5.2 Application of the two-box model
5.3.2 Effect of the exchange velocity
Figure 5.9 illustrates the effect of the exchange velocity (wt,0) on CNO,L
2 (ppb), i.e. the concentration in the lower box for (a) Case EX-LL (wt,0 =0.012 m s-1), (b) Case EX-L (wt,0 =0.016 m s-1), (c) Case BASE (wt,0 =0.02 m s-1), (d) Case EX-H (wt,0 =0.024 m s-1) and (e) Case EX-HH (wt,0 =0.028 m s-1). wt,0 can vary with the external wind turbulence above the street canyon, the street canyon geometry and the stability of the atmosphere. It is observed that CNO,L
2 is significantly influenced by wt,0. For Case EX-LL, levels of
L
CNO,
2 are extremely high (the maximum value could be up to 350 ppb). This is attributed to the lowest wt,0 adopted in Case EX-LL, which gives the worst (lowest) exchange between the lower and upper box (indicated by a lower value of wt,L in Figure 5.4).
Therefore, it is not effective for pollutants to be carried from the lower box to the overlying street canyon. It is interesting to notice that the solid red curve (i.e. representing the UK air quality standard of hourly NO2) shifts from the region with lower emissions to that with higher emissions as wt,0 increases. It means that even lower emissions under the worst dispersion conditions can result in very poor air quality inside street canyons. It is also observed that trajectory 2005-2020 falls entirely into the region exceeding the UK air quality standard of hourly NO2 for Case EX-LL with the lowest wt,0. With the increase of the exchange velocity, the solid red curve moves from the year 2020 towards the year 2005. This also indicates that the ventilation conditions should be improved to achieve better air quality for future years. It is also noted that TRES exceed the UK air quality standard of hourly NO2 for Case EX-LL, Case EX-L and Case BASE, but is within the air quality limit for Case EX-H and Case EX-HH. The detailed results along the selected lines for analysis, shown as Figure 5.5f, are presented below.
152 (a) CNO,L
2 (ppb) for Case EX-LL (b) CNO,L
2 (ppb) for Case EX-L
(c) CNO,L
2 (ppb) for Case BASE (d) CNO,L
2 (ppb) for Case EX-H
(e) CNO,L
2
(ppb) for Case EX-HH
Figure 5.9 CNO,L
2 (ppb), i.e. the concentration in the lower box derived from the “two-box” model, in the (a) Case EX-LL (wt,0 =0.012 m s-1), (b) Case EX-L (wt,0 =0.016 m s-1), (c) Case BASE (wt,0
=0.02 m s-1), (d) Case EX-H (wt,0 =0.024 m s-1) and (e) Case EX-HH (wt,0 =0.028 m s-1). EVOCs and ENOx are normalised by those of the Typical Real-world Emission Scenario (TRES, represented by), for the year of 2010. Trajectory 2005-2020 represents the emission scenarios for 2005 to 2020, assuming constant traffic volume and speed. The solid red curves denote the UK air quality standard of hourly NO2 (105 ppb).
153
(a) Fixed ENOx (b) Fixed EVOCs
(c) TRES-2010 (d) Trajectory 2005-2020
Figure 5.10 CNO,L
2 (ppb), i.e. the concentration in the lower box derived from the “two-box” model, for (a) “Fixed ENOx” at a fixed NOx emissions of TRES, (b) “Fixed EVOCs” at a fixed VOCs emissions of TRES, (c) “TRES-2010” varying the total traffic volume only and (d) “Trajectory 2005-2020”
assuming constant traffic volume and speed varying wt,0. EVOCs and ENOx are normalised by those of the Typical Real-world Emission Scenario (TRES, represented by), for the year of 2010. The dashed line indicates the UK air quality standard of hourly NO2 (105 ppb).
