Nitrogen dioxide is often described as a secondary pollutant because the majority of ambient NO₂ is formed by oxidation of NO that has been emitted into the atmosphere. An understanding of the oxidation processes that lead to the current measured concentrations of NO₂ and how these processes are likely to be influenced by changes in future emissions are therefore essential for predicting future concentrations of NO₂.

One of the main mechanisms by which NO is oxidised to NO₂ is by reaction with O₃. If NO_x emissions are reduced and O₃ concentrations remain approximately unchanged then this will increase the proportion of NO_x emissions that will be rapidly converted to NO₂. This means that annual mean concentrations are unlikely to respond in direct proportion to reductions in NO_x emissions.

The trimolecular reaction of NO with O₂ is thought to be one of the dominant oxidation routes on rare winter smog episode days (Bower et al, 1994). The rate of this reaction depends on the square of the NO concentration and a reduction in NO_x emissions to about half of the current values on these types of days will therefore tend to significantly reduce the efficiency of this pathway. Winter smog episode concentrations of NO₂ are therefore expected to be much reduced by 2005 and this is the reason why predictions in this report are focused on annual means.

Methods for predicting future NO₂ concentrations from predictions of NO_x have been and are currently the subject of considerable research, because of the importance of these predictions within the review of the NAQS. The methods that were used to predict background concentrations of NO₂ in the report prepared for the review of the NAQS (Stedman et al, 1998) were not fully consistent with the methods that were applied at the roadside. Site specific projections of background NO₂ for 2005 were derived from 1996 NO₂ measurements. The results of the modelling work of Derwent (1999) that a 50% reduction in NO_x emissions would be expected to lead to a 30% reduction in annual mean urban NO₂ concentrations were applied. Projections of roadside NO₂ concentrations for 2005 were derived from projections of roadside NO_x concentrations using an empirically derived non-linear relationship between measured annual mean NO_x and NO₂ concentrations.

Figure 1 shows a comparison of measured annual mean concentrations of NO_x and NO₂ for both background and roadside monitoring locations in 1997. The solid line on Figure 1 shows a non-linear function that has been fitted to the measurement data for background sites. The advantage of using this type of non-linear curve is that future NO₂ concentrations can be directly predicted from NO_x predictions by assuming that the curve will remain the same in future years.

Figure 1 also shows that roadside NO₂ concentration are generally lower than background concentrations for the same measured NO_x concentration. This is because of the limited time that is available for NO to be oxidised to NO₂ at the roadside and the limited amount of ozone that may be available in the roadside environment. The dashed line shows the non-linear function that has been fitted to current roadside measurement data.

Chapter 2         Chapter 4