| Topic: | Re:Putney High Street - disgraceful example for air quality inaction | |
| Posted by: | Richard Hodges | |
| Date/Time: | 07/02/13 16:34:00 |
| I'll work from the same report as you: "Trends in NOx and NO2 emissions and ambient measurements in the UK”, prepared fro DEFRA July 2011. Section 5: Emission Factor Comparison Considers three models, based on: - The standardised UK model - The German/Swiss model (best fit) - The authors modified model, based on roadside measurements of NOx. Figure 5.1 provides estimates of NOx emissions for each vehicle type, crucially, this is in the real-world measure of grammes per kilometre, in all three models based on urban usage. With some additional assumptions and estimates, we can model PHS. These are: - Traffic composition: estimated by observation, based on normal daytime periods (ie when the high emission events occur) - Modal selection of emissions class: based on a combination of observation (checking registration plate for typical vehicle age) combined with other known variables (impact of LEZ on hcv and lcv, model life of cars. For balance, a pessimistic view can be taken for cars, other classes are fairly clear. - For simplicity, we can assume PHS has a road length of 1km (Cross to Bridge) - Since traffic composition estimate is based on percentage, we can factor at every 100 vehicles to give an absolute weight value. - Emissions rate (g/km) taken from each part of figure 5.1 (best and worst of the three models) We then get: Petrol cars: 30% of traffic, modal as Euro 5. Therefore contributes 3.0gNOx per km, per 100 vehicles* Diesel cars: 25% of vehicles stock, modal as Euro 5. therefore 12.5-27.5gNOx per km, per 100 vehicles LCV: 15% of vehicle stock, modal Euro 4, therefore 7.5-19.5gNOx per km, per 100 vehicles HCV: 10% of stock, modal Euro V, therefore 10-30gNOx per km, per 100 vehicles London Bus: 10% of stock, modal Euro III, therefore 100gNOx per km, per 100 vehicles *spread based on the three different estimation models. So this rough and ready model indicates that for every 100 vehicles driving down PHS 133-180grammes of NOx are released. Clearly of note: - Buses are responsible for >50% - There is no engineering or operational reason why buses should be producing more than hcvs (that is trucks and coaches). Brining buses to the competitive standard would reduce NOx emissions by 30-50% - Despite being much more numerous, diesel cars produce a fraction of the emissions of buses. - Petrol cars produce just trace levels. More, the degree of difference supports the hypotheseis that petrol cars are actively reducing the pollutant levels, at least at certain times (ie NOx concentrations in the exhaust are lower than at the inlet). I have only had a chance to skim the TRL report - "Emissions Source Appointment", but it seems to adopt a very similar process, noting: - Traffic modelled over 24 hours rather than the specific time of emissions events (creates a slightly different apportionment of vehicle type) - Emissions class apportionment is much more advanced, using a fairly accurate vehicle-by-vehicle measure. - It categorises vehicles in a different way (I have bundled lcvs to include lgv, taxis and other light commercial and public service vehicles; hcv bundles hgvs, private coaches and heavy public service vehicles such as fire engines) It comes up with remarkably similar numbers, and the same conclusions. Whichever model you apply, one vehicle type stands out over all the others combined. If there is an error in the UK standard model for diesel-powered vehicles, note that the issue is in the model NOT with the engine and vehicle manufacturers, who are meeting and beating the standards set for them. Some of the effects (eg. and increase in relative concentration of NO2) were totally predictable. But above all, note how little influence diesel-engined cars have on total NOx production on Putney High Street. It remains limited however pessimistic the model. It is overwhelmed by both the actual contribution from the buses, and by reductions that are practically achievable. It may (and certainly is) different in other places around the country, but we're interested in Putney High Street. And all the evidence agrees. There's only one game in town, and it's called buses. Which is what I and others have been saying for nearly three years now. Now if you finally accept that, go back to the first report and see if you can find the UXB hiding in there, which has the potential to render current improvement plans useless. |