Figure 5.10 shows the transects of CNO,L
2 (ppb) for Case EX -LL, Case EX-L, Case BASE, Case EX-H and Case EX-HH through the selected lines for analysis in Figure 5.5f. It is also observed that CNO,L
2 increases with increase of EVOCs and ENOx, shown as Figure 5.10a-c. This indicates that the control of either EVOCs or ENOx is effective to reduce the NO2 levels. It is also interesting to notice that there is not significant change of CNO,L
2
154
while the EVOCs and ENOx are lower. The minimum and maximum differences of CNO,L
2
between Case EX-LL with wt,0=0.012 m s-1 and Case EX -HH with wt,0=0.028 m s-1 are 44 ppb and 201 ppb for Figure 5.10a, 15 ppb and 136 ppb for Figure 5.10b , and 17 ppb and 228 ppb for Figure 5.10c. This indicates the importance of controlling ventilation conditions of street canyons especially for highly polluted scenarios. Figure 5.10d shows that CNO,L
2 decreases significantly with year due to the control technologies of both EVOCs and ENOx. This indicates that the air quality will be improved in future years. However, for the worst ventilation condition (e.g. Case EX-LL), CNO,L
2 still exceeds the UK air quality standard over the year 2005 to 2020. This indicates that control of air ventilation together with control of vehicle emissions is also important in improving air quality within street canyons. Air ventilation is strongly influenced by the urban street design and deep street canyons could lead to poor ventilation.
Figure 5.11 shows the effect of the exchange velocity (wt,0 ) on NO ,L
2 (%), i.e. the percentage of overestimation for NO2 in the lower canyon by the ‘one-box’ model, compared with the two-box system. It is found that NO ,L
2 decreases slightly with increasing exchange velocity (wt,0), i.e. the range of (-37.49 %, -17.64 %) among all tested emission scenarios for Case EX-LL (-35.26 %, -17.22 %) for Case EX-L, (-33.49 %, -17.07 %) for Case BASE, (-31.89 %, -17.02 %) for Case EX-H and (-30.52 %, -17.01 %) for Case EX-HH. As =0.5 is adopted for all cases in Figure 10, the nonlinear patterns reflect the characteristics of scenarios with heterogeneity coefficient ( =0.5). This indicates that there is an underestimation of NO2 concentrations by the ‘one-box’ model and this underestimation changes significantly with the heterogeneity coefficient (Figure 5.5), to a much greater extent than the change with the exchange velocity (Figure 5.11).
155 (a) NO ,L
2 (%) for Case EX-LL (b) NO ,L
2 (%) for Case EX-L
(c) NO ,L
2 (%) for Case BASE (d) NO ,L
2 (%) for Case EX-H
(e)
L NO ,2
(%) for Case EX-HH
Figure 5.11 NO ,L
2 (%), i.e. the percentage of overestimation for NO2 in the lower canyon by the ‘one-box’ model compared with that by the “two-box” model, in the (a) Case EX-LL (wt,0 =0.012 m s-1), (b) Case EX-L (wt,0 =0.016 m s-1), (c) Case BASE (wt,0 =0.02 m s-1), (d) Case EX-H (wt,0 =0.024 m s-1) and (e) Case EX-HH (wt,0 =0.028 m s-1). EVOCs and ENOx are normalised by those of the Typical Real-world Emission Scenario (TRES, represented by), for the year of 2010. Trajectory 2005-2020 represents the emission scenarios for 2005 to 2020, assuming constant traffic volume and speed.
156
(a) Fixed ENOx (b) Fixed EVOCs
(c) TRES-2010 (d) Trajectory 2005-2020
Figure 5.12 NO ,L
2 (%), i.e. the percentage of overestimation for NO2 in the lower canyon by the ‘one-box’ model compared with that by the “two-box” model, for (a) “Fixed ENOx” at a fixed NOx emissions of TRES, (b) “Fixed EVOCs” at a fixed VOCs emissions of TRES, (c) “TRES-2010” varying the total traffic volume only and (d) “Trajectory 2005-2020” assuming constant traffic volume and speed varying wt,0. EVOCs and ENOx are normalised by those of the Typical Real-world Emission Scenario (TRES, represented by), for the year of 2010.
Figure 5.12 illustrates the transects of NO ,L
2 (ppb) for Case EX -LL, Case EX-L, Case BASE, Case EX-H and Case EX-HH through the selected lines for analysis in Figure 5.5f.
Figure 5.12a shows that NO ,L
2 slightly decreases with the increase of EVOCs, i.e. from -21.15 % to -26.86 % for Case EX-LL, from -19.26 % to -25.37 % for Case EX-L, from -18.14 % to -24.16 % for Case BASE, from -17.48 % to -23.16 % for Case EX-H and from
157