5.2 Application of the two-box model
5.3.3 Effect of the box height ratio
Figure 5.13 illustrates the effect of the box height ratio ( ) on CNO,L
2 (ppb), i.e. the concentration in the lower box for Case HB-LL ( =0.1), (b) Case HB-L ( =0.3), (c) Case BASE ( =0.5), (d) Case HB-H ( =0.7), and (e) Case HB-HH ( =0.9). can vary with the flow structure in a street canyon, which may be significantly influenced by the building geometry. A high-level circulation induced by pitched roof of the building will give a relatively smaller size of the upper vortex (Louka et al., 2000), suggesting an extremely higher value of (possibly equivalent to 0.9). There is clear evidence that
2 for smaller box height ratio, e.g. with a maximum value of about 520 ppb for Case HB-LL with =0.1. This small box height ratio represents the case that pollutants are highly trapped in the small lower part of the street canyon under poor ventilation conditions. This is similar to the secondary smaller eddies near the street corner, where levels of pollutants can extremely high. The exchange velocity between lower and upper box (indicated by a lower value of wt,Lin Figure 5.4) is the lowest for Case HB-LL. It is observed that almost all the scenarios (including trajectory 2005-2020) in Case HB-LL are expected to exceed the UK air quality standard of hourly NO2 except for scenarios with
158
extremely lower emissions, shown as Figure 5.13a. As the box height ratio increases, the solid red curve in Figure 5.13 shifts towards scenarios with higher emissions across trajectory 2005-2020. For Case HB-H and Case HB-HH, TRES is observed in the region below the UK air quality standard for NO2. The box height ratio is mainly determined by the flow structure in the street canyon. Therefore, understanding the flow characteristics in a street canyon is of vital importance and the numerical modelling approach can provide flow patterns in high spatial and temporal resolution within the street canyon.
159 (a) CNO,L
2 (ppb) for Case HB-LL (b) CNO,L
2 (ppb) for Case HB-L
(c) CNO,L
2 (ppb) for Case BASE (d) CNO,L
2 (ppb) for Case HB-H
(e) CNO,L
2 (ppb) for Case HB-HH
Figure 5.13 CNO,L
2 (ppb), i.e. the concentration in the lower box derived from the “two-box” model, in the (a) Case HB-LL ( =0.1), (b) Case HB-L ( =0.3), (c) Case BASE ( =0.5), (d) Case HB-H (
=0.7), and (e) Case HB-HH ( =0.9). 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).
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(a) Fixed ENOx (b) Fixed EVOCs
(c) TRES-2010 (d) Trajectory 2005-2020
Figure 5.14 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 . 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 5.14 shows the transects of CNO,L
2 (ppb) for Case HB-LL, Case HB-L, Case BASE, Case HB-H and Case HB-HH through the selected lines for analysis in Figure 5.5f. It can be seen that there is an increase of CNO,L
2 with the increase with increase of EVOCs and ENOx. This increasing tendency is extremely significant for Case HB-LL with the lowest box height ratio ( =0.1), i.e. 207 ppb difference for Figure 5.14a, 302 ppb difference for
161
Figure 5.14b and 461 ppb difference for Figure 5.14c. For other box height ratios in Figure 5.14a-c, the concentration difference is around 100 ppb, much lower than that for Case HB-LL. Figure 5.14d shows that there is a decrease of CNO,L
2 with years. However, the air quality is still worse for Case HB-LL and Case HB-L, i.e. about 4 times and 2 times of the UK air quality standard of hourly NO2 for the year 2005.
Figure 5.15 shows the effect of the box height ratio () on NO ,L
2 (%), i.e. the percentage of overestimation for NO2 in the lower canyon by the ‘one-box’ model. There are significant changes of NO ,L
2 with the changes of the box height ratio, i.e. (-82.22 %, -57.37 %) for Case HB-LL with =0.1, (-54.15 %, -30.26 %) for Case HB-L with =0.3, (-33.49 %, -17.07 %) for Case BASE with =0.5, (-17.71 %, -8.63 %) for Case HB-H with =0.7 and (-5.27 %, -2.59 %) for Case HB-HH with =0.9. This indicates that for higher box height ratio, the ‘one-box’ model predicts more accurate NO2 concentrations. It is also noted that NO ,L
2 is less sensitive to emissions of NOx and VOCs while the box height ratio is higher. For the extremely high box height ratio, the upper box plays a similar role as shear layer, where frequent exchange takes place. In such a situation, the two-box model can approximate to the one-box model.
162 (a) NO ,L
2 (%) for Case HB-LL (b) NO ,L
2 (%) for Case HB-L
(c) NO ,L
2 (%) for Case BASE (d) NO ,L
2 (%) for Case HB-H
(e) NO ,L
2 (%) for Case HB-HH
Figure 5.15 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 HB-LL (=0.1), (b) Case HB-L (=0.3), (c) Case BASE (=0.5), (d) Case HB-H (=0.7), and (e) Case HB-HH (=0.9). 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).
163
(a) Fixed ENOx (b) Fixed EVOCs
(c) TRES-2010 (d) Trajectory 2005-2020
Figure 5.16 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 . EVOCs and ENOx are normalised by those of the Typical Real-world Emission Scenario (TRES, represented by), for the year of 2010.
Figure 5.16 illustrates the transects of NO ,L
2 (ppb) for Case HB-LL, Case HB-L, Case BASE, Case HB-H and Case HB-HH through the selected lines for analysis in Figure 5.5f.
Figure 5.16a shows that the magnitude of NO ,L
2 slightly increases with the increase of EVOCs, i.e. from -64.94% to -72.29% for =0.1, from -33.18% to -41.62% for =0.3, from -18.14% to -24.16% for =0.5, from -8.98% to -12.37% for =0.7 and from -2.65%
164
to -3.65% for =0.9. This indicates that the difference of NO ,L
2 decreases with the increase of the box height ratio and the one box model performs better for the case with higher box height ratio. This finding is also indicated by Figure 5.16b, but the magnitude of NO ,L
2 is mainly influenced by the box height ratio (). Figure 5.16d also shows that NO ,L
2 do not change significantly over the year 2005 to 2020, but significant contrasts are found for the cases with different box height ratio.