In the background atmosphere the formation of O3 from NOx emissions is a non–linear process, however, it is acknowledged that the chemical reactions involved in the formation of ozone from aviation NOx take place on a much smaller scale that those of the back-ground atmosphere. This study was thus designed to test the bounds of the non-linearity of the NOx – O3 system when applied to aviation. At cruise flight levels, as a result of enhanced ultraviolet radiation, ozone forms more readily from NOxemissions than it does at ground level (see Chapter 3). Its formation is linked, highly sensitively, to precursor emissions and ambient background concentrations at the time and location of formation.
The relationship between background atmospheric NOx and ozone formation from avi-ation NOx is a complex one – in the troposphere NOx is involved in both the formation and destruction of ozone. The aviation scenarios (Table 5.2) were run on the MOZART–3 CTM as described in Chapter 4 and the results are presented in the following sections (Figure 5.7 Figure 5.21).
The results illustrate that, as aviation NOxemissions increase, so does ozone burden, this relationship appears linear up to around 3 Tg N Yr-1and then becomes non-linear in both the low NOx and high NOx background atmospheric states (Figure 5.7 and Figure 5.8).
At values of aviation NOxemissions release higher than 3 Tg N Yr-1ozone formation per NOxmolecule begins to reduce as aviation emissions increase, reflecting the non–linearity of ozone formation from NOxin the atmosphere.
Several other modelling studies focused on aviation NOx emissions have suggested that on the scale at which aviation emits NOx, its associated interactions with the background atmosphere and subsequent reactions respond linearly, in accordance with the NOx emis-sion rate. As long as the background atmosphere remains reasonably consistent, this appears true on a global scale (Grewe et al., 1999; IPCC 1999; Sausen and Schumann, 2000; Kohler et al., 2008; Khodayari et al., 2014). Grewe et al. (1999) suggest that, for a given scenario of aviation emissions, the NOx – O3 cycle only becomes non–linear at high latitudes during summer when convection enhances background atmospheric NOx
and therefore causes a decrease in ozone productivity, thus, the summer season alone
Figure 5.7: The ozone burden (Tg O3) resulting from the aviation NOxemissions in the low NOx
atmospheric background state. Each point represents one of the emissions scenarios described in Table5.2 The lowest of which was the React4c base case. The emissions scenarios were modelled on the MOZART CTM with the set up as described in Chapter 4. The bottom right panel shows the emissions scenarios up to 10 Tg N Yr-1 with a linear trend line formulated from the values up to 3 Tg N Yr-1to show where the linear relationship between aviation NOxemissions and the associated ozone burden breaks down.
Figure 5.8: The ozone burden (Tg O3) resulting from the aviation NOxemissions in the high NOx
atmospheric background state. Each point represents one of the emissions scenarios described in Table5.2 The lowest of which was the React4c base case. The emissions scenarios were modelled on the MOZART CTM with the set up as described in Chapter 4. The bottom right panel shows the emissions scenarios up to 10 Tg N Yr-1 with a linear trend line formulated from the values up to 3 Tg N Yr-1to show where the linear relationship between aviation NOxemissions and the associated ozone burden breaks down.
accounts for most of the non–linearity seen in the aviation NOx– O3cycle.
The non–linearity seen here can be explained through the chemistry involved. The overall production of ozone from NOxdepends on peroxy radicals – HO2and RO2(R = organinc radical) which oxidise NO to NO2 which then undergoes photodissociation and results in the formation of ozone through equation 5.1 to equation 5.5 (also discussed in Chapter 2).
NMHC + OH ! O2+RO2 (5.1)
RO2+NO + O2 ! NO2+HO2+CARB (5.2)
HO2+NO ! NO2+OH (5.3)
2NO2+hv + 2O2 ! 2NO + 2O3 (5.4)
Net NMHC + 4O2+hv ! 2O3+CARB (5.5)
Where NMHC is non-methyl hydrocarbons and CARB is carbonyl compounds
These reactions show that NOxand NMHC are ozone precursors and form ozone via HOx
and that NOx and HOx act as catalysts in the production of O3 . Therefore, generally ozone production increases as NOx and HOx emissions do. However, the destruction of NOx and HOx also depend on the background NOx and NMHC levels, thus the rate of ozone production does not respond linearly to the to the increase in ozone precursors, as the rate of the termination reactions of NOx and HOx (ozone precursors) also increase as background NOxemissions increase (Lin et al., 1988), explaining the non-linear reactions seen in figures 5.7 and 5.8.
The difference in average OPE for the two backgrounds, stems again from changes in the hydroxyl radical. A low NOxbackground will promote OH, allowing more production of ozone per NOx molecule than in a high NOxbackground where OH is inhibited. In a low NOx environment, such as the low background NOx atmospheric states used here, OH formation is greater than the emission rate of NOx , excess NOx is thus removed rapidly through equation 5.6 for example.
OH + NO2 ! HNO3 (5.6)
This results in an excess of free radicals in the atmosphere, which are available for other reactions such as the sequence of reactions that lead to the formation of ozone (above). In a high NOxbackground, NOxemission rate is greater than the free radical formation rate,
resulting in excess NOx , peroxide formation is then limited and NOx removal reactions are slow. In other words, in a low NOx background the additional input of aviation NOx
emissions are removed by oxidation, leading to the formation of ozone, however, the oxidising capacity of the atmosphere in a high NOx state is diminished reducing the rate of ozone production in comparison to a low NOxstate (Kleinman, 1994).