two objectives, the annual mean objective of 40µg/m3 is the more stringent. A monitoring station which records an annual mean of 40µg/m3 or less is unlikely to also record more than 18 hourly means greater than 200µg/m3 (i.e. an exceedence of the hourly objective) in the same year. However, predicting exceedances of the NO2 1-hour objective is not straightforward, as these will be highly variable from year to year, and from site to site. If monitoring is to be relied upon, then this should be carried out for an extended period (preferably a full calendar year) to ensure that the occurrence of occasional peaks is adequately captured. Dispersion models cannot predict short-term concentrations as reliably as annual mean concentrations. Moreover, model verification is likely to be challenging.
It is therefore useful to understand the relationship between the annual mean and the number of exceedences of the 1-hour objective, to establish whether the annual mean can be used to predict the likelihood of exceeding the hourly mean objective.
Previous research, based on analysis of data for the period 1980 to 2001, carried out on behalf of Defra and the Devolved Administrations, identified that exceedances of the NO2 1-hour mean are unlikely to occur where the annual mean is below 60µg/m3. This empirical relationship is still considered valid; therefore, local authorities should refer to it if NO2 1-hour mean monitoring data are not available (typically if monitoring NO2 using passive diffusion tubes).
It should be noted that this relationship is based upon observations made predominantly at roadside and kerbside monitoring sites where road traffic is the primary source of emissions; consequently, this relationship is not considered to be applicable in instances where industrial emissions impact on air quality, where the relationship with compliance on the hourly NO2 objective is more appropriately considered through dispersion modelling and the plume chemistry of NOx/NO2 conversion.
Relationship Between Annual Mean Concentration and 1-hour Mean Exceedences of the NO2 Air Quality Objective
Released: May 2008 (PDF, 500 KB, 19 pages)